N2 separation efficiencies of MOF-74(Ni, Co) by doping palladium-containing activated carbon

Chemical Engineering Journal 284 (2016) 1348–1360

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Improving CO2 adsorption capacities and CO2/N2 separation efficiencies of MOF-74(Ni, Co) by doping palladium-containing activated carbon Abhijit Krishna Adhikari, Kuen-Song Lin ⇑ Department of Chemical Engineering and Materials Science/Environmental Technology Research Center, Yuan Ze University, Chungli District 320, Taoyuan City, Taiwan, ROC

h i g h l i g h t s
MOF-74(Co, Ni) were modified with Pd-loaded activated carbon.

1 of CO2 at 298 K and 32 bar. respectively, at same condition after modification.
MOFs are capable to separate CO2 completely from the mixture of CO2/N2 gases.
The CO2/N2 selectivity for MOF-74(Ni)-Pd and MOF-74(Co)-Pd were calculated as 14.6 and 12.4, respectively.


Pristine MOF-74(Ni) and MOF-74(Co) were adsorbed 11.06 and 10.28 mmol g 1


CO2 adsorption capacities were enhanced to 12.24 and 11.42 mmol g

a r t i c l e

i n f o

Article history: Received 18 July 2015 Received in revised form 24 September 2015 Accepted 25 September 2015

Keywords: Metal organic frameworks MOF-74 Pd-doping CO2 storage CO2/N2 separation XANES/EXAFS

a b s t r a c t MOF-74(Ni, Co) were synthesized, characterized, modified with Pd-loaded activated carbon (AC) and evaluated for CO2 adsorption capacity and CO2/N2 separation efficiency. The BET specific surface areas of MOF-74(Ni) and MOF-74(Co) were measured as 1418 and 1404 m2 g1 with the pore volumes of 0.86 and 0.82 cm3 g1, respectively. CO2 adsorption capacity of these MOFs was enhanced with Pd containing AC doping. CO2 adsorption capacities of modified MOF-74(Ni)-Pd and MOF-74(Co)-Pd were 12.24 and 11.42 mmol g1 respectively, measured at 298 K and 32 bar. The breakthrough curves of mixed CO2/N2 gases demonstrate the complete separation of CO2 from N2 stream. The adsorption and separation of CO2 were facilitated due the interaction between partially negative charge oxygen atoms of polarized CO2 molecule and partially positive Pd metal due to its low electronegativity. The fine structures of synthesized MOFs were further characterized with XANES and EXAFS. The bond distances of Ni–O, and Co–O in synthesized MOF-74(Ni) and MOF-74(Co) are 1.96 and 1.97 Å with the coordination numbers of 5.4 and 5.3, respectively. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction CO2 is primary anthropogenic greenhouse gas and the foremost perpetrator in global climate change [1]. Intergovernmental Panel on Climate Change (IPCC) has warned about the possible serious impacts of CO2 level rising in atmosphere such as; the rise of temperature, natural calamities, the increase of sea levels, species extinction and so on [2]. Thus, it is crucial to develop effective techniques to rebalance the CO2 distribution in order to minimize the greenhouse effect. Carbon capture and storage (CCS) is one of the most promising approaches to alleviate this issue. This process may subdivide into three parts; CO2 capture and separation at stationary sources, compression and transport to an injection site, and permanent storage in geological reservoirs or oceans. There are several technologies have been employed for CO2 capture includ⇑ Corresponding author. Tel.: +886 34638800x2574; fax: +886 34559373. E-mail address: [email protected] (K.-S. Lin). http://dx.doi.org/10.1016/j.cej.2015.09.086 1385-8947/Ó 2015 Elsevier B.V. All rights reserved.

ing cryogenic distillation, chemical absorption and physical adsorption. Among them, physical adsorption by microporous materials is promising for cost effective and efficiency. Many porous materials have been used for physical adsorption such as zeolites [3–5], carbon nanotubes [6–8], nanoporous carbons [9–11], porous aromatic frameworks [12–14], covalent organic frameworks [15–17], metal organic frameworks [18–20], calcium oxides [21,22], titania nanotubes [23] etc. However, most of these conventional materials have some drawbacks like low adsorption capacity, low selectivity over other gases, chemical and physical stability etc. Ideal adsorbent materials should have high CO2 adsorption capacity, excellent adsorption selectivity over other gases, and good chemical and mechanical stability during cyclic sorption–desorption processes. Metal–organic frameworks (MOFs), also known as coordination polymers have attracted great interest in last two decades as adsorbents due to their extremely high surface area, low density, and chemical tunability [24–26]. These properties make them

