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A review on production of metal organic frameworks (MOF) for CO2 adsorption
Journal Pre-proofs Review A Review on Production of Metal Organic Frameworks (MOF) for CO2 Adsorption Taravat Ghanbari, Faisal Abnisa, Wan Mohd Ashri Wan Daud PII: DOI: Reference:
S0048-9697(19)35082-X https://doi.org/10.1016/j.scitotenv.2019.135090 STOTEN 135090
To appear in:
Science of the Total Environment
Received Date: Revised Date: Accepted Date:
8 September 2019 16 October 2019 19 October 2019
Please cite this article as: T. Ghanbari, F. Abnisa, W.M.A. Wan Daud, A Review on Production of Metal Organic Frameworks (MOF) for CO2 Adsorption, Science of the Total Environment (2019), doi: https://doi.org/10.1016/ j.scitotenv.2019.135090
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TITLE PAGE
Title: A Review on Production of Metal Organic Frameworks (MOF) for CO2 Adsorption
Author names and affiliations: Mrs. Taravat Ghanbari Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603, Kuala Lumpur, Malaysia. Email: [email protected] Associate Professor Dr. Faisal Abnisa (Corresponding Author) Department of Chemical and Materials Engineering, Faculty of Engineering, King Abdulaziz University, Rabigh, 21911, Saudi Arabia Email: [email protected]; [email protected] Professor Dr. Wan Mohd Ashri Wan Daud Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603, Kuala Lumpur, Malaysia. Email: [email protected]
A Review on Production of Metal Organic Frameworks (MOF) for CO2 Adsorption Taravat Ghanbaria, Faisal Abnisab,*, Wan Mohd Ashri Wan Dauda aDepartment
of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603, Kuala Lumpur, Malaysia.
bDepartment
of Chemical and Materials Engineering, Faculty of Engineering, King Abdulaziz University, Jeddah, Saudi Arabia
Abstract The environment sustenance and preservation of global climate are known as the crucial issues of the world today. Currently, the crisis of global warming due to CO2 emission has turned into a paramount
concern. To address such a concern, diverse CO2 capture and sequestration techniques (CCS) have been introduced so far. In line with this, Metal Organic Frameworks (MOFs) have been considered as the newest and most promising material for CO2 adsorption and separation. Due to their outstanding properties, this new class of porous materials a have exhibited a conspicuous potential for gas separation technologies especially for CO2 storage and separation. Thus, the present review paper is aimed to discuss the adsorption properties of CO2 on the MOFs based on the adsorption mechanisms and the design of the MOF structures. In addition, the main challenge associated with using this prominent porous material has been mentioned.
Keywords: CCS, CO2, MOFs, Synthesis, Selectivity, Capacity
_______________
*Corresponding author E-mail addresses: [email protected] (T. Ghanbari); [email protected] (F. Abnisa); [email protected] (W. M. A. W. Daud).
1. Introduction Along with the rapid rise of global population, energy consumption has been growing significantly. One of the main sources of energy in the world is fossil fuel, which has currently attracted a plethora of attention and demand as an energy source in worldwide. By now, beyond 85% of the global energy demand is being supported by fossil fuels burning, notwithstanding the emission of huge amounts of CO2 into the atmosphere (Rackley, 2009). Based on the literature, the CO2 concentration in the atmosphere has boosted dramatically from 340 ppm in 1980 to 408 ppm in 2019, leading to an adverse impact on the environment (ESRL, 2019). The high level of CO2 emission leads to an increase in the average temperature of the Earth. For that reason, effective CCS techniques are required in order to minimize the carbon dioxide emission, and this will then protect the environment and maintain the climate temperature (Yu et al., 2012). Besides, the captured CO2 can be potentially used for many applications such as refrigerants, polymers, chemicals, cosmetics, and fertilizer. Moreover, it also
becomes an important precursor in the petroleum industry especially in enhancing the oil recovery (Abdi-Khanghah et al., 2018; Muthuraj and Mekonnen, 2018). Various CO2 capture technologies are presently available including the absorption (chemical and physical absorptions), the adsorption, and the membrane technologies (Yu et al., 2012). Several experimental and computational studies have been accomplished concerning all the above techniques so far (Li et al., 2019; Li and Zhang, 2018). Of these technologies, the chemical absorption of amine scrubbing has been found to be a common technique used especially in industrial settings such as power plants (Rochelle, 2009). Although the use of amine technique can lessen CO2 up to 98 % (Yeh et al., 2005), this method is disadvantaged with high energy consumption, a large quantity of absorber requirements, and corrosiveness, etc. (Yu et al., 2012). As a result, to mitigate these inherent problems associated with the chemical absorption, solid adsorption processes have been highly recommended and studied. So far, considerable classes of porous materials have been scrutinized and introduced as adsorbents, as among which zeolites, activated carbon, metal-oxide molecular-sieves, aluminophosphates, activated alumina, silica gel, pillared clays, carbon nanotubes, inorganic and polymeric resins, porous organic materials, as well as porous metal–organic frameworks (Chue et al., 1995; Loureiro and Kartel, 2006; Paul and Wright, 2008; Warrendale, 2005; Xu et al., 2009). Of the mentioned materials, zeolites and activated carbons have been extensively studied, being found to be more efficient for the CO2 capture (Díaz et al., 2008; Zhang et al., 2008). Nevertheless, they suffer from some limitations such as low CO2 adsorption capacity at a high temperature as well as instability in the presence of water (zeolite based adsorbents). It is acknowledged that although activated carbon has a greater CO2 adsorption capacity than zeolites, CO2 molecules interaction in the activated carbon is very weak and thus making this adsorbent sensitive to temperature changes. Hence, by raising the temperature of the process, the CO2 adsorption capacity drops dramatically and becomes relatively poor in selectivity, especially in the case of CO2/N2 selectivity (Zhang et al., 2008).
To alleviate this issue, novel functional materials, known as metal-organic frameworks (MOFs), have been introduced as adsorbents. MOFs are also recognized as coordination polymers or coordination networks. These novel hybrid materials are formed by the combination of metal ions or clusters organic ligands which are linked via coordination bonding. This kind of adsorbent has numerous merits like having robust 3D structures, a remarkable high surface area, controllable pore structures, and tunable pore surface properties (Millward and Yaghi, 2005). Combination of these favorable properties makes MOFs ideal while being the most promising adsorbent for gas adsorption, particularly for CO2 capture and sequestration. Moreover, these novel materials have other applications in diverse areas such as sensors, drug delivery, and catalysis (Kuppler et al., 2009). So far, numerous review papers have addressed MOFs in different areas; yet, this review paper meticulously elaborates on the contemporary MOFs which have been used for CO2 adsorption and separation with respect to synthesis methods, CO2 uptake and selectivity, methods to improve CO2 adsorption and the water stability of MOFs in CO2 adsorption.
2. MOF as an adsorbent As novel functional materials, MOFs have grabbed the attention of many researchers for the past two decades. These novel hybrid porous materials are formed by combination of organic ligands and metal-containing clusters or metal nodes (Fig. 1). Nearly all metals and a large diversity of organic species can be used to create MOFs; therefore, a huge variety of MOFs materials having different structures and properties are then available. The main characteristics of the MOFs are their robust 3D structure, their permanent porosity, and their modular nature. These unique properties of MOFs assists these adsorbents in preventing their structure from removal of guest molecules like the solvents in their open channels and pores. Therefore, the MOFs can commonly maintain their structures with negligible damage and can be utilized for other guest adsorption (Kuppler et al., 2009; Millward and Yaghi, 2005; Yu et al., 2012). Besides, extraordinary surface areas, ultrahigh porosity (Furukawa et al., 2013; Furukawa et al., 2010; Rowsell and Yaghi, 2006; Tanabe and Cohen, 2011), flexibility in tuning the
porous structure (Yuan et al., 2010a; Zhao et al., 2009) and also surface functionality could be considered as the prominent characteristics of the MOFs.
Fig. 1. Simple schematic of forming metal organic frame work (MOF)
Due to the presence of functional groups in the organic ligand, MOFs have the capability of being tuned while being chemically modified (Furukawa et al., 2013). The most salient advantage of the MOFs over the other solid materials is the possibility of designing the pore size and the functionality by selection of the organic ligand, the functional group, the metal ion, and the activation method. These favorable properties make MOFs attractive porous materials with the capability of being used in different areas such as sensors, biomedical, catalysts as well as gas storage in particular for CO2 adsorption(Kuppler et al., 2009). This review paper investigates the usage of the MOFs for CO2 adsorption and separation from the synthesis routes. Furthermore, we address the separation and synthesis mechanisms as well as the limitation in using this kind of adsorbent for CO2 capture.
3. CO2 capture using the MOF 3.1. Selectivity of CO2 over other gases One of the most essential parts of gas adsorption is selectivity of the targeted gas over other components of a mixture. To clarify this subject, it is essential to initially enlighten the principles, methods, and mechanisms involved in adsorptive gas separation.
3.1.1. Principles and methods of adsorptive gas separation Separation is a method to detach the components of a mixture, whose procedure often requires large amounts of energy for dividing the substances of a mixture (Yang, 2003). Furthermore, separation can be attained via selective adsorption and this takes place when the components on the surface of the adsorbent have different affinities to adsorb the diffused guest molecule. Gas separation methods comprise membrane-based, cryogenic distillation, absorption and adsorption based technologies(King, 2013). Among these methods, adsorptive separation has a greater attraction due to potentially lower energy consumption, easier maintenance, flexibility in the design for different applications, a simple operation (Rouquerol et al., 2013; Yang, 1987) and the environment-friendly characteristic of this method (Rouquerol et al., 2013). Generally, in the process of adsorptive gas separation, two main steps are involved; adsorption and desorption processes. In the adsorption stage, a mixture of the gases is passed through a fixed-bed adsorber or a column packed with the adsorbents. Then this process is ensued by the desorption step to remove the component attached on the surface of the adsorbent for reuse (Yang, 2003). Therefore, the gas adsorption process is dependent on the desorption methods used for regeneration of the adsorbent. Several cyclic adsorption processes are used in the gas separation technology including the inert purge cycles, the displacement cycles, the temperature swing adsorption (Gao et al.), the pressure swing adsorption (PSA), the vacuum swing adsorption (VSA) (Fig. 2), and the electrical swing adsorption (ESA) (Ling et al., 2014; Yang, 1987). Although all these methods are expedient for gas separation and purification, the TSA and PSA are more commonly used in CO2 sequestration. The TSA desorption occurs by heating the adsorbent, has albeit its long cycle time due to the heating/cooling stage that requires a few hours up to over a day. On the other hand, the PSA is the process whereby regeneration is achieved by lowering the pressure of the adsorbent. This cycle has some advantages over the TSA such as a higher throughput of the cycle (the process usually occur in seconds or minutes) and a
temperature-independent process. The PSA is predominantly used for bulk separation. Another more recent method is the VSA that is of a greater interest for the researchers because it operates at near ambient pressure and most flue gas streams pressures are approximately atmospheric. As opposed to the other methods, , the PSA and VSA have typically more desirability thanks to fewer energy requirements of these cycles (Yang, 1987).
Fig. 2. Schematic of TSA, PSA and VSA cyclic processes for regeneration of MOFs in the fixed-bed column.
It should be noted that adsorptive gas separation is attained when different components of the adsorbent have different abilities to adsorb. Also, a promising adsorbent should contain acceptable mechanical properties such as high selectivity, capacity, regenerability as well as promising adsorption kinetics.
3.1.2. Mechanisms of selective adsorption
Four mechanisms exist for gas adsorptive separation on porous materials; 1) the molecular sieving effect (size and shape exclusion), 2) the thermodynamic equilibrium effect, 3) the kinetic effect and 4) the quantum sieving effect. As such, it is well documented that typically gas adsorption is obtained with one or more than one mechanisms (Keller and Staudt, 2005; Yang, 2003). In size/ shape exclusion (steric separation), the selective adsorption is achieved based on the shape and cross-sectional size (kinetic or collision diameter) of the adsorbate molecule. The Kinetic or collision diameter means the closest distance or diameter between two molecules with zero kinetic energy that can approach each other and have a collision. According to the literature, this effect is common in zeolite and molecular sieves adsorbent. Besides, it is noteworthy to acknowledge that the pore size of some adsorbents is sensitive to temperature; therefore, molecular sieving in these cases are affected by temperature (Li et al., 2009a). The second mechanism, i.e. equilibrium separation (dynamic separation) is useful when the pores size of the adsorbent is large enough; consequently, all the components of the gas mixture can pass inside the adsorbent. In this condition, the interaction between the surface of the adsorbent and the adsorbate molecules has a vital impact on the selective adsorption quality. It should be pointed out that the strength of this interaction is related to the properties of the adsorbate molecule and the surface characteristics of the adsorbent such as polarizability, magnetic susceptibility, the permanent dipole moment, and the quadrupole moment (Li et al., 2009a). Kinetic separation is utilized when equilibrium separation is not feasible. The most important issue in kinetic separation is that the pore size of the adsorbent should be controlled accurately between the kinetic diameters of the two molecules that require to be separated. As stated by the previous researches, this mechanism is used for separation of CH4 from CO2 by utilizing a carbon molecular sieve while another example is the separation of air by using zeolite as an adsorbent via the PSA method (Li et al., 2009a; Yang, 2003). For the quantum sieve effect, the adsorption is accomplished based on differences in diffusion speeds of the guest molecules and compatibility of the pore diameter with the de Broglie wavelength of these
molecules. This effect can be particularly useful for isotopic separation like H2 /D2 separation (Li et al., 2009a).
3.1.3. Selection of the adsorbent and its criteria One of the important parts of adsorptive separation is the adsorbent selection. Generally, the best adsorbent is the one that has both high selectivity and capacity of adsorbing the targeted molecules. Hence, for choosing the adsorbent, the followings are to be considered: the nature of the adsorbent (also nature of its pores) and the adsorption process. For instance, in the bulk gas separation process, the adsorbent must be selected based on the ease of desorption. If the separation process is not considered, the nature of the adsorbent is the key criterion for choosing the adsorbent including the size and the shape of the adsorbate molecule, the dipole moment, the quadrupole moment, and the polarizability. For example, if the targeted molecule has a high dipole moment, the adsorbents with high polarized surfaces are ideal. For the targeted molecule with a high quadrupole moment (like CO2 molecules), the desirable adsorbent should contain a surface with high electric field gradients whereas for the adsorption of a non-polar molecule with high polarizability, an adsorbent with a high surface area could be appropriate (Yang, 2003). Moreover, numerous important separations and purifications are accomplished due to the formation of 𝜋-complexation bonds and H-bonds (weak chemical bonding) between the adsorbates molecules and the adsorbent. Finally, the most important and basic point of selecting the adsorbent is the equilibrium isotherm, followed by diffusivity (Li et al., 2009a).
3.2. The impact of MOFs on selective adsorption of CO2 Hitherto, a wide range of porous materials has been introduced and used as an adsorbent in different fields, especially in the gas separation technology (Barton et al., 1999; Davis et al., 2002; Férey, 2008; Stein, 2003; Yu and Xu, 2006). As stated in the first section, the increase of the CO2 concentration contributes to some environmental disorders in the world (Jacobson, 2009). In this context, several studies have been accomplished and based on the literature, MOFs are the best porous materials for
selective trapping of CO2 (Bhattacharya and Ghoshal, 2015; Fukushima et al., 2010; McDonald et al., 2012; Nugent et al., 2013). The first key criterion of choosing a suitable MOF for CO2 adsorption is that the MOF’s pores have to be compatible with the kinetic diameter of the CO2 molecules. Another criterion is that the MOFs with polar (-OH, -N=N-, -NH2 and –N=C (R)-) pores have a higher CO2 adsorption capacity due to the quadrupole moments of CO2 molecules. Therefore, by designing the MOF structure based on these criteria, it is possible to significantly improve the CO2 adsorption capacity and selectivity of these outstanding adsorbents. By and large, the MOFs can be divided into two types, rigid and flexible (dynamic). The former possesses robust frameworks that make permanent pores like zeolites, while the latter possesses soft (dynamic) frameworks whose structure changes through external stimuli such as guest molecules, pressure, and temperature (Li et al., 2011). Majority of the MOFs have been used as a selective CO2 adsorption and most of them are in the rigid categories; yet, nowadays the flexible MOF has taken more attention due to their outstanding abilities. In the subsequent section, the types of the MOFs’ structure in selective CO2 sequestration are investigated. 3.2.1. Selective CO2 adsorption in rigid MOFs Various rigid MOFs have been used for selective gas adsorption to date. The mechanism of selective adsorption in rigid MOFs resembles greatly to the zeolites, meaning that selective adsorption in this kind of MOFs may be achieved based on the molecular sieving effect. In addition, it is probably attained according to the different strengths of the interaction between adsorbate-adsorbent and adsorbateadsorbate. The selective adsorption in the rigid MOFs is dependent on three main factors: size / shape exclusion, adsorbate–surface interactions, and simultaneous corporation of both these factors (Li et al., 2009a; Li et al., 2011).
3.2.1.1. Selective adsorption of CO2 on rigid MOFs based on size/shape exclusion According to the literature, many of the MOFs are proper and have been used to adsorb different gases based on the molecular sieving effect (Dybtsev et al., 2004; Loiseau et al., 2006; Xue et al., 2008;
Zhuang et al., 2011). This implies that only the molecules are able to pass through the pores whose Kinetic diameters are compatible with the diameters of the pores (Fig. 3).
Fig. 3. Schematic illustration of selective gas adsorption in rigid MOFs based on the molecular sieving effect (Zhuang et al., 2011).
The kinetic diameters of CO2, CH4, and N2 have been tabulated in Table 1. To elucidate this matter, it needs some structural analyses. For instance, for selective adsorption of CO2 over CH4, manganese MOFs are considered as one of the rigid and most evaluated groups of MOFs that work based on molecular sieving. These kinds of MOFs have a 3D robust structure with 1D channel which includes large cages. These cages are linked together through a small window. Dybtsev et. al. stated based on experimental works on these kinds of MOFs that CO2 can be adsorbed over CH4 at 195 K, at a lower temperatures (78 K) just small molecules like H2 can adsorb over Ar and N2 (Dybtsev et al., 2004). It is declared that in both cases, the adsorption capacities of the excluded gases (CH4, N2, and Ar) were near to zero. In this case, selectivity was achieved due to the presence of small windows in the channels (Dybtsev et al., 2004). Other examples are MIL-96 (Loiseau et al., 2006) and Zn2 (cnc)2(dpt) (Xue et al., 2008) that are also used for selective adsorption of CO2 over CH4 according to size/shape exclusion. These MOFs can be suitable for selective separation of CO2 over CH4 specialty for landfill gas separation and natural-gas purification. It should be noted that the selective adsorption behavior of some MOFs such as Zn2 (cnc)2(dpt) has been changed in the activation process. The former has the capability to adsorb H2 over N2 while the latter can adsorb CO2 over CH4. This behavior probably refers to pore temperature sensitivity of this kind of adsorbent (Xue et al., 2008).
Table1 Kinetic diameters of different gases. Gas molecules CO2 CH4 N2
Kinetic diameter Å 3.3 3.8 3.64
PCN-26 also known as a rigid MOF possesses a 3D structure. This MOF is proper for selective gas adsorption based on the molecular sieving effect due to the pores diameters (3.68 Å). According to Zhuang et al. (Zhuang et al., 2011) studies (experimental and molecular simulation), activated PCN-26 performed CO2 adsorption capacity of 109.1 cm3 g-1 at 298 K and 800 Torr. Also, the selective adsorption of CO2 over N2 and CH4 at 273 K was 49: 1, 8: 4: 1, respectively. As results indicated, this MOF is more useful for selective adsorption of CO2 over N2 than methane.
3.2.1.2. Selective adsorption of CO2 on rigid MOFs based on surface –adsorbate interactions The second imperative factor for selective gas adsorption is the nature of the guest molecules and their interaction with the surface of the adsorbent. Consequently, in some rigid MOFs, selective adsorption can be achieved based on the kinetic effect or the thermodynamic equilibrium effect in a given equilibrium time. Therefore, selectivity occurs based on the adsorbate properties such as Hbonding, polarity, the quadruple moment, and the surface properties of the pores. Regarding the kinetic effect, a wide range of rigid MOFs has been used in selective gas adsorption technology so far. In the case of CO2, the selective adsorption takes place based on the large quadrupole moment of CO2 whereas CH4 and N2 are nonpolar molecules. Bae et al. synthesized Zn2 (ndc)2(dpni) as a pillared-layer MOF via the microwave heating method. Their research revealed that the preferential adsorption of CO2 was much more than CH4 because of the kinetic effect and the quadrupole moment of CO2 (Bae et al., 2008b).
Moon et al. prepared a 3D porous MOF with 1D channel that consisted of coordinatively unsaturated MnII sites (Moon et al., 2006). The researchers found that this MOF had higher adsorption capabilities for CO2 than for CH4 at ambient temperature. The other rigid MOFs showing high selective CO2 adsorption over other gases especially over CO is ZIFs frameworks, which enjoy the outstanding feature of the presence of large cages which have connection via small pores (Banerjee et al., 2008). ZIF-68 to 70 are good example of these categories which possess large cages with 7.2, 10.2, and 15.9 Å diameters respectively which are connected through pores with 4.4, 7.5, and 13.1 Å diameters, respectively. All these ZIFs frameworks display high CO2 adsorption capacity and selectivity over CO at 273 K. This selective adsorption has been also strengthened via breakthrough experiments. The results indicated that in the 50:50 v/v binary mixture of CO and CO2 at 298 K, the CO2 molecules were retained completely while CO molecules passed through the column. According to their observation the pores size of the frameworks are large enough to pass all CO and CO2 molecules through the pores. Hence, this selectivity was attributed to the equilibrium effect and in the competition between CO2 and CO molecules to be adsorbed on the surface of the framework, CO2 molecules are a winner because of their more quadrupole moments characteristic (Banerjee et al., 2008).
3.2.1.3. Selective adsorption of CO2 on rigid MOFs based on contributing size/shape exclusion and surface– adsorbate interactions effects In addition to the effect of the pore size and the interaction of the guest molecules and the surface of the pores, some MOFs can selectively adsorb the guest molecules based on both these effects. In order to select CO2 adsorption through this effect, Er2 (pda)3 can be a good example which was introduced by Pan et al. (Pan et al., 2003). This MOF possesses a 3D framework with 1D channels which are decorated by unsaturated ErIII sites. The CO2/N2 selectivity was examined that the affinity to adsorb CO2 was much higher than N2 due to two different mechanisms; 1) compatibility of the pore sizes (3.4Å) by CO2 molecules diameters (3.3 Å) (for N2: 3.64 Å), and 2) the presence of functionality in the framework such as coordinatively
unsaturated metal sites (ErIII ions), 𝜋-electrons and polar groups. Further to these factors, CO2 molecules also have the quadrupole moment which can interact with this electrical field and thus enhance the potential of the adsorption energy. ZIF-95 and ZIF-100 (Wang et al., 2008) are the other examples of rigid frameworks which can selectively adsorb CO2 based on these effects. The MOFs possess large cavities (3.65 Å) as well as highly straitened pores (3.35 Å). The selective adsorption of CO2 over CH4, N2 and CO of these MOFs was investigated by Wang et al. (Wang et al., 2008). According to their observation, both frameworks indicated higher affinity of CO2 over the other examined gases and this can be attributed to the combination of the pore size and the strong interaction of CO2 molecules and the framework due to the quadrupole interaction of CO2 molecules and the presence of nitrogen atoms in the frameworks. Moreover, Xiang et al. investigated the CO2 adsorption capacity and selectivity of UTSA-16 via experimental and simulated breakthrough experiments. The results exhibited great CO2 adsorption capacity (160 cm3 cm-3 (volumetric)) and selectivity over N2 and CH4. The high CO2 adsorption capacity and selectivity of UTSA-16 are attributed to the strong binding sites to CO2 as well as the optimal size of the pores which are compatible with the kinetic diameter of the CO2 molecules (see Table 1) (Xiang et al., 2012) Likewise, the rigid MOFs hold a high potential for gas separation as well as CO2 sequestration due to the tenability of their pore sizes, shapes, and surface properties based on their application. These prominent properties are achieved through modification methods which are explicated in sections 4. Concerning the molecular sieving effect, the pore size must be compatible with the kinetic diameter of the targeted molecules (Dybtsev et al., 2004). For the surface properties, the presence of effective functional groups on the surface of the pores is essential (such as amine groups for CO2 separation). Besides, another outstanding method to enhance the CO2 adsorption is using open metal sites (unsaturated metal sites) on the frameworks (Dietzel et al., 2009). Some rigid MOFs which are used for CO2 selective adsorption have been tabularized in Table 2.
Table 2 Some rigid MOFs for selective CO2 adsorption.
MOF (MIL-96)
Structure Three types of cages connected through small widows Triangular 1D channels
Zn2(cnc)2(dpt) Zn2(ndc)2(dpni) Er2(pda)3 (ZIF-100)
MIL-102
(CUK-1)
Zn3(OH)(pcdc)2.5
Crossing channels
Pore size Å
1D channel with diamond-shaped contains corrugated walls Crossing rhombic channels contains coordinatively unsaturated metal sites ZnII
Selective adsorption
CO2 uptake
T
P
Ref. (Loiseau et al., 2006) (Xue et al., 2008) (Bae et al., 2008b) (Pan et al., 2003) (Wang et al., 2008)
2.5 ~ 3.5
size/ shape exclusion
CO2 over CH4
~3.7 mmol g-1
303 K
3.5 bar
~ 3.7
both
CO2 over CH4
~ 150 mL g-1
195 K
1 P/P1
4~5
adsorbateadsorbent
CO2 over CH4
4.3 mmol g-1
296 K
1750 kPa
~ 3.4
both
CO2 over N2 and Ar
273 K
760 torr
3.35
both
CO2 over CH4 , CO and N2
~ 0.95 mmol g-1
298 K
850 torr
adsorbateadsorbent
CO2 over CH4 and N2
~ 3.4 mmol g-1
304 K
3 MPa
(Surblé et al., 2006)
11.1
adsorbateadsorbent
CO2 over CH4
~ 88 mL g-1
298 K
760 torr
(Yoon et al., 2007)
3 × 5
adsorbateadsorbent
CO2 over CH4
~ 0.586 mmol g-1
298 K
0.5 bar
(Bae et al., 2008a)
rhombic
1D channel with unsaturated metal sites ErIII Large cages which connected via small apertures 1D channel which decorated by F-anions and coordinated terminal water molecules
Selectivity reason
~ 24 mg g-1
4.4
3.2.2 Selective CO2 adsorption in flexible/dynamic MOFs. The other generation of the MOFs that has recently received a conspicuous attention is the flexible (soft) MOFs. Due to their distinctive properties, the flexible MOFs differ from the rigid one in both the behavior and the nature (Chang et al., 2015; Schneemann et al., 2014). These unique MOFs enjoy a flexible framework and have shown dynamic behaviors during the adsorption/ desorption process. The most remarkable feature of the flexible MOFs is their structural transformability via external stimuli such as pressure, temperature, (Henke et al., 2013; Keene et al., 2013) and guest molecules (Alhamami et al., 2014; Llewellyn et al., 2006). Their flexible characteristic is related to regional movements of the organic ligands which comprise bending, twisting, tilting or rotating, and the environmental change of the secondary building units (SBUs) (Seo et al., 2011; Tian et al., 2012).
As stated above, the behavior of these kinds of MOF is different and more complicated than the rigid one. For example, the pore size of the flexible MOFs alters during the adsorption/desorption process based on the size of the guest molecules (Fig. 4). Moreover, most of the time, this change is accompanied by the hysterics behavior due to the rearrangement of the framework during the uptake/leaving of the guest molecules (Chen et al., 2007a; Chen et al., 2007b; Cheon and Suh, 2008; Ma et al., 2007b). Therefore, in gas selective adsorption of such MOFs, not only the size/shape exclusion and the adsorbate-surface interaction must be taken into account but also the structural rearrangement should be considered. Concerning the adsorption process and the structural feature of the flexible MOFs, their selective gas adsorption can be categorized into three groups mentioned below (Li et al., 2009a). Prior to the explanation of these categories, two prominent dynamic behaviors of the flexible MOF during the gas adsorption/desorption should be considered, which are the breathing phenomenon (Férey and Serre, 2009; Serre et al., 2007; Serre et al., 2002; Trung et al., 2008) and the gate-opening/closing (Fairen-Jimenez et al., 2011; Gücüyener et al., 2010; Kitaura et al., 2003; Seo et al., 2009). The breathing phenomenon occurs when an abrupt expansion/compression is applied to the unit cell of the MOF, which causes the structural transitions due to the interactive guest molecules. The gate opening/closing occurs via external stimuli in the close and nonporous MOFs and causes a transition to an open and porous phase (Coudert et al., 2009; Férey and Serre, 2009). The prominence and value of these two special phenomena have been emphasized by Kitagawa (Seo et al., 2009) and Ferey (Férey and Serre, 2009), that will be explained in more details in the following section.
Fig. 4. Schematic illustration of selective gas adsorption in a flexible MOF (Li et al., 2009a).
3.2.2.1. Selective CO2 adsorption on the flexible MOFs based on the size/shape exclusion accompanied by the change of the pore size/shape Resembling the rigid MOFs, the size/shape exclusion can result in gas adsorption in some flexible MOFs. This has been observed in Zn(ADC)(4,4΄-Bpe)0.5 which was composed of paddle-wheel dinuclear Zn2 units that were bridged by ADC dianions and further pillared by 4,4΄-Bpe to form a 3D interpenetrated structure (Chen et al., 2007a). As an adsorption study has demonstrated, at 195 K, this MOF adsorbs CO2 over CH4 (CO2: ~130mL, CH4: almost none). Evidently, this selectivity arises from the molecular sieving effect due to the compatibility of the pores size of the MOF (3.4×3.4 Å) and CO2 molecules (3.3Å) compared to CH4 molecules (3.8Å). A similar adsorption selectivity was observed for Ni2 (cyclam)2 (mtb) at 195 K and 1 atm for selective adsorption of CO2 (57 mLg-1) over CH4 (˂10mL) (Cheon and Suh, 2008). Likewise, Chen and co-workers investigated the selectivity of CO2 over N2 and CH4 by using flexible Cu (Bon et al.) (4,4ʹ-bpe)0.5. The results indicated that the MOF can take up CO2 over the other selected gases at 195 K (CO2: 100 mL g-1, CH4: 35 mL g-1 and N2: 15 mL g-1) because of similarity in the size of the CO2 molecules and the host MOF. It is worth noting that at lower temperature (77 K), none of the gases can be adsorbed on the MOF, suggesting that by increasing the temperature, the pores start to enlarge; consequently, at a higher temperature (195 K), all the gasses
can uptake on the surface of the MOF (Chen et al., 2007b). Size-exclusion relies on adsorption selectivity which was also recognized in flexible PCN-5 that possesses a 2-fold interpenetrated structure. This MOF can selectively adsorb CO2 over CH4 at 195 K and 760 Torr based on compatibility of the pore diameter of the host framework and the kinetic diameter of the gasses molecules (3.3 Å and 3.8 Å for CO2 and CH4 respectively). In a given condition, CO2 uptake can reach 210 mg g-1 (4.8 mmol/g), whereas that of CH4 was constrained to 30 mg g-1 (1.9 mmol/g) (Ma et al., 2007b).
