Recent advances in gas storage and separation using metal–organic frameworks

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RESEARCH: Review

Recent advances in gas storage and separation using metal–organic frameworks Hao Li, Kecheng Wang, Yujia Sun, Christina T. Lollar, Jialuo Li, Hong-Cai Zhou ⇑ Department of Chemistry, Texas A&M University, College Station, TX 77842-3012, United States

Gas storage and separation are closely associated with the alleviation of greenhouse effect, the widespread use of clean energy, the control of toxic gases, and various other aspects in human society. In this review, we highlight the recent advances in gas storage and separation using metal-organic frameworks (MOFs). In addition to summarizing the gas uptakes of some benchmark MOFs, we emphasize on the desired chemical properties of MOFs for different gas storage/separation scenarios. Greenhouse gases (CO2), energy-related gases (H2 and CH4), and toxic gases (CO and NH3) are covered in the review. Introduction Gas storage and separation are closely related to various aspects in human society, such as environmental protection, energy utilization and industrial production. Specifically, carbon dioxide separation is crucial to the alleviation of greenhouse effect; hydrogen and methane storage are indispensable for the widespread use of clean energy; the separation and storage of toxic gases, such as carbon monoxide and ammonia, are important for pollution control and the synthesis of industrial chemicals. Metal–organic frameworks (MOFs) are an emerging class of crystalline porous materials built from inorganic metal nodes and organic linkers [1–11]. Compared with other porous materials (e.g. activated carbons, silicas, and zeolites), the unique structural features of MOFs, such as high porosity, large surface area, tunable structure, and modifiable functionality, make them very promising to be applied in gas storage and separation [12–15]. Here, we review the progress that has been made recently in this arena, with an emphasis on the desired chemical properties of MOFs for required performance. The aforementioned energyrelated gases (CO2, H2 and CH4) and toxic gases (CO and NH3) are covered in this review. As for hydrocarbon separation, significant progress has been made in acetylene/ethylene separation and propylene/propane separation through judicious control of ⇑ Corresponding author: Zhou, H.-C. ([email protected])

pore size [16,17]. Furthermore, the efficient storage of acetylene at an ambient condition was also achieved with rationally designed MOFs [18]. Considering the diversity of hydrocarbon species, a comprehensive discussion of this topic is not included. Readers are directed to several reviews concerning this area [19,20].

Carbon dioxide separation Human society has developed into a stage where economic prosperity is predominantly dependent on the exploitation of fossil fuels, which are the main energy sources. However, combustion of fossil fuels for power has inevitably brought the emission of large amounts of CO2 into the atmosphere and led to global warming [7]. Therefore, it is urgent to develop materials that can effectively remove CO2 from gas mixtures. Current CO2 scrubbers are mostly aqueous alkanolamine solutions, which suffer from high regeneration energy due to the large portions of water in their composition [7]. This propels the investigation of solid sorbents that have low heat capacities as new materials for CO2 separation. As a new class of porous solids, MOFs are considered promising materials to perform CO2 separation. MOFs are primarily utilized as sorbents to achieve CO2 separation. This is based on differences in interaction between gas species and frameworks and the consequent selective adsorption of target gas. In recent years, a growing number of explorations

1369-7021/Ó 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). https://doi.org/10.1016/j.mattod.2017.07.006

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RESEARCH: Review FIGURE 1

(a) Structure of the amine-appended CO2 adsorbent mmen-Mg2(dobpdc). Atom color legend: gray (carbon), red (oxygen), and green (magnesium). Reprinted with permission from Ref. [28]. Copyright 2012 American Chemical Society. (b) Illustration of the cooperative insertion mechanism for CO2 adsorption in mmen-M2(dobpdc). Comparison of the idealized CO2 adsorption isotherms of (c) a phase-change adsorbent and (d) a classical microporous adsorbent at varied temperatures. Reprinted with permission from Ref. [30]. Copyright 2015 Nature Publishing Group.

have also been done to fabricate membranes with MOFs for CO2 separation, which rely on differences in either the diffusion rates of gas molecules within membranes or their molecular sizes [21,22]. There are generally four types of CO2 capture that can be performed using MOFs, namely post-combustion capture, pre-combustion capture, oxy-fuel combustion, and direct capture from air [7,23]. Their execution conditions and recent advances will be discussed as follows.

Table 1

CO2 uptakes of selected MOFs. Data are all extracted from CO2 adsorption isotherms. CO2 uptake (mmol/g)

Temperature (°C)

Pressure (mbar)

Ref.

Mg-MOF-74

5.28

40

150

[25]

mmen-Mg2(dobpdc)

3.5a 3.0a

40 25

150 0.4

[30]

Post-combustion capture

dmen-Mg2(dobpdc)

3.1a

40

150

[31]

The flue gas generated in current power plants is mainly comprised of N2 (73–77%) and CO2 (15–16%), with a total pressure of approximately 1 bar. After removal of SOx, the flue gas is anticipated to interact with the CO2 scrubber at temperatures between 40 and 60°C [7]. Thus, post-combustion capture is aimed at separating CO2 from N2 at 40°C or higher, with CO2 partial pressure equivalent to 0.15 bar. In the context of sorbent-based separation, Mg-MOF-74 (CPO27-Mg) is notable for its high CO2 uptakes at low pressure under dry conditions (5.28 mmol/g at 40°C and 0.15 bar), which stem primarily from the high density of open metal sites (OMSs) [24–26]. OMSs have strong interaction with CO2 molecules, but are occupied more preferably by H2O. This leads to a dramatically decreased CO2 uptake of Mg-MOF-74 when it is exposed to moisture. Considering the measurable amount of water vapor (5– 7%) in flue gas, it is definitely necessary to develop methods to endow MOFs with good selectivity toward CO2 even in the presence of water [7]. Attempts were made by introducing alkylamines into MOFs to mimic the chemisorption of CO2 in alkanolamine solutions [27–29]. In 2012, Long and coworkers reported functionalizing Mg2(dobpdc) (H4dobpdc = 4,40 -dihy droxy-3,30 -biphenyldicarboxylic acid) with N,N0 -dimethylethyle

en-Mg2(dobpdc)

3.53 2.83

40 25

150 0.39

[33] [32]

men-Mg2(dobpdc)

3.6

40

150

[33]

den-Mg2(dobpdc)

2.55a

40

150

[33]

Cr-MIL-101-SO3HTAEAb

2.28 1.12

40 20

150 0.4

[34]

[Mg2(dobdc)(N2H4)1.8]

4.9a 3.89

40 25

150 0.4

[35]

SIFSIX-3-Zn

2.3a

45

150

[36]

SIFSIX-3-Cu

2.4a 1.24

45 25

150 0.4

[37]

SIFSIX-3-Ni

2.2a

45

150

[38]

2

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Compounds

a

Estimations are made from CO2 adsorption isotherms in cases where specific values were not reported. b TAEA = tris(2-aminoethyl)amine.