A.K. Adhikari, K.-S. Lin / Chemical Engineering Journal 284 (2016) 1348–1360

promising candidates for adsorption and separations of various gases [27–31]. Recently, Furukawa et al. [25] reported the MOF210 with highest specific surface area to date (6240 m2 g1) and studied the CO2 adsorption performance. It can uptake 71 wt% of CO2 at 50 bar and 298 K. Another record holder MOFs reported by Millward and Yaghi [32], MOF-177 also showed high CO2 adsorption capacity which was about 60 wt% at 35 bar and ambient temperature. There are another two MOFs famous for their ultra large cages, MIL-100 and MIL-101 exhibited 40 and 18 mmol g1 CO2 adsorption capacity measured at 303 K and 50 bar [33]. Further approach was found to obtain higher CO2 adsorption by synthesizing open metal sites containing MOFs. Yan et al. [34] has reported 11.6 mmol g1 CO2 adsorption by HKUST-1 at 273 K and 1 atm which has open Cu2+ metal sites, though it was ethanol and ammonium chloride treated. Britt et al. [35] has reported the CO2 adsorption capacity of Mg-MOF-74 (open Mg2+ sites) as 35.2 wt% measured at 298 K and 1 bar. However, most other MOF materials still show relatively low CO2 uptakes. It is vital to develop new materials to enhance CO2 adsorption or modify MOFs or other adsorbent by using postsynthetic approaches [36–38] . Unfortunately except MOFs post synthetic approaches are not easy to perform for other adsorbent materials. This is one of the great advantages of MOFs that these materials can be modified using postsynthetic approach. Recent studies indicated that the doping of alkali-metal ions significantly enhanced the CO2 adsorption by MOFs [39,40]. Lan et al. [41] has performed multi-scale simulations for CO2 adsorption in different metal ions (Li, Na, K, Be, Mg, Ca, Sc, Ti, etc.) doped MOFs. Among them, Li modified MOFs showed very good performance on CO2 adsorption. Thus, researchers focused on Li doping into MOFs and performed CO2 adsorption. Mishra and Ramaprabhu [42] found that doping of Pd metal enhanced the CO2 adsorption of graphite nanoplatelets. However, there are very limited reports are available on Pd-doping MOFs to enhance the CO2 capture. Xiang et al. [43] reported the Li doping as well as carbon nanotubes incorporation in HKUST-1 in order to enhance the CO2 adsorption capacity and also improve the mechanical and chemical properties of the modified MOFs. Kang et al. [44] reported the enhanced carbon dioxide and methane adsorption enthalpy by incorporation of carbon nanotubes into hybrid MOFs. Anbia and Hoseini [45] also synthesized a hybrid composite of acid treated multi walled carbon nanotubes and metal organic framework MIL-101. This material has 60% higher CO2 adsorption (from 0.84 to 1.35 mmol g1) than that of pristine MIL-101 at 298 K and 10 bar. Motivated by these experimental and theoretical results, we have synthesized MOF-74(Ni) and MOF-74(Co) as these MOFs have open metal sites which may help to enhance the CO2 adsorption capacity. Afterward postsynthetic approach was implemented to these MOFs using two techniques. One of the modification approaches is incorporation of Pd metals into AC and subsequently doping of Pd containing AC into MOFs in order to achieve enhanced composite performance and higher CO2 adsorption/separation. Thus, Herein we report the enhanced CO2 adsorption mechanism and binary gas mixtures such as CO2/N2 separation performance of Pd modified MOF-74(Ni) and MOF-74(Co). The synthesized MOFs were also characterized with XANES/EXAFS techniques in order to understand their local fine structure.

2. Experimental 2.1. Synthesis of MOF-74 samples MOF-74 samples were prepared by following our previously reported procedure published elsewhere [46]. In detail, to synthesis MOF-74(Ni) 1.00 g of 2,5-dihydroxyterephthalic acid (DHTA)