3.2.2.2. Selective adsorption of CO2 on flexible MOFs based on interactions between the surface of the MOF and CO2 molecules accompanied by the pore size/shape change The surface property of the pores also plays a pivotal role in selective adsorption of flexible MOFs. To illustrate this, MIL-53 series (the flexible MOF) in both hydrated and dehydrated forms could be referred to. This MOF has a 3D structure comprising 1D channels with the chemical formulation of MO4 (OH)2 linked by 1,4- benzenedicarboxylic acid (BDC). This MOF demonstrated the breathing phenomenon during the hydration/ dehydration process (Alhamami et al., 2014; Bourrelly et al., 2005; Llewellyn et al., 2006). In the hydrated form, the size of the pore altered and became narrower (np) due to hydrogen bonding between the O atoms of the carboxylated group of the ligands and the hydrogen of the H2O molecules, whereas in the dehydrated form, the pore was larger (lp) (Fig. 5).
Fig. 5. Illustration of the breathing behaviour in MIL-53. Right: dehydrated form; left: hydrated form (Bourrelly et al., 2005)
Hydrated MIL-53s have shown an unprecedented adsorption performance for CO2 uptake. At low pressure (below 5 bar), the CO2 uptake was low due to the presence of water molecules that make the pores narrower (np) (Alhamami et al., 2014; Llewellyn et al., 2006). Hence, by increasing the gas pressure, the framework starts to tune itself into a large pore (np→ coexistence of np and lp → lp). This framework extension resulted in an increase in CO2 uptake to about 7.2 mmol g-1 at 18 bar. In addition, the pore volume increased from 1012.8 Å3 for the hydrated one to 1522.5 Å3 for the hydrated + CO2. The Adsorption behavior of dehydrated MIL-53s ostensibly contrasts with that of the hydrated form (Alhamami et al., 2014; Bourrelly et al., 2005). In the case of CO2 uptake, dehydrated MIL-53s have shown two steps for the structural transition (Fig. 6). The first step occurs at low pressure (below 5 bar) in which the CO2 uptake capacity increases dramatically (3 mmol g-1). The second step starts at about 8 bar in which CO2 uptake was around 7.7 mmol g-1. This cold be attributed to the structure of the large pore changing to narrower ones due to the CO2 adsorption. It is also attributed to high pressure of the gas causing the structural transition again to the large pore (lp→ 𝑛𝑝→𝑙𝑝). This might be due to steric hindrance of the prior water molecules in the structure of MIL-53s (Llewellyn et al., 2006; Murray et al., 2009) CH4 adsorption in dehydrated MIL-53 was reported to be approximately 4.6 mmol g-1 at 20 bar while in the hydrated MIL-53 the adsorption was almost zero (around 0.2mmol g-1). This difference was due to the repulsive effect polarity of the water molecules in the host framework with nonpolar molecules of CH4. Based on this study, the CH4 adsorption on both MIL-53 (hydrated and dihydrated) was less than CO2. Accordingly, this project proves a high potential in the application of such an MOF for gas storage and separation, exclusively for selective adsorption of CO2 over CH4 (Llewellyn et al., 2006; Murray et al., 2009). Another worthy illustration of this category is [Ni(bpe)2(N(CN)2)](N(CN)2) (Maji et al., 2007) which has a stable interpenetrated 3D framework with 1D channels. This flexible MOF exhibited a unique and exceptional selective adsorption behavior. For example, even though the framework has an adequate
pore size, no O2 and N2 molecules could diffuse into the microspores at 77K. Surprisingly, a remarkable amount of CO2 can be adsorbed into the framework at 195 K, whilst the size of the CO2 molecules resembles that of the O2. This uncommon selective adsorption (at 77 K) might be explained by the strong interaction between surface of the pore windows and the primer molecules of O2 / N2. Furthermore, they blocked the pores so that the other molecules were unable to pass through them and be adsorbed. The CO2 uptake at 195 K was attributed to the rise in the temperature and the powerful interaction between the host framework and the CO2 molecules. The mentioned robust interaction was attributable to the presence of NiII atoms, polar groups, and π-electron clouds in the framework. On the other hand, CO2 molecules possess high quadrupole moments, contributing to a notable interaction occurring between the host and guest molecules (Maji et al., 2007).
Fig. 6. Gas adsorption isotherms of dehydrated MIL-53(Cr) at 304 K (illustration of the ‘‘breathing’’ effect) (Bourrelly et al., 2005).
3.2.2.3. Selective adsorption of CO2 on flexible MOFs according to gate-opening or structural rearrangement induced by adsorbate–surface interactions
As previously underscored in section 3.2.2, one of the attention-grabbing functions of some flexible MOFs during the adsorption process is the gate-opening phenomenon. This process takes place in the MOFs comprising small pores or even no pores to pass the guest molecules through the adsorbent. During this mechanism, when the framework is exposed to the gust molecules (certain gas), the pores begin to expand, and such an expansion is directly dependent on the adsorbate-surface interactions with the framework (Fig. 7).
Fig. 7. Schematic of selective gas adsorption in the flexible (dynamic) framework based on gate opening phenomena and guest specific pressures. Purple triangle and red circles represent nonsorbable and sorbable guest molecules.
ZIF-20 is the best instance of the flexible MOF where the gate opening occurs in the selective adsorption of CO2 over CH4 (Hayashi et al., 2007). This MOF has a 3D structure including large cages which are connected via small windows. The amount of CO2 adsorption at 273 K and 760 torr was five times greater than CH4. This difference of adsorption is seemingly attributed to a stronger interaction between the CO2 molecules and the pore surface. The other thought-provoking fact is the size of the pore window which is 2.8 Å (measured by the crystal structure) whereas the kinetic diameters of CO2 and CH4 are larger than this (3.24 Å and 3.8 Å, respectively). As cited above, the large cages are accessible via the small windows that are expanded due to the window-widening process; accordingly, the gas molecules are able to pass through the framework and this process is accomplished based on the ligands swing. Moreover, the quadrupole moment of CO2 exerts a distinctive impact on the flexibility of the framework in some MOFs such as ZIF-20.
In the gate opening phenomenon, the specific pressure of the guest molecules needs to be considered, implying that the guest molecules are allowed to pass through the adsorbent at this specific pressure (Fig. 7). Interestingly enough, the gate opening pressure of every gas is specified to its own. Therefore, the gas entrance pressure should be the same or above the gas gate-opening pressure. Cu (dhbc)2(4,4ʹbpy) (Kitaura et al., 2003) is one of the best examples of this kind of MOFs, where the hydrated form has a 2D sheet shape with 1D channels. Its framework includes rigidly interconnected sheets and its pores are furnished by dhbc benzene which is located in a vertical form into those sheets and mutually interdigitated. Also, water molecules are embedded in the cross section of the framework (3.6 * 4.2 Å). Besides, there are some interactions between the neighboring dhbc ligands via π-π stacking causing a consolidation of the 3D structure. As Fig. 8 depicts, the isotherm adsorption of CO2, CH4, O2, and N2 starts at a certain pressure (0.4, 9, 35 and50 bar, respectively) having an immediate increase at the pressure specified to each gas. Another remarkable point is the hysteresis behavior for each gas due to the gate-closing pressure. Therefore, different gases would have exclusive gate-opening and gate-closing pressures that are related to the distinctive intermolecular interaction force between the gas molecules and the surface of the adsorbent. Another MOF revealing a similar dynamic behavior with Cu (dhbc)2(4,4ʹ-bpy) (Kitaura et al., 2003) is Cu (pyrdc)(bpp) (Maji et al., 2005) which has a 2D pillared-bilayer framework with channels comprising the nonporous structure. This MOF indicates a sponge-like dynamic behavior with the bond formation via inclusion as well as a breaking triggered by the guest removal (the adsorption/desorption process). Moreover, the adsorption properties of this MOF was scrutinized by Maji et al. for N2, O2, and CO2 gases. In the case of N2 and O2 having kinetic diameters of 3.6 Å and 3.4 Å, respectively, no inclusion was discerned at 77 K and no pores were discovered in the temperature. On the contrary, CO2 with a similar kinetic diameter (3.3 Å) to N2 and O2 can pass through the framework and can be adsorbed. This behavior refers to the specific opening-gate pressure of each gas that causes the CO2 to be accommodated in the framework as compared with N2 and O2. Furthermore, this adsorption directly depends on the strength of the H-bound interaction (O-H…O) between the CO2 molecules and the ligand
parts which are located in the walls of the framework channels. Nonetheless, such a phenomenon is rare and has only been observed in a few flexible MOFs thus far. Some flexible MOFs used for CO2 adsorptions have been enlisted in Table 3.
Fig. 8. Isotherm adsorption (filled circles) and desorption (open circles) process of different gases based on opening gate pressure at 298 K (Kitaura et al., 2003).
Table 3 Some flexible MOFs for CO2 adsorption. MOF
Structure
Selectivity reason
(MIL-53)
3D framework with 1D channels(diamondshaped ) comprising Hbonding sites
Different guest adsorption interaction
Selectivity of CO2 over other gases CO2 over CH4
CO2 Uptake ~ 7.5 mmol g1
T (K)
P
Ref.
304
20 bar
(Llew ellyn et al., 2006)
Zn(adc)(4, 4'-bpe)0.5
Interpenetrated 3D framework (pillared-layer) with crossing 1D open channels
Size & shape exclusion
Ni2(cyclam )2(mtb)
Interpenetrated 3D framework containing wide inside pockets linked via narrow entrances Interpenetrated 3D framework ?(pillared-layer) with crossing 1D open channels Interpenetrated 3D framework which stabilized via H-bonding including 1D channels which have three types of crossing in three directions Interdigitated 2D framework, stacking through p–p interactions to form 1D channels Interpenetrated 3D pillaredlayer framework with intersecting 1D potential open channels 3D framework with 1D diamond-shaped channels containing H-bonding sites
Size & shape exclusion
CO2 over CH4 and N2
57 mL g-
Size & shape exclusion
CO2 over CH4 and N2
Size & shape exclusion
[Ni(bpe)2( N(CN)2)] (N(CN)2) (ZIF-20)
Interpenetrated 3D framework with bimodal functionality 3D framework with large cages connected by small distensible apertures
Different guest adsorption interaction Guest-framework interaction induced gate opening
CO2 over N2
Cu(pyrdc)( bpp)
2D pillared-bilayer framework without pores
Different dependent opening
gest gate
Zn2(tcom)( 4,4'-bpy)
Interpenetrated 3D PtS-type framework with intersecting channels
Different adsorption interaction
guest
Cu(fam)(4, 4'-bpe)0.5 (PCN-5)
Cu(dhbc)2( 4,4'-bpy) Cu(bdc)(4, 4'-bpy)0.5 MIL-53 M = Al, Cr
CO2 over CH4
~ 130 mmol g-
195
1 p/p
(Chen et al., 2007a )
195
1 atm
100 mL g-1
195
760 torr
CO2 over CH4
~ 210 mg g-1
195
760 torr
(Cheo n and Suh, 2008) (Chen et al., 2007b ) (Ma et al., 2007b )
1
1
Size & exclusion
shape
CO2 over CH4, O2, and N2
~ 70 mL g-1
298
0.4 ~ 8 atm
Different dependent opening
gest gate
CO2 over CH4 and N2
~ 70 mL g-1
298
0.1 ~ 0.2 MPa
Different adsorption interaction
guest
CO2 over CH4
~ 10 mmol g-
304
30 bar
~ 35 mL g-1
195
1 p/p
CO2 over CH4
~70 mL g-1
273
760 torr
CO2 over CH4
Differed adsorpti on capacity 5 wt %
195
Differen t pressure
298
1 bar
1
CO2 over N2
Notwithstanding an ambiguous rearrangement mechanism of the flexible MOFs, this kind of adsorbent provides the opportunity of finding a novel material for a particular application thanks to their exclusive capability. The most distinguished factor that differentiates the flexible MOF from the rigid one or other adsorbents is the combination of weak and strong intermolecular interactions in their
(Kitau ra et al., 2003) (Kitau ra et al., 2003) (Bour relly et al., 2005) (Maji et al., 2007) (Haya shi et al., 2007) (Maji et al., 2005) (Thall apally et al., 2008)
structure. The framework integrity and porosity are obtained from the strong bond while the flexibility and dynamic behaviors are as a result of the weak interactions. 3.3. CO2 Adsorption Capacity One of the most imperative features of the adsorption technology is the adsorbate capacity which is mostly reliant on the adsorbent surface area. Thanks to their high and ultra-high surface area, the MOFs have shown the best CO2 uptake capacity as compared to the other adsorbates such as zeolites and activated carbons. Besides, the surface area, the adsorbate-adsorbent interaction, the pressure and the adsorption temperature also exert an impact on the CO2 uptake capacity (Liu et al., 2012a; Liu et al., 2012b). Numerous studies have been carried out in this area to dates, and most of them are related to the gravimetric capacity which is based on the weight percentage of the uptake gas over the total weight of the system (Liu et al., 2012b). The first systematic survey on the correlation between the surface area of the MOF and the capacity of CO2 adsorption was accomplished by Millward and Yaghi (Millward and Yaghi, 2005) on nine MOFs with different characteristics and geometrics including MOF-2, MOF505, Cu3 (BTC)2, MOF-74, IRMOF-11, IRMOFs-3, IRMOF-6, IRMOF-1 and MOF-177 (Table 4). All the capacities were measured under the same condition (T: 298 K and P: 35 bar). The results established that of all the studied MFOs, MOF-177 had the highest surface area (4,508 m2g-1) and possessed a greater CO2 uptake capacity (60.0 wt %). One of the best reported amount of CO2 uptakes belongs to MOF-210 (~71 wt%) at 298 K and 50 bar. This outstanding CO2 uptake is related to the ultrahigh surface area of these MOFs (BET and Langmuir surface areas for this MOF is 6240 and 10,400 m2g-1 respectively). This result was presented by Furukawa and co-workers who investigated 5 MOFs with same metal sites with one or two ligands (the ligand structures are tabularized in Table 5). Consistent with the results and the ligand structure, it was acknowledged that by expanding the organic linker of the framework, the MOF capacity can be boosted. For example, just by enlarging the ligand of MOF-177 from BTB: 1,3,5 benzenetribenzoate to
BBC: 4,4’,4’’-(benzene-1,3,5- triyl-tris(benzene-4,1-iyl)) tribenzoate, MOF-200 was gained and the great enhancement was observed in CO2 uptake from 60.0 to 71wt % (Furukawa et al., 2010).
Table 4 Capacity of 9 MOFa for the CO2 uptake based on the surface area. MOF Name
Formulation
Surface area
Capacity
Pressure
Temperature
(m2*g-1)
(wt %)
(Bar)
(K)
Ref.
MOF-177
Zn4O(BTB)3
4,508
60.0
35
298
(Furukawa et al., 2010)
IRMOF-1
Zn4O(BDC)3
2,833
48.8
35
298
(Millward and Yaghi, 2005)
IRMOF-6
Zn4O(C2H4BDC)3
2,516
46.2
35
298
(Millward and Yaghi, 2005)
IRMOF-3
Zn4O(NH2BDC)3
2,160
45.1
35
298
(Millward and Yaghi, 2005)
IRMOF-11
Zn4O(HPDC)3
2,096
39.3
35
298
(Millward and Yaghi, 2005)
HKUST-1
Cu3(BTC)2
1,781
32.0
35
298
(Millward and Yaghi, 2005)
Zn-MOF-74
Zn2(DOBDC)
816
31.4
35
298
(Millward and Yaghi, 2005)
MOF-505
Cu2(BPTC)
1,547
31.0
35
298
(Millward and Yaghi, 2005)
MOF-2
Zn2(BDC)2
345
12.3
35
298
(Millward and Yaghi, 2005)
Li et al. (Li et al., 2014a) represented UiO(bpdc) with the highest value of CO2 uptake; 79.7 %wt at 20 bar and 293 K. The BET surface area and the Langmuir surface were 2646 m2 g-1 and 2965 m2 g-1 respectively and the pore value of 1.057 cm3 g-1 was found. It is worth noting that this MOF has exhibited the highest CO2 uptake in the MOFs materials so far. Obviously, at high pressure, the CO2 capacity adsorption relies on the surface area and the pore volume of the MOF. Therefore, one of the effective ways of boosting the capacity of CO2 uptake is to enhance the pore volume and the surface area of the MOFs. This could be achieved by using the large organic ligand such as the BTB and the BTC or extending the MOF structure in the synthesis part
(Furukawa et al., 2010; Li et al., 2014a). Some of the new MOFs that are expedient for CO2 capture at high pressure are listed in Table 6. In addition, measuring the CO2 capacity adsorption at low pressure is also highly paramount. Because the partial pressure of CO2 generated by the power-plant flue gas is less than the atmospheric pressure, MOFs with a high storage capacity at low pressure should be considered for separation and storage of CO2 from the power plant(Zou et al., 2010). Some of the new MOFs which are proper for CO2 uptake at low pressure are specified in Table 7. In addition to the capacity and selectivity which are the most essential parts for choosing the MOF in the CO2 adsorption process, the adsorption heat has to be considered as another vital factor which is explained in the next section.
Table 5 The structure and the chemical formula of the organic ligand of in some MOFs (Furukawa et al., 2010). Name of MOF MOF-177
Organic Ligand BTB: 4,4’,4’’-benzene-1,3,5-triyltribenzoate
Ligand Chemical Formula O
OH
MOF-180
BTE: 4,4’,4’’-benzene-1,3,5-triyltris(ethyne-2,1-diyl)tribenzoate
Ref. (Furukawa et al., 2010)
OH
OH O
O
O O
HO O
(Furukawa et al., 2010)
OH
HO
MOF-200
BBC: 4,4’,4’’-benzene-1,3,5-triyltris(benzene-4,1diyl)tribenzoate
(Furukawa et al., 2010)
COO
COO
COO
MOF-205
BTB +2,6naphtalenedicarboxylate (NDC)
OOC COO
COO
COO OOC
(Furukawa et al., 2010)
MOF-210
BTE + biphenyl-4,4’-dicarboxilate (BPDC)
(Furukawa et al., 2010)
COO
COO
COO
COO COO
Table 6 Adsorption capacities of CO2 in different MOFs at high pressure. Surface Area (m2/g) MOF
BET
UiO(bpdc)
2646
NU-111
Langmuir 2965
Capacity (wt %)
Pressure (Bar)
Temp. (K)
Selectivity
Qst (kJ mol-1)
79.7
20
303
4932
61.8
30
298
DGC-MIL-101
4198
59.8
40
298
HTS-MIL-101
3482
52.8
40
298
ZJU-32
3831
49
40
300
UTSA-62a
2190
43.7
55
298
16
Basolite® C 300
1706.42
41.9
224.99
318
18
Basolite@ A100
1524.8
26.9
224.99
318
9
DMOF
1980
38.1
20
298
DMOF-NO2
1310
32
20
298
DMOF-DM1/2
1500
27.5
20
298
DMOF-Cl2
1180
26.4
20
DMOF-DM
1120
25.4
DMOF-OH
1130
DMOF-Br DMOF-TM1/2
23
12
20
298
17
21
20
298
23
23
24.8
20
298
1320
24.3
20
298
1210
23.9
20
298
Ref. (Li et al., 2014a) (Luo et al., 2013) (Kim et al., 2013) (Kim et al., 2013) (Cai et al., 2014a) (He et al., 2013) (Deniz et al., 2013) (Deniz et al., 2013) (Burtch et al., 2013) (Burtch et al., 2013) (Burtch et al., 2013) (Burtch et al., 2013) (Burtch et al., 2013) (Burtch et al., 2013) (Burtch et al., 2013) (Burtch et al., 2013)
DMOF-A
760
17.1
20
298
MIL-101(Cr)
2549
24.2
30
303
HKUST-1
1326
26.3
30
303
IRMOF-8
1599
51.2
30
298
21.1
IRMOF-8-NO2
832
31.3
30
298
35.4
(Burtch et al., 2013) (Ye et al., 2013) (Ye et al., 2013) (Orefuwa et al., 2013) (Orefuwa et al., 2013)
Table 7 Adsorption capacities of CO2 in different MOFs at low pressure.
UiO(bpdc)
Surface Area (m2/g) BET Langmuir 2646 2965
DGC-MIL-101
8
P. (Bar) 1
Temp. (K) 303
4164
14.5
1
298
HTS-MIL-101
3482
12.3
1
298
UTSA-62a
2190
8.1
1
298
DMOF-TM
1050
13.3
1
298
DMOF-A
760
10.6
1
298
DMOF-Cl2
1180
8.8
1
298
HKUST-1
1326
13.2
1
303
Mg-MOF-74
1416
2085
30.1
1
298
Ni-MOF-74
1252
1841
19.4
1
298
Mg/DOBDC
1415.1
25
1
298
MOF
Capacity (wt %)
Selectivity (IAST)
Qst (kJ mol-1)
16 28
29
17
21
47
Ref. (Li et al., 2014a ) (Kim et al., 2013) (Kim et al., 2013) (He et al., 2013) (Burtc h et al., 2013) (Burtc h et al., 2013) (Burtc h et al., 2013) (Ye et al., 2013) (Wu et al., 2013) (Wu et al., 2013) (Li et al., 2014b )
Co/DOBDC
1089.3
21.6
1
298
37
Ni/DOBDC
1017.5
20.5
1
298
42
MIL-100(Cr)
1528.7
9.5
1
298
23.8
1
313
15.8
1
313
21.2
1
313
7.3
1
313
12.8
1
313
mmen-CuBTTri
11.3
1
313
[Zn2(BME-bdc)x(DBbdc) 2.xdabco]n
21.7
0.91
195
Mg2(dobpdc)
1940
mmen-Mg2(dobpdc)
Ni2(dobpdc)
1593
mmen-Ni2(dobpdc)
HKUST-1
2203
CPM-33b
808
1119
19.9
1
298
25
CPM-33a
966
1257
12.6
1
298
22.5
Cu-TDPAH
1762
18.4
1
298
Ni-DOBDC
798
18.2
1
298
Py-Ni-DOBDC
409
12
1
298
ZJNU-40
2209
16.4
1.01
296
18.4
JLU-Liu22
1487
15.6
1
298
30
200
33.8
(Li et al., 2014b ) (Li et al., 2014b ) (Li et al., 2014b ) (Maso n et al., 2015) (Maso n et al., 2015) (Maso n et al., 2015) (Maso n et al., 2015) (Maso n et al., 2015) (Maso n et al., 2015) (Bon et al., 2015) (Zhao et al., 2015) (Zhao et al., 2015) (Liu et al., 2014) (Bae et al., 2014) (Bae et al., 2014) (Song et al., 2014a ) (Wan g et al., 2015)
UNLPF-1
1046.6
13.9
1
273
13.6
1
298
UTSA-49
710.5
rht-MOF-pyr
2100
12.7
1
298
28
rht-MOF-1
2100
11
1
298
29
ZnAcBPDC
920
11.7
0.9
293
ZnBuBPDC
850
7.6
0.89
293
{[H2N(CH3)2]4[Zn9O2(BTC).6( H2O)3].3DMA}cn
844
10.9
0.91
298
CuBTTri
1700
10.8
1
293
IRMOF-74-III-CH2NH2
2310
10.8
1
298
IRMOF-74-III-NH2
2720
10.4
1
298
IRMOF-74-IIICH2NHMe
2250
9.6
1
298
SIFSIX-3-Ni
368
10.3
1
298
51
SIFSIX-3-Co
223
10
1
298
47
8.9
1
298
750
9.9
1
298
1132
SIFSIX-3-Zn
Zn/Ni-ZIF-8-1000
ZIF-7
312
355
9.1
1
298
UiO-66(Ti56)
1844
2200
8.8
1
298
95.8
29
30
61.2
37
(Johns on et al., 2013) (Xion g et al., 2014) (Gao et al., 2015) (Gao et al., 2015) (Kece li et al., 2014) (Kece li et al., 2014) (Li et al., 2015) (Das et al., 2013) (Fraca roli et al., 2014) (Fraca roli et al., 2014) (Fraca roli et al., 2014) (Elsai di et al., 2015) (Elsai di et al., 2015) (Elsai di et al., 2015) (Li et al., 2014c ) (Wu et al., 2014) (Lau et al., 2013)
CPM-5
2187
8.8
1
298
ZIF-7-R
5
8.7
1
303
Zn(5-mtz)(2-eim) .(guest)[ZTIF-1]
1430
8.2
1
295
81
22.5
7.3
1
293
25
36.0
7.2
1
298
94.2 (cm3 g-1)
1
273
1981
[Cu(tba)2]n Zn-DABCO
1870
[Zn2(TMTA)(DMF)2].NO 3.2H2O.3DMF
788
1902
-
16.1
36.1
34
22.4
39.3
28.8
3.3.1. Enthalpy of adsorption (Qst) The adsorption heat, the isosteric heat, or adsorption enthalpy is a critical factor in appraising the gas adsorption on the MOFs and other solid sorbents. This value demonstrates the MOFs’ affinity to adsorb CO2. It shows the strength of the interaction between the host and guest molecules. Indeed, the magnitude of the Qst is a function of the binding strength. Moreover, this value indicates the amount of the required energy for the regeneration process. If the Qst of adsorption is very high, it implies that the adsorbent has a very high affinity to adsorb CO2. Nevertheless, its regeneration process requires high energy to regenerate the adsorbent. Thus, the optimal MOF for CO2 uptake should possess an ideal enthalpy of adsorption; furthermore, this value is measured based on kJ mol-1 unite (Sumida et al., 2012).
4. Synthesis of Metal Organic Frameworks 4.1. MOF preparation 4.1.1. Metal
(Sabo uni et al., 2013) (Cai et al., 2014b ) (Wan g et al., 2014) (Du et al., 2014) (Chae mchu en et al., 2015) (Sayar i et al., 2011)
All the MOFs are constructed from the metal (inorganic) and the ligand (organic) which are linked to each other via a coordination bond. Almost all of the transition metals as the ions or clusters can be used to form the MOFs. The transition metals are the group of metals whose block d is not completed yet; therefore, they are very active and possess outstanding characteristics such as a variety of coordination numbers and oxidation states. Based on these values, numerous MOFs with a diversity of geometrics such a square–planer, tetrahedron, octahedron, trigonal, pentagonal , etc. can be constructed. Besides metal sites and their values, ligands, solvents and even counter-inion (reaction condition) have influenced the MOF geometrics (Li et al., 2012). Typically, the coordination reaction takes place between the metal salt and the ligand in the presence of one or more organic solvents such as the dimethylformamide (DMF), ethanol, methanol, etc. Based on the reaction condition, the end-product (MOF) is produced in different geometric shapes.
4.1.2. Ligand The organic part of the MOFs is the ligand that comprises Lewis base–binding atoms (functional groups) such as halides, nitriles, cyanides, anionic organic molecules (benzenedicarboxylic acid), and neutral organic molecules (4,4′-bipyridine) (Furukawa et al., 2013). Some of the ligands structures used for constructing the MOFs are shown in Table 8. Resembling the metal sites, the ligands also exert a prominent impact on CO2 adsorption into MOFs. The tunable characteristic of the MOFs mostly relies on this part of the framework due to the capability of the organic linkers to have variable functional groups in different conditions. Even the length of the ligand can alter the gas storage capacity. Indeed, larger storage spaces are achieved via longer organic linkers by increasing the functional groups of the ligand.. Nonetheless, a very long ligand contributes to formation of the interpenetrating structure and then intertwining of the chains (Furukawa et al., 2013). Therefore, the ligand part of the framework exerts a great impact on the improvement of the gas adsorption capacity of the MOFs due to their functionalization. The effect of the ligand site on the enhancement of CO2 adsorption properties are detailed in sections 5.2 and 5.4.
Table 8 Organic ligands used in the MOF for CO2 adsorption. Ligand H2 BDC (Benzene di carboxylate acid)
Molecular structure of the ligands OH
Active sites Oxygen of Hydroxyl groups.
Ref. (Furukawa et al., 2013)
Multi-functional ligands: Amine groups and free nitrogen groups are active to attracted CO2 molecules.
(Song et al., 2014b)
Multi-functional ligands: Free nitrogen, amine group, oxygen and OH groups all are active to adsorb CO2 molecules.
(Pachfule et al., 2011)
Multi-functional ligands: Amide units and oxygen of carboxylic groups.
(Duan et al., 2012)
Mixt Linker: Open N sites of bipy and P-O in the phosphonate ligands.
(Taddei et al., 2013)
Mixt Linker: amino group and three N-atoms of the triazole ring as well as the oxygen atoms of oxalate ligands.
(Furukawa et al., 2013; Garcı́a-Ochoa and Genesca, 2004)
Amine group and open nitrogen site.
(Mighell and Reimann, 1967)
OH
NH2
Adenine
H N
N
OH
ANIC: 2-Amino-isonicotinic acid
N
N O
N O
H3L: 5-(4carboxybenzoylamino)isophthalic acid
H
NH2
OH
C N
O
OH
O
O
OH
H2L2 and bipy N
N
(HO)2OP
N N
(HO)2OP
3-amino-1,2,4-triazole oxalic acid
NH2
and
PO(OH)2
O
N
C N
PO(OH)2
OH C
HO
N
O
Pyrazole
N N H
4.2. Synthesis methods In the past decade, a conspicuous emphasis has been laid on the synthesis of MOFs due to the possibility of designing their structure based on their applications. Different synthesis methods such as conventional, microwave assisted, sonochemical, mechanochemical, and electrochemical have been adopted for producing the MOFs and the main difference in these methods is the form of the energy input which will be explained in the succeeding sections.
4.2.1. Conventional Synthesis routes (Ca) It is of note that the most popular method for synthesizing the MOFs is the conventional route and the required energy is introduced to the system via conventional heating sources. Since the temperature plays a critical role in chemical reactions, this synthesis route is distinguished from the solvothermal (hydrothermal) and non-solvothermal reactions. While the solvothermal reactions occur in the closed reactor (usually Teflon sealed) under autogenous pressure and the temperature of the reaction must be higher than the boiling point of the solvent, the non-solvothermal reaction is accomplished under ambient pressure and below or at the boiling point of the solvent (Byrappa and Yoshimura, 2012). Typically, the temperature range for the synthesis of the MOFs varies from the room temperature to approximately 250 C. By far, most MOFs are synthesized via the solvothermal reactions owing to the ease of synthesis conditions. As mentioned above, this reaction normally takes place via the coordination bond between the organic ligand and the metal salt in the solvent under the above condition and the end product will be in crystal or powder forms. Although the temperature is an essential parameter for most syntheses, some well-known MOFs comprising HKUST-1, MOF-5, MOF-177, MOF-74, and ZIF-8 are prepared just by mixing the primer materials at room temperature. The merit associated with such reactions is that the precipitation occurs in a short time-scale, sometimes known as the direct precipitation reaction;
therefore, there will be a considerable diminishment in the reaction time (David J. Tranchemontagne, 2008; Janosch Cravillon, 2009). Notably, ZIF-8 exhibits good chemical and thermal stabilities compared to the others. In general, synthesis of the MOFs is highly dependent on the reaction temperature, even an MOF having various properties such as different surface areas and crystallinity can be achieved at different temperatures while dense MOFs are commonly obtained at a high reaction temperature (Bauer and Stock, 2007; Forster et al., 2005). Further to this, not only high synthesis temperature has an influence on crystallinity but also leads to improvement of the reaction rates, particularly if kinetically more inert ions are utilized (Biemmi et al., 2009). It is worth asserting that prolonging the reaction times can lead to degradation of the frameworks (Millange et al., 2011). The long reaction time and the subsequent high energy consumption have led to the introduction of alternative methods for synthesis. Additionally, alternative methods can not only reduce the reaction time and save energy but also exert great effects on morphology, size distribution and particles size of the pores, thus significantly affecting the MOFs properties. Regarding the separation and gas storage technology, the pore size in porous materials such as MOFs have an impact on the diffusion of the guest molecules as well as the adsorption properties. Due to these objectives, alternative synthesis methods such as microwave-assisted, sonochemical, mechanochemical and electrochemical routes have been applied.