nediamine (mmen) via the coordination bonds formed between the OMSs on Mg2(dobpdc) and amine groups [28]. For each mmen molecule, one end is coordinated to an unsaturated Mg center, while the other end is dangling in the channel to capture CO2 in a chemisorptive manner (Fig. 1a). The high density of amine groups in mmen-Mg2(dobpdc) results in its good selectiv-

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FIGURE 2

Structures of alkylamines in amine-modified Mg2(dobpdc).

ity for CO2 even in the presence of moisture [30]. Its CO2 uptake is 3.5 mmol/g at 40°C and 0.15 bar (Table 1). Particularly, a steep step is observed in the low-pressure region of the isotherm. Spectroscopic and diffraction measurements indicate that the unique step in the CO2 isotherm of mmen-M2(dobpdc) is attributed to a cooperative CO2 insertion mechanism that involves a phasechange in the capture process (Fig. 1b). The CO2 pressure at which the phase-change occurs can be tuned by altering temperature and the metal species in mmen-M2(dobpdc). For a specific mmen-M2(dobpdc), the phase-change pressure would increase from below 0.15 bar to above 1 bar when the temperature is elevated approximately 50°C [30]. Thus, a large working capacity can be achieved with only a small temperature swing when the adsorption and desorption temperatures are tuned to make the phase-change pressures in the two isotherms right before the adsorption pressure and after the desorption pressure, respectively (Fig. 1c). Moreover, the identity of the metal species in mmen-M2(dobpdc) affects the bond strength of mmen-M, and influences the phase-change temperature accordingly. Compared with regular MOF sorbents, the adjustable phase-change step in the CO2 isotherm of mmen-M2(dobpdc) help it successfully circumvent the disadvantage of large temperature swings in regeneration (Fig. 1d), making it extremely favorable for industrial CO2 capture. Works on integrating other alkylamines into Mg2(dobpdc) were also reported subsequently, such as dmen-Mg2(dobpdc) (dmen = N,N-dimethylethylenediamine) [31], en-Mg2(dobpdc) (en = ethylenediamine) [32,33], men-Mg2(dobpdc) (men = 1methylethylenediamine) [33], and den-Mg2(dobpdc) (den = 1,1dimethylethylenediamine) (Fig. 2) [33]. These works mainly focus on tuning the alkylamine structure to optimize the performance of the functionalized MOF in terms of its working capacity, regeneration energy, recyclability, water stability, among others. Different alkylamine structures resulted in aminemodified MOFs with different abilities to adsorb CO2. However, more in-depth investigations are still required to elucidate the relationship between amine structure and the CO2 capture capability of the functionalized MOF. The foregoing examples all concern alkylamine modification on Mg2(dobpdc), an expanded variant of Mg-MOF-74, because the sizes of the alkylamines molecules necessitate an isostructure with larger channels to avoid steric hindrance or intermolecular hydrogen bond formation. Nevertheless, amine functionalization was still achieved in Mg-MOF-74 in 2016 by introducing hydrazine into the framework [35]. Consequently, an ultrahigh density of free amine groups (6.01 mmol/g) was attained in the generated new material [Mg2(dobdc)(N2H4)1.8] (H4dobdc = 2,5-d ihydroxy-1,4-benzenedicarboxylic acid), which exhibited exceptionally high CO2 uptakes in both single- or multi-component gas adsorption tests.

Apart from utilizing OMSs in MOFs to tether amines to enhance the affinity toward CO2, other functional moieties can also be employed to achieve this goal. For example, the Brønsted acidic sites in MOFs, like sulfonic acid groups, can be harnessed to fix amine through a Brønsted acid-base reaction for CO2 capture [34]. In addition, monodentate hydroxide is also demonstrated to have strong yet reversible interaction with CO2 in [CoIICoIII(OH)Cl2(bbta)] (MAF-X27ox, H2bbta = 1H, 5H-benzo (1,2-d:4,5-d0 )bistriazole, MAF = metal azolate framework) via the formation and decomposition of bicarbonate [39]. It enables the framework to selectively adsorb CO2 from gas mixtures even at humid conditions. Some MOFs, such as SIFSIX-3-M (M = Cu, Zn, Ni), are designed with suitable pore sizes and favorable arrays of inorganic anions to afford enhanced CO2 binding affinity. Although the interaction between CO2 and these MOFs are physical interaction, SIFSIX-3-M still exhibit good selectivity for CO2 at humid conditions [36–38]. Explorations were also made in the realm of MOF-derived membranes for post-combustion CO2 capture [21,40–43]. Compared with synthesizing MOF adsorbents, developing MOF membranes to separate CO2 from N2 is more effortful. The main difficulty lies in the comparable kinetic diameters of CO2 (3.30 Å) and N2 (3.64 Å), making it challenging to prepare a MOF-derived membrane with simultaneous high permeance and high CO2/N2 selectivity.

Pre-combustion capture Pre-combustion CO2 capture requires the decarbonation of fuels into H2 and concomitant CO2. CO2 is to be separated from highpressure (5–40 bar) gas mixtures for the purpose of emitting zero CO2 during the subsequent combustion step [7]. Because of the evident difference between the kinetic diameters of H2 (2.89 Å) and CO2 (3.30 Å) [7], separating the two species based on their distinctive sizes using molecular sieve membranes are mechanistically favorable. However, there exists a long-term bottleneck in the fabrication of membranes with evenly distributed and uniformly sized pores. In 2014, a breakthrough was made by Li, Yang and coworkers in this area [22]. They successfully prepared molecular sieve nanosheets from a layered MOF, Zn2(bIm)4 (bIm = benzimidazolate), by wet ball milling and exfoliation. The apertures on a single-layered Zn2(bIm)4 sheet are estimated to be 2.9 Å, which would function as channels for small-sized H2 but also as filters to impede the passage of CO2. In addition to the good CO2/H2 selectivity, the straightforward apertures, constructed from four flat bIm molecules, would also facilitate the fast transport of passing H2 for a high H2 permeance. Besides these inherent structural advantages, the temperature under which Zn2(bIm)4 nanosheets were coated onto a-Al2O3 support was fine tuned to minimize ordered restacking between the nanosheets because lamellar restacking would block the pathway for H2 and significantly affect its permeance. Accordingly, the generated membrane achieves a simultaneous high H2 permeance (2700 GPU) and high H2/CO2 selectivity (291) in the binary CO2/H2 separation test [22]. It successfully overcomes the Robeson's upper limit on previous MOF membranes whose separation effects rely upon the different diffusion rates of the gas species [44–47]. 3

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RESEARCH: Review

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RESEARCH: Review FIGURE 3

(a) Structure of Co-BTTri. Atom color legend: purple (cobalt), gray (carbon), blue (nitrogen), green (chlorine). (b) Molecular structures of BTTri3 and BDTriP3. (c) O2 and N2 adsorption isotherms of Co-BTTri at 195 K. Solid lines represent corresponding dual-site Langmuir fit curves. (d) O2 adsorption isotherms of CoBDTriP and Co-BTTri at 195 K. Solid lines represent corresponding Langmuir fits. Reprinted with permission from Ref. [55].