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and 4.51 g of Ni(NO3)26H2O were dissolved in 250 mL dimethyl formamide (DMF) with sonication in a 500 mL Duran bottle. In case of MOF-74(Co) 4.50 g of Co(NO3)26H2O were used instead of Ni (NO3)26H2O. The rest of the procedures are identical for both samples. Then 5 mL of water and 5 mL of ethanol were added to the solution and sonication was continued for 20 mins. The bottle was capped tightly and placed in a 110 °C oven for 20 h. After cooling to room temperature, the mother liquor was decanted and the products were washed several times with DMF, three times with methanol, and finally immersed into methanol. The methanol solvent was decanted and replaced once per day over the next six days. The products were then evacuated to dryness and heated under vacuum to150 °C over a period of 1 h. After 12 h at 150 °C, the heat was increased to different temperatures over a period of 1 h and hold in that temperature for another 12 h. Finally, the sample was cooled to room temperature and stored for analyses. 2.2. Preparation of Pd-doped MOF-74 samples 5.0 wt% palladium doped AC was used to modify the pristine MOF-74 samples. Carbon bridges between the MOFs (receptor) and AC (source) were formed by carbonization of sucrose (precursor) that was previously introduced into a physical mixture with Pd doped AC. The receptor/precursor/source ratio was fixed at 4:1:1 on the basis of the complete carbonization of the precursor. The resultant mixture was ground together for 1 h in ball mill and then subjected to the heating treatment procedures as described by Li et al. [47]. After the modification of the MOF-74(Ni) and MOF-74(Co) were termed as MOF-74(Ni)-Pd and MOF-74(Co)-Pd, respectively. 2.3. Characterization The morphologies, microstructures, and particle size of the synthesized MOF-74 samples were determined by FE-SEM/EDS (Hitachi, S-4700 Type II). Structure of the solid phase and crystallinities were carried out by XRD with a scan rate of 4°(2h) min1 using monochromatic CuKa radiation (MAC Science, MXP18) at 30 kV and 20 mA. Thermal decomposition was investigated using a TGA (model SDT 2960 & Thermal Analyst 2000, TA Instruments). Reaction temperatures and sample weights were recorded at 10 s intervals. About 20 mg of samples were heated from 298 to 873 K at a heating rate of 10 K min1. The surface area and pore diameter were measured using Brunauer–Emmett–Teller (BET) equation from nitrogen adsorption isotherms (Micromeritics ASAP 2020 Instrument). Low pressure CO2 adsorption isotherms were also collected from the same Instrument. Fourier transform infrared (FT-IR) spectroscopy was carried out using a Nicolet MagnaIR 830 spectrometer using the attenuated total reflectance method. The spectrum was generated, collected 16 times and corrected for the back-ground noise. 2.4. XAS data analysis The XANES/EXAFS spectra were collected at the Wiggler beam line 17C1 in the National Synchrotron Radiation and Research Center (NSRRC) of Taiwan. The electron storage ring was operated with the energy of 1.5 GeV and the ring current was 100–200 mA. The double crystal monochromator (DCM) of Si(111) employed at beamline selected X-rays with energy resolving power (E/DE) better than 7000, sufficient for most XANES/EXAFS measurements. Data were collected in transmission mode with a Lytle detector in the regions of the Ni and Co K edges at room temperature [48]. The spectra were measured with a step size equivalent to less than 0.5 eV in the near-edge and with a count time weighted to be proportional to k3 at high energy. Data were normalised using the

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program Athena (vi) with a linear pre-edge and polynomial postedge background subtracted from the raw ln(It/I0) data, and then analysed using the Artemis (vi) software, which makes use of the FFEF code-8 [49,50]. The spectra were first energy-calibrated by simultaneous measurements of the transmission spectra of Ni(0) and Co(0) in Athena (vi), where the energy of the first inflection point for the reference sample absorption edge were defined as 8333 and 7709 eV, respectively. After calibration, the samples were then background corrected using a linear pre-edge region and a polynomial for the post-edge region, and the samples were normalized. The EXAFS energy spectra were then converted to wavevector K space. The resulting scatter curve was weighed by K3 to enhance dampened scattering oscillations. This curve was followed by Fourier transformation to yield the radial structure function [51]. These data directly reflect the average local environment around the absorption atoms. Spectra were analyzed using the software package IFEFFIT [50]. The theoretical paths for Ni–O and Co–O species used for fitting the first coordination shell of the experimental data were generated using the FEFF-8 program based on the crystallographic data of individual species [52]. The coordination number, interatomic distance, Debye–Waller factor and inner potential correction were used as variable parameters for the fitting procedures.

absolute adsorption. When an adsorbent is exposed to gas molecules, some of the gas molecules are physisorbed on the surface and some present in the void space of the adsorbent as bulk density. The physisorbed process is an equilibrium between adsorption and desorption processes. Hence, the bulk density of the adsorbate shouldn’t be zero. Therefore, during the adsorption of gas molecules on porous materials, the measured mass change in the sample can be represented by the difference between the total adsorbed amount and the bulk density of the adsorbate. To measure the total adsorbed amount of gas, we have to know the actual size and structure of the adsorbed phase, as well as the adsorbate density and composition profiles within that phase. The density profile of the adsorbed phase becomes more diffused at high temperatures and pressures. Therefore, it is not possible to distinguish or measure the adsorbed and bulk phases with the present techniques. Hence, the actual adsorbed amount of gas molecules is not meaningful experimental variable. This problem is resolved by using an model of adsorption systems proposed by Gibbs [55]. With the help of this model Gibbsian surface excesses adsorbed amount can be measured [56] and subsequently, the experimental measurements in this work are reported as surface excess amount. In this study, we have also used the Langmuir and Freundlich isotherm as the model to fit both low and high pressure CO2 adsorption data by using Eqs. (1) and (2), respectively:

2.5. High pressure CO2 adsorption measurement

1 1 1 ¼ þ Q Q max Q max K L C

ð1Þ

1 Ln Q ¼ Ln K F þ Ln C n

ð2Þ

CO2 isotherms were measured gravimetrically at high pressure using Cahn Thermax 500 microgravimetric balance [53]. Changes in mass of samples suspended within a glass enclosure under a certain atmosphere were measured, which had a sensitivity of 1 lg. A pressuresensor of 0.011 atm sensitivity with the range of ambient to 100 bar at 298 K and ambient to 69 bar at 1273 K was used to measure the CO2 pressure in the chamber. Prior to admittance of the analyte gas the entire chamber and manifold were evacuated at room temperature and measured the wight of the sample holder which is made of glass (round shape with 20 mm long, 14 mm outer and 10 mm inner diameter). After loading the MOF-74 samples (180 mg) into sample holder at room temperature it was purged three times with helium and outgassed. Samples were outgassed for 10 h at 150 °C with the pump (EDWARDS, model RV3, czech republic). The entire chamber and manifold were evacuated (4  103 torr) during the outgass process. All samples showed almost constant mass detection after 4–5 h. The outgass process was prolonged to 10 h to make sure the maximum possible elimination of residual gases from the sample pores during each experiment. Then the adsorbate gas was admitted into the chamber incrementally and the data points were recorded at 298 K when no further change in mass was observed. The analyte (CO2) and purge (He) gases were passed through a molecular sieve trap immersed in ice bath to remove any condensable impurities or moisture before being exposed to the samples. The data points were recorded until the desired value of CO2 pressure and high purity CO2 gas was used for these experiments. The excess adsorbed amount of CO2 was calculated after the buoyancy correction. During the high pressure CO2 adsorption, the buoyancy effect exhibited by the sample holder and the components associated with the balance. The buoyancy of the adsorbent was corrected on the basis of the change in mass of the empty bucket within the analyte gas at 298 K. The weight loss due to the buoyancy of the adsorbent was determined using a helium measurement followed by the procedure described by Furukawa et al. [54], assuming that helium adsorption at room temperature can be neglected. During the gas adsorption process, the adsorbed amount can be divided into two components which are the surface excess and the

where, C is CO2 concentration (bar1) and Q is the adsorbed amount (mmol g1 or wt.%), Qmax is the maximum amount of sorbent in equilibrium. KL and KF are Langmuir and Freundlich equilibrium constant, respectively. 2.6. Breakthrough experiments The breakthrough experiments were performed by using a selfassembly experimental set-up as shown in Fig. 1. High purity gases were used to carry out the experiments. The gases flow rate of N2 and CO2 were controlled by the multi-component mass flow controller. The flow-rate of gas mixture (20% CO2 and 80% N2) through the adsorption column was kept fixed at 5.0 mL/min. The gas flow rate was chosen lower so that no channeling occurs during the experiment in packed bed. The adsorption column we used for this experiment was 26 cm long with 1.0 cm outer and 0.8 cm inner diameter. The detail dimensions and geometry of the adsorption column is presented in Fig. 1. It was specially designed with the screen in the middle so that the powder materials can be easily packed inside the column. Only the gas molecules can pass through this screen and the MOF particles are not allowed to pass due to its particle size. The adsorption column was packed exactly 8 cm in length in each experiment. The column was placed inside the heating chamber vertically and the upper end (the gas entrance end) was filled with glass fiber so that no MOF particle can enter into the gas tube. Normally, during the adsorption experiment as gas enters through the upper end and exits at bottom and due to gravity force it is assumed that no MOF particle can go back into gas tube against the gas flow. The composition of exit gas stream from the adsorption column was analyzed by the in situ FTIR instrument and the compositions were determined using a mass spectrometer (CHINA CHROMATOGRAPHY, Model: 9800). All the breakthrough curves of binary mixture were obtained at 298 K and ambient pressure.

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Fig. 1. Top: schematic diagram of the experimental set-up used for breakthrough experiments. (A) N2 gas bottle, (B) CO2 gas bottle, (C) multi-component mass flow controller, (D) adsorption column chamber, (E) temperature controller, (F) FT-IR, (G) computer, (H) mass spectrometer. Bottom: the geometry and dimensions of the adsorption column.