4.2.2. Microwave Synthesis (MW) Microwave irradiation is a distinguished technique for materials synthesis (Mingos and Baghurst, 1991). This synthetic method occurs based on the interaction of the mobile electric charge with electromagnetic waves. The electric charge can be supplied by electrons or ions in solid materials and the existence of the polar solvent, molecules/ ions in the solution. Therefore, the electrical current and heating are produced based on the above mentioned distinguish mechanism in solid and solution forms. For the case of solid, the electrical current is generated based on the electrical resistance of the solid materials. In the solution, the electrical current is created by aligning the polar molecules / ions when
they are exposed in the electromagnetic field, where their orientation is changed permanently. Hence, to achieve the best end-product, adequate frequency is required to create a proper collision between the reactants contributing to enhancement of the kinetic energy and the reaction temperature. In addition, choosing the appropriate solvent and the selective energy input is highly vital and must be considered due to the possibility of the interaction between the starting materials and the MW radiation (Klinowski et al., 2011). The synthesis reaction takes place in the microwave oven and the condition of the reaction should be accurately controlled, especially in terms of temperature and pressure. For MOFs, the reaction has been accomplished above 100 C and less than 1h which is indeed faster than the conventional methods. The MW assisted method has also led to an increase in the crystallization speed as well as a reduction in the crystals size (mostly in the nanoscale size). High reaction speed, low reaction time, a direct interaction between the reactant and the radiation make this synthesis route as a good heating energy efficient method. Accordingly, the synthesis reaction can take place at a high heating rate as well as homogeneous heating throughout the reactants (Klinowski et al., 2011). Since MOF-5 is one of the basic MOFs, the effect of the MW irradiation on its structure has been also investigated by Choi et al. (Choi et al., 2008). They synthesized MOF-5 via the MW irradiation method and based on their observation, the texture properties of this MOF was similar to the Ca. route (Langmuir surface area: 3800, 3200 m2 g-1 for MW and Ca. respectively). It was found that the reaction time and the average pore size decreased dramatically for the MW synthesized MOF-5 (reaction time: 1h to 30 mins, pore size: 400-500 to 20-25 𝜇m). In practice, by prolonging the reaction time, several surface defects and deterioration of crystals could be created. Therefore, optimization of the reaction time is quite critical in the MW assist synthesis route. As regards the effect of this synthesis route on the MOFs’ structure, several studies have been undertaken to this date. To illustrate this, Blanita et al. (Blanita et al., 2016) investigated the effect of concentrating the starting materials, solvents, temperature, reaction time and MW power on the texture properties of HKUST-1. According to their statement, the best BET surface area (1863 m2 g-1) was related to the concentration of 4c which was dissolved in DMF, EtOH and H2O at 70 C during 10 mins
with MW power of 360 w. Moreover, Tari et al. (Tari et al., 2016) synthesized MCM-41/Cu(BDC) composite via the MW irradiation for selective CO2 adsorption. Practically, they functionalized MCM41 by Cu (BDC) MOF with cyanotripropyltriethoxysilane linker. The MOF showed a great CO2 capacity and selectivity adsorption over CH4 at 298 K and 4-40 bar. These enhancements were attributed to the quadrupole moments of CO2 and the presence of negative and positive charges of COO- and Cu2+ ions in the framework. The BET surface area of the composite was also improved as compared to the parents MOFs (1084, 1063 and 624 m2 g-1 for composite, MCM-41 and Cu(BDC), respectively). In addition, after three times of cyclic adsorption of CO2 and CH4, the composite lost only 1 % of its capacity suggesting the high stability of its structure. Another study in this area was accomplished by Cabello and coworkers (Cabello et al., 2015). They prepared (Na,Cd)-MOF through the MW irradiation which possessed open metal sites in the framework structure (sodium ions). As expected, the reaction time declined as compared to conventional synthesis route from 5 days to 1 h. The MW synthesis of (Na,Cd)-MOF indicated improvement in CO2 adsorption properties (Table 9). Furthermore, Cd-NaMOF (MW) exhibited long term water stability even under 95 % humidity. Additionally, after 15 cycles of CO2 uptake, it still maintained the CO2 adsorption capacity that is very indispensable factor, especially in case of industrial usage.
Table 9 CO2 adsorption properties of (Na,Cd)-MOF based on different synthesis methods. Methods
BET surface area (cm3 g-1)
CO2 uptake (mmol g-1) at 298 K and 1 bar
Qst (kJ mol-1)
Conventional
418
2.25
42
MW
526
2.71
-
Ref. (Cabello et al., 2015) (Cabello et al., 2015)
4.2.3. Sonochemical Synthesis In this method, the synthesis reaction takes place by applying ultrasound waves having very high levels of energy. Actually, it is a cyclic mechanical vibration with a frequency range of 20 kHz (higher than the human hearing range) to 10 MHz. By channeling the high ultrasonic energy into the solution, different pressure (compression and rarefaction) regions are created in the reactor which leads to
formation of cyclic alternating regions (bubbles). As a result, in the low pressure area, the pressure drops below the vapor pressure of the reactants and the solvent; for that reason, small bubbles or cavities would be created. These cavities (bubbles) have the ability to expand to around tens of micrometers due to diffusion of the solution vapor into the cavities. The bubbles will reach their critical size which is an unstable state that subsequently leads to implosion. The process of formation, growing and collapsing of the bubbles is referred to as the cavitation procedure. Hence, the reaction occurs within a very short time. Several factors such as the starting materials (viscosity, reactivity, vapor pressure and selection of the solvent) and equipment (acoustic frequency and intensity, temperature and gas atmosphere) should be taken into consideration in this synthesis method. Moreover, attention must be paid in choosing the solvents, i.e. the organic solvents are not typically proper for the sonochemical synthesis. The high vapor pressure of the organic solvents would shrink the intensity of the collapsing cavities, consequently affecting the reaction pressure and temperature(Shono et al., 2000). For solid particles in the solution, the microjet phenomenon takes place in preference to cavitation. When the bubbles (with very high speed) come in the vicinity of the solid particles, erosion of the bubbles occurs, resulting in solid materials’ clean and activated surface. Likewise, dispersion of smaller particles in the solution will be accomplished. The chemical reaction can occur inside the cavities (the best conditions), in the bulk media, or at the interface of the medium (intermediate pressure and temperature). This method is extensively used for synthesis of nanomaterials and organic materials (Bang and Suslick, 2010; Fillion and Luche, 1998). The ultrasound synthesis method is advantaged with very fast reaction, while being accomplished easily, being environment-friendly, plus its being accomplished at room temperature and being energy efficient. Not many MOFs were synthesized through this method so far than solvothermal and MV but the results of that one which reported were acceptable.
HKUST-1 was the first MOF being synthesized via the ultrasound route (Li et al., 2009b). This synthesis was accomplished by adding H3BTC (ligand) in the mixture of DMF and ethanol (solvents); then a cupric acetate dihydrate (aqueous solution) was added to the mixture in the ultrasonic bath at the ambient condition. The prominent difference of HKUST-1 synthesized through ultrasound and solvothermal routes was the reaction time which decreased from 24 h to 5- 60 minutes, the reaction temperature of 25 C for the ultrasound and the dimensions of the pores which decreased to nanosize (10–200 nm). However, the physicochemical properties of the MOF have no difference with the solvothermal one. The morphology and size of the end product were indeed related to the reaction time. For example, spherical particles with the range of 100-200 nm were resulted from the short time reaction, whereas the long reaction time (30 and 90 min) generated long needle particles with the size of up to 900 nm. MOF-5 was synthesized via the sonochemical route for the first time by Son et al. (Son et al., 2008) who examined different factors having effects on the end product. As expected, the reaction time decreased dramatically from 24 h to 30 mins (ultrasound route). MOF-5 is usually synthesized in diethylformamide (solvent), albeit they used 1-methyl-2pyrrolidone (NMP) as the solvent because it is more economical than diethylformamide. Likewise, to gain the best result, different amounts of NMP were investigated (25-75 molar ratio) confirming that the best crystals were obtained with an appropriate amount of solvent. The textural properties such as the specific surface area and the pores volume of S-MOF-5 and C-MOF-5 (conventional route) were identical (320 m2 g-1 and 1.21 m3 g-1). For the CO2 adsorption capacity, a small improvement was obtained (790 and 820 mg g-1 for C-MOF-5 and S-MOF-5, respectively). The effect of the solvent on the textural properties on the MOF in the ultrasound synthesis was investigated by Israr and associates (Israr et al., 2016). They synthesized Cu-BTC in various solvents (DMF, H2O, EtOH) with different ratios. Among them, the exceptional Cu-BTC crystals yield of 86% was obtained from EtOH, H2O, and, the DMF mixture with the ratio of 20:40:40. All the samples displayed the nanoporous nature and the highest BET value was associated with the samples which were obtained from water/DMF and water/ethanol/DMF solvent (1430 m2/g and 1400 m2/g, respectively).
Kim et al. (Kim et al., 2011) synthesized catenated Cu-TATBn (n=30-60) via a sonochemical route. On the word of their observation, catenation in CuTATB-60 yielded a positive effect on CO2 adsorption properties, including an improvement in the surface area, stability of the framework, a high CO2 adsorption capacity (189 mg CO2 g-1) as well as outstanding selectivity over nitrogen (>20: 1) at 298 K. The heat of the adsorption was also scrutinized which was 35 kJ mol-1at under the same condition. Further to these properties, the catenated sample retained its structure after 5 consecutive adsorption/desorption processes (high purity CO2).
4.2.4. Mechanochemical Synthesis As opposed to the previous methods, the mechanochemical synthesis is a solvent-free route and the end material (MOFs) is produced by neat mixing and grinding of the metal salt and the organic linker via the ball mill mixture (The schematic of this synthesis method is shown in Fig. 9). This method has numerous advantages, namely their being environment-friendly due to the solvent free reaction (Garay et al., 2007), a short reaction time (usually 10-60 min), a small particle product. Moreover, the reaction normally occurs at ambient temperature. In addition, in some occasions, the metals site can be changed with metal oxide and this leads to formation of water as a side product (Friščić and Fábián, 2009; Friščić et al., 2010; Yuan et al., 2010b).
Fig. 9. Schematic illustration of mechanochemical route.
In order to promote the reactivity of the reactant, using the linkers with lower melting points and metals which release the solvent or the ones hydrated during the reaction can be highly effective. One of the mechanochemical synthesis methods is the liquid assisted grinding (LAG) route (Friščić and Fábián, 2009) wherein the reaction takes place by adding a very small amount of solvent to the reaction medium. The advantage of LAG over the original one is acceleration of the mechanochemical procedure due to an increase in the mobility of the reactant molecules in the presence of the solvent. Friscic and Fabian adopted this method to synthesize six coordination polymers for the first time by adding variable amounts of solvent in the mixture of ZnO (metal oxide) and fumaric acid. Hence, different MOFs were obtained having various dimensions (1D, 2D, and 3D). They also constructed new pillared MOFs by adding bpe or bipy (pillared linkers) in the presence of methanol, ethanol, DMF or isopropanol as the solvents (Friščić and Fábián, 2009). This research enclosed the ability of the LAG methods to producing a variety of MOFs with different structures and geometries. Furthermore, they observed that by adding the catalytic amount of metal salts, several structures could be acquired. For instance, usage of the sulfates salts promoted a hexagonal structure while the nitrile salts induced a tetragonal structure. For CO2 adsorption on MOFs, Masoomi et al. (Masoomi et al., 2014)introduced a new framework (Zn(II)-MOF based MOF) by utilizing the azinefunctionalized linker via the mechanochemical route. They synthesized TMU-4 and TMU-5 by using H2oba and n-donor ligands with different topologies and pore sizes. Based on their results, both frameworks indicated a high affinity towards CO2 molecules (almost same) at different temperatures (195, 298 and 273 K). Although the mechanochemical method is one of the promising synthesis routes, it still requires supplementary investigations.
4.2.5. Electrochemical Synthesis In this synthesis method, the chemical reaction occurs through the electrons transfer which generates the electrical current. The reaction medium contains anode, cathode, and the electrolyte solution which comprises a conducting salt and dissolved linker molecules. Rather than utilizing the metal salts, the metal ions are introduced continuously to the reactor via anodic dissolution. The striking point is that the cathode electrode must be protected due to the tendency of the metal ions deposition on its surface. Hence, the protic solvents and compounds such as acrylic, maleic esters and acrylonitrile (preferentially reduced) can be used in the reaction medium to tackle this issue. At large scale productions, the electrochemical method would most probably have a continuous procedure which leads to a higher end product yield (scale up). This method was represented for synthesizing the MOFs by researchers of BASF in 2005, who synthesized some Zn- and Cu- based MOFs (Mueller et al., 2007). In this research, various experimental setups and different combinations of linkers (H2BDC, H2BDC.(OH)2, 1,2,3H3BTC and 1,3,5-H3BTC) and anode materials (Mg, Zn, Co, and Cu) were explored. It was established the MOFs which contained Zn- or Cu- compounds exhibited higher porosity. Since then, this synthesis method has been applied in the construction of the MOF. For instance, Muller et al. (Mueller et al., 2007) produced HKUST-1 for hydrogen storage and purification from natural gas. The effect of the synthesis process on the properties of HKUST-1 was investigated by Schlesinger et al. (Schlesinger et al., 2010) who generated this MOF via the conventional, electrochemical, microwave routes at ambient pressure. The results indicated that HKUST-1 which was produced via the electrochemical method had an inferior quality among them. This was indeed attributed to incorporation of the conducting salt and the linker molecules during crystallization. Another instance is MIL-100 (Fe) which was initially synthesized via the electrochemical route by Campagnol et al. (Campagnol et al., 2013), where the reaction took place at high temperature and pressure. The electrochemical cell contained Fe as the anode and the solution of H3BTC in the mixture of ethanol and water (2:1). The crystals of MIL-100 (Fe) were obtained at different temperature ranges of 110–190 °C on the top of the anode electrode and in the solution simultaneously. According to their report, an improvement was gained on the morphology of
this MOF. Generally, this method is claimed to be very useful when producing MOFs in the form of thin films is intended.
5. Methods to improve CO2 adsorption As far as this, different methods have been introduced to improve the CO2 adsorption capacity and selectivity on MOFs, including the open metal site (OMS), pre-synthesis modifications and postsynthesis modifications and tuning the pore size. Numerous research has been also accomplished on these effective modification methods while there are a plethora of review papers in this context (Liu et al., 2012a; Liu et al., 2012b; Tanabe and Cohen, 2011). For this reason, the current review paper just elaborates on these modification methods succinctly while casting further emphasis on new achievements in this area.
5.1. Open Metal Sites (OMS) and Its Effect on CO 2 adsorption Properties. Metals site exerts a significant impact on increasing the capacity and selectivity of CO2 over the other gases. During the synthesis procedure, some metal sites in the MOFs structure are involved with solvent molecules. Therefore, the capability of the MOFs to adsorb the guest molecules would be diminished. To overcome this issue, the best way is the activation process which can be achieved via applying the temperature and/ or the vacuum. In this process, the exceeded solvents are removed from the metal sites leading to creation of the open metal sites (OMS). Therefore, there will be an upsurge in the CO2 (guest molecules) adsorption capacity of the MOF. The OMS has a great influence on the binding energy between the surface of the MOF and the adsorbed CO2 molecules. Indeed, these metal centers make the surface of the framework as active sites to trap CO2 molecules and bond them
by inducing dipole–quadrupole interactions (Liu et al., 2012b). Some MOFs used for CO2 adsorption via their OMS sites are indicated in Table 10.
Table 10 The MOFs with open metal sites used for CO2 adsorption.
Cu-TDPAT
BET surface area (m2g-1) 1938
132 cm3/g
CO2/N2 (79)
298
1
MIL-101 (Cr)
2800
28 mmol/g
CO2/CH4
300
< 50
MIL-101 (Cr)
3780
34 mmol/g
CO2/CH4
300
< 50
MIL-101 (Cr)
4230
40 mmol/ g
CO2/CH4
300
< 50
MOP: metal– organic polyhedron (Cu II)
12.1 mmol/g
CO2/N2
196
10-5
M/DOBDC M: Mg, Ni, Co, Zn
Mg/DOBD C: ~250 mg/g
293298
1
298
10
MOF
M-DABCO M = Ni, Co, Cu, Zn
Ni: 2120 Zn: 1870 Cu:1616 Co: 2022
CO2 uptake
Ni: 2.4 mmol / g Ni> Zn > Cu >Co
Selectivity
CO2/CH4
T (K)
P (bar)
Reason for enhancement of CO2 adsorption properties Lewis basic sites and the dual functionalization of the framework Enhancement of OMS due to activation method (hot ethanol) Higher density of Oms due to effective activation method (hot ethanol+ NH4F) strong interaction between coordinativelyunsaturated Cu(II) sites and CO2 molecules Mg/ DOBDC showed excellent CO2 adsorption among them due to, shorter length of Mg-O because of the smaller atomic weight of Mg comparing to other metals.Binding length: Mg-O (1.969 Å) < Ni-O (2.003 Å) < Co-O (2.031 Å)< Zn-O (2.083 Å). Ni-DABCO because Ni cation as a metal center possesses the highest charge density
Ref. (Li et al., 2012) (Llewellyn et al., 2008) (Llewellyn et al., 2008) (Llewellyn et al., 2008) (LópezOlvera et al., 2017) (Yazaydın et al., 2009)
(Cabello et al., 2014)
As stated in section 4.1.1, almost all the transition metal in the periodic table can be used to create the MOFs. Since these metals interact with other atoms and molecules through their d atomic orbital, recognizing their bounding mechanism with other molecules is interesting and essential. This condition could be explained by the interaction of MOF frameworks through OMS with small guest
molecules based on the d band center theory. To produce the MOFs with superior adsorption and separation properties, it is important to know the nature of the interaction of the guest molecules with the OMS sites. Besides, the difference of these cations should be considered. Also, the binding between the OMS sites and the adsorbate is the result of an appropriate balance between hybridization of the molecular orbitals, electrostatics, van der Waals forces, and Pauli repulsion. Therefore, the transition metal cation plays an important role in the adsorption properties of the MOFs. Lee et al. performed a comprehensive study on M-MOF-74 with different metal sites (M: Mg, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) (Lee et al., 2015). They investigated the binding enthalpies of the flue-gas components in the transition-metal of M-MOF-74s based on the density functional theory (DFT). The binding enthalpies of the flue gas on the M-MOF- 74s are summarized in Table 11. The highest binding enthalpy represents a stronger binding between the OMS cations and the guest molecules. As the data shows V-MOF-74 has a stronger binding with CO2 and CH4, while N2 molecules have a strong binding with Ni cation of MOF-74. Based on the obtained result in this research, it can be concluded that the transition metals show different adsorption behaviors due to the diversity of the electrons in d orbitals, accordingly, to improve the adsorption properties of the MOFs, their structure can be designed based on the guest molecules.
Table 11. Calculated Binding Enthalpies (in kJ/mol at 297 K) of the flue gas in the Isostructural M-MOF-74s (Lee et al., 2015). Flue gas H2 CH4 N2 CO2 H2O
d0 Mg 10 19 28 41 62
d2 Ti 17 23 58 46 71
d3 V 18 26 52 51 77
d4 Cr 6 14 14 27 31
d5 Mn 8 19 21 34 51
d6 Fe 9 19 21 34 49
d7 Co 9 18 21 34 48
d8 Ni 10 19 24 37 56
d9 Cu 6 14 13 27 24
d10 Zn 8 19 19 30 41
Since the MOFs possess the transition metal cation in their frameworks, the influence of the spin and magnetic states of the metal ion on the MOFs performance should be taken into account. In order
to do gas adsorption, although gas sorption and spin-crossover can co-exist in the framework, no appreciable interaction takes place between the adsorbed gas molecules and the magnetic host network (Ohba et al., 2009; Southon et al., 2009). Therefore, in most adsorption cases, adsorbed gas molecules do not exert any effects on the spin transition temperature (Espallargas and Coronado, 2018). However, in case of using MOFs as sensors, in magnetic refrigeration, in electronic applications, or in quantum computing, spin and magnetic state must be strongly considered.
5.2. Pre-synthetic procedure
Ligand functionalization exerts a tremendous effect on improving the MOFs’ affinity toward CO2 adsorption and separation. Ease of the modification procedure and the variety of functional groups make this method as one of the best and applicable processes to achieve high CO2 storage properties. CO2 molecules possess the quadrupole moment; thus, the presence of polar groups in the backbone of framework will improve CO2 adsorption and selectivity among other gases such as CH4, N2, and H2 (non-polar). The functionalization process commonly occurs through pre-synthesis and post-synthesis modification procedures. So far, different functionalization methods have been introduced based on functional groups which are attached on the backbone of the frameworks. as the methods include amine, phosphonate, sulfonate, and multi functionalization processes.
5.2.1. Amine functionalization Amine functionalized ligands have more attraction among other modification methods because of the high affinity of the amine molecules toward CO2 adsorption and selectivity. Without a doubt, the amine molecules act as the Lewis basic and CO2 molecule as the Lewis acid (Liu et al., 2012b). For instance, Pachfule et al. introduced two isostructural interpenetrated amine functionalized MOFs; CoANIC-1 and Cd-ANIC-1 (ANIC: 2-amino-isonicotinic acid). Both MOFs exhibited a high CO2 uptake
capacity of 4.22 and 4.72 (mmol g-1) for Co-ANIC-1 and Cd- ANIC-1, respectively. This desirable CO2 uptake is attributed to the Lewis basic effect of amine groups which was presented in the ligand (ANIC) (Pachfule et al., 2011). Moreover, a comparison study between MIL-101(Al) and NH2-MIL-101 (Al) was performed by Serra et al. (Serra-Crespo et al., 2011). They reported that both CO2 capacity and selectivity of amine functionalized MIL-101 (Al) increased compared to the parent MOF. In addition, –NH2-MIL-101(Al) performed fast regenerability and high thermal stability making this MOF high potential adsorbent for separation of natural gas. Apart from the positive effects of amine groups on improving the CO2 capacity and selectivity into MOFs, amine groups have some limitations concerning the number of this functional group on the framework. If the ligand contains a high number of amine groups, the excessive amine cluster can reduce the CO2 uptake due to interwove of them on the backbone. An experimental and theoretical study of grand canonical monte carlo (GCMC) simulation on NJUBai3 ([Cu3L2(H2O)5]. x Guest) that contained polar amide groups was accomplished by Duan and coworkers (Duan et al., 2012). Both experimental and theoretical studies exhibited a high CO2 uptake (6.21 mmol g-1 at 1 bar and 273 K) and selectivity. Also, the high isosteric heat confirmed a strong affinity of this MOF toward CO2 adsorption (36.5 kJ mol-1). In order to modify the ligand via amine groups, the length of the chain and the activation methods must be considered as they have effects on the surface area, porosity and CO2 uptake capacity. With regard to the prominence of these parameters, Keceli and coworkers (Keceli et al., 2014) modified biphenyl ligand with the amide group which possessed different chain lengths (R: CH3, C2H5, C3H7, and C4H9) and employed two activation methods (the exchange with acetone by vacuum drying at 80 C for 24 h and supercritical CO2 drying). The results exhibited that the MOF functionalized having the amide with a shorter chain had more affinity to adsorb CO2 (4.65 and 2.95 (mmol/g) for R: CH3 and C4H9 respectively). In case of activation methods, different impacts were observed; for instance, in the short chains, supercritical CO2 increased the pore volume and the surface of the MOFs. In contrast, for the long chains, the exchange with acetone by vacuum system was increased the pore volume and surface area of MOFs. Accordingly, the activation methods have a critical effect on the
adsorption properties of the MOFs, and this is congruent with the previous literature (Bae et al., 2009; Hafizovic et al., 2007).
5.2.2. Open nitrogen sites Generally, the existence of open nitrogen sites in the framework has a positive effect on CO2 adsorption. Triazoles and tetrazoles are the best examples of this category. CPF-6 is the MOF that possesses a 3D structure with high concentration of uncoordinated nitrogen sites from tetrazole ligand (1,3,5-tris(2H-tetrazol-5-yl)benzene). This framework has indeed demonstrated high CO2 adsorption (98 cm3g-1) and selectivity over N2, even without any open metal sites (OMS). Superior CO2 adsorption properties are attributed to the dense of the open nitrogen sites in the pores surface (Lin et al., 2011). Although usage of open nitrogen sites in reinforcement of CO2 adsorption properties has a great impact, it may not be necessary at all. According to the results of a comparison study on tetrazolate base rthMOF and pyrazolate base rth-MOf, the pyrazolate form which has less uncoordinated nitrogen site (Table 8 ) displayed a higher CO2 adsorption capacity. This behavior is attributed to the strong electric field of the pyrazol group (Gao et al., 2015). Another illustration of tetrazolate functionalization MOFs is UTSA-49 [Zn(mtz)2] whose ligand is decorated by open nitrogen sites via Hmtz groups (5-methyl1H-tetrazole) and methyl groups. The Zinc atom attached to the mtz- ligand only forms two free nitrogen atoms which are neighboring the methyl groups on the ligand rings. Therefore, the other nitrogen atoms are uncoordinated on the surface of the pore and ready to trap the CO2 molecules. The amount of the CO2 uptake for this MOF is 13.6 wt.% at 298 K and 1 bar. Moreover, the reaction enthalpy was 27 KJ mol-1 revealing the affinity of framework to trap the CO2 molecules (Xiong et al., 2014). Adenine groups are also a good candidate for ligand functionalization due to uncoordinated nitrogen sites in their structure (Table 8). Adenine has been utilized in constructing many MOFs for various reasons, summarized as follows: 1) different nitrogen positions on its structure allow diversity of MOFs, 2) it inherently has a rigid structure whereby robust frameworks will be obtained 3) all of the adenine atoms are coplanar, meaning that an inter-ligand interaction (π–π) would be possible during formation
of the MOF(An et al., 2009; Song et al., 2014b). These striking properties make adenine an effective building block that can be used in constructing MOFs for CO2 adsorption. It is substantiated by a computational study that determined that the interaction energy between adenine and CO2 molecules is higher than other MOFs which comprise nitrogen linkers. The effect of adenine groups on bio-MOF-11 (Co2(ad)2(CO2CH3)2.2DMF·0.5H2O) was investigated by Rosi group (An et al., 2009). The results demonstrated a high CO2 adsorption capacity (4.1 mmol g-1), isosteric heat (45 kJ mol-1) and selectivity over N2 (75:1) at 298 K and 1bar. These promising CO2 uptake properties are attributed to the narrow dimension of the pores (compatible with CO2) as well as Lewis basic pyrimidine and amino groups of adenine in the framework. The effect of the long chain on the CO2 adsorption capacity on Co-adenine series MOFs (Bio-MOF-11 to 14) was examined as well. According to the results, by increasing the aliphatic chain length (acetate, propionate, butyrate, and valerate, respectively) the CO2 capacity declined at 1 bar and 273 as well as 298 K. This reduction was attributable to the entanglement of the long chain. Among them, Bio-MOF-11 showed the highest adsorption capacity (105 cm3 g-1) at both temperatures and 1 bar (Li et al., 2013). These promising CO2 uptake properties are attributed to the narrow dimension of the pores (compatible with CO2) as well as Lewis basic pyrimidine and amino groups of the adenine in the framework. The most prominent subclasses of the MOFs are Zeolitic imidazolate frameworks (ZIFs) which are attention-grabbing because of their unique features and their capabilities of being utilized in several applications. (Al-Maythalony et al., 2015; Eddaoudi et al., 2015). The gas adsorption capacity of these materials can also be improved as illustrated by Wang et al (Wang et al., 2014). They successfully synthesized new ZIF (ZTIF) by applying the tetrazolates ligand into the Zn-Imidazolate framework. The result disclosed a significant surge in the CO2 adsorption capacity (86 cm3 g-1 at 273 K), isosteric heat (Qst: is 22.6 kJ mol-1) as well as selectivity of CO2 over CH4 and N2 (6.3 and 59 at 273 K respectively). This enhancement was as a result of the presence of the uncoordinated N-heteroatom sites in the frameworks of this MOF which was created via the tetrazolate ligands sites.
5.2.3. Phosphonate and sulfonate functionalization Ligand functionalization can both exert a great impact on CO2 adsorption and selectivity over the other gases and improve the framework stability. In this context, phosphonate, and sulfonate organic ligands should be taken into account. These functional groups have a substantial effect on the water stability of the framework (Shimizu et al., 2009). The flue gas often contains a small quantity of water vapor which remarkably impacts CO2 adsorption, especially in the MOFs. Therefore, reinforcing the MOFs’ water stability plays a pivotal role in gas adsorption technology (Taddei et al., 2013). The first study based on water stable phosphonate-ligand for CO2 capture into MOFs was executed by Marco and coworkers (Taddei et al., 2013). They synthesized phosphonate co-ligand MOF [(Cu3 (H2L2) (bipy)2. 11H2O), H2L2: [N,N,N΄,N΄tetrakis(phosphonomethyl)hexamethylenediamine]. According to their observation, this MOF took up 77 cm3 g-1 of CO2 at ambient temperature and 10 bar, demonstrating a high value of adsorption. In addition, it has acceptable selectivity of CO2/N2 and this denotes two implications: firstly the polarity of the P–O groups on the ligand may boost the affinity of the CO2 uptake due to its quadrupole moment; secondly, after the activation process, no pores would remain to take up N2 molecules. In this respect, CALF-25 and CALF-30 are the other MOFs exhibiting good water stability (Gelfand et al., 2015; Taylor et al., 2012). In practice, both reported MOFs demonstrated a considerable CO2 uptake. In addition, the frameworks structures were still retained even after exposing them to 90% of humidity at 273K and 298 K for both MOFs. The water stability arises from the steric group on the ligand parts of the MOF, leading to creation of the hydrophobic framework. Recently, an experimental and computational study was accomplished on the CALF class of the MOFs by Gelfand et al. (Gelfand et al., 2017). They produced CALF-33 via coordination of Cu(II) and Phosphonate monoesters (PMEs) ligand. This new MOF not only led to a boost in the water stability of the framework but also improved the CO2 adsorption capacity of the MOF. This improvement is justified by the presence of the ester group in the ligand sites (CALF-33-Et3) which leads to enhancing the active site to trapping CO2 molecules.