This MOF-derived membrane serves as a good example to illustrate the advantages of membrane-based separation, such as simple separating procedure, energy economy, and high productivity. By contrast, there is scarce experimental research concerning pre-combustion CO2 capture with MOF sorbents [48– 51]. In fact, high CO2/H2 selectivity can be achieved in most aforementioned MOFs in post-combustion CO2 capture section. The restrictions in this area do not exist in CO2/H2 separation itself but in the expensive large-scale gasification of fuels. Nevertheless, this field still holds promise if the drawbacks can be overcome.

Oxy-fuel combustion Oxy-fuel combustion is the ignition of fossil fuels in nearly pure O2. The flue gas generated is thus almost completely CO2 after removal of water, rendering a more facile capture step of CO2 [7]. In this approach, the separation target is transformed into producing pure O2 from air. Traditional cryogenic distillation requires a large energy input, which fails to be a viable solution if CO2 capture from oxy-fuel combustion is to be implemented extensively [7]. Therefore, alternative strategies that consume less energy but still separate O2/N2 effectively is highly desired. Membrane-based separation would not be feasible in this scenario, because the kinetic diameters of O2 (3.46 Å) and N2 (3.64 Å) are extremely close [7]. In the context of sorbent-based separation, selective adsorption of one gas species with an absolute physisorption mechanism is also not applicable, due to the strong resemblance between the physical properties of O2 and N2 (such as quadruple moment and polarizability). By contrast, separation that harnesses the different chemisorptive behaviors of the two gases would be very effective. This is because O2 has a high propensity to accept electrons from redox-active metal sites, while N2 is not endowed with such a feature [7]. Much progress has been made in this aspect to date. 4

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Cr2+ and Fe2+ are metal species with relatively strong reducing ability. With these unsaturated redox-active metal centers, Cr3(btc)2 (H3btc = 1,3,5-benzenetricarboxylic acid), Cr-BTT (H3BTT = 1,3,5-Tris(2H-tetrazol-5-yl)benzene), and Fe2(dobdc) have the tendency to donate electrons to O2 to have strong O2 affinity and marked O2/N2 selectivity [52–54]. However, CrIIMOFs display gradual decreases in O2 capacity over multiple cycles [53,54], while Fe2(dobdc) would lose its recyclability at temperatures above 222 K for irreversible oxidation [52]. Subsequently, more explorations were made with CoII-MOFs. Unlike previous Co-MOFs built with weak-field carboxylate-terminated ligands, strong-field N-donor ligands are adopted in the cases of Co-BTTri (H3BTTri = 1,3,5-tri(1H-1,2,3-triazol-5-yl)benzene) and Co-BDTriP (H3BDTriP = 5,50 -(5-(1H-pyrazol-4-yl)-1,3phenylene)bis(1H-1,2,3-triazole)) (Fig. 3a and b) [55]. The rorbitals in BTTri3 and BDTriP3 are higher in energy compared with those in weak-field ligands. Upon their formation of molecular orbitals with d orbitals from the metal, eg orbitals would be lifted to a higher energy level, making the electrons in these elevated eg orbitals have a stronger inclination to transfer to O2 (Fig. 4). This gives rise to stronger O2 binding and a higher O2/ N2 selectivity in MOFs. Consequently, Co-BTTri has an O2 loading of 3.3 mmol/g at 0.21 bar and 195 K, with an IAST (IAST = ideal adsorbed solution theory) selectivity of 41 at the same temperature (Fig. 3c) [55]. Its isostructure, Co-BDTriP, in which one triazolate terminal is replaced by a more electron-donating pyrazolate group, exhibits greater O2/N2 selectivities at temperatures between 195 and 240 K (Fig. 3d). This further demonstrates the fact that a coordination environment created by strongerfield ligands around the metal center would lead to a MOF with higher O2 affinity. In addition to the enhanced selectivities, CoIIMOFs also have good recyclability and water stability, which are also important criteria to consider for the design of future O2 adsorbents.

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FIGURE 4

r-Bond molecular orbital diagrams of Co2+ coordinated by strong- (left) and weak-field ligands (right) in Oh symmetry. The coordination sphere around Co2+ is simplified into Oh symmetry for easy illustration. Only r-orbitals of SALCs with eg symmetry are shown for clarity.

Direct capture from air All the previous scenarios are CO2 capture from stationary point sources, which are intended for slowing the increasing rate of atmospheric CO2 concentration. To scavenge the CO2 that has already been released into air, a concept called ‘negative carbon technology’ was recently put forward, which necessitates the practice of direct CO2 capture from air (or direct air capture, DAC) [23]. In addition, DAC can also contribute to maintaining a low CO2 level in submarines or spacecraft, so that a habitable environment can be guaranteed in these confined spaces [23]. The current atmospheric CO2 level is 400 ppm, so the operational condition of DAC is 0.4 mbar and 25°C [7,23]. The methods to endow MOFs with the ability to scavenge CO2 at extremely low pressure are similar to those discussed in the section of post-combustion capture, including but not limited to, incorporating functional groups that have strong yet reversible chemical interaction with CO2, and rational tailoring of the size and shape of internal pores for enhanced physical interaction between MOFs and CO2. The DAC performance of some notable MOFs is summarized in Table 1. As for DAC with MOF-derived membranes, few examples have been reported. The similar molecular sizes of CO2, N2 and O2, extremely low atmospheric CO2 concentration, and complex air composition all make this research area very challenging.

Hydrogen storage Hydrogen is an excellent alternative energy source for coal and gasoline because of its ultrahigh gravimetric combustion heat and benign combustion products [56]. The environmental problems caused by extensive use of fossil fuels could be significantly relieved if hydrogen becomes the fuel for automobiles [57]. This necessitates the investigation of hydrogen storage. However, the boiling point and crucial temperature of hydrogen are only 20 K and 38 K, making it notoriously difficult to liquefy or compress. Thus, two exploratory techniques to store hydrogen involve the

utilization of cryogenic liquid hydrogen tanks and high-pressure tanks, respectively [58]. For the former method, large quantities of energy are consumed to liquefy hydrogen and keep the tanks cool [56]. While for the latter, high-pressure techniques entail the use of extremely heavy apparatus to prevent hydrogen leakage, which severely reduces the ‘real gravimetric capacity’ of the tanks. Currently, the hydrogen storage capacities of all existing tanks are 3.4–4.7 wt% and 14–28 kg/m3 [58]. To enhance the capacities, efforts should be devoted to storing a considerable amount of hydrogen at comparatively low pressure and high temperature. Much attention has been directed to highly porous MOFs to achieve this goal, because their large surface areas and tunable structures can provide high densities of relatively strong interaction sites to adsorb hydrogen. Attempts are primarily made in two directions: cryo-temperature and room temperature hydrogen storage, detailed below.