5.2.4. Mixed ligand-functionalization and multi-functional ligands The idea of using mixed ligand in the MOFs’ structure creates a fantastic clue, especially in tuning the frameworks and boosting the adsorption properties. Combinations of several functional groups within the framework not only reinforces the CO2 capacity and selectivity but also causes an improvement in the MOFs’ stability (Zhang et al., 2013). Consequently, presence of multiple functional sites on the framework leads to an improvement in the CO2 uptake ability of the MOF. In recent years, these functionalization methods have attracted a plethora of attention due to outstanding results that can be achieved. For example, Shimizu et al. (Vaidhyanathan et al., 2009) employed oxalic acid (ox), 3amino-1,2,4-triazole(Atz) as a mixed ligand and zinc carbonate to synthesize multi-functional MOF [Zn2(Atz)2(ox)] through the solvotheraml synthesis technique. According to the findings, apart from the small pore size (pore volume: 0.19 cm3/g) and the surface area of 782 m2/g, the CO2 adsorption capacity of this functionalized MOF was high (4.35 mmol/g at 273 K and 1.2 bar). In addition, it showed a high enthalpy of adsorption (40.8 KJ mol-1) at zero coverage that represented the affinity of the MOF to take up CO2 molecules. Furthermore, the modified MOF does not have the capability of uptaking other gases such as Ar, H2, and N2. Having considered all these observations, it can be inferred that this modified MOF is a good candidate for CO2 adsorption and selectivity over the other gases. In the subsequent year, Shimizu and his group (Vaidhyanathan et al., 2010) investigated the activated sites on the backbone of the framework via a computational study and an in-depth X-ray diffraction analysis. They identified two active sites for CO2 interaction; free amine groups via hydrogen bonding between the oxygen of CO2 and the hydrogen of the amine (N-H…O) and the interaction between the lone pair electrons of the nitrogen and the C atom of the CO2 molecules. Moreover, the oxygen site of the oxalic acid ligand was very active to interact with the CO2 molecules via the carbon site as well as the adjacent molecules with each other. As a result, a combination of these multi-functional sites on a framework
contributes to enhancement of the adsorption capacity, the selectivity, and the affinity of this MOF for CO2. A comparison study on 5 isomorphs MOFs was undertaken by Luo et al. (Luo et al., 2012). These MOFs were constructed by Zn (NO3)2, L (N4,N4΄-di(pyridin-4-yl)biphenyl-4,4΄-dicarboxamide) and isophthalic acid which were decorated by different functional groups such as –H (1), -NH2 (2), - OH (3), - NO2 (4) as well as –COOH (5). The results demonstrated that the MOF (2) possessed the highest amount of CO2 adsorption capacity and selectivity among the other functional groups (2 > 1 > 3 > 4 > 5) at 1 bar and 273K. Moreover, the CO2 binding affinity of Sample 2 was higher than the others (based on the DFT). These high CO2 adsorption capacity and selectivity for Sample 2 were attributed to the high affinity of the CO2 to both amide and acylamide groups on the pores wall of the framework. The Cobalt-based MOF with triazole and benzenetricaboxilic based ligands were decorated by Liu et al. via the solvothermal synthesis reaction(Liu et al., 2013a). The end-products adsorbed 77.3 cm3 g−1 of CO2 at 1 bar and 295 K also displayed significant selectivity over N2. The high CO2 uptake and selectivity of this MOF were attributed to the presence of free amine groups, triazolate’s uncoordinated nitrogen atoms, unsaturated metal sites (Co2+) as well as -C=O/ -COOH sites on the pore walls of the framework (several functional groups). Lately, a comparison study on {[Zn4(bpydb)3(datz)2(H2O)]-(DMF)4(EtOH)5(H2O)8}n (1) and {Zn(bpydb)(bpy)}n (2) as a mixed ligand functionalization was performed by Chen et al. The pore walls of activated MOF (1) was decorated by pyridine and amino groups and demonstrated a higher CO2 uptake capacity (80.9 cm3 g -1 at 273 K and 1 bar) than MOF 2 (40 cm3 g-1 under the same condition) as well as outstanding selectivity over CH4 and N2 . Combination of free pyridine-N atoms and amino functional groups led to improvement of CO2 adsorption properties in MOF 1. The turning point in this research was the heat of the adsorption value which indicated an enhancing trend (30.33 kJ/mol) at zero coverage revealing a strong interaction between CO2 molecules and the framework (Fig. 10) (Chen et al., 2015).
Fig. 10. Qst of CO2 for MOF (1) (Chen et al., 2015).
5.2.5. Other functionalization Alkyl groups (CH3), Hydroxyl (OH), and (COOH) groups are other effective functional groups used in the ligand functionalization as reported by several studies. For the alkyl group, the best example is methyl functionalization on the pillared ligand of [Zn3 (bpdc)3 (bpy)].(DMF)4· (H2O) and produced [Zn3 (bpdc)3(2,2’-dmbpy).(DMF)x (H2O)y (1) and [Zn3 (bpdc)3(3,3’dmbpy)]·(DMF)4(H2O)0.5 (2). Methyl functionalization in this study brought two opposing results based on the competitive effects happening for these MOFs. After modification, both the pore volume and the BET surface area of MOFs (1) and (2) fell as compared to the parent MOF. On the other hand, Sample (1) possessed the highest amount of CO2 uptake and affinity to CO2 adsorption (based on Qst) whereas Sample (2) demonstrated the lowest amounts (Table 12). This different adsorption behavior is due to two contrary effects that occurred simultaneously throughout the CO2 uptake. If the upsurge of CO2 affinity is dominated over the space-loss during CO2 adsorption, CO2 uptake will then decline. For Sample 2, the enhancement in CO2 affinity was not high enough to tackle the loss of the surface area and the pore volume. Therefore, the CO2 uptake diminishes during the adsorption process (Liu et al., 2013b).
Table 12 CO2 adsorption properties of functionalized and parent Zn-MOFs. MOF
Pore volume (cm3g-1)
BET surface area (m2g-1)(a)
CO2 Uptake (wt %) (T: 298 K) (P: 1 atm)
Qst (kJmo l-1)(b)
Parent
804 (457)
0.21
10.2
30.5–28.7
1
466 (308)
0.18
11.5
31.6–29.1
2
378 (195)
0.13
8.6
30.8–25.2
Ref. (Liu et al., 2013b) (Liu et al., 2013b) (Liu et al., 2013b)
Hydroxyl functionalization is the other method used for improvement of the CO2 uptake capacity. This method is highly effective owing to polarity of the hydroxyl groups (OH) which can increase the affinity of CO2 adsorption based on the quadruple moment of the CO2 molecules. Zhao et al. (Zhao et al., 2011) modified Zn(BDC) via Hydroxyl groups and the results demonstrated a significant improvement in the CO2 adsorption capacity (13.1 wt. %) at ambient temperature and 1 atm as compared to the parent MOF (7.4 wt. %). In this case, the role of the surface chemistry was more effective than porosity in Zn (BDC-OH) which was due to the strong interaction between CO2 and OH groups on the backbone of the framework. Furthermore, selectivity of CO2 over CH4 and N2 was remarkably higher in Zn (BDC-OH).
5.3. Post-Synthetic procedure (PSM) Another method for MOFs structure tuning with the intention of improving the CO2 adsorption performance is the post-synthesis modification (PSM). The main objective of this method is to decorate MOFs with a high CO2 capacity and selectivity as well as lower energy consumption for the regeneration process. This method differs from the pre-synthesis modification as in the PSM, the functional groups are inserted into the preexisting MOF. In some cases, the PSM has some advantages over the pre-synthesis methods such as the possibility of the reaction between the functional groups
of the ligand with the metal site in the synthesis procedure of the primer framework which may alter the crystal structures; therefore, the end product will be changed. In addition, an accurate synthesis condition should be considered, particularly in the solvothermal synthesis reaction due to instability of some functional groups under the synthesis condition. Additionally, some other parameters are involved in pre-synthesis functionalization such as the hindrance effects, the solubility of the functional groups in the solvent where all these parameters might produce side-products. Thus, the PSM is an attractive route to improve the CO2 adsorption properties (Cohen, 2011; Deng et al., 2010). As mentioned formerly, the amine groups have a great affinity to adsorb CO2 molecules. Ethylenediamine (en) is the most common functional group which is used in PSM. A good example of en grafted MOF is en-MOF-74 with amine loading of 16.7 wt. %. This modified MOF exhibited an exceptional CO2 uptake (13.7 wt. %) at 298 K and 0.15 bar. The value of the isosteric heat was also very high (49 to 51 kJ mol-1) that exhibited chemisorption between CO2 molecules and amine groups resulting in formation of carbamic acid. This MOF just lost 3 % of the CO2 uptake capacity after 5 cycles of adsorption-desorption notwithstanding chemisorption of CO2 and amine groups. Besides, en-MOF74 exhibited high water stability in different amounts of moisture. Hence, en modified MOF possessed a great potential for CO2 adsorption (Lee et al., 2014). In 2014, Gadipelli. S et.al introduced the thermal annealing method for the first time as the post-synthesis route on MOF-5 (Zn4O[BDC]3). In this method, the framework is heated close to but below the decomposition temperature of the MOF to eliminate the solvents from the surface of the pores. The results showed high CO2 adsorption (up to 2 mmol g−1) at 298 K and 1 bar which was almost two times higher than the unmodified MOF-5. This indicated high stability for cyclic CO2 adsorption as well as moisture (Gadipelli and Guo, 2014). Some MOFs being treated via the post-synthesis methods are concisely mentioned in Table 13. Several post-synthesis modifications of the MOFs have been executed to date. Despite their efficiency, the PSM reactions require an accurate control due to formation of single crystals without any collapse and degradation of the framework. Yet, this area needs further investigations to improve
this method to create ideal frameworks especially in case of amine functionalization as the best functional group for MOFs treatment (Cohen, 2011). So far, numerous researches have been performed on ligand functionalization and comprehensive investigation is accessible in the literature in this area (Du et al., 2013; Li et al., 2016; Zhao and Sun, 2014) although more investigations are needed for optimization of this method.
Table 13 Some modified MOFs via the post-synthesis procedure. All adsorptions take place at 298 K, P (bar), Qst kJmol-1. Modified MOF Ni-DOBDCPyridine IRMOF-74CH2-NH2-
en-Cu-BTTir
BET(m2 g-1) Befor After e
P
Qst
798
409
-
-
low
-
2440
2310
-
3.2 (mmol g1)
1
-
345
0.277 (mmol g1)
0.366 (mmol g1)
up to 0.06
90
870
0.37 (mmol g1)
2.38 (mmol g1)
0.15
-96
608
0.33 (mmol g1)
4.2 (mmol g1)
0.15
-
-
23.4 wt %
26.9 wt %
760 torr
-
1770
Mmen- CuBTTir
1770
MIL-101- PEI (Polyethylen imine)
3125
Mg-MOF-74 TEPA (Tetraethylenep entamine)
CO2 Uptake Before After
1628
Remarks
Ref.
-Increased water stability also CO2 capacity was maintained. - Showed the highest CO2 uptake -After regeneration maintain its structure completely (at 65 % humidity). -Regarding to affinity to adsorb CO2 were very high which leads to enhancing CO2 adsorption
(Bae et al., 2014)
-Drastically enhance CO2 adsorption and selectivity over N2 (327) -Raised CO2 uptake capacity and selectivity --dramatically (selectivity CO2/N2: up to 770). - raised CO2 uptake due to higher amine groups (15 wt %) on the framework. Increased water stability due to protecting effect amine group on the surface of MOF.
(Fracaroli et al., 2014)
(Lee et al., 2014) (Gadipelli and Guo, 2014) (Lin et al., 2013)
(Su et al., 2017)
5.4. Tuning pore Size
As mentioned in section 3.1.2, the pore size of MOFs has affected their adsorption behaviors especially in terms of selectivity; this is named the size-exclusive effect. Since each molecule
has unique kinetic diameters (Table 1), the adsorption selectivity occurs based on the pores size compatibility of the MOF and the kinetic diameter of the targeted molecules. Therefore, only smaller molecules can pass through the pores and the other ones are blocked. The ideal pore diameters of the frameworks for selective adsorption of CO2 over N2 and CH4 are 3.3 Å - 3.6 Å. Customarily, this method is expedient for selective adsorption of small gas molecules such as H2, N2, CO2, and CH4. The desirable pore size is achieved through the ligand or the metal exchange. According to the literature, the bulky linker, the short ligand, the interpenetrated networks, and smaller metal molecules are the best candidates in this respect (Kim et al., 2012; Ma et al., 2007a; Zhao et al., 2010). The pore tuning modification takes place through the postsynthetic exchange (PSE) process. The effect of the pores size on the adsorption behavior of the MOFs based on the ligand and the metal exchange has been examined by several researchers. In this aspect, zirconium-based MOFs such as UiO-66 (Zr) are good examples. According to the literature, these MOFs perform moderate CO2 adsorption at the ambient condition (Cmarik et al., 2012). The metal ion exchange in UiO-66 (Zr) to Ti (IV) was accomplished by Kim et al. for the first time (Kim et al., 2012). Also, the CO2 adsorption performance of the exchanged UiO-66 (Ti) was scrutinized by Lau et al. The results showed two times enhancement in both the CO2 uptake (81%) and the heat of adsorption. The smaller size of Ti ions in the exchanged framework afforded a smaller pores size which may be closer to the kinetic diameter of the CO2 molecules. Therefore, the CO2 properties of the new framework increase (Lau et al., 2013). As stated above, another option to reduce the pore size is using the interpenetration network (Zhao et al., 2010). Using elongated linkers leads to entanglement in the crystals structure, and this is called interpenetration. It means that the second or more identical frameworks are formed within each other (self-assemble) (Han et al., 2012). Hence, these networks are interwoven with one other and create the interpenetration network. As a result, the stability of these frameworks will increase while the available pore volume will decrease. Indeed, by creating more frameworks and their entanglements, the size of
the pores in these kinds of frameworks will be declined. Hence, the MOFs with this structure are the ideal frameworks for the gas separation and purification technology. This has been also confirmed by Han and Heo. They evaluated the effect of interpenetrated and porosity of 14 frameworks via the GCMC simulations method. As anticipated, the pore size of the interpenetrated frameworks reduced. Unexpectedly, the CO2 uptake capacity decreased at high pressure because of the pores volume reduction. The CO2 adsorption increased at lower pressure due to creating new sites to absorb the CO2 molecules. Indeed, this high adsorption capacity and selectivity at low pressure (up to 2 bar) are related to creating new CO2 adsorption sites with a high affinity in the interpenetrated frameworks. These sites have been obtained through the generation of spaces with two organic linkers of adjacent MOFs(Han et al., 2012). PCN-5 (H2 [Ni3O (H2O) 3(tatb)2] . 5DMF . 2H2O) is a double-interpenetrated framework which is suitable for selective adsorption of CO2 over CH4 based on the size-exclusive effect. This framework is constructed by 3 connected ligands (tatb) which are linked to the tri-nuclear Ni SBUs. The pore size of PCN-5 is 3.3 Å which is compatible with the kinetic diameter of the CO2 molecules (3.3 Å). Thus, this framework is proper for molecular sieving of CO2 over other gases such as CH4 and N2 (Zhao et al., 2010). Another interpenetrated framework which has been introduced recently is Cd-MOF which was synthesized via the solvotheraml method as a new 2-fold interpenetrated framework by Qin and coworkers. This framework demonstrated selective adsorption of CO2 over CH4, due to both the CO2 molecules strong interaction with the framework and the size-exclusive effect of this framework (Qin et al., 2014). Gao et al. presented a 4-folded interpenetrated framework MMPF-18 (metal−metalloporphyrin framework) by using a linear organic linker H2bcpp (5,15-bis(4carboxyphenyl)porphyrin) and tetra nuclear zinc cluster through the solvothermal method. They investigated the CO2 adsorption properties of this interpenetrated MOF at the ambient condition. The results showed a moderate CO2 adsorption capacity (11.8 wt %) and the heat of CO2 adsorption was around 23 kJ mol-1. In spite of a moderate CO2 uptake, the framework showed good CO2 / CH4 selectivity. This behavior is not only attributed to the narrow pores in the interpenetrated network but also to the presence of the epoxy groups of the ligand which afford to the size-selective chemical
transformation of CO2 and formation of cyclic carbonates (Gao et al., 2016). The highest amount of CO2 uptake on the interpenetrated MOF was reported by Chen et al. They synthesized a triple selfinterpenetrated MOF with nanoscale cages. This MOF showed the highest CO2 adsorption capacity of 2.23 mmol g-1 that has been reported by far among the interpenetrated frameworks. This framework also performed high Qst for CO2 adsorption (52.5 kJ mol-1) which confirmed the framework’s high affinity toward CO2 molecules. This MOF’s superior adsorption behavior is due to the contribution of nanoscopic cages and the charge of the frameworks (Chen et al., 2014). To gain high CO2 adsorption in the interpenetrated frameworks besides usage of effective functional groups, their accessibility plays a crucial role. In this regard, Safarifard et al. synthesized novel 3-fold interpenetrated MOFs (TMU22, -23 and -24) which were functionalized with different amide groups. Based on kinetics and breakthrough experiments, the selectivity of CO2 over N2 in TMU-24 was much higher than TMU-22 and -23 (70 % for kinetic experiment). Furthermore, TMU-24 showed the highest amount of Qst for CO2 in different temperatures 24-26 (kJ mol-1). More accessibility of the amide functional groups in the TMU-24 leads to high CO2 selective adsorption in this MOF. It means that the amide groups in this framework were more directed into the pores than the other isoreticular frameworks (TMU-22, -23) (Safarifard et al., 2016). In 2016, Chen et al. generated the supramolecular isomerism frameworks (Qc-5-M-dia and Qc-5Cu-sql-α) to improve selective adsorption of CO2/N2 and CO2/CH4 via tuning the pore-size. Both frameworks possess the same composition but with different topologies and pores sizes (Qc-5-M-dia: 4.8 Å and Qc-5-Cu-sql-α: 3.8Å). Interestingly, in the desolvation process of Qc-5-Cu-sql-α, the framework underwent an irreversible change to the Qc-5-Cu-sql-β which had a smaller pore size (3.3). According to their observation, Qc-5-Cu-sql-β showed the best selective adsorption of CO2 over N2 and CH4 which was attributed to the compatibility of the framework’s pore size (3.3 Å) with the kinetic diameter of CO2 molecules. Additionally, Qc-5-Cu-sql-β exhibited high water stability. This research demonstrated the effect of desolvation on the tuning of the framework’s pore size(Chen et al., 2016). Generally, tuning the pore size is fairly daunting for both flexible and rigid MOFs. For flexible MOFs, it is more complicated due to the dynamic behavior of the framework. For the rigid ones, the
accurate design of the pore size has been achieved rarely(Chen et al., 2016), which is because the control of the pore size between the ranges of 3–4 Å (appropriate for gas separations) is demanding (Zhang et al., 2014).
6. A new strategy for enhancement of CO2 adsorption properties on MOFs Rather than the previous reinforcement methods, some new strategies have been introduced to this technology. One of the best methods of improving CO2 adsorption which has enthused the researchers nowadays is producing MOF composites. The key criterion of using MOF-composites is their synergistic effects on their adsorption behaviors. According to the literature, usage of some adsorbents with high surface areas such as graphite, graphene, and carbon nanotube (CNT) has an outstanding effect on the adsorption behavior of the MOF-composites. For graphite and graphene, these materials should be added to the MOF structure in the form of graphite oxide (GO) or graphene oxide (GrO) due to the strong binding between the functional groups of GO and GrO (epoxy, carboxylic, hydroxylic) and the metal site of the framework. These materials have been introduced as one of the best adsorbents with the ultra-high surface area and the robust structure. Several MOFs have been produced by this great adsorbent as the MOF-composites which are applicable in different areas. For CO2 adsorption, some investigations have been accomplished to date. For instance, GO@MOF-505 composite which is produced from graphite oxide and copper-based MOF via the hydrothermal (solvothermal) route (Chen et al., 2017). This composite has shown good selectivity of CO2 over CH4 and N2. The CO2 adsorption capacity of the composite has 37.3% increase as compared to MOF-505 (parents). This remarkable enhancement of CO2 adsorption can be related to the presence of the unsaturated metal sites, creating new micropores, and improvement of dispersive forces in the surface of the composite framework. In addition, GO@MOF-505 has shown high moisture stability at 80 % RH. This stability is attributed to the strong coordination interaction between Cu ions and carboxylate groups in GO.
The effect of graphene oxide (GrO) on CO2 adsorption properties of Cu-BTC was investigated by Huang et al.(Huang et al., 2014). They synthesized GrO@Cu-BTC as a MOF composite via the hydrothermal reaction. The result indicated dramatic enhancement in selective adsorption of CO2 over CH4 rather than Cu-BTC (2.6 times that of parents). This improvement takes place due to a stronger interaction of CO2 molecules to the GrO@Cu-BTC framework. Since this method is new especially for the CO2 adsorption technology not, there is a dearth of research and a need for further investigations is felt because of their prominent effects on the CO2 adsorption capacity and selectivity. Some MOFcomposites which are used for CO2 adsorption are mentioned in Table 14. Similar to GrO, carbon nanotube (CNT) is an excellent candidate to produce the MOF-composite. Anbia andHoseini designed an MOF-composite (MWCNT@MIL-101 (Cr)) by using a multi-wall carbon nanotube (MWCNT) and MIL-101(Cr) (Anbia and Hoseini, 2012). The results indicated 60 % improvement in CO2 adsorption for the composite which is related to the higher micropore volume in the composite in comparison with the parent MOF. The high volume of porosity was achieved due to the presence of the MWCNT. Moreover, the composite showed high thermal stability. The effect of functionalized CNT (hydroxyl-CNT) on the MOF as a new composite was performed by Yang et al (Yang et al., 2014). They produced ZIF-8/CNT by using ZIF-8 and hydroxyl-CNT. According to their observation, not only CO2 adsorption had a rise but also the thermal stability underwent an improvement due to the presence of hydroxyl-CNT in the composite framework. It is striking to note that the morphology and the crystal structure of ZIF-8 in the composite were the same as the parents. In addition, a computational study has also been undertaken to investigate the selective adsorption of CO2 over N2 on single-walled carbon nanotube by Du et al.. They demonstrated that the combination of a van der Waals correction with ab initio DFT for explaining long-range dispersive interactions of CO2/N2 in the CNTs structure is very effective. Their results demonstrated enhancement in the selectivity of CO2 over N2 by doping of an Fe atom on the CNT surface. This improvement occurred due to quatherapole moment of CO2 molecules. Interestingly, none of the N2 molecules was adsorbed on the framework (Du et al., 2009).
Table14 Composites-MOFs for CO2 adsorption and selectivity. Composite-MOF
BET surface area (m2 g-1) Parents Composite
CO2 uptake (mmol/g) Parents Composite 7.09
Selectivity
T (K)
P (bar)
298
15
(Policicchio et al., 2013)
298
1
(Chen et al., 2017)
CO2/CH4: 32
298
25
CO2/N2: 24.77 CO2/CH4: 9.57 CO2/N2: 37.2 CO2/CH4: 8.6
Aminated Cu-BTCgraphite oxide
892
1367
GO@MOF-505
1101
1279
2.87
GrO@MIL-101
2670
2950
14.6
GrO@Cu-BTC
1382
1677
6.49
8.19
CO2/CH4: 14
273
1
GO@HKUST-1
1193
1554
6.85
9.02
CO2/N2: 186
273
1
MWCNT@MIL-101
1270
1243
0.84
1.35
298
10
ZIF-8/CNT
1839
1997
2.176
2.210
273
1
13.41
3.94 22.4
CO2/N2: 12.47
Ref.
(Zhou et al., 2015) (Huang et al., 2014) (Xu et al., 2016) (Anbia and Hoseini, 2012) (Yang et al., 2014)
In 2015, Kang and coworkers produced MWCNTs@JUC32-1 and MWCNTs@JUC32-2 as new composite-MOFs (Kang et al., 2015). These composites were prepared by using nano-sized JUC-32 and MWCNTs. According to the results, composite-MOFs captured more CO2 and CH4 per specific surface area as compared to their parents. Moreover, Qst of CO2 and CH4 in both composites had a significant improvement which was 32.3 kJ·mol-1, 33.8 kJ·mol-1 and 23.4 kJ·mol-1 for MWCNTs@JUC32-1, MWCNTs@JUC32-2 and parents MOF, respectively. In order to achieve a desirable MOF-composite, an optimum amount of the filler (GO, GrO and CNT) must be considered (optimization). Several MOF-composites have been reported so far, but in order to CO2 capture and storage, just some articles are available. Regarding these prominent effects of the fillers on the adsorption behavior of the Composites-MOFs, more investigations are needed in this area of research for future working.
7. The effect of water on the CO2 adsorption behavior of the MOFs
The most challenging part of the CO2 capture via the MOFs is the presence of water molecules during the adsorption procedure. 5-10 % of the flue gas from the power plant contains water molecules and most frameworks have a high affinity to react with the water molecules. Therefore, this would lead to distortion or even collapse of the framework. Subsequently, the pores are destroyed and the CO2 capacity and selectivity experience a decline. The most important and the weakest part of the framework is the coordination bond between the metal site and the ligand. Thus, the water molecules break this bond via the hydrolysis reaction. As shown by Equation 1, the ligand bond undergoes a displacement; therefore, the structure of the framework is destroyed (Sumida et al., 2012).
ML + H2O → M (OH) + LH
(1)
One of the prominent factors is the pKa value of the ligand which indicates the strength of the coordination bond between the ligand and the metal. If the ligand possesses high pKa, the basicity of that ligand is also higher. Therefore, the metal–ligand interaction will be stronger. Further to this, the kinetic and thermodynamic factors should be considered due to the Gibbs free energy of the reactant via the hydrolysis reaction (Burtch et al., 2014). As mentioned above, the kinetic and thermodynamic studies play important roles in improving MOFs’ water stability. In line with this, several studies have been accomplished in this area up to the present time. Jasuja and coworkers (Jasuja et al., 2013) conducted several studies on water stability in different MOFs. They evaluated the water stability of isostructural Zn(BDC-X)-(DABCO)0.5 series (DMOFs) via ligand functionalization with a polar group (fluorine) and nonpolar groups (methyl). While the structure of the DMOF was unstable in the presence of water, DMOF-TM2 (tetramethyl-BDC ligand) maintained its structure during the cyclic water adsorption procedure. Moreover, the molecular simulation showed improvement in DMOF-TM2’s kinetic stability because of the shielding effect of the methyl groups (nonpolar) on the ligand site. Therefore, no water molecules can come in the vicinity of the BDC to create the hydrogen bond, leading to the displacement of the ligand-metal interaction. Hence, there is
no chance for the electrophilic zinc atoms of the frameworks to react with the nucleophilic oxygen of the water which has caused the hydrolysis reaction. In the succeeding year, they appraised the impact of catenation and pKa of the ligand on the water stability of the framework. They chose two sets of MOFs; DMOF and MOF-58 (parents) also DMOFTM and MOF-508-TM (modified with the tetramethyl group). The results indicated that in the parents set, MOF-508 was completely water stable after being exposed to 90 % relative humidity (RH) while possessing a lesser pKa value. This result was attributed to the two-fold catenation effect of this MOF. In the TM modified set, DMOF-TM remained stable after 90 % RH which was related to the pKa value of the pillared ligand in this MOF (pKa for DMOF-TM and MOF-508-TM are 3.80, 8.86 and 3.80, 4.60, respectively). Moreover, the MOF-508-TM framework was not penetrated because of the steric hindrance of four –CH3 groups on its linker (Jasuja et al., 2014). Accordingly, utilizing the interpenetrated MOFs with a combination of the ligands having high basicity displayed good water stability. Based on their previous study, they synthesized new pillared MOFs with different metal sites (Cu, Ni, Zn, and Co) and a similar topology. All MOFs showed good water stability in the humid condition (90 % RH). Two main factors were involved in the great water stability of these MOFs including the presence of the methyl groups (in the BDC structure) and catenation in the framework due to the BTTB linker. The methyl groups preserve the metal site via the steric factors; therefore, the water molecule cannot react with the metal site. For the case of the BTTB ligand, the presence of catenation in its framework improved its water stability even with less basicity than the DABCO linker (Jasuja and Walton, 2013). Bae and coworkers conducted a research on the pyridine modified Ni-DOBDC to explore the CO2 capacity of the MOFs in the presence of water. According to their result, the modified MOF maintained the CO2 adsorption capacity whereas the water adsorption was reduced due to an increase in hydrophobicity of the framework (the presence of the pyridine group into the framework structure) (Bae et al., 2014). In 2016, Benoit et al. (Benoit et al., 2016) synthesized the titanium based MOF (MIL-91(Ti)) which possessed small pores. This hydrophilic MOF showed high water stability after several cyclic water
adsorption/ desorption processes due to the presence of the phosphonate group in the ligand site. Additionally, it showed a high CO2 uptake (~3 mmol g-1 at 303 K and 1 bar) which was the highest amount among the other small pore MOFs. High selectivity over other gases, rapid kinetics, and high Qst: (-47 kJ mol-1 at zero coverage) are other great properties of this metal bisphosphonate framework. Recently, Pal et al. (Pal et al., 2016) improved the water stability of the carboxylate MOF via fluorine, and amine ligand functionalization as well as transmetalation (Zn to Cu). They synthesized {[Zn2(L)(H2O)2]·(5DMF)·(2H2O)}n (Zn(II)-MOF) through the solvothermal reaction which was then followed by the metal exchanging process to produce the Cu(II)-MOF without any changes in the crystallinity of the framework. The new MOF performed higher water stability, CO2 uptake and selectivity of CO2 over N2. In their report, an increase of water stability was attributed to the lining effect of the fluorine molecule which made the surface of the pores hydrophobic. In addition, the higher stability of the Cu (II) than Zn (II) reinforced the metal-ligand bond which in turn increased the strength of the frame in the humid condition. Regarding all the studies in this area, the ligand–metal strength, kinetic and thermodynamic factors of the MOFs are the main keys in improving water stability of the framework that should be further investigated.
Conclusion To be laconic, a sheer of research in CO2 capture and separation has been accomplished on the MOFs during last two decades. This is due to their exceptional properties such as large pores volumes, high surface areas, tenability, diversity in their structures, and their environment-friendly characteristic. In the present review paper, we investigated most new MOFs which are specifically used in the CCS technology. Different synthesis routes, selectivity over other gases, the CO2 adsorption capacity and limitations in using the MOFs were all reviewed thoroughly. As mentioned above, the key criteria in the CO2 capture technology by using the MOFs are a high surface area and the presence of effective functional groups on the surface of the framework. In addition, pressure and temperature play a major role. At the low-pressure region, the CO2 adsorption capacity is subordinate to the heat of adsorption while at high pressure, the CO2 adsorption capacity is subordinate to the surface area of the MOFs.
Regarding the presence of the other gases in the gas stream, we also surveyed the selective adsorption of CO2 over other gases based on the selective adsorption mechanism. Besides, the selectivity synthesis routes and their effects on the CO2 adsorption capacity were investigated. As stated previously, the synthesis methods have more effects on the size of the pores (nano-size) and consequently on the surface area of the framework. Further to the MOFs’ outstanding CO2 adsorption properties , their structure can be optimized via the activation process, the pre-synthesis, the post-synthesis procedure, and producing the MOF-Composites by using additives such as graphite, graphene and CNT. Although MOFs exhibit a high CO2 adsorption capacity and selectivity, further investigations are still required to improve the ability of these excellent adsorbents. Usage of the MOFs in the CCS technology also faces some challenges such as low water, thermal and mechanical stabilities in harsh conditions. In terms of water stability, some researches are available as mentioned in section 5 but still more investigations are required. For thermal stability, the output gas flow from the power plant has high temperature while most MOFs examined so far show high CO2 adsorption and selectivity at ambient temperature. Hence, production of the MOFs with high thermal and mechanical stabilities are essential and need much attention for future studies.
Acknowledgements This project was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under grant No. (DF-434-829-1441). The authors, therefore, gratefully acknowledge DSR technical and financial support.