Cryo-temperature hydrogen storage Cryo-temperature hydrogen storage refers to retaining hydrogen in a tank filled with MOFs at usually 77 K and relatively low pressure (less than 100 bar) [58]. Compared to the traditional method using cryogenic liquid hydrogen tanks (20 K), much less energy is required to ‘liquefy’ hydrogen and keep the tanks cool in this scenario due to the interaction between MOFs and hydrogen. Although the strongest interaction sites for hydrogen in MOFs are OMSs, weaker interaction positions are more crucial in determining the storage capacity at extremely low temperatures. These positions are at the surfaces of cages and channels, which have van der Waals interactions with hydrogen. This conclusion is based on the analysis of adsorption data. For instance, MOF74-Mg has the highest OMS density (8.3 mmol/g) among all reported MOFs [59]. Even if each OMS adsorbs one H2 molecule, its H2 uptake is still as low as 1.7 wt%. By contrast, MOFs with much lower OMS density are capable to adsorb 6–10 wt% hydrogen at 77 K and 100 bar [60,61], substantiating the point that H2 molecules are mostly adsorbed not on OMSs, but on other 5

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Table 2

Hydrogen uptakes of selected MOFs. Compound

H2 uptake (wt%)

Temperature (K)

Pressure (bar)

Ref.

MOF-5

10

MOF-177 a

RESEARCH: Review a

77

100

[60]

7.5

77

70

[61]

IRMOF-6

4.63

77

45

[61]

IRMOF-8

3.4

77

33.7

[61]

SNU-4

3.7

77

50

[67]

Mn-BTT

6.9

77

90

[74]

Cu-BTT

3.7

77

90

[75]

Fe-BTT

4.1

77

95

[76]

Ni-MOF-74

2.95

77

10

[77]

Co-MOF-74

3.15

77

10

[77]

HKUST-1

3.6

77

50

[66]

SNU-5

5.22

77

50

[67]

SNU-6

10.0

77

70

[78]

PCN-6

1.9

77

1

[72]

0

PCN-6

1.1

77

1

[73]

PCN-10

6.84

30

3.5

[69]

PCN-12

3.05

77

1

[68]

PCN-120

2.4

77

1

[68]

PCN-20

6

77

35

[79]

PCN-46

7.2

77

60

[70]

PCN-66

6.65

77

45

[6]

NOTT-140

6

77

20

[71]

IRMOF = isoreticular metal–organic framework.

weaker interaction sites. The latter type of sites is mainly determined by the size and shape of internal pores [62]. MOFs with large pores are not desirable for hydrogen storage, because H2 molecules in the void central space of large pores do not experience the attractive forces from pore surfaces [63]. A calculation suggests that in carbon materials, pores with a width of 9 Å are optimal to maximize the hydrogen capacities at 77 K and 100 bar [64]. With this pore size, the overlap of the potential fields from multiple walls leads to the highest H2 affinity [64]. This conclusion is applicable in MOFs as well, considering the walls of cages or channels in MOFs are also mainly constructed by carbon-based ligands [62]. To obtain MOFs with suitable pore sizes, directing synthesis with a bottom-up strategy is a good choice [65]. For example, combining copper paddlewheels with ligands containing 3,5-benezedicarboxylic acid moieties often yields frameworks with rhombicuboctahedral supermolecular building blocks (SBBs) that are 9 Å in diameter [65]. Many MOFs with suitable pores for H2 are constructed in this approach [6,66– 71]. Their high H2 storage capacities are summarized in Table 2. In addition, synthesizing MOFs with interpenetration can also achieve the goal by dividing the large voids in frameworks and generate suitable micropores to store hydrogen. In 2006, Zhou and coworkers reported a twofold interpenetrated MOF named

6

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PCN-6 (PCN = porous coordination network) and its noninterpenetrated counterpart, PCN-60 , which are constructed by Cu2+ and 4,40 ,400 -s-triazine-2,4,6-triyltribenzoate acid (TATB). The effect of interpenetration is well demonstrated by comparing the hydrogen uptakes of PCN-6 and PCN-60 . At 77 K and low pressure (1.2 bar), the hydrogen uptake of PCN-6 is 1.9 wt% [72], 70% higher than that of PCN-60 [73].

Room-temperature hydrogen storage By introducing MOF adsorbents into the tanks, the aforementioned cryo-temperature storage method successfully increases the temperature at which hydrogen is ‘liquefied’. Nevertheless, substantial energy and extra equipment beside the tanks are still indispensable to maintain the tanks at a temperature that is much lower than the environment's. The ultimate goal for hydrogen storage is to compact hydrogen at room temperature and under reasonable pressure [58]. The U.S. Department of Energy (DOE) set a series of targets to guide the development of on-board hydrogen storage systems: 6.0 wt% and 45 g/L by 2010, and 9.0 wt% and 81 g/L by 2015 [58]. The working temperature of the tank should be 40–85°C, and the inner pressure should be lower than 100 bar. Considering H2 has no dipole moment, its interactions with the surfaces of cages or channels built by organic ligands are too weak to result in much adsorption at room temperature. Thus, unlike cryo-temperature H2 storage, the density of strong rather than weak interaction sites determines the H2 capacity in this scenario [74]. Although no MOF has met the DOE's targets to date, some methods have been explored to improve the density and strength of H2 interaction sites for high H2 uptakes at ambient temperature.

OMSs with low coordination number As is discussed previously (cryo-temperature hydrogen storage), the OMSs in MOFs are strong H2 binding sites, but the amount of H2 adsorbed on the OMSs is very limited. One reason is that most OMSs are five-coordinated and can take only one H2 molecule before their coordination sphere are saturated. However, if the coordination number (CN) of OMSs further decreases, they will have the potential to bind multiple H2 molecules. As a result, the H2 uptakes of MOFs featuring these OMSs with low CN will increase dramatically at ambient temperature. In 2016, Long and coworkers reported Mn2(dsbdc), which is constructed by Mn2+ and 2,5-disulfido-1,4-benzenedicarboxylate (dsbdc4) [80]. In Mn2(dsbdc), there are two types of crystallographically distinguishable Mn2+ ions. One type of the Mn2+ ions is sixcoordinated with an octahedral geometry while the other is four-coordinated adopting a see-saw geometry. The neutron powder diffraction result of Mn2(dsbdc) suggests that a single four-coordinated Mn2+ ion can bind up to two D2 molecules with an overall occupancy of 0.7 when dosed with large amounts of D2. Thus, four-coordinated OMSs are demonstrated to be beneficial for enhanced H2 adsorption capacities. Although the hydrogen uptake (1.6 wt% at 77 K and 1.2 bar) of Mn2(dsbdc) does not break the record, this study unveils a new strategy for investigation.