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Graphical Abstract
Highlights
The synthesis and applications of MOFs for CO2 adsorption was reviewed. Comparison among different synthesis routes. The synthesis routes effects on the CO2 adsorption capacity. High CO2 adsorption and selectivity found at ambient temperature.
S0048-9697(19)35082-X https://doi.org/10.1016/j.scitotenv.2019.135090 STOTEN 135090
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Science of the Total Environment
Received Date: Revised Date: Accepted Date:
8 September 2019 16 October 2019 19 October 2019
Please cite this article as: T. Ghanbari, F. Abnisa, W.M.A. Wan Daud, A Review on Production of Metal Organic Frameworks (MOF) for CO2 Adsorption, Science of the Total Environment (2019), doi: https://doi.org/10.1016/ j.scitotenv.2019.135090
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TITLE PAGE
Title: A Review on Production of Metal Organic Frameworks (MOF) for CO2 Adsorption
Author names and affiliations: Mrs. Taravat Ghanbari Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603, Kuala Lumpur, Malaysia. Email: [email protected] Associate Professor Dr. Faisal Abnisa (Corresponding Author) Department of Chemical and Materials Engineering, Faculty of Engineering, King Abdulaziz University, Rabigh, 21911, Saudi Arabia Email: [email protected]; [email protected] Professor Dr. Wan Mohd Ashri Wan Daud Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603, Kuala Lumpur, Malaysia. Email: [email protected]
A Review on Production of Metal Organic Frameworks (MOF) for CO2 Adsorption Taravat Ghanbaria, Faisal Abnisab,*, Wan Mohd Ashri Wan Dauda aDepartment
of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603, Kuala Lumpur, Malaysia.
bDepartment
of Chemical and Materials Engineering, Faculty of Engineering, King Abdulaziz University, Jeddah, Saudi Arabia
Abstract The environment sustenance and preservation of global climate are known as the crucial issues of the world today. Currently, the crisis of global warming due to CO2 emission has turned into a paramount
concern. To address such a concern, diverse CO2 capture and sequestration techniques (CCS) have been introduced so far. In line with this, Metal Organic Frameworks (MOFs) have been considered as the newest and most promising material for CO2 adsorption and separation. Due to their outstanding properties, this new class of porous materials a have exhibited a conspicuous potential for gas separation technologies especially for CO2 storage and separation. Thus, the present review paper is aimed to discuss the adsorption properties of CO2 on the MOFs based on the adsorption mechanisms and the design of the MOF structures. In addition, the main challenge associated with using this prominent porous material has been mentioned.
Keywords: CCS, CO2, MOFs, Synthesis, Selectivity, Capacity
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*Corresponding author E-mail addresses: [email protected] (T. Ghanbari); [email protected] (F. Abnisa); [email protected] (W. M. A. W. Daud).
1. Introduction Along with the rapid rise of global population, energy consumption has been growing significantly. One of the main sources of energy in the world is fossil fuel, which has currently attracted a plethora of attention and demand as an energy source in worldwide. By now, beyond 85% of the global energy demand is being supported by fossil fuels burning, notwithstanding the emission of huge amounts of CO2 into the atmosphere (Rackley, 2009). Based on the literature, the CO2 concentration in the atmosphere has boosted dramatically from 340 ppm in 1980 to 408 ppm in 2019, leading to an adverse impact on the environment (ESRL, 2019). The high level of CO2 emission leads to an increase in the average temperature of the Earth. For that reason, effective CCS techniques are required in order to minimize the carbon dioxide emission, and this will then protect the environment and maintain the climate temperature (Yu et al., 2012). Besides, the captured CO2 can be potentially used for many applications such as refrigerants, polymers, chemicals, cosmetics, and fertilizer. Moreover, it also
becomes an important precursor in the petroleum industry especially in enhancing the oil recovery (Abdi-Khanghah et al., 2018; Muthuraj and Mekonnen, 2018). Various CO2 capture technologies are presently available including the absorption (chemical and physical absorptions), the adsorption, and the membrane technologies (Yu et al., 2012). Several experimental and computational studies have been accomplished concerning all the above techniques so far (Li et al., 2019; Li and Zhang, 2018). Of these technologies, the chemical absorption of amine scrubbing has been found to be a common technique used especially in industrial settings such as power plants (Rochelle, 2009). Although the use of amine technique can lessen CO2 up to 98 % (Yeh et al., 2005), this method is disadvantaged with high energy consumption, a large quantity of absorber requirements, and corrosiveness, etc. (Yu et al., 2012). As a result, to mitigate these inherent problems associated with the chemical absorption, solid adsorption processes have been highly recommended and studied. So far, considerable classes of porous materials have been scrutinized and introduced as adsorbents, as among which zeolites, activated carbon, metal-oxide molecular-sieves, aluminophosphates, activated alumina, silica gel, pillared clays, carbon nanotubes, inorganic and polymeric resins, porous organic materials, as well as porous metal–organic frameworks (Chue et al., 1995; Loureiro and Kartel, 2006; Paul and Wright, 2008; Warrendale, 2005; Xu et al., 2009). Of the mentioned materials, zeolites and activated carbons have been extensively studied, being found to be more efficient for the CO2 capture (Díaz et al., 2008; Zhang et al., 2008). Nevertheless, they suffer from some limitations such as low CO2 adsorption capacity at a high temperature as well as instability in the presence of water (zeolite based adsorbents). It is acknowledged that although activated carbon has a greater CO2 adsorption capacity than zeolites, CO2 molecules interaction in the activated carbon is very weak and thus making this adsorbent sensitive to temperature changes. Hence, by raising the temperature of the process, the CO2 adsorption capacity drops dramatically and becomes relatively poor in selectivity, especially in the case of CO2/N2 selectivity (Zhang et al., 2008).
To alleviate this issue, novel functional materials, known as metal-organic frameworks (MOFs), have been introduced as adsorbents. MOFs are also recognized as coordination polymers or coordination networks. These novel hybrid materials are formed by the combination of metal ions or clusters organic ligands which are linked via coordination bonding. This kind of adsorbent has numerous merits like having robust 3D structures, a remarkable high surface area, controllable pore structures, and tunable pore surface properties (Millward and Yaghi, 2005). Combination of these favorable properties makes MOFs ideal while being the most promising adsorbent for gas adsorption, particularly for CO2 capture and sequestration. Moreover, these novel materials have other applications in diverse areas such as sensors, drug delivery, and catalysis (Kuppler et al., 2009). So far, numerous review papers have addressed MOFs in different areas; yet, this review paper meticulously elaborates on the contemporary MOFs which have been used for CO2 adsorption and separation with respect to synthesis methods, CO2 uptake and selectivity, methods to improve CO2 adsorption and the water stability of MOFs in CO2 adsorption.
2. MOF as an adsorbent As novel functional materials, MOFs have grabbed the attention of many researchers for the past two decades. These novel hybrid porous materials are formed by combination of organic ligands and metal-containing clusters or metal nodes (Fig. 1). Nearly all metals and a large diversity of organic species can be used to create MOFs; therefore, a huge variety of MOFs materials having different structures and properties are then available. The main characteristics of the MOFs are their robust 3D structure, their permanent porosity, and their modular nature. These unique properties of MOFs assists these adsorbents in preventing their structure from removal of guest molecules like the solvents in their open channels and pores. Therefore, the MOFs can commonly maintain their structures with negligible damage and can be utilized for other guest adsorption (Kuppler et al., 2009; Millward and Yaghi, 2005; Yu et al., 2012). Besides, extraordinary surface areas, ultrahigh porosity (Furukawa et al., 2013; Furukawa et al., 2010; Rowsell and Yaghi, 2006; Tanabe and Cohen, 2011), flexibility in tuning the
porous structure (Yuan et al., 2010a; Zhao et al., 2009) and also surface functionality could be considered as the prominent characteristics of the MOFs.
Fig. 1. Simple schematic of forming metal organic frame work (MOF)
Due to the presence of functional groups in the organic ligand, MOFs have the capability of being tuned while being chemically modified (Furukawa et al., 2013). The most salient advantage of the MOFs over the other solid materials is the possibility of designing the pore size and the functionality by selection of the organic ligand, the functional group, the metal ion, and the activation method. These favorable properties make MOFs attractive porous materials with the capability of being used in different areas such as sensors, biomedical, catalysts as well as gas storage in particular for CO2 adsorption(Kuppler et al., 2009). This review paper investigates the usage of the MOFs for CO2 adsorption and separation from the synthesis routes. Furthermore, we address the separation and synthesis mechanisms as well as the limitation in using this kind of adsorbent for CO2 capture.
3. CO2 capture using the MOF 3.1. Selectivity of CO2 over other gases One of the most essential parts of gas adsorption is selectivity of the targeted gas over other components of a mixture. To clarify this subject, it is essential to initially enlighten the principles, methods, and mechanisms involved in adsorptive gas separation.
3.1.1. Principles and methods of adsorptive gas separation Separation is a method to detach the components of a mixture, whose procedure often requires large amounts of energy for dividing the substances of a mixture (Yang, 2003). Furthermore, separation can be attained via selective adsorption and this takes place when the components on the surface of the adsorbent have different affinities to adsorb the diffused guest molecule. Gas separation methods comprise membrane-based, cryogenic distillation, absorption and adsorption based technologies(King, 2013). Among these methods, adsorptive separation has a greater attraction due to potentially lower energy consumption, easier maintenance, flexibility in the design for different applications, a simple operation (Rouquerol et al., 2013; Yang, 1987) and the environment-friendly characteristic of this method (Rouquerol et al., 2013). Generally, in the process of adsorptive gas separation, two main steps are involved; adsorption and desorption processes. In the adsorption stage, a mixture of the gases is passed through a fixed-bed adsorber or a column packed with the adsorbents. Then this process is ensued by the desorption step to remove the component attached on the surface of the adsorbent for reuse (Yang, 2003). Therefore, the gas adsorption process is dependent on the desorption methods used for regeneration of the adsorbent. Several cyclic adsorption processes are used in the gas separation technology including the inert purge cycles, the displacement cycles, the temperature swing adsorption (Gao et al.), the pressure swing adsorption (PSA), the vacuum swing adsorption (VSA) (Fig. 2), and the electrical swing adsorption (ESA) (Ling et al., 2014; Yang, 1987). Although all these methods are expedient for gas separation and purification, the TSA and PSA are more commonly used in CO2 sequestration. The TSA desorption occurs by heating the adsorbent, has albeit its long cycle time due to the heating/cooling stage that requires a few hours up to over a day. On the other hand, the PSA is the process whereby regeneration is achieved by lowering the pressure of the adsorbent. This cycle has some advantages over the TSA such as a higher throughput of the cycle (the process usually occur in seconds or minutes) and a
temperature-independent process. The PSA is predominantly used for bulk separation. Another more recent method is the VSA that is of a greater interest for the researchers because it operates at near ambient pressure and most flue gas streams pressures are approximately atmospheric. As opposed to the other methods, , the PSA and VSA have typically more desirability thanks to fewer energy requirements of these cycles (Yang, 1987).
Fig. 2. Schematic of TSA, PSA and VSA cyclic processes for regeneration of MOFs in the fixed-bed column.
It should be noted that adsorptive gas separation is attained when different components of the adsorbent have different abilities to adsorb. Also, a promising adsorbent should contain acceptable mechanical properties such as high selectivity, capacity, regenerability as well as promising adsorption kinetics.
3.1.2. Mechanisms of selective adsorption
Four mechanisms exist for gas adsorptive separation on porous materials; 1) the molecular sieving effect (size and shape exclusion), 2) the thermodynamic equilibrium effect, 3) the kinetic effect and 4) the quantum sieving effect. As such, it is well documented that typically gas adsorption is obtained with one or more than one mechanisms (Keller and Staudt, 2005; Yang, 2003). In size/ shape exclusion (steric separation), the selective adsorption is achieved based on the shape and cross-sectional size (kinetic or collision diameter) of the adsorbate molecule. The Kinetic or collision diameter means the closest distance or diameter between two molecules with zero kinetic energy that can approach each other and have a collision. According to the literature, this effect is common in zeolite and molecular sieves adsorbent. Besides, it is noteworthy to acknowledge that the pore size of some adsorbents is sensitive to temperature; therefore, molecular sieving in these cases are affected by temperature (Li et al., 2009a). The second mechanism, i.e. equilibrium separation (dynamic separation) is useful when the pores size of the adsorbent is large enough; consequently, all the components of the gas mixture can pass inside the adsorbent. In this condition, the interaction between the surface of the adsorbent and the adsorbate molecules has a vital impact on the selective adsorption quality. It should be pointed out that the strength of this interaction is related to the properties of the adsorbate molecule and the surface characteristics of the adsorbent such as polarizability, magnetic susceptibility, the permanent dipole moment, and the quadrupole moment (Li et al., 2009a). Kinetic separation is utilized when equilibrium separation is not feasible. The most important issue in kinetic separation is that the pore size of the adsorbent should be controlled accurately between the kinetic diameters of the two molecules that require to be separated. As stated by the previous researches, this mechanism is used for separation of CH4 from CO2 by utilizing a carbon molecular sieve while another example is the separation of air by using zeolite as an adsorbent via the PSA method (Li et al., 2009a; Yang, 2003). For the quantum sieve effect, the adsorption is accomplished based on differences in diffusion speeds of the guest molecules and compatibility of the pore diameter with the de Broglie wavelength of these
molecules. This effect can be particularly useful for isotopic separation like H2 /D2 separation (Li et al., 2009a).
3.1.3. Selection of the adsorbent and its criteria One of the important parts of adsorptive separation is the adsorbent selection. Generally, the best adsorbent is the one that has both high selectivity and capacity of adsorbing the targeted molecules. Hence, for choosing the adsorbent, the followings are to be considered: the nature of the adsorbent (also nature of its pores) and the adsorption process. For instance, in the bulk gas separation process, the adsorbent must be selected based on the ease of desorption. If the separation process is not considered, the nature of the adsorbent is the key criterion for choosing the adsorbent including the size and the shape of the adsorbate molecule, the dipole moment, the quadrupole moment, and the polarizability. For example, if the targeted molecule has a high dipole moment, the adsorbents with high polarized surfaces are ideal. For the targeted molecule with a high quadrupole moment (like CO2 molecules), the desirable adsorbent should contain a surface with high electric field gradients whereas for the adsorption of a non-polar molecule with high polarizability, an adsorbent with a high surface area could be appropriate (Yang, 2003). Moreover, numerous important separations and purifications are accomplished due to the formation of 𝜋-complexation bonds and H-bonds (weak chemical bonding) between the adsorbates molecules and the adsorbent. Finally, the most important and basic point of selecting the adsorbent is the equilibrium isotherm, followed by diffusivity (Li et al., 2009a).
3.2. The impact of MOFs on selective adsorption of CO2 Hitherto, a wide range of porous materials has been introduced and used as an adsorbent in different fields, especially in the gas separation technology (Barton et al., 1999; Davis et al., 2002; Férey, 2008; Stein, 2003; Yu and Xu, 2006). As stated in the first section, the increase of the CO2 concentration contributes to some environmental disorders in the world (Jacobson, 2009). In this context, several studies have been accomplished and based on the literature, MOFs are the best porous materials for
selective trapping of CO2 (Bhattacharya and Ghoshal, 2015; Fukushima et al., 2010; McDonald et al., 2012; Nugent et al., 2013). The first key criterion of choosing a suitable MOF for CO2 adsorption is that the MOF’s pores have to be compatible with the kinetic diameter of the CO2 molecules. Another criterion is that the MOFs with polar (-OH, -N=N-, -NH2 and –N=C (R)-) pores have a higher CO2 adsorption capacity due to the quadrupole moments of CO2 molecules. Therefore, by designing the MOF structure based on these criteria, it is possible to significantly improve the CO2 adsorption capacity and selectivity of these outstanding adsorbents. By and large, the MOFs can be divided into two types, rigid and flexible (dynamic). The former possesses robust frameworks that make permanent pores like zeolites, while the latter possesses soft (dynamic) frameworks whose structure changes through external stimuli such as guest molecules, pressure, and temperature (Li et al., 2011). Majority of the MOFs have been used as a selective CO2 adsorption and most of them are in the rigid categories; yet, nowadays the flexible MOF has taken more attention due to their outstanding abilities. In the subsequent section, the types of the MOFs’ structure in selective CO2 sequestration are investigated. 3.2.1. Selective CO2 adsorption in rigid MOFs Various rigid MOFs have been used for selective gas adsorption to date. The mechanism of selective adsorption in rigid MOFs resembles greatly to the zeolites, meaning that selective adsorption in this kind of MOFs may be achieved based on the molecular sieving effect. In addition, it is probably attained according to the different strengths of the interaction between adsorbate-adsorbent and adsorbateadsorbate. The selective adsorption in the rigid MOFs is dependent on three main factors: size / shape exclusion, adsorbate–surface interactions, and simultaneous corporation of both these factors (Li et al., 2009a; Li et al., 2011).
3.2.1.1. Selective adsorption of CO2 on rigid MOFs based on size/shape exclusion According to the literature, many of the MOFs are proper and have been used to adsorb different gases based on the molecular sieving effect (Dybtsev et al., 2004; Loiseau et al., 2006; Xue et al., 2008;
Zhuang et al., 2011). This implies that only the molecules are able to pass through the pores whose Kinetic diameters are compatible with the diameters of the pores (Fig. 3).
Fig. 3. Schematic illustration of selective gas adsorption in rigid MOFs based on the molecular sieving effect (Zhuang et al., 2011).
The kinetic diameters of CO2, CH4, and N2 have been tabulated in Table 1. To elucidate this matter, it needs some structural analyses. For instance, for selective adsorption of CO2 over CH4, manganese MOFs are considered as one of the rigid and most evaluated groups of MOFs that work based on molecular sieving. These kinds of MOFs have a 3D robust structure with 1D channel which includes large cages. These cages are linked together through a small window. Dybtsev et. al. stated based on experimental works on these kinds of MOFs that CO2 can be adsorbed over CH4 at 195 K, at a lower temperatures (78 K) just small molecules like H2 can adsorb over Ar and N2 (Dybtsev et al., 2004). It is declared that in both cases, the adsorption capacities of the excluded gases (CH4, N2, and Ar) were near to zero. In this case, selectivity was achieved due to the presence of small windows in the channels (Dybtsev et al., 2004). Other examples are MIL-96 (Loiseau et al., 2006) and Zn2 (cnc)2(dpt) (Xue et al., 2008) that are also used for selective adsorption of CO2 over CH4 according to size/shape exclusion. These MOFs can be suitable for selective separation of CO2 over CH4 specialty for landfill gas separation and natural-gas purification. It should be noted that the selective adsorption behavior of some MOFs such as Zn2 (cnc)2(dpt) has been changed in the activation process. The former has the capability to adsorb H2 over N2 while the latter can adsorb CO2 over CH4. This behavior probably refers to pore temperature sensitivity of this kind of adsorbent (Xue et al., 2008).
Table1 Kinetic diameters of different gases. Gas molecules CO2 CH4 N2
Kinetic diameter Å 3.3 3.8 3.64
PCN-26 also known as a rigid MOF possesses a 3D structure. This MOF is proper for selective gas adsorption based on the molecular sieving effect due to the pores diameters (3.68 Å). According to Zhuang et al. (Zhuang et al., 2011) studies (experimental and molecular simulation), activated PCN-26 performed CO2 adsorption capacity of 109.1 cm3 g-1 at 298 K and 800 Torr. Also, the selective adsorption of CO2 over N2 and CH4 at 273 K was 49: 1, 8: 4: 1, respectively. As results indicated, this MOF is more useful for selective adsorption of CO2 over N2 than methane.
3.2.1.2. Selective adsorption of CO2 on rigid MOFs based on surface –adsorbate interactions The second imperative factor for selective gas adsorption is the nature of the guest molecules and their interaction with the surface of the adsorbent. Consequently, in some rigid MOFs, selective adsorption can be achieved based on the kinetic effect or the thermodynamic equilibrium effect in a given equilibrium time. Therefore, selectivity occurs based on the adsorbate properties such as Hbonding, polarity, the quadruple moment, and the surface properties of the pores. Regarding the kinetic effect, a wide range of rigid MOFs has been used in selective gas adsorption technology so far. In the case of CO2, the selective adsorption takes place based on the large quadrupole moment of CO2 whereas CH4 and N2 are nonpolar molecules. Bae et al. synthesized Zn2 (ndc)2(dpni) as a pillared-layer MOF via the microwave heating method. Their research revealed that the preferential adsorption of CO2 was much more than CH4 because of the kinetic effect and the quadrupole moment of CO2 (Bae et al., 2008b).
Moon et al. prepared a 3D porous MOF with 1D channel that consisted of coordinatively unsaturated MnII sites (Moon et al., 2006). The researchers found that this MOF had higher adsorption capabilities for CO2 than for CH4 at ambient temperature. The other rigid MOFs showing high selective CO2 adsorption over other gases especially over CO is ZIFs frameworks, which enjoy the outstanding feature of the presence of large cages which have connection via small pores (Banerjee et al., 2008). ZIF-68 to 70 are good example of these categories which possess large cages with 7.2, 10.2, and 15.9 Å diameters respectively which are connected through pores with 4.4, 7.5, and 13.1 Å diameters, respectively. All these ZIFs frameworks display high CO2 adsorption capacity and selectivity over CO at 273 K. This selective adsorption has been also strengthened via breakthrough experiments. The results indicated that in the 50:50 v/v binary mixture of CO and CO2 at 298 K, the CO2 molecules were retained completely while CO molecules passed through the column. According to their observation the pores size of the frameworks are large enough to pass all CO and CO2 molecules through the pores. Hence, this selectivity was attributed to the equilibrium effect and in the competition between CO2 and CO molecules to be adsorbed on the surface of the framework, CO2 molecules are a winner because of their more quadrupole moments characteristic (Banerjee et al., 2008).
3.2.1.3. Selective adsorption of CO2 on rigid MOFs based on contributing size/shape exclusion and surface– adsorbate interactions effects In addition to the effect of the pore size and the interaction of the guest molecules and the surface of the pores, some MOFs can selectively adsorb the guest molecules based on both these effects. In order to select CO2 adsorption through this effect, Er2 (pda)3 can be a good example which was introduced by Pan et al. (Pan et al., 2003). This MOF possesses a 3D framework with 1D channels which are decorated by unsaturated ErIII sites. The CO2/N2 selectivity was examined that the affinity to adsorb CO2 was much higher than N2 due to two different mechanisms; 1) compatibility of the pore sizes (3.4Å) by CO2 molecules diameters (3.3 Å) (for N2: 3.64 Å), and 2) the presence of functionality in the framework such as coordinatively
unsaturated metal sites (ErIII ions), 𝜋-electrons and polar groups. Further to these factors, CO2 molecules also have the quadrupole moment which can interact with this electrical field and thus enhance the potential of the adsorption energy. ZIF-95 and ZIF-100 (Wang et al., 2008) are the other examples of rigid frameworks which can selectively adsorb CO2 based on these effects. The MOFs possess large cavities (3.65 Å) as well as highly straitened pores (3.35 Å). The selective adsorption of CO2 over CH4, N2 and CO of these MOFs was investigated by Wang et al. (Wang et al., 2008). According to their observation, both frameworks indicated higher affinity of CO2 over the other examined gases and this can be attributed to the combination of the pore size and the strong interaction of CO2 molecules and the framework due to the quadrupole interaction of CO2 molecules and the presence of nitrogen atoms in the frameworks. Moreover, Xiang et al. investigated the CO2 adsorption capacity and selectivity of UTSA-16 via experimental and simulated breakthrough experiments. The results exhibited great CO2 adsorption capacity (160 cm3 cm-3 (volumetric)) and selectivity over N2 and CH4. The high CO2 adsorption capacity and selectivity of UTSA-16 are attributed to the strong binding sites to CO2 as well as the optimal size of the pores which are compatible with the kinetic diameter of the CO2 molecules (see Table 1) (Xiang et al., 2012) Likewise, the rigid MOFs hold a high potential for gas separation as well as CO2 sequestration due to the tenability of their pore sizes, shapes, and surface properties based on their application. These prominent properties are achieved through modification methods which are explicated in sections 4. Concerning the molecular sieving effect, the pore size must be compatible with the kinetic diameter of the targeted molecules (Dybtsev et al., 2004). For the surface properties, the presence of effective functional groups on the surface of the pores is essential (such as amine groups for CO2 separation). Besides, another outstanding method to enhance the CO2 adsorption is using open metal sites (unsaturated metal sites) on the frameworks (Dietzel et al., 2009). Some rigid MOFs which are used for CO2 selective adsorption have been tabularized in Table 2.
Table 2 Some rigid MOFs for selective CO2 adsorption.
MOF (MIL-96)
Structure Three types of cages connected through small widows Triangular 1D channels
Zn2(cnc)2(dpt) Zn2(ndc)2(dpni) Er2(pda)3 (ZIF-100)
MIL-102
(CUK-1)
Zn3(OH)(pcdc)2.5
Crossing channels
Pore size Å
1D channel with diamond-shaped contains corrugated walls Crossing rhombic channels contains coordinatively unsaturated metal sites ZnII
Selective adsorption
CO2 uptake
T
P
Ref. (Loiseau et al., 2006) (Xue et al., 2008) (Bae et al., 2008b) (Pan et al., 2003) (Wang et al., 2008)
2.5 ~ 3.5
size/ shape exclusion
CO2 over CH4
~3.7 mmol g-1
303 K
3.5 bar
~ 3.7
both
CO2 over CH4
~ 150 mL g-1
195 K
1 P/P1
4~5
adsorbateadsorbent
CO2 over CH4
4.3 mmol g-1
296 K
1750 kPa
~ 3.4
both
CO2 over N2 and Ar
273 K
760 torr
3.35
both
CO2 over CH4 , CO and N2
~ 0.95 mmol g-1
298 K
850 torr
adsorbateadsorbent
CO2 over CH4 and N2
~ 3.4 mmol g-1
304 K
3 MPa
(Surblé et al., 2006)
11.1
adsorbateadsorbent
CO2 over CH4
~ 88 mL g-1
298 K
760 torr
(Yoon et al., 2007)
3 × 5
adsorbateadsorbent
CO2 over CH4
~ 0.586 mmol g-1
298 K
0.5 bar
(Bae et al., 2008a)
rhombic
1D channel with unsaturated metal sites ErIII Large cages which connected via small apertures 1D channel which decorated by F-anions and coordinated terminal water molecules
Selectivity reason
~ 24 mg g-1
4.4
3.2.2 Selective CO2 adsorption in flexible/dynamic MOFs. The other generation of the MOFs that has recently received a conspicuous attention is the flexible (soft) MOFs. Due to their distinctive properties, the flexible MOFs differ from the rigid one in both the behavior and the nature (Chang et al., 2015; Schneemann et al., 2014). These unique MOFs enjoy a flexible framework and have shown dynamic behaviors during the adsorption/ desorption process. The most remarkable feature of the flexible MOFs is their structural transformability via external stimuli such as pressure, temperature, (Henke et al., 2013; Keene et al., 2013) and guest molecules (Alhamami et al., 2014; Llewellyn et al., 2006). Their flexible characteristic is related to regional movements of the organic ligands which comprise bending, twisting, tilting or rotating, and the environmental change of the secondary building units (SBUs) (Seo et al., 2011; Tian et al., 2012).
As stated above, the behavior of these kinds of MOF is different and more complicated than the rigid one. For example, the pore size of the flexible MOFs alters during the adsorption/desorption process based on the size of the guest molecules (Fig. 4). Moreover, most of the time, this change is accompanied by the hysterics behavior due to the rearrangement of the framework during the uptake/leaving of the guest molecules (Chen et al., 2007a; Chen et al., 2007b; Cheon and Suh, 2008; Ma et al., 2007b). Therefore, in gas selective adsorption of such MOFs, not only the size/shape exclusion and the adsorbate-surface interaction must be taken into account but also the structural rearrangement should be considered. Concerning the adsorption process and the structural feature of the flexible MOFs, their selective gas adsorption can be categorized into three groups mentioned below (Li et al., 2009a). Prior to the explanation of these categories, two prominent dynamic behaviors of the flexible MOF during the gas adsorption/desorption should be considered, which are the breathing phenomenon (Férey and Serre, 2009; Serre et al., 2007; Serre et al., 2002; Trung et al., 2008) and the gate-opening/closing (Fairen-Jimenez et al., 2011; Gücüyener et al., 2010; Kitaura et al., 2003; Seo et al., 2009). The breathing phenomenon occurs when an abrupt expansion/compression is applied to the unit cell of the MOF, which causes the structural transitions due to the interactive guest molecules. The gate opening/closing occurs via external stimuli in the close and nonporous MOFs and causes a transition to an open and porous phase (Coudert et al., 2009; Férey and Serre, 2009). The prominence and value of these two special phenomena have been emphasized by Kitagawa (Seo et al., 2009) and Ferey (Férey and Serre, 2009), that will be explained in more details in the following section.
Fig. 4. Schematic illustration of selective gas adsorption in a flexible MOF (Li et al., 2009a).
3.2.2.1. Selective CO2 adsorption on the flexible MOFs based on the size/shape exclusion accompanied by the change of the pore size/shape Resembling the rigid MOFs, the size/shape exclusion can result in gas adsorption in some flexible MOFs. This has been observed in Zn(ADC)(4,4΄-Bpe)0.5 which was composed of paddle-wheel dinuclear Zn2 units that were bridged by ADC dianions and further pillared by 4,4΄-Bpe to form a 3D interpenetrated structure (Chen et al., 2007a). As an adsorption study has demonstrated, at 195 K, this MOF adsorbs CO2 over CH4 (CO2: ~130mL, CH4: almost none). Evidently, this selectivity arises from the molecular sieving effect due to the compatibility of the pores size of the MOF (3.4×3.4 Å) and CO2 molecules (3.3Å) compared to CH4 molecules (3.8Å). A similar adsorption selectivity was observed for Ni2 (cyclam)2 (mtb) at 195 K and 1 atm for selective adsorption of CO2 (57 mLg-1) over CH4 (˂10mL) (Cheon and Suh, 2008). Likewise, Chen and co-workers investigated the selectivity of CO2 over N2 and CH4 by using flexible Cu (Bon et al.) (4,4ʹ-bpe)0.5. The results indicated that the MOF can take up CO2 over the other selected gases at 195 K (CO2: 100 mL g-1, CH4: 35 mL g-1 and N2: 15 mL g-1) because of similarity in the size of the CO2 molecules and the host MOF. It is worth noting that at lower temperature (77 K), none of the gases can be adsorbed on the MOF, suggesting that by increasing the temperature, the pores start to enlarge; consequently, at a higher temperature (195 K), all the gasses
can uptake on the surface of the MOF (Chen et al., 2007b). Size-exclusion relies on adsorption selectivity which was also recognized in flexible PCN-5 that possesses a 2-fold interpenetrated structure. This MOF can selectively adsorb CO2 over CH4 at 195 K and 760 Torr based on compatibility of the pore diameter of the host framework and the kinetic diameter of the gasses molecules (3.3 Å and 3.8 Å for CO2 and CH4 respectively). In a given condition, CO2 uptake can reach 210 mg g-1 (4.8 mmol/g), whereas that of CH4 was constrained to 30 mg g-1 (1.9 mmol/g) (Ma et al., 2007b).
3.2.2.2. Selective adsorption of CO2 on flexible MOFs based on interactions between the surface of the MOF and CO2 molecules accompanied by the pore size/shape change The surface property of the pores also plays a pivotal role in selective adsorption of flexible MOFs. To illustrate this, MIL-53 series (the flexible MOF) in both hydrated and dehydrated forms could be referred to. This MOF has a 3D structure comprising 1D channels with the chemical formulation of MO4 (OH)2 linked by 1,4- benzenedicarboxylic acid (BDC). This MOF demonstrated the breathing phenomenon during the hydration/ dehydration process (Alhamami et al., 2014; Bourrelly et al., 2005; Llewellyn et al., 2006). In the hydrated form, the size of the pore altered and became narrower (np) due to hydrogen bonding between the O atoms of the carboxylated group of the ligands and the hydrogen of the H2O molecules, whereas in the dehydrated form, the pore was larger (lp) (Fig. 5).
Fig. 5. Illustration of the breathing behaviour in MIL-53. Right: dehydrated form; left: hydrated form (Bourrelly et al., 2005)
Hydrated MIL-53s have shown an unprecedented adsorption performance for CO2 uptake. At low pressure (below 5 bar), the CO2 uptake was low due to the presence of water molecules that make the pores narrower (np) (Alhamami et al., 2014; Llewellyn et al., 2006). Hence, by increasing the gas pressure, the framework starts to tune itself into a large pore (np→ coexistence of np and lp → lp). This framework extension resulted in an increase in CO2 uptake to about 7.2 mmol g-1 at 18 bar. In addition, the pore volume increased from 1012.8 Å3 for the hydrated one to 1522.5 Å3 for the hydrated + CO2. The Adsorption behavior of dehydrated MIL-53s ostensibly contrasts with that of the hydrated form (Alhamami et al., 2014; Bourrelly et al., 2005). In the case of CO2 uptake, dehydrated MIL-53s have shown two steps for the structural transition (Fig. 6). The first step occurs at low pressure (below 5 bar) in which the CO2 uptake capacity increases dramatically (3 mmol g-1). The second step starts at about 8 bar in which CO2 uptake was around 7.7 mmol g-1. This cold be attributed to the structure of the large pore changing to narrower ones due to the CO2 adsorption. It is also attributed to high pressure of the gas causing the structural transition again to the large pore (lp→ 𝑛𝑝→𝑙𝑝). This might be due to steric hindrance of the prior water molecules in the structure of MIL-53s (Llewellyn et al., 2006; Murray et al., 2009) CH4 adsorption in dehydrated MIL-53 was reported to be approximately 4.6 mmol g-1 at 20 bar while in the hydrated MIL-53 the adsorption was almost zero (around 0.2mmol g-1). This difference was due to the repulsive effect polarity of the water molecules in the host framework with nonpolar molecules of CH4. Based on this study, the CH4 adsorption on both MIL-53 (hydrated and dihydrated) was less than CO2. Accordingly, this project proves a high potential in the application of such an MOF for gas storage and separation, exclusively for selective adsorption of CO2 over CH4 (Llewellyn et al., 2006; Murray et al., 2009). Another worthy illustration of this category is [Ni(bpe)2(N(CN)2)](N(CN)2) (Maji et al., 2007) which has a stable interpenetrated 3D framework with 1D channels. This flexible MOF exhibited a unique and exceptional selective adsorption behavior. For example, even though the framework has an adequate
pore size, no O2 and N2 molecules could diffuse into the microspores at 77K. Surprisingly, a remarkable amount of CO2 can be adsorbed into the framework at 195 K, whilst the size of the CO2 molecules resembles that of the O2. This uncommon selective adsorption (at 77 K) might be explained by the strong interaction between surface of the pore windows and the primer molecules of O2 / N2. Furthermore, they blocked the pores so that the other molecules were unable to pass through them and be adsorbed. The CO2 uptake at 195 K was attributed to the rise in the temperature and the powerful interaction between the host framework and the CO2 molecules. The mentioned robust interaction was attributable to the presence of NiII atoms, polar groups, and π-electron clouds in the framework. On the other hand, CO2 molecules possess high quadrupole moments, contributing to a notable interaction occurring between the host and guest molecules (Maji et al., 2007).
Fig. 6. Gas adsorption isotherms of dehydrated MIL-53(Cr) at 304 K (illustration of the ‘‘breathing’’ effect) (Bourrelly et al., 2005).
3.2.2.3. Selective adsorption of CO2 on flexible MOFs according to gate-opening or structural rearrangement induced by adsorbate–surface interactions
As previously underscored in section 3.2.2, one of the attention-grabbing functions of some flexible MOFs during the adsorption process is the gate-opening phenomenon. This process takes place in the MOFs comprising small pores or even no pores to pass the guest molecules through the adsorbent. During this mechanism, when the framework is exposed to the gust molecules (certain gas), the pores begin to expand, and such an expansion is directly dependent on the adsorbate-surface interactions with the framework (Fig. 7).
Fig. 7. Schematic of selective gas adsorption in the flexible (dynamic) framework based on gate opening phenomena and guest specific pressures. Purple triangle and red circles represent nonsorbable and sorbable guest molecules.
ZIF-20 is the best instance of the flexible MOF where the gate opening occurs in the selective adsorption of CO2 over CH4 (Hayashi et al., 2007). This MOF has a 3D structure including large cages which are connected via small windows. The amount of CO2 adsorption at 273 K and 760 torr was five times greater than CH4. This difference of adsorption is seemingly attributed to a stronger interaction between the CO2 molecules and the pore surface. The other thought-provoking fact is the size of the pore window which is 2.8 Å (measured by the crystal structure) whereas the kinetic diameters of CO2 and CH4 are larger than this (3.24 Å and 3.8 Å, respectively). As cited above, the large cages are accessible via the small windows that are expanded due to the window-widening process; accordingly, the gas molecules are able to pass through the framework and this process is accomplished based on the ligands swing. Moreover, the quadrupole moment of CO2 exerts a distinctive impact on the flexibility of the framework in some MOFs such as ZIF-20.
In the gate opening phenomenon, the specific pressure of the guest molecules needs to be considered, implying that the guest molecules are allowed to pass through the adsorbent at this specific pressure (Fig. 7). Interestingly enough, the gate opening pressure of every gas is specified to its own. Therefore, the gas entrance pressure should be the same or above the gas gate-opening pressure. Cu (dhbc)2(4,4ʹbpy) (Kitaura et al., 2003) is one of the best examples of this kind of MOFs, where the hydrated form has a 2D sheet shape with 1D channels. Its framework includes rigidly interconnected sheets and its pores are furnished by dhbc benzene which is located in a vertical form into those sheets and mutually interdigitated. Also, water molecules are embedded in the cross section of the framework (3.6 * 4.2 Å). Besides, there are some interactions between the neighboring dhbc ligands via π-π stacking causing a consolidation of the 3D structure. As Fig. 8 depicts, the isotherm adsorption of CO2, CH4, O2, and N2 starts at a certain pressure (0.4, 9, 35 and50 bar, respectively) having an immediate increase at the pressure specified to each gas. Another remarkable point is the hysteresis behavior for each gas due to the gate-closing pressure. Therefore, different gases would have exclusive gate-opening and gate-closing pressures that are related to the distinctive intermolecular interaction force between the gas molecules and the surface of the adsorbent. Another MOF revealing a similar dynamic behavior with Cu (dhbc)2(4,4ʹ-bpy) (Kitaura et al., 2003) is Cu (pyrdc)(bpp) (Maji et al., 2005) which has a 2D pillared-bilayer framework with channels comprising the nonporous structure. This MOF indicates a sponge-like dynamic behavior with the bond formation via inclusion as well as a breaking triggered by the guest removal (the adsorption/desorption process). Moreover, the adsorption properties of this MOF was scrutinized by Maji et al. for N2, O2, and CO2 gases. In the case of N2 and O2 having kinetic diameters of 3.6 Å and 3.4 Å, respectively, no inclusion was discerned at 77 K and no pores were discovered in the temperature. On the contrary, CO2 with a similar kinetic diameter (3.3 Å) to N2 and O2 can pass through the framework and can be adsorbed. This behavior refers to the specific opening-gate pressure of each gas that causes the CO2 to be accommodated in the framework as compared with N2 and O2. Furthermore, this adsorption directly depends on the strength of the H-bound interaction (O-H…O) between the CO2 molecules and the ligand
parts which are located in the walls of the framework channels. Nonetheless, such a phenomenon is rare and has only been observed in a few flexible MOFs thus far. Some flexible MOFs used for CO2 adsorptions have been enlisted in Table 3.
Fig. 8. Isotherm adsorption (filled circles) and desorption (open circles) process of different gases based on opening gate pressure at 298 K (Kitaura et al., 2003).
Table 3 Some flexible MOFs for CO2 adsorption. MOF
Structure
Selectivity reason
(MIL-53)
3D framework with 1D channels(diamondshaped ) comprising Hbonding sites
Different guest adsorption interaction
Selectivity of CO2 over other gases CO2 over CH4
CO2 Uptake ~ 7.5 mmol g1
T (K)
P
Ref.
304
20 bar
(Llew ellyn et al., 2006)
Zn(adc)(4, 4'-bpe)0.5
Interpenetrated 3D framework (pillared-layer) with crossing 1D open channels
Size & shape exclusion
Ni2(cyclam )2(mtb)
Interpenetrated 3D framework containing wide inside pockets linked via narrow entrances Interpenetrated 3D framework ?(pillared-layer) with crossing 1D open channels Interpenetrated 3D framework which stabilized via H-bonding including 1D channels which have three types of crossing in three directions Interdigitated 2D framework, stacking through p–p interactions to form 1D channels Interpenetrated 3D pillaredlayer framework with intersecting 1D potential open channels 3D framework with 1D diamond-shaped channels containing H-bonding sites
Size & shape exclusion
CO2 over CH4 and N2
57 mL g-
Size & shape exclusion
CO2 over CH4 and N2
Size & shape exclusion
[Ni(bpe)2( N(CN)2)] (N(CN)2) (ZIF-20)
Interpenetrated 3D framework with bimodal functionality 3D framework with large cages connected by small distensible apertures
Different guest adsorption interaction Guest-framework interaction induced gate opening
CO2 over N2
Cu(pyrdc)( bpp)
2D pillared-bilayer framework without pores
Different dependent opening
gest gate
Zn2(tcom)( 4,4'-bpy)
Interpenetrated 3D PtS-type framework with intersecting channels
Different adsorption interaction
guest
Cu(fam)(4, 4'-bpe)0.5 (PCN-5)
Cu(dhbc)2( 4,4'-bpy) Cu(bdc)(4, 4'-bpy)0.5 MIL-53 M = Al, Cr
CO2 over CH4
~ 130 mmol g-
195
1 p/p
(Chen et al., 2007a )
195
1 atm
100 mL g-1
195
760 torr
CO2 over CH4
~ 210 mg g-1
195
760 torr
(Cheo n and Suh, 2008) (Chen et al., 2007b ) (Ma et al., 2007b )
1
1
Size & exclusion
shape
CO2 over CH4, O2, and N2
~ 70 mL g-1
298
0.4 ~ 8 atm
Different dependent opening
gest gate
CO2 over CH4 and N2
~ 70 mL g-1
298
0.1 ~ 0.2 MPa
Different adsorption interaction
guest
CO2 over CH4
~ 10 mmol g-
304
30 bar
~ 35 mL g-1
195
1 p/p
CO2 over CH4
~70 mL g-1
273
760 torr
CO2 over CH4
Differed adsorpti on capacity 5 wt %
195
Differen t pressure
298
1 bar
1
CO2 over N2
Notwithstanding an ambiguous rearrangement mechanism of the flexible MOFs, this kind of adsorbent provides the opportunity of finding a novel material for a particular application thanks to their exclusive capability. The most distinguished factor that differentiates the flexible MOF from the rigid one or other adsorbents is the combination of weak and strong intermolecular interactions in their
(Kitau ra et al., 2003) (Kitau ra et al., 2003) (Bour relly et al., 2005) (Maji et al., 2007) (Haya shi et al., 2007) (Maji et al., 2005) (Thall apally et al., 2008)
structure. The framework integrity and porosity are obtained from the strong bond while the flexibility and dynamic behaviors are as a result of the weak interactions. 3.3. CO2 Adsorption Capacity One of the most imperative features of the adsorption technology is the adsorbate capacity which is mostly reliant on the adsorbent surface area. Thanks to their high and ultra-high surface area, the MOFs have shown the best CO2 uptake capacity as compared to the other adsorbates such as zeolites and activated carbons. Besides, the surface area, the adsorbate-adsorbent interaction, the pressure and the adsorption temperature also exert an impact on the CO2 uptake capacity (Liu et al., 2012a; Liu et al., 2012b). Numerous studies have been carried out in this area to dates, and most of them are related to the gravimetric capacity which is based on the weight percentage of the uptake gas over the total weight of the system (Liu et al., 2012b). The first systematic survey on the correlation between the surface area of the MOF and the capacity of CO2 adsorption was accomplished by Millward and Yaghi (Millward and Yaghi, 2005) on nine MOFs with different characteristics and geometrics including MOF-2, MOF505, Cu3 (BTC)2, MOF-74, IRMOF-11, IRMOFs-3, IRMOF-6, IRMOF-1 and MOF-177 (Table 4). All the capacities were measured under the same condition (T: 298 K and P: 35 bar). The results established that of all the studied MFOs, MOF-177 had the highest surface area (4,508 m2g-1) and possessed a greater CO2 uptake capacity (60.0 wt %). One of the best reported amount of CO2 uptakes belongs to MOF-210 (~71 wt%) at 298 K and 50 bar. This outstanding CO2 uptake is related to the ultrahigh surface area of these MOFs (BET and Langmuir surface areas for this MOF is 6240 and 10,400 m2g-1 respectively). This result was presented by Furukawa and co-workers who investigated 5 MOFs with same metal sites with one or two ligands (the ligand structures are tabularized in Table 5). Consistent with the results and the ligand structure, it was acknowledged that by expanding the organic linker of the framework, the MOF capacity can be boosted. For example, just by enlarging the ligand of MOF-177 from BTB: 1,3,5 benzenetribenzoate to
BBC: 4,4’,4’’-(benzene-1,3,5- triyl-tris(benzene-4,1-iyl)) tribenzoate, MOF-200 was gained and the great enhancement was observed in CO2 uptake from 60.0 to 71wt % (Furukawa et al., 2010).
Table 4 Capacity of 9 MOFa for the CO2 uptake based on the surface area. MOF Name
Formulation
Surface area
Capacity
Pressure
Temperature
(m2*g-1)
(wt %)
(Bar)
(K)
Ref.
MOF-177
Zn4O(BTB)3
4,508
60.0
35
298
(Furukawa et al., 2010)
IRMOF-1
Zn4O(BDC)3
2,833
48.8
35
298
(Millward and Yaghi, 2005)
IRMOF-6
Zn4O(C2H4BDC)3
2,516
46.2
35
298
(Millward and Yaghi, 2005)
IRMOF-3
Zn4O(NH2BDC)3
2,160
45.1
35
298
(Millward and Yaghi, 2005)
IRMOF-11
Zn4O(HPDC)3
2,096
39.3
35
298
(Millward and Yaghi, 2005)
HKUST-1
Cu3(BTC)2
1,781
32.0
35
298
(Millward and Yaghi, 2005)
Zn-MOF-74
Zn2(DOBDC)
816
31.4
35
298
(Millward and Yaghi, 2005)
MOF-505
Cu2(BPTC)
1,547
31.0
35
298
(Millward and Yaghi, 2005)
MOF-2
Zn2(BDC)2
345
12.3
35
298
(Millward and Yaghi, 2005)
Li et al. (Li et al., 2014a) represented UiO(bpdc) with the highest value of CO2 uptake; 79.7 %wt at 20 bar and 293 K. The BET surface area and the Langmuir surface were 2646 m2 g-1 and 2965 m2 g-1 respectively and the pore value of 1.057 cm3 g-1 was found. It is worth noting that this MOF has exhibited the highest CO2 uptake in the MOFs materials so far. Obviously, at high pressure, the CO2 capacity adsorption relies on the surface area and the pore volume of the MOF. Therefore, one of the effective ways of boosting the capacity of CO2 uptake is to enhance the pore volume and the surface area of the MOFs. This could be achieved by using the large organic ligand such as the BTB and the BTC or extending the MOF structure in the synthesis part
(Furukawa et al., 2010; Li et al., 2014a). Some of the new MOFs that are expedient for CO2 capture at high pressure are listed in Table 6. In addition, measuring the CO2 capacity adsorption at low pressure is also highly paramount. Because the partial pressure of CO2 generated by the power-plant flue gas is less than the atmospheric pressure, MOFs with a high storage capacity at low pressure should be considered for separation and storage of CO2 from the power plant(Zou et al., 2010). Some of the new MOFs which are proper for CO2 uptake at low pressure are specified in Table 7. In addition to the capacity and selectivity which are the most essential parts for choosing the MOF in the CO2 adsorption process, the adsorption heat has to be considered as another vital factor which is explained in the next section.
Table 5 The structure and the chemical formula of the organic ligand of in some MOFs (Furukawa et al., 2010). Name of MOF MOF-177
Organic Ligand BTB: 4,4’,4’’-benzene-1,3,5-triyltribenzoate
Ligand Chemical Formula O
OH
MOF-180
BTE: 4,4’,4’’-benzene-1,3,5-triyltris(ethyne-2,1-diyl)tribenzoate
Ref. (Furukawa et al., 2010)
OH
OH O
O
O O
HO O
(Furukawa et al., 2010)
OH
HO
MOF-200
BBC: 4,4’,4’’-benzene-1,3,5-triyltris(benzene-4,1diyl)tribenzoate
(Furukawa et al., 2010)
COO
COO
COO
MOF-205
BTB +2,6naphtalenedicarboxylate (NDC)
OOC COO
COO
COO OOC
(Furukawa et al., 2010)
MOF-210
BTE + biphenyl-4,4’-dicarboxilate (BPDC)
(Furukawa et al., 2010)
COO
COO
COO
COO COO
Table 6 Adsorption capacities of CO2 in different MOFs at high pressure. Surface Area (m2/g) MOF
BET
UiO(bpdc)
2646
NU-111
Langmuir 2965
Capacity (wt %)
Pressure (Bar)
Temp. (K)
Selectivity
Qst (kJ mol-1)
79.7
20
303
4932
61.8
30
298
DGC-MIL-101
4198
59.8
40
298
HTS-MIL-101
3482
52.8
40
298
ZJU-32
3831
49
40
300
UTSA-62a
2190
43.7
55
298
16
Basolite® C 300
1706.42
41.9
224.99
318
18
Basolite@ A100
1524.8
26.9
224.99
318
9
DMOF
1980
38.1
20
298
DMOF-NO2
1310
32
20
298
DMOF-DM1/2
1500
27.5
20
298
DMOF-Cl2
1180
26.4
20
DMOF-DM
1120
25.4
DMOF-OH
1130
DMOF-Br DMOF-TM1/2
23
12
20
298
17
21
20
298
23
23
24.8
20
298
1320
24.3
20
298
1210
23.9
20
298
Ref. (Li et al., 2014a) (Luo et al., 2013) (Kim et al., 2013) (Kim et al., 2013) (Cai et al., 2014a) (He et al., 2013) (Deniz et al., 2013) (Deniz et al., 2013) (Burtch et al., 2013) (Burtch et al., 2013) (Burtch et al., 2013) (Burtch et al., 2013) (Burtch et al., 2013) (Burtch et al., 2013) (Burtch et al., 2013) (Burtch et al., 2013)
DMOF-A
760
17.1
20
298
MIL-101(Cr)
2549
24.2
30
303
HKUST-1
1326
26.3
30
303
IRMOF-8
1599
51.2
30
298
21.1
IRMOF-8-NO2
832
31.3
30
298
35.4
(Burtch et al., 2013) (Ye et al., 2013) (Ye et al., 2013) (Orefuwa et al., 2013) (Orefuwa et al., 2013)
Table 7 Adsorption capacities of CO2 in different MOFs at low pressure.
UiO(bpdc)
Surface Area (m2/g) BET Langmuir 2646 2965
DGC-MIL-101
8
P. (Bar) 1
Temp. (K) 303
4164
14.5
1
298
HTS-MIL-101
3482
12.3
1
298
UTSA-62a
2190
8.1
1
298
DMOF-TM
1050
13.3
1
298
DMOF-A
760
10.6
1
298
DMOF-Cl2
1180
8.8
1
298
HKUST-1
1326
13.2
1
303
Mg-MOF-74
1416
2085
30.1
1
298
Ni-MOF-74
1252
1841
19.4
1
298
Mg/DOBDC
1415.1
25
1
298
MOF
Capacity (wt %)
Selectivity (IAST)
Qst (kJ mol-1)
16 28
29
17
21
47
Ref. (Li et al., 2014a ) (Kim et al., 2013) (Kim et al., 2013) (He et al., 2013) (Burtc h et al., 2013) (Burtc h et al., 2013) (Burtc h et al., 2013) (Ye et al., 2013) (Wu et al., 2013) (Wu et al., 2013) (Li et al., 2014b )
Co/DOBDC
1089.3
21.6
1
298
37
Ni/DOBDC
1017.5
20.5
1
298
42
MIL-100(Cr)
1528.7
9.5
1
298
23.8
1
313
15.8
1
313
21.2
1
313
7.3
1
313
12.8
1
313
mmen-CuBTTri
11.3
1
313
[Zn2(BME-bdc)x(DBbdc) 2.xdabco]n
21.7
0.91
195
Mg2(dobpdc)
1940
mmen-Mg2(dobpdc)
Ni2(dobpdc)
1593
mmen-Ni2(dobpdc)
HKUST-1
2203
CPM-33b
808
1119
19.9
1
298
25
CPM-33a
966
1257
12.6
1
298
22.5
Cu-TDPAH
1762
18.4
1
298
Ni-DOBDC
798
18.2
1
298
Py-Ni-DOBDC
409
12
1
298
ZJNU-40
2209
16.4
1.01
296
18.4
JLU-Liu22
1487
15.6
1
298
30
200
33.8
(Li et al., 2014b ) (Li et al., 2014b ) (Li et al., 2014b ) (Maso n et al., 2015) (Maso n et al., 2015) (Maso n et al., 2015) (Maso n et al., 2015) (Maso n et al., 2015) (Maso n et al., 2015) (Bon et al., 2015) (Zhao et al., 2015) (Zhao et al., 2015) (Liu et al., 2014) (Bae et al., 2014) (Bae et al., 2014) (Song et al., 2014a ) (Wan g et al., 2015)
UNLPF-1
1046.6
13.9
1
273
13.6
1
298
UTSA-49
710.5
rht-MOF-pyr
2100
12.7
1
298
28
rht-MOF-1
2100
11
1
298
29
ZnAcBPDC
920
11.7
0.9
293
ZnBuBPDC
850
7.6
0.89
293
{[H2N(CH3)2]4[Zn9O2(BTC).6( H2O)3].3DMA}cn
844
10.9
0.91
298
CuBTTri
1700
10.8
1
293
IRMOF-74-III-CH2NH2
2310
10.8
1
298
IRMOF-74-III-NH2
2720
10.4
1
298
IRMOF-74-IIICH2NHMe
2250
9.6
1
298
SIFSIX-3-Ni
368
10.3
1
298
51
SIFSIX-3-Co
223
10
1
298
47
8.9
1
298
750
9.9
1
298
1132
SIFSIX-3-Zn
Zn/Ni-ZIF-8-1000
ZIF-7
312
355
9.1
1
298
UiO-66(Ti56)
1844
2200
8.8
1
298
95.8
29
30
61.2
37
(Johns on et al., 2013) (Xion g et al., 2014) (Gao et al., 2015) (Gao et al., 2015) (Kece li et al., 2014) (Kece li et al., 2014) (Li et al., 2015) (Das et al., 2013) (Fraca roli et al., 2014) (Fraca roli et al., 2014) (Fraca roli et al., 2014) (Elsai di et al., 2015) (Elsai di et al., 2015) (Elsai di et al., 2015) (Li et al., 2014c ) (Wu et al., 2014) (Lau et al., 2013)
CPM-5
2187
8.8
1
298
ZIF-7-R
5
8.7
1
303
Zn(5-mtz)(2-eim) .(guest)[ZTIF-1]
1430
8.2
1
295
81
22.5
7.3
1
293
25
36.0
7.2
1
298
94.2 (cm3 g-1)
1
273
1981
[Cu(tba)2]n Zn-DABCO
1870
[Zn2(TMTA)(DMF)2].NO 3.2H2O.3DMF
788
1902
-
16.1
36.1
34
22.4
39.3
28.8
3.3.1. Enthalpy of adsorption (Qst) The adsorption heat, the isosteric heat, or adsorption enthalpy is a critical factor in appraising the gas adsorption on the MOFs and other solid sorbents. This value demonstrates the MOFs’ affinity to adsorb CO2. It shows the strength of the interaction between the host and guest molecules. Indeed, the magnitude of the Qst is a function of the binding strength. Moreover, this value indicates the amount of the required energy for the regeneration process. If the Qst of adsorption is very high, it implies that the adsorbent has a very high affinity to adsorb CO2. Nevertheless, its regeneration process requires high energy to regenerate the adsorbent. Thus, the optimal MOF for CO2 uptake should possess an ideal enthalpy of adsorption; furthermore, this value is measured based on kJ mol-1 unite (Sumida et al., 2012).
4. Synthesis of Metal Organic Frameworks 4.1. MOF preparation 4.1.1. Metal
(Sabo uni et al., 2013) (Cai et al., 2014b ) (Wan g et al., 2014) (Du et al., 2014) (Chae mchu en et al., 2015) (Sayar i et al., 2011)
All the MOFs are constructed from the metal (inorganic) and the ligand (organic) which are linked to each other via a coordination bond. Almost all of the transition metals as the ions or clusters can be used to form the MOFs. The transition metals are the group of metals whose block d is not completed yet; therefore, they are very active and possess outstanding characteristics such as a variety of coordination numbers and oxidation states. Based on these values, numerous MOFs with a diversity of geometrics such a square–planer, tetrahedron, octahedron, trigonal, pentagonal , etc. can be constructed. Besides metal sites and their values, ligands, solvents and even counter-inion (reaction condition) have influenced the MOF geometrics (Li et al., 2012). Typically, the coordination reaction takes place between the metal salt and the ligand in the presence of one or more organic solvents such as the dimethylformamide (DMF), ethanol, methanol, etc. Based on the reaction condition, the end-product (MOF) is produced in different geometric shapes.
4.1.2. Ligand The organic part of the MOFs is the ligand that comprises Lewis base–binding atoms (functional groups) such as halides, nitriles, cyanides, anionic organic molecules (benzenedicarboxylic acid), and neutral organic molecules (4,4′-bipyridine) (Furukawa et al., 2013). Some of the ligands structures used for constructing the MOFs are shown in Table 8. Resembling the metal sites, the ligands also exert a prominent impact on CO2 adsorption into MOFs. The tunable characteristic of the MOFs mostly relies on this part of the framework due to the capability of the organic linkers to have variable functional groups in different conditions. Even the length of the ligand can alter the gas storage capacity. Indeed, larger storage spaces are achieved via longer organic linkers by increasing the functional groups of the ligand.. Nonetheless, a very long ligand contributes to formation of the interpenetrating structure and then intertwining of the chains (Furukawa et al., 2013). Therefore, the ligand part of the framework exerts a great impact on the improvement of the gas adsorption capacity of the MOFs due to their functionalization. The effect of the ligand site on the enhancement of CO2 adsorption properties are detailed in sections 5.2 and 5.4.
Table 8 Organic ligands used in the MOF for CO2 adsorption. Ligand H2 BDC (Benzene di carboxylate acid)
Molecular structure of the ligands OH
Active sites Oxygen of Hydroxyl groups.
Ref. (Furukawa et al., 2013)
Multi-functional ligands: Amine groups and free nitrogen groups are active to attracted CO2 molecules.
(Song et al., 2014b)
Multi-functional ligands: Free nitrogen, amine group, oxygen and OH groups all are active to adsorb CO2 molecules.
(Pachfule et al., 2011)
Multi-functional ligands: Amide units and oxygen of carboxylic groups.
(Duan et al., 2012)
Mixt Linker: Open N sites of bipy and P-O in the phosphonate ligands.
(Taddei et al., 2013)
Mixt Linker: amino group and three N-atoms of the triazole ring as well as the oxygen atoms of oxalate ligands.
(Furukawa et al., 2013; Garcı́a-Ochoa and Genesca, 2004)
Amine group and open nitrogen site.
(Mighell and Reimann, 1967)
OH
NH2
Adenine
H N
N
OH
ANIC: 2-Amino-isonicotinic acid
N
N O
N O
H3L: 5-(4carboxybenzoylamino)isophthalic acid
H
NH2
OH
C N
O
OH
O
O
OH
H2L2 and bipy N
N
(HO)2OP
N N
(HO)2OP
3-amino-1,2,4-triazole oxalic acid
NH2
and
PO(OH)2
O
N
C N
PO(OH)2
OH C
HO
N
O
Pyrazole
N N H
4.2. Synthesis methods In the past decade, a conspicuous emphasis has been laid on the synthesis of MOFs due to the possibility of designing their structure based on their applications. Different synthesis methods such as conventional, microwave assisted, sonochemical, mechanochemical, and electrochemical have been adopted for producing the MOFs and the main difference in these methods is the form of the energy input which will be explained in the succeeding sections.
4.2.1. Conventional Synthesis routes (Ca) It is of note that the most popular method for synthesizing the MOFs is the conventional route and the required energy is introduced to the system via conventional heating sources. Since the temperature plays a critical role in chemical reactions, this synthesis route is distinguished from the solvothermal (hydrothermal) and non-solvothermal reactions. While the solvothermal reactions occur in the closed reactor (usually Teflon sealed) under autogenous pressure and the temperature of the reaction must be higher than the boiling point of the solvent, the non-solvothermal reaction is accomplished under ambient pressure and below or at the boiling point of the solvent (Byrappa and Yoshimura, 2012). Typically, the temperature range for the synthesis of the MOFs varies from the room temperature to approximately 250 C. By far, most MOFs are synthesized via the solvothermal reactions owing to the ease of synthesis conditions. As mentioned above, this reaction normally takes place via the coordination bond between the organic ligand and the metal salt in the solvent under the above condition and the end product will be in crystal or powder forms. Although the temperature is an essential parameter for most syntheses, some well-known MOFs comprising HKUST-1, MOF-5, MOF-177, MOF-74, and ZIF-8 are prepared just by mixing the primer materials at room temperature. The merit associated with such reactions is that the precipitation occurs in a short time-scale, sometimes known as the direct precipitation reaction;
therefore, there will be a considerable diminishment in the reaction time (David J. Tranchemontagne, 2008; Janosch Cravillon, 2009). Notably, ZIF-8 exhibits good chemical and thermal stabilities compared to the others. In general, synthesis of the MOFs is highly dependent on the reaction temperature, even an MOF having various properties such as different surface areas and crystallinity can be achieved at different temperatures while dense MOFs are commonly obtained at a high reaction temperature (Bauer and Stock, 2007; Forster et al., 2005). Further to this, not only high synthesis temperature has an influence on crystallinity but also leads to improvement of the reaction rates, particularly if kinetically more inert ions are utilized (Biemmi et al., 2009). It is worth asserting that prolonging the reaction times can lead to degradation of the frameworks (Millange et al., 2011). The long reaction time and the subsequent high energy consumption have led to the introduction of alternative methods for synthesis. Additionally, alternative methods can not only reduce the reaction time and save energy but also exert great effects on morphology, size distribution and particles size of the pores, thus significantly affecting the MOFs properties. Regarding the separation and gas storage technology, the pore size in porous materials such as MOFs have an impact on the diffusion of the guest molecules as well as the adsorption properties. Due to these objectives, alternative synthesis methods such as microwave-assisted, sonochemical, mechanochemical and electrochemical routes have been applied.