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FIGURE 5

(a) The H2 adsorption isotherms of Mn-BTT and its cation-exchanged variants at 77 K. (b) H2 adsorption enthalpies of Mn-BTT and the cation-exchanged counterparts. Reprinted with permission from Ref. [81]. Copyright 2007 American Chemical Society.

Extraframework cations To increase the density of strong H2 interaction sites, synthesizing MOFs with extraframework cations also serves the purpose. These cations function as additional strong binding sites besides OMSs through their electrostatic interaction with H2. In 2006, Long and coworkers reported Mn-BTT, a MOF constructed by Mn2+ ions and H3BTT [74]. Notably, a hydrogen capacity of 12.1 g/L is achieved in Mn-BTT at 298 K and 90 bar, which is much higher than that of a high-pressure hydrogen tank at the same conditions. The chemical formula of Mn-BTT after solvent exchange and activation is Mn3[(Mn4Cl)3(BTT)8]2. The Mn2+ ions outside the square bracket, acting as the counterions for the anionic framework, can be substituted by many other transition metal ions to produce M3[(Mn4Cl)3(BTT)8]2(MCl2)x (M = Fe2+, Co2+, Ni2+, Cu2+, Zn2+, x = 0–2) (Fig. 5a) [81]. These cationexchanged materials exhibit apparently different zero-coverage H2 adsorption enthalpies (ranging from 8.5 to 10.5 kJ/mol) (Fig. 5b), suggesting that their H2 binding affinities and consequent H2 uptakes are closely associated with the identity of the counterion. This indicates that extraframework cations contribute to an enhanced strong interaction between MOFs and H2. Furthermore, the extraframework Mn2+ ions in Mn-BTT can also be replaced by Li+ to give Li3.2Mn1.4[(Mn4Cl)3(BTT)8]2 (LiCl)0.4. According to the simulation results reported by HeadGordon and coworkers [82], the introduction of Li+ into a MOF would significantly raise its H2 adsorption enthalpy via the strong electrostatic interaction induced by Li+. However, the lowest heat of H2 adsorption was observed in Li3.2Mn1.4[(Mn4Cl)3 (BTT)8]2(LiCl)0.4 among Mn-BTT and all the cation-exchanged counterparts [81], which could be ascribed to the extremely difficult desolvation of Li+ ions. Nonetheless, this work reveals the important role of extraframework cations in the H2 uptake of a MOF, and is inspiring for future study.

Methane storage Apart from hydrogen, natural gas, whose main component is methane, is another energy source with the potential to replace vehicular liquid hydrocarbon fuel owing to its natural abundance [83], and highest H/C ratio among all fossil fuels for reduced CO2 emission [84]. Since methane is in the gaseous state at ambient temperature and pressure, the volumetric energy den-

sity of natural gas is rather low (0.04 MJ/L) [85]. Thus, for largescale transportation and utilization of natural gas, enhancing its energy density is very crucial. Although conventional compression and liquefaction can serve this purpose, both methods have unavoidable drawbacks, which are similar to the cases in hydrogen storage. Compressed natural gas (CNG) requires costly multi-stage compressors and heavy high-pressure tanks for storage, while liquefied natural gas (LNG) can only be stored with complicated cryogenic cooling systems [86]. By contrast, using adsorbents enables the storage of natural gas at ambient temperature and moderate pressures. According to the targets set by DOE in 2012 for on-board methane storage systems, the gravimetric capacities of adsorbents should reach 0.5 g/g (700 cm3/ g) at 298 K, while their volumetric capacities should be 263 cm3/cm3, which is equivalent to that of CNG at 298 K and 250 bar [87,88]. In view of an approximately 25% loss in volumetric energy density during a packing process, a volumetric capacity of 350 cm3/cm3 is required for an adsorbed natural gas (ANG) system [85,86]. As a type of highly porous material, MOFs have been investigated extensively for ANG systems [85,86,88– 90]. The in-depth study suggests that several factors, namely the electrostatic and van der Waals interaction between MOFs and methane, and framework flexibility, are closely associated with the methane capacities of MOFs. Their detailed discussions are as follows.

Electrostatic interaction The electrostatic interaction between MOFs and CH4 usually occurs at the OMSs in frameworks. A typical MOF series with high densities of OMSs is M-MOF-74 (M = Mg, Mn, Co, Ni, Zn) [85,101,106]. These OMSs act as strong CH4 interaction sites and lead to the notable CH4 uptakes of M-MOF-74 (Table 3). Among the series, Ni-MOF-74 has the highest total volumetric CH4 capacity (230 cm3/cm3 at 298 K and 35 bar), which can be attributed to the strongest polarizing ability of Ni2+ ions and the consequent strongest electrostatic interaction between OMSs and CH4. This point is corroborated by the highest initial isosteric heat of CH4 adsorption of Ni-MOF-74 (20.6 kJ/mol) [85,101]. Furthermore, a volumetric uptake of 172 cm3/cm3 is estimated to be attained by its open nickel sites considering each OMS is occupied by one CH4 molecule [85,101]. The above 7

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Table 3

Methane uptakes of selected MOFs.

RESEARCH: Review

Compound

Total uptake (cm3/cm3)

Working capacity (cm3/cm3)

Temperature (K)

Pressure (bar)

Initial heat of adsorption (kJ/mol)

Ref.

HKUST-1

227 267

150 190

298 298

35 65

17.0

[91]

Ni-MOF-74

230 260

115 142

298 298

35 65

20.6

[85]

PCN-14

230 202 239

290 298 298

35 35 65

17.6

[85,92]

125 160

MAF-38

226 263

150 187

298 298

35 65

21.6

[93]

Co-MOF-74

221 249

110 136

298 298

35 65

19.5

[85]

UTSA-76a

211 257

151 197

298 298

35 65

15.44

[94]

Mg-MOF-74

200

113

18.5

PCN-250(Fe2Co)

200

NJU-Bai10

199

NOTT-107

196

NOTT-109 UTSA-20

298

35

298

35

107

290

35

110

298

35

196

125

300

35

17.1

[98]

195

101

300

35

17.7

[99]

PCN-11

194

125

298

35

14.6

[69]

ZJU-5

190

130

300

35

15.3

[100]

Zn-MOF-74

188

102

298

35

18.3

[101]

NU-135

187

127

298

35

16.6

[102]

NU-125

182

133

298

35

15.1

[91]

IRMOF-6

177

134

298

36.5

Mn-MOF-74

176

100

298

35

Co(bdp)

163 205

155 197

298 298

35 65

[104]

Fe(bdp)

156 196

150 190

298 298

35 65

[104]

Al-soc-MOF-1

125 198

176

298 298

35 65

calculation fully clarifies the contribution from OMSs in the total CH4 uptake of Ni-MOF-74. Nevertheless, the strong electrostatic interaction also results in a considerable portion of methane remaining in the framework as the pressure drops to 5 bar, which significantly limits the working capacity of M-MOF-74. Thus, to enhance the amount of deliverable CH4, MOFs featuring moderate interaction sites are highly desired.