4.2.2. Microwave Synthesis (MW) Microwave irradiation is a distinguished technique for materials synthesis (Mingos and Baghurst, 1991). This synthetic method occurs based on the interaction of the mobile electric charge with electromagnetic waves. The electric charge can be supplied by electrons or ions in solid materials and the existence of the polar solvent, molecules/ ions in the solution. Therefore, the electrical current and heating are produced based on the above mentioned distinguish mechanism in solid and solution forms. For the case of solid, the electrical current is generated based on the electrical resistance of the solid materials. In the solution, the electrical current is created by aligning the polar molecules / ions when
they are exposed in the electromagnetic field, where their orientation is changed permanently. Hence, to achieve the best end-product, adequate frequency is required to create a proper collision between the reactants contributing to enhancement of the kinetic energy and the reaction temperature. In addition, choosing the appropriate solvent and the selective energy input is highly vital and must be considered due to the possibility of the interaction between the starting materials and the MW radiation (Klinowski et al., 2011). The synthesis reaction takes place in the microwave oven and the condition of the reaction should be accurately controlled, especially in terms of temperature and pressure. For MOFs, the reaction has been accomplished above 100 C and less than 1h which is indeed faster than the conventional methods. The MW assisted method has also led to an increase in the crystallization speed as well as a reduction in the crystals size (mostly in the nanoscale size). High reaction speed, low reaction time, a direct interaction between the reactant and the radiation make this synthesis route as a good heating energy efficient method. Accordingly, the synthesis reaction can take place at a high heating rate as well as homogeneous heating throughout the reactants (Klinowski et al., 2011). Since MOF-5 is one of the basic MOFs, the effect of the MW irradiation on its structure has been also investigated by Choi et al. (Choi et al., 2008). They synthesized MOF-5 via the MW irradiation method and based on their observation, the texture properties of this MOF was similar to the Ca. route (Langmuir surface area: 3800, 3200 m2 g-1 for MW and Ca. respectively). It was found that the reaction time and the average pore size decreased dramatically for the MW synthesized MOF-5 (reaction time: 1h to 30 mins, pore size: 400-500 to 20-25 𝜇m). In practice, by prolonging the reaction time, several surface defects and deterioration of crystals could be created. Therefore, optimization of the reaction time is quite critical in the MW assist synthesis route. As regards the effect of this synthesis route on the MOFs’ structure, several studies have been undertaken to this date. To illustrate this, Blanita et al. (Blanita et al., 2016) investigated the effect of concentrating the starting materials, solvents, temperature, reaction time and MW power on the texture properties of HKUST-1. According to their statement, the best BET surface area (1863 m2 g-1) was related to the concentration of 4c which was dissolved in DMF, EtOH and H2O at 70 C during 10 mins
with MW power of 360 w. Moreover, Tari et al. (Tari et al., 2016) synthesized MCM-41/Cu(BDC) composite via the MW irradiation for selective CO2 adsorption. Practically, they functionalized MCM41 by Cu (BDC) MOF with cyanotripropyltriethoxysilane linker. The MOF showed a great CO2 capacity and selectivity adsorption over CH4 at 298 K and 4-40 bar. These enhancements were attributed to the quadrupole moments of CO2 and the presence of negative and positive charges of COO- and Cu2+ ions in the framework. The BET surface area of the composite was also improved as compared to the parents MOFs (1084, 1063 and 624 m2 g-1 for composite, MCM-41 and Cu(BDC), respectively). In addition, after three times of cyclic adsorption of CO2 and CH4, the composite lost only 1 % of its capacity suggesting the high stability of its structure. Another study in this area was accomplished by Cabello and coworkers (Cabello et al., 2015). They prepared (Na,Cd)-MOF through the MW irradiation which possessed open metal sites in the framework structure (sodium ions). As expected, the reaction time declined as compared to conventional synthesis route from 5 days to 1 h. The MW synthesis of (Na,Cd)-MOF indicated improvement in CO2 adsorption properties (Table 9). Furthermore, Cd-NaMOF (MW) exhibited long term water stability even under 95 % humidity. Additionally, after 15 cycles of CO2 uptake, it still maintained the CO2 adsorption capacity that is very indispensable factor, especially in case of industrial usage.
Table 9 CO2 adsorption properties of (Na,Cd)-MOF based on different synthesis methods. Methods
BET surface area (cm3 g-1)
CO2 uptake (mmol g-1) at 298 K and 1 bar
Qst (kJ mol-1)
Conventional
418
2.25
42
MW
526
2.71
-
Ref. (Cabello et al., 2015) (Cabello et al., 2015)
4.2.3. Sonochemical Synthesis In this method, the synthesis reaction takes place by applying ultrasound waves having very high levels of energy. Actually, it is a cyclic mechanical vibration with a frequency range of 20 kHz (higher than the human hearing range) to 10 MHz. By channeling the high ultrasonic energy into the solution, different pressure (compression and rarefaction) regions are created in the reactor which leads to
formation of cyclic alternating regions (bubbles). As a result, in the low pressure area, the pressure drops below the vapor pressure of the reactants and the solvent; for that reason, small bubbles or cavities would be created. These cavities (bubbles) have the ability to expand to around tens of micrometers due to diffusion of the solution vapor into the cavities. The bubbles will reach their critical size which is an unstable state that subsequently leads to implosion. The process of formation, growing and collapsing of the bubbles is referred to as the cavitation procedure. Hence, the reaction occurs within a very short time. Several factors such as the starting materials (viscosity, reactivity, vapor pressure and selection of the solvent) and equipment (acoustic frequency and intensity, temperature and gas atmosphere) should be taken into consideration in this synthesis method. Moreover, attention must be paid in choosing the solvents, i.e. the organic solvents are not typically proper for the sonochemical synthesis. The high vapor pressure of the organic solvents would shrink the intensity of the collapsing cavities, consequently affecting the reaction pressure and temperature(Shono et al., 2000). For solid particles in the solution, the microjet phenomenon takes place in preference to cavitation. When the bubbles (with very high speed) come in the vicinity of the solid particles, erosion of the bubbles occurs, resulting in solid materials’ clean and activated surface. Likewise, dispersion of smaller particles in the solution will be accomplished. The chemical reaction can occur inside the cavities (the best conditions), in the bulk media, or at the interface of the medium (intermediate pressure and temperature). This method is extensively used for synthesis of nanomaterials and organic materials (Bang and Suslick, 2010; Fillion and Luche, 1998). The ultrasound synthesis method is advantaged with very fast reaction, while being accomplished easily, being environment-friendly, plus its being accomplished at room temperature and being energy efficient. Not many MOFs were synthesized through this method so far than solvothermal and MV but the results of that one which reported were acceptable.
HKUST-1 was the first MOF being synthesized via the ultrasound route (Li et al., 2009b). This synthesis was accomplished by adding H3BTC (ligand) in the mixture of DMF and ethanol (solvents); then a cupric acetate dihydrate (aqueous solution) was added to the mixture in the ultrasonic bath at the ambient condition. The prominent difference of HKUST-1 synthesized through ultrasound and solvothermal routes was the reaction time which decreased from 24 h to 5- 60 minutes, the reaction temperature of 25 C for the ultrasound and the dimensions of the pores which decreased to nanosize (10–200 nm). However, the physicochemical properties of the MOF have no difference with the solvothermal one. The morphology and size of the end product were indeed related to the reaction time. For example, spherical particles with the range of 100-200 nm were resulted from the short time reaction, whereas the long reaction time (30 and 90 min) generated long needle particles with the size of up to 900 nm. MOF-5 was synthesized via the sonochemical route for the first time by Son et al. (Son et al., 2008) who examined different factors having effects on the end product. As expected, the reaction time decreased dramatically from 24 h to 30 mins (ultrasound route). MOF-5 is usually synthesized in diethylformamide (solvent), albeit they used 1-methyl-2pyrrolidone (NMP) as the solvent because it is more economical than diethylformamide. Likewise, to gain the best result, different amounts of NMP were investigated (25-75 molar ratio) confirming that the best crystals were obtained with an appropriate amount of solvent. The textural properties such as the specific surface area and the pores volume of S-MOF-5 and C-MOF-5 (conventional route) were identical (320 m2 g-1 and 1.21 m3 g-1). For the CO2 adsorption capacity, a small improvement was obtained (790 and 820 mg g-1 for C-MOF-5 and S-MOF-5, respectively). The effect of the solvent on the textural properties on the MOF in the ultrasound synthesis was investigated by Israr and associates (Israr et al., 2016). They synthesized Cu-BTC in various solvents (DMF, H2O, EtOH) with different ratios. Among them, the exceptional Cu-BTC crystals yield of 86% was obtained from EtOH, H2O, and, the DMF mixture with the ratio of 20:40:40. All the samples displayed the nanoporous nature and the highest BET value was associated with the samples which were obtained from water/DMF and water/ethanol/DMF solvent (1430 m2/g and 1400 m2/g, respectively).
Kim et al. (Kim et al., 2011) synthesized catenated Cu-TATBn (n=30-60) via a sonochemical route. On the word of their observation, catenation in CuTATB-60 yielded a positive effect on CO2 adsorption properties, including an improvement in the surface area, stability of the framework, a high CO2 adsorption capacity (189 mg CO2 g-1) as well as outstanding selectivity over nitrogen (>20: 1) at 298 K. The heat of the adsorption was also scrutinized which was 35 kJ mol-1at under the same condition. Further to these properties, the catenated sample retained its structure after 5 consecutive adsorption/desorption processes (high purity CO2).
4.2.4. Mechanochemical Synthesis As opposed to the previous methods, the mechanochemical synthesis is a solvent-free route and the end material (MOFs) is produced by neat mixing and grinding of the metal salt and the organic linker via the ball mill mixture (The schematic of this synthesis method is shown in Fig. 9). This method has numerous advantages, namely their being environment-friendly due to the solvent free reaction (Garay et al., 2007), a short reaction time (usually 10-60 min), a small particle product. Moreover, the reaction normally occurs at ambient temperature. In addition, in some occasions, the metals site can be changed with metal oxide and this leads to formation of water as a side product (Friščić and Fábián, 2009; Friščić et al., 2010; Yuan et al., 2010b).
Fig. 9. Schematic illustration of mechanochemical route.
In order to promote the reactivity of the reactant, using the linkers with lower melting points and metals which release the solvent or the ones hydrated during the reaction can be highly effective. One of the mechanochemical synthesis methods is the liquid assisted grinding (LAG) route (Friščić and Fábián, 2009) wherein the reaction takes place by adding a very small amount of solvent to the reaction medium. The advantage of LAG over the original one is acceleration of the mechanochemical procedure due to an increase in the mobility of the reactant molecules in the presence of the solvent. Friscic and Fabian adopted this method to synthesize six coordination polymers for the first time by adding variable amounts of solvent in the mixture of ZnO (metal oxide) and fumaric acid. Hence, different MOFs were obtained having various dimensions (1D, 2D, and 3D). They also constructed new pillared MOFs by adding bpe or bipy (pillared linkers) in the presence of methanol, ethanol, DMF or isopropanol as the solvents (Friščić and Fábián, 2009). This research enclosed the ability of the LAG methods to producing a variety of MOFs with different structures and geometries. Furthermore, they observed that by adding the catalytic amount of metal salts, several structures could be acquired. For instance, usage of the sulfates salts promoted a hexagonal structure while the nitrile salts induced a tetragonal structure. For CO2 adsorption on MOFs, Masoomi et al. (Masoomi et al., 2014)introduced a new framework (Zn(II)-MOF based MOF) by utilizing the azinefunctionalized linker via the mechanochemical route. They synthesized TMU-4 and TMU-5 by using H2oba and n-donor ligands with different topologies and pore sizes. Based on their results, both frameworks indicated a high affinity towards CO2 molecules (almost same) at different temperatures (195, 298 and 273 K). Although the mechanochemical method is one of the promising synthesis routes, it still requires supplementary investigations.
4.2.5. Electrochemical Synthesis In this synthesis method, the chemical reaction occurs through the electrons transfer which generates the electrical current. The reaction medium contains anode, cathode, and the electrolyte solution which comprises a conducting salt and dissolved linker molecules. Rather than utilizing the metal salts, the metal ions are introduced continuously to the reactor via anodic dissolution. The striking point is that the cathode electrode must be protected due to the tendency of the metal ions deposition on its surface. Hence, the protic solvents and compounds such as acrylic, maleic esters and acrylonitrile (preferentially reduced) can be used in the reaction medium to tackle this issue. At large scale productions, the electrochemical method would most probably have a continuous procedure which leads to a higher end product yield (scale up). This method was represented for synthesizing the MOFs by researchers of BASF in 2005, who synthesized some Zn- and Cu- based MOFs (Mueller et al., 2007). In this research, various experimental setups and different combinations of linkers (H2BDC, H2BDC.(OH)2, 1,2,3H3BTC and 1,3,5-H3BTC) and anode materials (Mg, Zn, Co, and Cu) were explored. It was established the MOFs which contained Zn- or Cu- compounds exhibited higher porosity. Since then, this synthesis method has been applied in the construction of the MOF. For instance, Muller et al. (Mueller et al., 2007) produced HKUST-1 for hydrogen storage and purification from natural gas. The effect of the synthesis process on the properties of HKUST-1 was investigated by Schlesinger et al. (Schlesinger et al., 2010) who generated this MOF via the conventional, electrochemical, microwave routes at ambient pressure. The results indicated that HKUST-1 which was produced via the electrochemical method had an inferior quality among them. This was indeed attributed to incorporation of the conducting salt and the linker molecules during crystallization. Another instance is MIL-100 (Fe) which was initially synthesized via the electrochemical route by Campagnol et al. (Campagnol et al., 2013), where the reaction took place at high temperature and pressure. The electrochemical cell contained Fe as the anode and the solution of H3BTC in the mixture of ethanol and water (2:1). The crystals of MIL-100 (Fe) were obtained at different temperature ranges of 110–190 °C on the top of the anode electrode and in the solution simultaneously. According to their report, an improvement was gained on the morphology of
this MOF. Generally, this method is claimed to be very useful when producing MOFs in the form of thin films is intended.
5. Methods to improve CO2 adsorption As far as this, different methods have been introduced to improve the CO2 adsorption capacity and selectivity on MOFs, including the open metal site (OMS), pre-synthesis modifications and postsynthesis modifications and tuning the pore size. Numerous research has been also accomplished on these effective modification methods while there are a plethora of review papers in this context (Liu et al., 2012a; Liu et al., 2012b; Tanabe and Cohen, 2011). For this reason, the current review paper just elaborates on these modification methods succinctly while casting further emphasis on new achievements in this area.
5.1. Open Metal Sites (OMS) and Its Effect on CO 2 adsorption Properties. Metals site exerts a significant impact on increasing the capacity and selectivity of CO2 over the other gases. During the synthesis procedure, some metal sites in the MOFs structure are involved with solvent molecules. Therefore, the capability of the MOFs to adsorb the guest molecules would be diminished. To overcome this issue, the best way is the activation process which can be achieved via applying the temperature and/ or the vacuum. In this process, the exceeded solvents are removed from the metal sites leading to creation of the open metal sites (OMS). Therefore, there will be an upsurge in the CO2 (guest molecules) adsorption capacity of the MOF. The OMS has a great influence on the binding energy between the surface of the MOF and the adsorbed CO2 molecules. Indeed, these metal centers make the surface of the framework as active sites to trap CO2 molecules and bond them
by inducing dipole–quadrupole interactions (Liu et al., 2012b). Some MOFs used for CO2 adsorption via their OMS sites are indicated in Table 10.
Table 10 The MOFs with open metal sites used for CO2 adsorption.
Cu-TDPAT
BET surface area (m2g-1) 1938
132 cm3/g
CO2/N2 (79)
298
1
MIL-101 (Cr)
2800
28 mmol/g
CO2/CH4
300
< 50
MIL-101 (Cr)
3780
34 mmol/g
CO2/CH4
300
< 50
MIL-101 (Cr)
4230
40 mmol/ g
CO2/CH4
300
< 50
MOP: metal– organic polyhedron (Cu II)
12.1 mmol/g
CO2/N2
196
10-5
M/DOBDC M: Mg, Ni, Co, Zn
Mg/DOBD C: ~250 mg/g
293298
1
298
10
MOF
M-DABCO M = Ni, Co, Cu, Zn
Ni: 2120 Zn: 1870 Cu:1616 Co: 2022
CO2 uptake
Ni: 2.4 mmol / g Ni> Zn > Cu >Co
Selectivity
CO2/CH4
T (K)
P (bar)
Reason for enhancement of CO2 adsorption properties Lewis basic sites and the dual functionalization of the framework Enhancement of OMS due to activation method (hot ethanol) Higher density of Oms due to effective activation method (hot ethanol+ NH4F) strong interaction between coordinativelyunsaturated Cu(II) sites and CO2 molecules Mg/ DOBDC showed excellent CO2 adsorption among them due to, shorter length of Mg-O because of the smaller atomic weight of Mg comparing to other metals.Binding length: Mg-O (1.969 Å) < Ni-O (2.003 Å) < Co-O (2.031 Å)< Zn-O (2.083 Å). Ni-DABCO because Ni cation as a metal center possesses the highest charge density
Ref. (Li et al., 2012) (Llewellyn et al., 2008) (Llewellyn et al., 2008) (Llewellyn et al., 2008) (LópezOlvera et al., 2017) (Yazaydın et al., 2009)
(Cabello et al., 2014)
As stated in section 4.1.1, almost all the transition metal in the periodic table can be used to create the MOFs. Since these metals interact with other atoms and molecules through their d atomic orbital, recognizing their bounding mechanism with other molecules is interesting and essential. This condition could be explained by the interaction of MOF frameworks through OMS with small guest
molecules based on the d band center theory. To produce the MOFs with superior adsorption and separation properties, it is important to know the nature of the interaction of the guest molecules with the OMS sites. Besides, the difference of these cations should be considered. Also, the binding between the OMS sites and the adsorbate is the result of an appropriate balance between hybridization of the molecular orbitals, electrostatics, van der Waals forces, and Pauli repulsion. Therefore, the transition metal cation plays an important role in the adsorption properties of the MOFs. Lee et al. performed a comprehensive study on M-MOF-74 with different metal sites (M: Mg, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) (Lee et al., 2015). They investigated the binding enthalpies of the flue-gas components in the transition-metal of M-MOF-74s based on the density functional theory (DFT). The binding enthalpies of the flue gas on the M-MOF- 74s are summarized in Table 11. The highest binding enthalpy represents a stronger binding between the OMS cations and the guest molecules. As the data shows V-MOF-74 has a stronger binding with CO2 and CH4, while N2 molecules have a strong binding with Ni cation of MOF-74. Based on the obtained result in this research, it can be concluded that the transition metals show different adsorption behaviors due to the diversity of the electrons in d orbitals, accordingly, to improve the adsorption properties of the MOFs, their structure can be designed based on the guest molecules.
Table 11. Calculated Binding Enthalpies (in kJ/mol at 297 K) of the flue gas in the Isostructural M-MOF-74s (Lee et al., 2015). Flue gas H2 CH4 N2 CO2 H2O
d0 Mg 10 19 28 41 62
d2 Ti 17 23 58 46 71
d3 V 18 26 52 51 77
d4 Cr 6 14 14 27 31
d5 Mn 8 19 21 34 51
d6 Fe 9 19 21 34 49
d7 Co 9 18 21 34 48
d8 Ni 10 19 24 37 56
d9 Cu 6 14 13 27 24
d10 Zn 8 19 19 30 41
Since the MOFs possess the transition metal cation in their frameworks, the influence of the spin and magnetic states of the metal ion on the MOFs performance should be taken into account. In order
to do gas adsorption, although gas sorption and spin-crossover can co-exist in the framework, no appreciable interaction takes place between the adsorbed gas molecules and the magnetic host network (Ohba et al., 2009; Southon et al., 2009). Therefore, in most adsorption cases, adsorbed gas molecules do not exert any effects on the spin transition temperature (Espallargas and Coronado, 2018). However, in case of using MOFs as sensors, in magnetic refrigeration, in electronic applications, or in quantum computing, spin and magnetic state must be strongly considered.
5.2. Pre-synthetic procedure
Ligand functionalization exerts a tremendous effect on improving the MOFs’ affinity toward CO2 adsorption and separation. Ease of the modification procedure and the variety of functional groups make this method as one of the best and applicable processes to achieve high CO2 storage properties. CO2 molecules possess the quadrupole moment; thus, the presence of polar groups in the backbone of framework will improve CO2 adsorption and selectivity among other gases such as CH4, N2, and H2 (non-polar). The functionalization process commonly occurs through pre-synthesis and post-synthesis modification procedures. So far, different functionalization methods have been introduced based on functional groups which are attached on the backbone of the frameworks. as the methods include amine, phosphonate, sulfonate, and multi functionalization processes.
5.2.1. Amine functionalization Amine functionalized ligands have more attraction among other modification methods because of the high affinity of the amine molecules toward CO2 adsorption and selectivity. Without a doubt, the amine molecules act as the Lewis basic and CO2 molecule as the Lewis acid (Liu et al., 2012b). For instance, Pachfule et al. introduced two isostructural interpenetrated amine functionalized MOFs; CoANIC-1 and Cd-ANIC-1 (ANIC: 2-amino-isonicotinic acid). Both MOFs exhibited a high CO2 uptake
capacity of 4.22 and 4.72 (mmol g-1) for Co-ANIC-1 and Cd- ANIC-1, respectively. This desirable CO2 uptake is attributed to the Lewis basic effect of amine groups which was presented in the ligand (ANIC) (Pachfule et al., 2011). Moreover, a comparison study between MIL-101(Al) and NH2-MIL-101 (Al) was performed by Serra et al. (Serra-Crespo et al., 2011). They reported that both CO2 capacity and selectivity of amine functionalized MIL-101 (Al) increased compared to the parent MOF. In addition, –NH2-MIL-101(Al) performed fast regenerability and high thermal stability making this MOF high potential adsorbent for separation of natural gas. Apart from the positive effects of amine groups on improving the CO2 capacity and selectivity into MOFs, amine groups have some limitations concerning the number of this functional group on the framework. If the ligand contains a high number of amine groups, the excessive amine cluster can reduce the CO2 uptake due to interwove of them on the backbone. An experimental and theoretical study of grand canonical monte carlo (GCMC) simulation on NJUBai3 ([Cu3L2(H2O)5]. x Guest) that contained polar amide groups was accomplished by Duan and coworkers (Duan et al., 2012). Both experimental and theoretical studies exhibited a high CO2 uptake (6.21 mmol g-1 at 1 bar and 273 K) and selectivity. Also, the high isosteric heat confirmed a strong affinity of this MOF toward CO2 adsorption (36.5 kJ mol-1). In order to modify the ligand via amine groups, the length of the chain and the activation methods must be considered as they have effects on the surface area, porosity and CO2 uptake capacity. With regard to the prominence of these parameters, Keceli and coworkers (Keceli et al., 2014) modified biphenyl ligand with the amide group which possessed different chain lengths (R: CH3, C2H5, C3H7, and C4H9) and employed two activation methods (the exchange with acetone by vacuum drying at 80 C for 24 h and supercritical CO2 drying). The results exhibited that the MOF functionalized having the amide with a shorter chain had more affinity to adsorb CO2 (4.65 and 2.95 (mmol/g) for R: CH3 and C4H9 respectively). In case of activation methods, different impacts were observed; for instance, in the short chains, supercritical CO2 increased the pore volume and the surface of the MOFs. In contrast, for the long chains, the exchange with acetone by vacuum system was increased the pore volume and surface area of MOFs. Accordingly, the activation methods have a critical effect on the
adsorption properties of the MOFs, and this is congruent with the previous literature (Bae et al., 2009; Hafizovic et al., 2007).
5.2.2. Open nitrogen sites Generally, the existence of open nitrogen sites in the framework has a positive effect on CO2 adsorption. Triazoles and tetrazoles are the best examples of this category. CPF-6 is the MOF that possesses a 3D structure with high concentration of uncoordinated nitrogen sites from tetrazole ligand (1,3,5-tris(2H-tetrazol-5-yl)benzene). This framework has indeed demonstrated high CO2 adsorption (98 cm3g-1) and selectivity over N2, even without any open metal sites (OMS). Superior CO2 adsorption properties are attributed to the dense of the open nitrogen sites in the pores surface (Lin et al., 2011). Although usage of open nitrogen sites in reinforcement of CO2 adsorption properties has a great impact, it may not be necessary at all. According to the results of a comparison study on tetrazolate base rthMOF and pyrazolate base rth-MOf, the pyrazolate form which has less uncoordinated nitrogen site (Table 8 ) displayed a higher CO2 adsorption capacity. This behavior is attributed to the strong electric field of the pyrazol group (Gao et al., 2015). Another illustration of tetrazolate functionalization MOFs is UTSA-49 [Zn(mtz)2] whose ligand is decorated by open nitrogen sites via Hmtz groups (5-methyl1H-tetrazole) and methyl groups. The Zinc atom attached to the mtz- ligand only forms two free nitrogen atoms which are neighboring the methyl groups on the ligand rings. Therefore, the other nitrogen atoms are uncoordinated on the surface of the pore and ready to trap the CO2 molecules. The amount of the CO2 uptake for this MOF is 13.6 wt.% at 298 K and 1 bar. Moreover, the reaction enthalpy was 27 KJ mol-1 revealing the affinity of framework to trap the CO2 molecules (Xiong et al., 2014). Adenine groups are also a good candidate for ligand functionalization due to uncoordinated nitrogen sites in their structure (Table 8). Adenine has been utilized in constructing many MOFs for various reasons, summarized as follows: 1) different nitrogen positions on its structure allow diversity of MOFs, 2) it inherently has a rigid structure whereby robust frameworks will be obtained 3) all of the adenine atoms are coplanar, meaning that an inter-ligand interaction (π–π) would be possible during formation
of the MOF(An et al., 2009; Song et al., 2014b). These striking properties make adenine an effective building block that can be used in constructing MOFs for CO2 adsorption. It is substantiated by a computational study that determined that the interaction energy between adenine and CO2 molecules is higher than other MOFs which comprise nitrogen linkers. The effect of adenine groups on bio-MOF-11 (Co2(ad)2(CO2CH3)2.2DMF·0.5H2O) was investigated by Rosi group (An et al., 2009). The results demonstrated a high CO2 adsorption capacity (4.1 mmol g-1), isosteric heat (45 kJ mol-1) and selectivity over N2 (75:1) at 298 K and 1bar. These promising CO2 uptake properties are attributed to the narrow dimension of the pores (compatible with CO2) as well as Lewis basic pyrimidine and amino groups of adenine in the framework. The effect of the long chain on the CO2 adsorption capacity on Co-adenine series MOFs (Bio-MOF-11 to 14) was examined as well. According to the results, by increasing the aliphatic chain length (acetate, propionate, butyrate, and valerate, respectively) the CO2 capacity declined at 1 bar and 273 as well as 298 K. This reduction was attributable to the entanglement of the long chain. Among them, Bio-MOF-11 showed the highest adsorption capacity (105 cm3 g-1) at both temperatures and 1 bar (Li et al., 2013). These promising CO2 uptake properties are attributed to the narrow dimension of the pores (compatible with CO2) as well as Lewis basic pyrimidine and amino groups of the adenine in the framework. The most prominent subclasses of the MOFs are Zeolitic imidazolate frameworks (ZIFs) which are attention-grabbing because of their unique features and their capabilities of being utilized in several applications. (Al-Maythalony et al., 2015; Eddaoudi et al., 2015). The gas adsorption capacity of these materials can also be improved as illustrated by Wang et al (Wang et al., 2014). They successfully synthesized new ZIF (ZTIF) by applying the tetrazolates ligand into the Zn-Imidazolate framework. The result disclosed a significant surge in the CO2 adsorption capacity (86 cm3 g-1 at 273 K), isosteric heat (Qst: is 22.6 kJ mol-1) as well as selectivity of CO2 over CH4 and N2 (6.3 and 59 at 273 K respectively). This enhancement was as a result of the presence of the uncoordinated N-heteroatom sites in the frameworks of this MOF which was created via the tetrazolate ligands sites.
5.2.3. Phosphonate and sulfonate functionalization Ligand functionalization can both exert a great impact on CO2 adsorption and selectivity over the other gases and improve the framework stability. In this context, phosphonate, and sulfonate organic ligands should be taken into account. These functional groups have a substantial effect on the water stability of the framework (Shimizu et al., 2009). The flue gas often contains a small quantity of water vapor which remarkably impacts CO2 adsorption, especially in the MOFs. Therefore, reinforcing the MOFs’ water stability plays a pivotal role in gas adsorption technology (Taddei et al., 2013). The first study based on water stable phosphonate-ligand for CO2 capture into MOFs was executed by Marco and coworkers (Taddei et al., 2013). They synthesized phosphonate co-ligand MOF [(Cu3 (H2L2) (bipy)2. 11H2O), H2L2: [N,N,N΄,N΄tetrakis(phosphonomethyl)hexamethylenediamine]. According to their observation, this MOF took up 77 cm3 g-1 of CO2 at ambient temperature and 10 bar, demonstrating a high value of adsorption. In addition, it has acceptable selectivity of CO2/N2 and this denotes two implications: firstly the polarity of the P–O groups on the ligand may boost the affinity of the CO2 uptake due to its quadrupole moment; secondly, after the activation process, no pores would remain to take up N2 molecules. In this respect, CALF-25 and CALF-30 are the other MOFs exhibiting good water stability (Gelfand et al., 2015; Taylor et al., 2012). In practice, both reported MOFs demonstrated a considerable CO2 uptake. In addition, the frameworks structures were still retained even after exposing them to 90% of humidity at 273K and 298 K for both MOFs. The water stability arises from the steric group on the ligand parts of the MOF, leading to creation of the hydrophobic framework. Recently, an experimental and computational study was accomplished on the CALF class of the MOFs by Gelfand et al. (Gelfand et al., 2017). They produced CALF-33 via coordination of Cu(II) and Phosphonate monoesters (PMEs) ligand. This new MOF not only led to a boost in the water stability of the framework but also improved the CO2 adsorption capacity of the MOF. This improvement is justified by the presence of the ester group in the ligand sites (CALF-33-Et3) which leads to enhancing the active site to trapping CO2 molecules.