Van der Waals interaction Modest methane interaction sites in MOFs are usually located at places where CH4 molecules have van der Waals interaction with the frameworks. The interaction strength can be fine-tuned by pore size and shape. In 2015, Eddaoudi and coworkers reported a MOF named Alsoc-MOF-1 for methane storage. Al-soc-MOF-1 is constructed by trigonal prismatic Al3O(OOC)6 clusters and tetratopic TCPT linkers (H4TCPT = 3,300 ,5,500 -tetrakis(4-carboxyphenyl)-p-terphenyl) 8

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[85] [95]

14.9

[96] [97]

[103] 19.1

11

[101]

[105]

[105]. Its gravimetric total methane uptake and working capacity at 298 K are 0.42 g/g (65 bar) and 0.37 g/g (5–65 bar), respectively (Fig. 6a). The impressively high working capacity/total uptake ratio of Al-soc-MOF-1 indicates that the interaction between adsorbed methane and the framework is rather weak. This is confirmed by the relatively low enthalpy of methane adsorption of Al-soc-MOF-1 (Table 3). Furthermore, the grand canonical Monte Carlo (GCMC) simulation results suggest that at high pressures, most CH4 adsorption occurs on the surfaces of the cages and channels of Al-soc-MOF-1 (Fig. 6b), which substantiates the point that van der Waals interactions are the dominant force accounting for its methane storage. Similar conclusions have also been arrived at in the cases of HKUST-1, PCN-250 (Fe2Co), and UTSA-76a where most methane adsorption sites at high pressure are primarily located at window sites, corners of large cages and centers of small cages by means of X-ray/ neutron diffraction or simulation [94,95,107–109].

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FIGURE 6

(a) CH4 adsorption isotherms of Al-soc-MOF-1 at different temperatures. (b) Screenshots of simulated CH4 adsorption in Al-soc-MOF-1 at 298 K and 5, 35, 65, and 80 bar, respectively. The purple spheres represent CH4. Reprinted with permission from Ref. [105].

FIGURE 7

(a) Total and excess methane adsorption (filled cycles) and desorption (open cycles) isotherms of MAF-38 at 298 K. (b) Heat of CH4 adsorption of MAF-38. Triangles represents the Qst calculated with the Clausius–Clapeyron equation using isotherms measured at 273, 288, and 298 K. Circles represent Qst fitted by the Langmuir–Freundlich model. (c) Primary (green), (d) secondary (black), (e) ternary (orange) methane adsorption sites, and the methane dimer/hexamer in MAF-38. Atom color legend: purple (zinc), gray (carbon), blue (nitrogen), red (oxygen), white (hydrogen). Dashed lines represent framework-methane (yellow) and methane-methane (black/orange) interactions shorter than 5
A. Reprinted with permission from Ref. [93]. Copyright 2016 John Wiley and Sons.

The rational design of the pore size of MOFs can not only tune the van der Waals interaction between frameworks and CH4, but also enhance the forces between the adsorbed CH4. In 2016, Chen and coworkers reported an OMS-free MOF named MAF38. It is assembled from Zn2+, Pypz (HPypz = 4-(1H-pyrazol-4yl)pyridine) and btc3. MAF-38 exhibits exceptionally high methane uptake (263 cm3/cm3 at 298 K and 65 bar), which is comparable to those of the current benchmark MOFs (Fig. 7a) [93]. Considering there is no OMS in MAF-38, van der Waals interaction is the only possible contributor to its methane uptake. The primary, secondary and ternary van der Waals adsorption sites of MAF-38 are located by GCMC simulation, which are labeled as Site I, Site II and Site III, respectively (Fig. 7c, d and e). Notably, the calculation suggests the CH4 molecules at Sites II experience appreciable interaction from the same

site and Site I. This situation is also true for CH4 at Site III. With these guest-guest interactions, the binding energies of Site II and Site III can be increased by 8.5% and 31.7%, respectively. Thus, an increased Qst is anticipated in MAF-38 at high CH4 loading when most Sites II and III are occupied. These simulation results are confirmed by the abnormal heat of CH4 adsorption curve of MAF-38. Its zero-coverage Qst is 21.6 kJ/mol. As the CH4 coverage increases, its Qst value climbs gradually to 30.8 kJ/mol at a loading amount of 16.1 mmol/g (Fig. 7b). This affords a ‘positive feedback’ in the CH4 adsorption of MAF-38. Therefore, compared with other MOFs with ‘normal’ Qst curves, a great deal of CH4 adsorption occurs at high pressure, which endows MAF-38 with enhanced working capacity. Take MAF-38 and Ni-MOF-74 for a comparison. The two MOFs are very close in both initial Qst and total methane uptake (Table 3). The only difference lies in 9

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RESEARCH: Review FIGURE 8

Total CH4 adsorption isotherms of (a) Co(bdp) and (b) Fe(bdp) at different temperatures (filled circles belong to adsorption curves, open circles belong to desorption curves). (c) Phase transition of Co(bdp) during adsorption/desorption of CH4. Reprinted with permission from Ref. [104]. Copyright 2015 Nature Publishing Group.

their heat of CH4 adsorption curves. The Qst of Ni-MOF-74 is almost a constant during the increase of CH4 dosing, while the Qst curve of MAF-38 is ‘monotonically increasing’. Accordingly, the final working capacity of MAF-38 is 32% higher than that of Ni-MOF-74. Therefore, strengthening the guest-guest interactions through judicious design of MOF structures stands out to be an effective strategy for high CH4 working capacities.

all reported MOFs, although its total CH4 uptake is much lower than those of many other MOFs (Table 3). Furthermore, the ‘triggering’ pressure of the phase transition can be adjusted by altering the temperature, replacing the metal species in the framework [104], and introducing different substituents on the ligand [13]. This enables the design of MOF materials for CH4 storage with a specific working temperature and adsorption/desorption pressure.

Framework flexibility To achieve high working capacity, the ideal CH4 sorption isotherm of a MOF should have a steep step so that the material can release most of the stored CH4 before the pressure drops to 5 bar. In 2015, two isostructural MOFs bearing such a feature were reported by Long and coworkers [104]. These two MOFs, namely Co(bdp) and Fe(bdp), are fabricated by Co2+/Fe2+ and bdp2 (H2bdp = 1,4-benzenedi(40 -pyrazole)). Dramatic decreases are observed in the sorption isotherms of Co(bdp) and Fe(bdp) when the pressure approaches 10–15 bar at ambient temperature (Fig. 8a and b). As a result, more than 96% of the adsorbed CH4 can be released as the pressure drops from 65 bar to 5 bar at 298 K [104]. This unusual sorption behavior is attributed to the flexibility of these two MOFs. Taking Co(bdp) as an example, its channels are able to switch between the ‘collapsed and expanded state’ in response to varied CH4 pressures. At high pressure, the channels are open to facilitate the entry and adsorption of CH4 in the framework. While at low pressure, the close state significantly reduces the space to accommodate CH4 molecules so that only a little amount of CH4 is left in the pores (Fig. 8c) [104]. This phenomenon contributes to an increased amount of deliverable CH4 (197 cm3/cm3, 5–65 bar and 298 K). To date, Co(bdp) still holds the record for the highest CH4 working capacity among 10

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Toxic gases Besides the aforementioned energy-related species (CO2, H2 and CH4), the storage and separation of toxic gases, such as CO, NOx, NH3, SO2 and H2S, also arouse considerable concern [110–116]. This research branch is closely related to human health as well as industrial production. Compared with CO2 separation and H2/CH4 storage, the adsorption of toxic gases with MOFs is less extensively investigated, which is partially caused by the limited stability of MOFs toward these corrosive gases. As a result, only a few MOFs exhibit satisfying uptakes and recyclabilities under the atmosphere of toxic gases. Nonetheless, some exciting achievements were made in the adsorption of CO and NH3 (Table 4) [110].

Carbon monoxide separation Carbon monoxide is a crucial starting material for the production of a variety of basic chemicals in C1 chemistry, which necessitates the generation of purified CO [121]. CO is mainly produced by the partial oxidation of carbon-containing compounds, appearing in the mixtures including H2, CO2, N2, and hydrocarbons [122]. Currently, CO purification is primarily achieved via cryogenic distillation, which consumes a large

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Table 4

a

Compound

Toxic gas

Mg-MOF-74 Mn-MOF-74 Fe-MOF-74 Co-MOF-74 Ni-MOF-74 Zn-MOF-74

CO CO CO CO CO CO

Fe-BTTri Mn2Cl2(BTDD)-(H2O)2 Co2Cl2(BTDD)-(H2O)2 Ni2Cl2(BTDD)-(H2O)2

Gas uptake(mmol/g)

Condition

Ref.

4.58 3.24 6.04 5.95 5.79 1.95

298 K, 298 K, 298 K, 298 K, 298 K, 298 K,

[117]

CO

1.45

298 K, 0.1 mbar

[118]

NH3 NH3 NH3

15.47 12.00 12.02

298 K, 1 bar 298 K, 1 bar 298 K, 1 bar

[119]

Fe-MIL-101-SO3H

NH3 NH3

18a 5a

298 K, 1 bar 298 K, 5 mbar

[120]

UiO-66-NH2

NH3 NH3

10.5a 2a

298 K, 1 bar 298 K, 5 mbar

[120]

UiO-66-NH3Cl

NH3 NH3

12a 4a

298 K, 1 bar 298 K, 5 mbar

[120]

1.2 bar 1.2 bar 1.2 bar 1.2 bar 1.2 bar 1.2 bar

RESEARCH: Review

Toxic gas uptakes of MOFs.

Estimations are made from adsorption isotherms in cases where specific values were not reported.

amount of energy. By contrast, a more energy-economic CO purification at ambient temperature can be attained with the assistance of MOF adsorbents. In 2014, Long and coworkers reported their study on CO adsorption with M-MOF-74 (M = Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Zn2+) [117]. Among this series, Fe-, Co-, Ni-MOF-74 display excellent CO uptakes at 298 K and 1.2 bar (Table 4). Moreover, Ni-MOF-74 and Co-MOF-74 exhibit very high CO/H2 and CO/N2 selectivities, suggesting their potential in CO purification. By contrast, there are also cases where CO is strictly avoided. For example, CO is highly poisonous to the expensive catalysts used in ammonia production and fuel cells. Thus, the removal of trace CO from H2 generated from syngas is extremely important for these applications [123]. However, the CO uptakes of M-MOF-74 materials at low pressure are not very high, which severely limits their performance in trace CO removal. Extensive study into carbonyl complexes demonstrates that the strong M ? CO p back-donation is the key to strengthening the MCO coordination bond [124]. In M-MOF-74, the metal ions are coordinated to weak-field ligands, H4dobdc. As a result, the OMSs in these MOFs are high-spin and electron-poor, which leads to very weak M ? CO p back-donation [117]. An efficient method to construct MOFs with increased electron density around the metal center is to use strong-field ligands, like azolateterminated ones [118]. In 2016, Long and coworkers reported a triazolate-based MOF named Fe-BTTri, constructed by Fe2+ ions and BTTri3 [118]. The stretching frequency of the CO fixed on the OMSs of Fe-BTTri displays an apparent red-shift [118], which is characteristic of the M ? CO p back-donation in the FeACO coordination bonds. Besides, the crystallography data show the FeACO distance is exceptionally short (Fig. 9d). All these results suggest the strong interaction between the OMSs of Fe-BTTri and CO. The high CO affinity of Fe-BTTri is confirmed by its CO uptakes (Fig. 9a and b). Its CO uptake at 298 K and 0.3 bar (2.75 mmol/g) is merely half of that of Ni-MOF-74 (5.5 mmol/ g). Nevertheless, at very low pressure, the CO uptake of Fe-

BTTri (2.5 mmol/g at 0.5 mbar) is three times higher than that of Ni-MOF-74 (0.75 mmol/g). Furthermore, Fe-BTTri exhibits very high selectivity toward CO over H2, N2, CO2 and hydrocarbons, especially in low pressure ranges, indicating the good potential of Fe-BTTri in trace CO removal (Fig. 9c).

Ammonia storage and separation NH3 is a highly toxic and corrosive gas species. The U.S. Occupational Safety and Health Administration (OSHA) set a 15-min exposure limit for gaseous ammonia of 35 ppm, and an 8-hour exposure limit of 25 ppm [120]. Nevertheless, ammonia is widely used in pharmaceutical and chemical industries, especially in fertilizer production. It has also been explored as an on-board energy source for fuel cells [125]. Many explorations were made to adsorb NH3 with activated carbons previously, but these materials usually suffer from low ammonia affinities and capacities [126,127]. MOFs, with highly tunable structure, have great potential in ammonia capture and storage, because both their OMSs and functional groups can be effective NH3 adsorption sites [128,129]. In disparate application scenarios, the requirements for the ammonia adsorbents are different. For instance, to remove trace NH3 from air or other gas mixtures, high affinity and selectivity for NH3 are crucial to the adsorbents. While for the materials designed to store ammonia, good resistance to NH3 shall be the top concern, because the material must be able to sustain its corrosivity during multiple adsorption cycles [120]. In 2016, Dinca and coworkers reported the adsorption of ammonia with three triazolate-based MOFs, namely M2Cl2(BTDD)-(H2O)2 (M = Mn, Co, Ni, H2BTDD = 1H-1,2,3-triazolo[4,5 -b],[40 ,50 -i])dibenzo-[1,4]dioxin). The highest NH3 uptake at 298 K and 1 bar is achieved by Mn2Cl2(BTDD) (15.47 mmol/g), which is superior to the state-of-the-art activated carbons currently used commercially [119]. The high NH3 capacities are largely ascribed to the high densities of OMSs in the frameworks. Moreover, all three MOFs show no obvious decrease in ammonia uptakes after three cycles. The excellent resistance of

11

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(a) CO adsorption isotherms of Fe-BTTri at various temperatures with the low pressure region expanded in (b). (c) Different gas adsorption isotherms of FeBTTri at 298 K. (d) Coordination sphere of Fe2+ ion in CO-dosed Fe-BTTri, with selected bond lengths highlighted. Atom color legend: yellow (iron), gray (carbon), red (oxygen), blue (nitrogen), green (chlorine). Reprinted with permission from Ref. [118]. Copyright 2016 American Chemical Society.