5.2.4. Mixed ligand-functionalization and multi-functional ligands The idea of using mixed ligand in the MOFs’ structure creates a fantastic clue, especially in tuning the frameworks and boosting the adsorption properties. Combinations of several functional groups within the framework not only reinforces the CO2 capacity and selectivity but also causes an improvement in the MOFs’ stability (Zhang et al., 2013). Consequently, presence of multiple functional sites on the framework leads to an improvement in the CO2 uptake ability of the MOF. In recent years, these functionalization methods have attracted a plethora of attention due to outstanding results that can be achieved. For example, Shimizu et al. (Vaidhyanathan et al., 2009) employed oxalic acid (ox), 3amino-1,2,4-triazole(Atz) as a mixed ligand and zinc carbonate to synthesize multi-functional MOF [Zn2(Atz)2(ox)] through the solvotheraml synthesis technique. According to the findings, apart from the small pore size (pore volume: 0.19 cm3/g) and the surface area of 782 m2/g, the CO2 adsorption capacity of this functionalized MOF was high (4.35 mmol/g at 273 K and 1.2 bar). In addition, it showed a high enthalpy of adsorption (40.8 KJ mol-1) at zero coverage that represented the affinity of the MOF to take up CO2 molecules. Furthermore, the modified MOF does not have the capability of uptaking other gases such as Ar, H2, and N2. Having considered all these observations, it can be inferred that this modified MOF is a good candidate for CO2 adsorption and selectivity over the other gases. In the subsequent year, Shimizu and his group (Vaidhyanathan et al., 2010) investigated the activated sites on the backbone of the framework via a computational study and an in-depth X-ray diffraction analysis. They identified two active sites for CO2 interaction; free amine groups via hydrogen bonding between the oxygen of CO2 and the hydrogen of the amine (N-H…O) and the interaction between the lone pair electrons of the nitrogen and the C atom of the CO2 molecules. Moreover, the oxygen site of the oxalic acid ligand was very active to interact with the CO2 molecules via the carbon site as well as the adjacent molecules with each other. As a result, a combination of these multi-functional sites on a framework
contributes to enhancement of the adsorption capacity, the selectivity, and the affinity of this MOF for CO2. A comparison study on 5 isomorphs MOFs was undertaken by Luo et al. (Luo et al., 2012). These MOFs were constructed by Zn (NO3)2, L (N4,N4΄-di(pyridin-4-yl)biphenyl-4,4΄-dicarboxamide) and isophthalic acid which were decorated by different functional groups such as –H (1), -NH2 (2), - OH (3), - NO2 (4) as well as –COOH (5). The results demonstrated that the MOF (2) possessed the highest amount of CO2 adsorption capacity and selectivity among the other functional groups (2 > 1 > 3 > 4 > 5) at 1 bar and 273K. Moreover, the CO2 binding affinity of Sample 2 was higher than the others (based on the DFT). These high CO2 adsorption capacity and selectivity for Sample 2 were attributed to the high affinity of the CO2 to both amide and acylamide groups on the pores wall of the framework. The Cobalt-based MOF with triazole and benzenetricaboxilic based ligands were decorated by Liu et al. via the solvothermal synthesis reaction(Liu et al., 2013a). The end-products adsorbed 77.3 cm3 g−1 of CO2 at 1 bar and 295 K also displayed significant selectivity over N2. The high CO2 uptake and selectivity of this MOF were attributed to the presence of free amine groups, triazolate’s uncoordinated nitrogen atoms, unsaturated metal sites (Co2+) as well as -C=O/ -COOH sites on the pore walls of the framework (several functional groups). Lately, a comparison study on {[Zn4(bpydb)3(datz)2(H2O)]-(DMF)4(EtOH)5(H2O)8}n (1) and {Zn(bpydb)(bpy)}n (2) as a mixed ligand functionalization was performed by Chen et al. The pore walls of activated MOF (1) was decorated by pyridine and amino groups and demonstrated a higher CO2 uptake capacity (80.9 cm3 g -1 at 273 K and 1 bar) than MOF 2 (40 cm3 g-1 under the same condition) as well as outstanding selectivity over CH4 and N2 . Combination of free pyridine-N atoms and amino functional groups led to improvement of CO2 adsorption properties in MOF 1. The turning point in this research was the heat of the adsorption value which indicated an enhancing trend (30.33 kJ/mol) at zero coverage revealing a strong interaction between CO2 molecules and the framework (Fig. 10) (Chen et al., 2015).
Fig. 10. Qst of CO2 for MOF (1) (Chen et al., 2015).
5.2.5. Other functionalization Alkyl groups (CH3), Hydroxyl (OH), and (COOH) groups are other effective functional groups used in the ligand functionalization as reported by several studies. For the alkyl group, the best example is methyl functionalization on the pillared ligand of [Zn3 (bpdc)3 (bpy)].(DMF)4· (H2O) and produced [Zn3 (bpdc)3(2,2’-dmbpy).(DMF)x (H2O)y (1) and [Zn3 (bpdc)3(3,3’dmbpy)]·(DMF)4(H2O)0.5 (2). Methyl functionalization in this study brought two opposing results based on the competitive effects happening for these MOFs. After modification, both the pore volume and the BET surface area of MOFs (1) and (2) fell as compared to the parent MOF. On the other hand, Sample (1) possessed the highest amount of CO2 uptake and affinity to CO2 adsorption (based on Qst) whereas Sample (2) demonstrated the lowest amounts (Table 12). This different adsorption behavior is due to two contrary effects that occurred simultaneously throughout the CO2 uptake. If the upsurge of CO2 affinity is dominated over the space-loss during CO2 adsorption, CO2 uptake will then decline. For Sample 2, the enhancement in CO2 affinity was not high enough to tackle the loss of the surface area and the pore volume. Therefore, the CO2 uptake diminishes during the adsorption process (Liu et al., 2013b).
Table 12 CO2 adsorption properties of functionalized and parent Zn-MOFs. MOF
Pore volume (cm3g-1)
BET surface area (m2g-1)(a)
CO2 Uptake (wt %) (T: 298 K) (P: 1 atm)
Qst (kJmo l-1)(b)
Parent
804 (457)
0.21
10.2
30.5–28.7
1
466 (308)
0.18
11.5
31.6–29.1
2
378 (195)
0.13
8.6
30.8–25.2
Ref. (Liu et al., 2013b) (Liu et al., 2013b) (Liu et al., 2013b)
Hydroxyl functionalization is the other method used for improvement of the CO2 uptake capacity. This method is highly effective owing to polarity of the hydroxyl groups (OH) which can increase the affinity of CO2 adsorption based on the quadruple moment of the CO2 molecules. Zhao et al. (Zhao et al., 2011) modified Zn(BDC) via Hydroxyl groups and the results demonstrated a significant improvement in the CO2 adsorption capacity (13.1 wt. %) at ambient temperature and 1 atm as compared to the parent MOF (7.4 wt. %). In this case, the role of the surface chemistry was more effective than porosity in Zn (BDC-OH) which was due to the strong interaction between CO2 and OH groups on the backbone of the framework. Furthermore, selectivity of CO2 over CH4 and N2 was remarkably higher in Zn (BDC-OH).
5.3. Post-Synthetic procedure (PSM) Another method for MOFs structure tuning with the intention of improving the CO2 adsorption performance is the post-synthesis modification (PSM). The main objective of this method is to decorate MOFs with a high CO2 capacity and selectivity as well as lower energy consumption for the regeneration process. This method differs from the pre-synthesis modification as in the PSM, the functional groups are inserted into the preexisting MOF. In some cases, the PSM has some advantages over the pre-synthesis methods such as the possibility of the reaction between the functional groups
of the ligand with the metal site in the synthesis procedure of the primer framework which may alter the crystal structures; therefore, the end product will be changed. In addition, an accurate synthesis condition should be considered, particularly in the solvothermal synthesis reaction due to instability of some functional groups under the synthesis condition. Additionally, some other parameters are involved in pre-synthesis functionalization such as the hindrance effects, the solubility of the functional groups in the solvent where all these parameters might produce side-products. Thus, the PSM is an attractive route to improve the CO2 adsorption properties (Cohen, 2011; Deng et al., 2010). As mentioned formerly, the amine groups have a great affinity to adsorb CO2 molecules. Ethylenediamine (en) is the most common functional group which is used in PSM. A good example of en grafted MOF is en-MOF-74 with amine loading of 16.7 wt. %. This modified MOF exhibited an exceptional CO2 uptake (13.7 wt. %) at 298 K and 0.15 bar. The value of the isosteric heat was also very high (49 to 51 kJ mol-1) that exhibited chemisorption between CO2 molecules and amine groups resulting in formation of carbamic acid. This MOF just lost 3 % of the CO2 uptake capacity after 5 cycles of adsorption-desorption notwithstanding chemisorption of CO2 and amine groups. Besides, en-MOF74 exhibited high water stability in different amounts of moisture. Hence, en modified MOF possessed a great potential for CO2 adsorption (Lee et al., 2014). In 2014, Gadipelli. S et.al introduced the thermal annealing method for the first time as the post-synthesis route on MOF-5 (Zn4O[BDC]3). In this method, the framework is heated close to but below the decomposition temperature of the MOF to eliminate the solvents from the surface of the pores. The results showed high CO2 adsorption (up to 2 mmol g−1) at 298 K and 1 bar which was almost two times higher than the unmodified MOF-5. This indicated high stability for cyclic CO2 adsorption as well as moisture (Gadipelli and Guo, 2014). Some MOFs being treated via the post-synthesis methods are concisely mentioned in Table 13. Several post-synthesis modifications of the MOFs have been executed to date. Despite their efficiency, the PSM reactions require an accurate control due to formation of single crystals without any collapse and degradation of the framework. Yet, this area needs further investigations to improve
this method to create ideal frameworks especially in case of amine functionalization as the best functional group for MOFs treatment (Cohen, 2011). So far, numerous researches have been performed on ligand functionalization and comprehensive investigation is accessible in the literature in this area (Du et al., 2013; Li et al., 2016; Zhao and Sun, 2014) although more investigations are needed for optimization of this method.
Table 13 Some modified MOFs via the post-synthesis procedure. All adsorptions take place at 298 K, P (bar), Qst kJmol-1. Modified MOF Ni-DOBDCPyridine IRMOF-74CH2-NH2-
en-Cu-BTTir
BET(m2 g-1) Befor After e
P
Qst
798
409
-
-
low
-
2440
2310
-
3.2 (mmol g1)
1
-
345
0.277 (mmol g1)
0.366 (mmol g1)
up to 0.06
90
870
0.37 (mmol g1)
2.38 (mmol g1)
0.15
-96
608
0.33 (mmol g1)
4.2 (mmol g1)
0.15
-
-
23.4 wt %
26.9 wt %
760 torr
-
1770
Mmen- CuBTTir
1770
MIL-101- PEI (Polyethylen imine)
3125
Mg-MOF-74 TEPA (Tetraethylenep entamine)
CO2 Uptake Before After
1628
Remarks
Ref.
-Increased water stability also CO2 capacity was maintained. - Showed the highest CO2 uptake -After regeneration maintain its structure completely (at 65 % humidity). -Regarding to affinity to adsorb CO2 were very high which leads to enhancing CO2 adsorption
(Bae et al., 2014)
-Drastically enhance CO2 adsorption and selectivity over N2 (327) -Raised CO2 uptake capacity and selectivity --dramatically (selectivity CO2/N2: up to 770). - raised CO2 uptake due to higher amine groups (15 wt %) on the framework. Increased water stability due to protecting effect amine group on the surface of MOF.
(Fracaroli et al., 2014)
(Lee et al., 2014) (Gadipelli and Guo, 2014) (Lin et al., 2013)
(Su et al., 2017)
5.4. Tuning pore Size
As mentioned in section 3.1.2, the pore size of MOFs has affected their adsorption behaviors especially in terms of selectivity; this is named the size-exclusive effect. Since each molecule
has unique kinetic diameters (Table 1), the adsorption selectivity occurs based on the pores size compatibility of the MOF and the kinetic diameter of the targeted molecules. Therefore, only smaller molecules can pass through the pores and the other ones are blocked. The ideal pore diameters of the frameworks for selective adsorption of CO2 over N2 and CH4 are 3.3 Å - 3.6 Å. Customarily, this method is expedient for selective adsorption of small gas molecules such as H2, N2, CO2, and CH4. The desirable pore size is achieved through the ligand or the metal exchange. According to the literature, the bulky linker, the short ligand, the interpenetrated networks, and smaller metal molecules are the best candidates in this respect (Kim et al., 2012; Ma et al., 2007a; Zhao et al., 2010). The pore tuning modification takes place through the postsynthetic exchange (PSE) process. The effect of the pores size on the adsorption behavior of the MOFs based on the ligand and the metal exchange has been examined by several researchers. In this aspect, zirconium-based MOFs such as UiO-66 (Zr) are good examples. According to the literature, these MOFs perform moderate CO2 adsorption at the ambient condition (Cmarik et al., 2012). The metal ion exchange in UiO-66 (Zr) to Ti (IV) was accomplished by Kim et al. for the first time (Kim et al., 2012). Also, the CO2 adsorption performance of the exchanged UiO-66 (Ti) was scrutinized by Lau et al. The results showed two times enhancement in both the CO2 uptake (81%) and the heat of adsorption. The smaller size of Ti ions in the exchanged framework afforded a smaller pores size which may be closer to the kinetic diameter of the CO2 molecules. Therefore, the CO2 properties of the new framework increase (Lau et al., 2013). As stated above, another option to reduce the pore size is using the interpenetration network (Zhao et al., 2010). Using elongated linkers leads to entanglement in the crystals structure, and this is called interpenetration. It means that the second or more identical frameworks are formed within each other (self-assemble) (Han et al., 2012). Hence, these networks are interwoven with one other and create the interpenetration network. As a result, the stability of these frameworks will increase while the available pore volume will decrease. Indeed, by creating more frameworks and their entanglements, the size of
the pores in these kinds of frameworks will be declined. Hence, the MOFs with this structure are the ideal frameworks for the gas separation and purification technology. This has been also confirmed by Han and Heo. They evaluated the effect of interpenetrated and porosity of 14 frameworks via the GCMC simulations method. As anticipated, the pore size of the interpenetrated frameworks reduced. Unexpectedly, the CO2 uptake capacity decreased at high pressure because of the pores volume reduction. The CO2 adsorption increased at lower pressure due to creating new sites to absorb the CO2 molecules. Indeed, this high adsorption capacity and selectivity at low pressure (up to 2 bar) are related to creating new CO2 adsorption sites with a high affinity in the interpenetrated frameworks. These sites have been obtained through the generation of spaces with two organic linkers of adjacent MOFs(Han et al., 2012). PCN-5 (H2 [Ni3O (H2O) 3(tatb)2] . 5DMF . 2H2O) is a double-interpenetrated framework which is suitable for selective adsorption of CO2 over CH4 based on the size-exclusive effect. This framework is constructed by 3 connected ligands (tatb) which are linked to the tri-nuclear Ni SBUs. The pore size of PCN-5 is 3.3 Å which is compatible with the kinetic diameter of the CO2 molecules (3.3 Å). Thus, this framework is proper for molecular sieving of CO2 over other gases such as CH4 and N2 (Zhao et al., 2010). Another interpenetrated framework which has been introduced recently is Cd-MOF which was synthesized via the solvotheraml method as a new 2-fold interpenetrated framework by Qin and coworkers. This framework demonstrated selective adsorption of CO2 over CH4, due to both the CO2 molecules strong interaction with the framework and the size-exclusive effect of this framework (Qin et al., 2014). Gao et al. presented a 4-folded interpenetrated framework MMPF-18 (metal−metalloporphyrin framework) by using a linear organic linker H2bcpp (5,15-bis(4carboxyphenyl)porphyrin) and tetra nuclear zinc cluster through the solvothermal method. They investigated the CO2 adsorption properties of this interpenetrated MOF at the ambient condition. The results showed a moderate CO2 adsorption capacity (11.8 wt %) and the heat of CO2 adsorption was around 23 kJ mol-1. In spite of a moderate CO2 uptake, the framework showed good CO2 / CH4 selectivity. This behavior is not only attributed to the narrow pores in the interpenetrated network but also to the presence of the epoxy groups of the ligand which afford to the size-selective chemical
transformation of CO2 and formation of cyclic carbonates (Gao et al., 2016). The highest amount of CO2 uptake on the interpenetrated MOF was reported by Chen et al. They synthesized a triple selfinterpenetrated MOF with nanoscale cages. This MOF showed the highest CO2 adsorption capacity of 2.23 mmol g-1 that has been reported by far among the interpenetrated frameworks. This framework also performed high Qst for CO2 adsorption (52.5 kJ mol-1) which confirmed the framework’s high affinity toward CO2 molecules. This MOF’s superior adsorption behavior is due to the contribution of nanoscopic cages and the charge of the frameworks (Chen et al., 2014). To gain high CO2 adsorption in the interpenetrated frameworks besides usage of effective functional groups, their accessibility plays a crucial role. In this regard, Safarifard et al. synthesized novel 3-fold interpenetrated MOFs (TMU22, -23 and -24) which were functionalized with different amide groups. Based on kinetics and breakthrough experiments, the selectivity of CO2 over N2 in TMU-24 was much higher than TMU-22 and -23 (70 % for kinetic experiment). Furthermore, TMU-24 showed the highest amount of Qst for CO2 in different temperatures 24-26 (kJ mol-1). More accessibility of the amide functional groups in the TMU-24 leads to high CO2 selective adsorption in this MOF. It means that the amide groups in this framework were more directed into the pores than the other isoreticular frameworks (TMU-22, -23) (Safarifard et al., 2016). In 2016, Chen et al. generated the supramolecular isomerism frameworks (Qc-5-M-dia and Qc-5Cu-sql-α) to improve selective adsorption of CO2/N2 and CO2/CH4 via tuning the pore-size. Both frameworks possess the same composition but with different topologies and pores sizes (Qc-5-M-dia: 4.8 Å and Qc-5-Cu-sql-α: 3.8Å). Interestingly, in the desolvation process of Qc-5-Cu-sql-α, the framework underwent an irreversible change to the Qc-5-Cu-sql-β which had a smaller pore size (3.3). According to their observation, Qc-5-Cu-sql-β showed the best selective adsorption of CO2 over N2 and CH4 which was attributed to the compatibility of the framework’s pore size (3.3 Å) with the kinetic diameter of CO2 molecules. Additionally, Qc-5-Cu-sql-β exhibited high water stability. This research demonstrated the effect of desolvation on the tuning of the framework’s pore size(Chen et al., 2016). Generally, tuning the pore size is fairly daunting for both flexible and rigid MOFs. For flexible MOFs, it is more complicated due to the dynamic behavior of the framework. For the rigid ones, the
accurate design of the pore size has been achieved rarely(Chen et al., 2016), which is because the control of the pore size between the ranges of 3–4 Å (appropriate for gas separations) is demanding (Zhang et al., 2014).
6. A new strategy for enhancement of CO2 adsorption properties on MOFs Rather than the previous reinforcement methods, some new strategies have been introduced to this technology. One of the best methods of improving CO2 adsorption which has enthused the researchers nowadays is producing MOF composites. The key criterion of using MOF-composites is their synergistic effects on their adsorption behaviors. According to the literature, usage of some adsorbents with high surface areas such as graphite, graphene, and carbon nanotube (CNT) has an outstanding effect on the adsorption behavior of the MOF-composites. For graphite and graphene, these materials should be added to the MOF structure in the form of graphite oxide (GO) or graphene oxide (GrO) due to the strong binding between the functional groups of GO and GrO (epoxy, carboxylic, hydroxylic) and the metal site of the framework. These materials have been introduced as one of the best adsorbents with the ultra-high surface area and the robust structure. Several MOFs have been produced by this great adsorbent as the MOF-composites which are applicable in different areas. For CO2 adsorption, some investigations have been accomplished to date. For instance, GO@MOF-505 composite which is produced from graphite oxide and copper-based MOF via the hydrothermal (solvothermal) route (Chen et al., 2017). This composite has shown good selectivity of CO2 over CH4 and N2. The CO2 adsorption capacity of the composite has 37.3% increase as compared to MOF-505 (parents). This remarkable enhancement of CO2 adsorption can be related to the presence of the unsaturated metal sites, creating new micropores, and improvement of dispersive forces in the surface of the composite framework. In addition, GO@MOF-505 has shown high moisture stability at 80 % RH. This stability is attributed to the strong coordination interaction between Cu ions and carboxylate groups in GO.
The effect of graphene oxide (GrO) on CO2 adsorption properties of Cu-BTC was investigated by Huang et al.(Huang et al., 2014). They synthesized GrO@Cu-BTC as a MOF composite via the hydrothermal reaction. The result indicated dramatic enhancement in selective adsorption of CO2 over CH4 rather than Cu-BTC (2.6 times that of parents). This improvement takes place due to a stronger interaction of CO2 molecules to the GrO@Cu-BTC framework. Since this method is new especially for the CO2 adsorption technology not, there is a dearth of research and a need for further investigations is felt because of their prominent effects on the CO2 adsorption capacity and selectivity. Some MOFcomposites which are used for CO2 adsorption are mentioned in Table 14. Similar to GrO, carbon nanotube (CNT) is an excellent candidate to produce the MOF-composite. Anbia andHoseini designed an MOF-composite (MWCNT@MIL-101 (Cr)) by using a multi-wall carbon nanotube (MWCNT) and MIL-101(Cr) (Anbia and Hoseini, 2012). The results indicated 60 % improvement in CO2 adsorption for the composite which is related to the higher micropore volume in the composite in comparison with the parent MOF. The high volume of porosity was achieved due to the presence of the MWCNT. Moreover, the composite showed high thermal stability. The effect of functionalized CNT (hydroxyl-CNT) on the MOF as a new composite was performed by Yang et al (Yang et al., 2014). They produced ZIF-8/CNT by using ZIF-8 and hydroxyl-CNT. According to their observation, not only CO2 adsorption had a rise but also the thermal stability underwent an improvement due to the presence of hydroxyl-CNT in the composite framework. It is striking to note that the morphology and the crystal structure of ZIF-8 in the composite were the same as the parents. In addition, a computational study has also been undertaken to investigate the selective adsorption of CO2 over N2 on single-walled carbon nanotube by Du et al.. They demonstrated that the combination of a van der Waals correction with ab initio DFT for explaining long-range dispersive interactions of CO2/N2 in the CNTs structure is very effective. Their results demonstrated enhancement in the selectivity of CO2 over N2 by doping of an Fe atom on the CNT surface. This improvement occurred due to quatherapole moment of CO2 molecules. Interestingly, none of the N2 molecules was adsorbed on the framework (Du et al., 2009).
Table14 Composites-MOFs for CO2 adsorption and selectivity. Composite-MOF
BET surface area (m2 g-1) Parents Composite
CO2 uptake (mmol/g) Parents Composite 7.09
Selectivity
T (K)
P (bar)
298
15
(Policicchio et al., 2013)
298
1
(Chen et al., 2017)
CO2/CH4: 32
298
25
CO2/N2: 24.77 CO2/CH4: 9.57 CO2/N2: 37.2 CO2/CH4: 8.6
Aminated Cu-BTCgraphite oxide
892
1367
GO@MOF-505
1101
1279
2.87
GrO@MIL-101
2670
2950
14.6
GrO@Cu-BTC
1382
1677
6.49
8.19
CO2/CH4: 14
273
1
GO@HKUST-1
1193
1554
6.85
9.02
CO2/N2: 186
273
1
MWCNT@MIL-101
1270
1243
0.84
1.35
298
10
ZIF-8/CNT
1839
1997
2.176
2.210
273
1
13.41
3.94 22.4
CO2/N2: 12.47
Ref.
(Zhou et al., 2015) (Huang et al., 2014) (Xu et al., 2016) (Anbia and Hoseini, 2012) (Yang et al., 2014)
In 2015, Kang and coworkers produced MWCNTs@JUC32-1 and MWCNTs@JUC32-2 as new composite-MOFs (Kang et al., 2015). These composites were prepared by using nano-sized JUC-32 and MWCNTs. According to the results, composite-MOFs captured more CO2 and CH4 per specific surface area as compared to their parents. Moreover, Qst of CO2 and CH4 in both composites had a significant improvement which was 32.3 kJ·mol-1, 33.8 kJ·mol-1 and 23.4 kJ·mol-1 for MWCNTs@JUC32-1, MWCNTs@JUC32-2 and parents MOF, respectively. In order to achieve a desirable MOF-composite, an optimum amount of the filler (GO, GrO and CNT) must be considered (optimization). Several MOF-composites have been reported so far, but in order to CO2 capture and storage, just some articles are available. Regarding these prominent effects of the fillers on the adsorption behavior of the Composites-MOFs, more investigations are needed in this area of research for future working.
7. The effect of water on the CO2 adsorption behavior of the MOFs
The most challenging part of the CO2 capture via the MOFs is the presence of water molecules during the adsorption procedure. 5-10 % of the flue gas from the power plant contains water molecules and most frameworks have a high affinity to react with the water molecules. Therefore, this would lead to distortion or even collapse of the framework. Subsequently, the pores are destroyed and the CO2 capacity and selectivity experience a decline. The most important and the weakest part of the framework is the coordination bond between the metal site and the ligand. Thus, the water molecules break this bond via the hydrolysis reaction. As shown by Equation 1, the ligand bond undergoes a displacement; therefore, the structure of the framework is destroyed (Sumida et al., 2012).
ML + H2O → M (OH) + LH
(1)
One of the prominent factors is the pKa value of the ligand which indicates the strength of the coordination bond between the ligand and the metal. If the ligand possesses high pKa, the basicity of that ligand is also higher. Therefore, the metal–ligand interaction will be stronger. Further to this, the kinetic and thermodynamic factors should be considered due to the Gibbs free energy of the reactant via the hydrolysis reaction (Burtch et al., 2014). As mentioned above, the kinetic and thermodynamic studies play important roles in improving MOFs’ water stability. In line with this, several studies have been accomplished in this area up to the present time. Jasuja and coworkers (Jasuja et al., 2013) conducted several studies on water stability in different MOFs. They evaluated the water stability of isostructural Zn(BDC-X)-(DABCO)0.5 series (DMOFs) via ligand functionalization with a polar group (fluorine) and nonpolar groups (methyl). While the structure of the DMOF was unstable in the presence of water, DMOF-TM2 (tetramethyl-BDC ligand) maintained its structure during the cyclic water adsorption procedure. Moreover, the molecular simulation showed improvement in DMOF-TM2’s kinetic stability because of the shielding effect of the methyl groups (nonpolar) on the ligand site. Therefore, no water molecules can come in the vicinity of the BDC to create the hydrogen bond, leading to the displacement of the ligand-metal interaction. Hence, there is
no chance for the electrophilic zinc atoms of the frameworks to react with the nucleophilic oxygen of the water which has caused the hydrolysis reaction. In the succeeding year, they appraised the impact of catenation and pKa of the ligand on the water stability of the framework. They chose two sets of MOFs; DMOF and MOF-58 (parents) also DMOFTM and MOF-508-TM (modified with the tetramethyl group). The results indicated that in the parents set, MOF-508 was completely water stable after being exposed to 90 % relative humidity (RH) while possessing a lesser pKa value. This result was attributed to the two-fold catenation effect of this MOF. In the TM modified set, DMOF-TM remained stable after 90 % RH which was related to the pKa value of the pillared ligand in this MOF (pKa for DMOF-TM and MOF-508-TM are 3.80, 8.86 and 3.80, 4.60, respectively). Moreover, the MOF-508-TM framework was not penetrated because of the steric hindrance of four –CH3 groups on its linker (Jasuja et al., 2014). Accordingly, utilizing the interpenetrated MOFs with a combination of the ligands having high basicity displayed good water stability. Based on their previous study, they synthesized new pillared MOFs with different metal sites (Cu, Ni, Zn, and Co) and a similar topology. All MOFs showed good water stability in the humid condition (90 % RH). Two main factors were involved in the great water stability of these MOFs including the presence of the methyl groups (in the BDC structure) and catenation in the framework due to the BTTB linker. The methyl groups preserve the metal site via the steric factors; therefore, the water molecule cannot react with the metal site. For the case of the BTTB ligand, the presence of catenation in its framework improved its water stability even with less basicity than the DABCO linker (Jasuja and Walton, 2013). Bae and coworkers conducted a research on the pyridine modified Ni-DOBDC to explore the CO2 capacity of the MOFs in the presence of water. According to their result, the modified MOF maintained the CO2 adsorption capacity whereas the water adsorption was reduced due to an increase in hydrophobicity of the framework (the presence of the pyridine group into the framework structure) (Bae et al., 2014). In 2016, Benoit et al. (Benoit et al., 2016) synthesized the titanium based MOF (MIL-91(Ti)) which possessed small pores. This hydrophilic MOF showed high water stability after several cyclic water
adsorption/ desorption processes due to the presence of the phosphonate group in the ligand site. Additionally, it showed a high CO2 uptake (~3 mmol g-1 at 303 K and 1 bar) which was the highest amount among the other small pore MOFs. High selectivity over other gases, rapid kinetics, and high Qst: (-47 kJ mol-1 at zero coverage) are other great properties of this metal bisphosphonate framework. Recently, Pal et al. (Pal et al., 2016) improved the water stability of the carboxylate MOF via fluorine, and amine ligand functionalization as well as transmetalation (Zn to Cu). They synthesized {[Zn2(L)(H2O)2]·(5DMF)·(2H2O)}n (Zn(II)-MOF) through the solvothermal reaction which was then followed by the metal exchanging process to produce the Cu(II)-MOF without any changes in the crystallinity of the framework. The new MOF performed higher water stability, CO2 uptake and selectivity of CO2 over N2. In their report, an increase of water stability was attributed to the lining effect of the fluorine molecule which made the surface of the pores hydrophobic. In addition, the higher stability of the Cu (II) than Zn (II) reinforced the metal-ligand bond which in turn increased the strength of the frame in the humid condition. Regarding all the studies in this area, the ligand–metal strength, kinetic and thermodynamic factors of the MOFs are the main keys in improving water stability of the framework that should be further investigated.
Conclusion To be laconic, a sheer of research in CO2 capture and separation has been accomplished on the MOFs during last two decades. This is due to their exceptional properties such as large pores volumes, high surface areas, tenability, diversity in their structures, and their environment-friendly characteristic. In the present review paper, we investigated most new MOFs which are specifically used in the CCS technology. Different synthesis routes, selectivity over other gases, the CO2 adsorption capacity and limitations in using the MOFs were all reviewed thoroughly. As mentioned above, the key criteria in the CO2 capture technology by using the MOFs are a high surface area and the presence of effective functional groups on the surface of the framework. In addition, pressure and temperature play a major role. At the low-pressure region, the CO2 adsorption capacity is subordinate to the heat of adsorption while at high pressure, the CO2 adsorption capacity is subordinate to the surface area of the MOFs.
Regarding the presence of the other gases in the gas stream, we also surveyed the selective adsorption of CO2 over other gases based on the selective adsorption mechanism. Besides, the selectivity synthesis routes and their effects on the CO2 adsorption capacity were investigated. As stated previously, the synthesis methods have more effects on the size of the pores (nano-size) and consequently on the surface area of the framework. Further to the MOFs’ outstanding CO2 adsorption properties , their structure can be optimized via the activation process, the pre-synthesis, the post-synthesis procedure, and producing the MOF-Composites by using additives such as graphite, graphene and CNT. Although MOFs exhibit a high CO2 adsorption capacity and selectivity, further investigations are still required to improve the ability of these excellent adsorbents. Usage of the MOFs in the CCS technology also faces some challenges such as low water, thermal and mechanical stabilities in harsh conditions. In terms of water stability, some researches are available as mentioned in section 5 but still more investigations are required. For thermal stability, the output gas flow from the power plant has high temperature while most MOFs examined so far show high CO2 adsorption and selectivity at ambient temperature. Hence, production of the MOFs with high thermal and mechanical stabilities are essential and need much attention for future studies.
Acknowledgements This project was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under grant No. (DF-434-829-1441). The authors, therefore, gratefully acknowledge DSR technical and financial support.
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Graphical Abstract
Highlights
The synthesis and applications of MOFs for CO2 adsorption was reviewed. Comparison among different synthesis routes. The synthesis routes effects on the CO2 adsorption capacity. High CO2 adsorption and selectivity found at ambient temperature.