FIGURE 10

(a) NH3 adsorption isotherms of Mn2Cl2(BTDD) (red), Co2Cl2(BTDD) (blue), Ni2Cl2(BTDD) (green), and UiO-66-NH2 (gray) at 298 K. (b) NH3 uptakes of Mn2Cl2(BTDD) (red), Co2Cl2(BTDD) (blue), Ni2Cl2(BTDD) (green), and UiO-66-NH2 (gray) at 293 K. Bars represent iterative cycles on the same sample of each material. Reprinted with permission from Ref. [119]. Copyright 2016 American Chemical Society.

M2Cl2(BTDD) toward ammonia is attributed to its triazolatebased ligands. Compared to carboxylate groups, triazolate groups are stronger-field r-donors, which strengthens the coordination bonds between metal ions and ligands. This endows M2Cl2(BTDD) with higher ammonia resistance than many other previously reported stable MOFs, like UiO-66-NH2 (Fig. 10). The high NH3 uptakes as well as the good NH3 resistance of M2Cl2(BTDD) indicate their eligibility as durable NH3 storage materials Besides the OMSs of MOFs, functional groups in frameworks can also act as strong ammonia capture sites. Since NH3 is a Brønsted base, it has high affinity to carboxylic and sulfonic acids. Thus, attempts were made to adsorb ammonia with ‘acid-bearing’ MOFs. In 2011, Yaghi and coworkers reported the modification of UiO-66-NH2 with anhydrous hydrochloric acid. Compared to the pristine MOF, the resulting UiO-66NH3Cl exhibits a significantly increased NH3 uptake at ambient 12

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temperature, especially in the low pressure regions (Fig. 11) [130]. In 2014, Long and coworkers reported the NH3 adsorption performance of Fe-MIL-101-SO3H, which contains a high density of sulfonic acid groups. This material shows both high ammonia affinity (5 mmol/g at 5 mbar) and a notable NH3 adsorption amount at high pressure (18 mmol/g at 1 bar) [120]. There are three types of NH3 adsorption sites in Fe-MIL-101-SO3H: the pore surface, the OMSs, and the –SO3H groups. Because the NH3 uptake of Fe-MIL-101 was not reported, it is difficult to clarify the contribution to NH3 uptake from respective sites. However, the knot can be unraveled by the NH3 uptakes of several ‘acidbearing’ porous polymers, namely PPN-6-SO3H, BPP-5(COOH) and BPP-7(COOH). Compared with Fe-MIL-101-SO3H, none of these three materials has OMSs and their surface areas are much lower. However, they all exhibit excellent NH3 uptakes at 298 K. Especially for BPP-5(COOH), its NH3 adsorption amount at both

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FIGURE 11

(a) NH3 adsorption isotherms of Fe-MIL-101-SO3H (red), UiO-NH3Cl (blue) and UiO-66-NH2 (green) at 298 K, with low pressure region expanded in (b). Reprinted with permission from Ref. [120]. Copyright 2014 American Chemical Society.

low pressure (6.5 mmol/g at 5 mbar) and high pressure (18 mmol/g at 1 bar) are as high as those of Fe-MIL-101-SO3H. These results strongly indicated the crucial role of acidic groups in porous materials for ammonia adsorption.

Summary and outlook Rapid progress has been made recently in the realm of gas storage and separation with MOFs. For CO2 separation, MOFs are either utilized as adsorbents or membrane materials. In the context of MOF-based adsorbents, efforts are primarily devoted to increasing the CO2 or O2 affinity of MOFs. This gives rise to enhanced selectivities in post-combustion capture (CO2/N2), direct capture from air (CO2/N2) and oxy-fuel combustion (O2/N2). Notably, some CO2-selective MOFs that are modified by alkylamines display exceptionally high CO2 working capacities with good recyclabilities at the required capture conditions. Particularly, their performances are not affected by the presence of moisture, which strongly suggests their potential to be applied in real industrial cases. With improvements on their long-term stabilities and resistance to minor components (SOx and NOx) in flue gas, as well as reductions in their regeneration energies and massive synthesis costs, these MOF adsorbents are likely to play a vital role in the arena of CO2 capture. By contrast, MOF-derived molecular sieving membranes are extremely suitable for the CO2/H2 separation involved in pre-combustion capture, considering the simple composition of ‘shifted syngas’ and the significantly different kinetic diameters of CO2 and H2. The previous bottleneck of membrane fabrication can be overcome by milling and exfoliating rationally selected layered MOFs. If the expenditure on fuel gasification can be curtailed for scale-up, MOF membranes are expected to have satisfying performances in pre-combustion CO2 capture. As for H2 storage, some MOF-filled cryo-temperature H2 storage tanks are demonstrated to have higher H2 capacities and working temperatures than conventional cryogenic liquid H2 tanks, while room-temperature H2 storage with MOFs is still a formidable challenge. This is because the weak interaction between H2 and MOFs, such as van der Waals interaction, is too weak to yield much H2 uptake of MOFs due to the zero dipole moment of H2. Therefore, to produce MOFs with high room-

temperature H2 capacities, synthesizing MOFs with high densities of strong H2 adsorption sites shall be the future direction. By comparison, room-temperature methane storage with MOFs is much closer to being realized. The total methane uptakes of some reported MOFs have reached the DOE targets. Future work may focus on more practical problems, such as the kinetics of CH4 adsorption/desorption in MOF-filled tanks, the packing efficiency of MOF particles, and how to lower the cost and increase the reusability of MOFs. For the adsorption of toxic gases with MOFs, the area is still in its infancy. Since most toxic gases are highly corrosive, the main target at this stage is to increase the resistance of MOFs to these gases as well as explore high-affinity binding sites within MOFs for high uptakes.

Acknowledgements This work was supported as part of the Center for Gas Separations Relevant to Clean Energy Technologies, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award No. DESC0001015, and by U.S. Department of Energy, Office of Fossil Energy, National Energy Technology Laboratory, under Award No. DE-FE0026472. We thank NSF Graduate Research Fellowship Program and Texas A&M University Graduate Diversity Fellowship for support of C.T.L. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

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