3. Results and discussion 3.1. Morphology of MOF-74 samples The morphology and microstructure of synthesized MOF-74(Ni) samples were investigated by FE-SEM images shown in Fig. 2. It has spherical shape with irregular sizes and average diameters of approximately 1–3 lm. The size of the particles didn’t change much with increasing the calcination temperature though some particles were agglomerated with each other. Hence, the diameter of the particles slightly increased. At high temperature >300 °C calcination, (Fig. 2e and f) some ash-like substances are found which are the symptoms of the destruction of the structure. The organic bridges between the structure may burned out and form carbon on the surface of the frameworks. On the other hand, in case of MOF-74(Co) the size and shape of the particles are quite different than MOF-74(Ni) as shown in Fig. 3. It has mixed block and rod type shape particles with different diameters approximately ranges from 1–5 lm. At around 250 °C the particles agglomerated and some sphere like particles can be observed as shown in Fig. 3d. At 300 °C the frameworks collapsed that can be clearly observed from the Fig. 3f. After modification of the pristine MOF-74 samples with Pd doped AC, TEM was used to investigate in order to confirm the presence of the Pd particles in the structure. The doped Pd particles are uniformly distributed on the surface of the both MOF-74 samples as shown in Fig. 4. The diameter of the doped Pd particles are ranges from 3–5 nm. The uniform distribution of Pd particles confirms the equal dispersion of AC throughout the modified MOF-74 samples. 3.2. Crystal Structure of MOF-74 samples The crystalline phases of the synthesized MOF-74(Ni) and MOF-74(Co) samples were characterized by PXRD as shown in Fig. 5a and b, respectively. Both samples show the unique crystalline phases. The calcination process didn’t change the crystal structure of MOF-74(Ni) under 300°C. However, this process is beneficial to obtain the pure MOF structure that can be observed from the N2 adsorption isotherms and BET specific surface area of different samples. The detail of N2 adsorption is discussed in pore textural properties part. However, the crystal structure of the MOF-74(Ni) has been changed when it is calcined in 350 °C. In such high temperature the organic part was burned out and the formation of NiO was confirmed from the XRD patterns (JCPDS card No. 71-1179). In case of MOF-74(Co), the structure was

unchanged until 250 °C. The structure of MOF-74(Co) was destroyed after calcined at 270 °C and converted into CoO which was confirmed with JCPDS card No. 43-1004. The XRD patterns were indexed on the basis of a face-centered hexagonal unit cell of MOF-74 for both samples. They are in very good agreement with the patterns reported by different groups [32,57] and showed almost similar diffraction patterns at identical 2h values ascribed the formation of similar crystallographic phases. 3.3. Thermal analysis The TG/DTA curves of synthesized MOF-74(Ni) and MOF-74(Co) are shown in Fig. 6 where it can be seen that they are consistent with notable weight-loss steps. Though the thermal stability are not same for these two materials, the trend of losing weight with temperature is very homologous. At early stage (25–125 °C) the weight loss is due to the elimination of adsorbed water and guest molecules. This value is about 23 wt% and similar for both samples. It means the fully hydrated samples contain about 23 wt% of adsorbed water and guest molecules which may incorporated during the hydrothermal synthesis. It should be noted that the amount of water content may vary significantly depending on the humidity of the laboratory or store place of the samples. In case of MOF-74 (Co) the second weight loss step contains about 7 wt% within the range of 125–255 °C. This loss corresponds to the organic ligands (DHTA) and hence indicates the thermal stability of the frameworks. Finally, at 258 °C the frameworks totally collapse and about 35 wt% lost was observed. However, though the initial weight loss trends of both samples are identical, in case of MOF-74(Ni), the second step weight loss is much longer than MOF-74(Co). Before final collapse of MOF-74(Ni) frameworks at 344 °C, there was about 18 wt% weight loss observed. Hence, MOF-74(Ni) shows much better thermal stability than MOF-74(Co) which was also confirmed from the XRD patterns. It may be due to the strong coordination bonds between the Ni and COO groups of organic ligands than Co atoms. 3.4. FTIR study FTIR spectra of as-synthesized and calcined samples of MOF-74 (Ni) and MOF-74(Co) are plotted in Fig. 7. It should be noted that the most of the vibration bands are absence in case of higher temperature calcined samples. It is in good agreement with the TGA data as the organic ligands of the frameworks were destroyed in higher temperature. For the as-synthesized samples, there are

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(a)

(b)

(c)

(d)

(e)

(f)

Fig. 2. FE-SEM micrographs of MOF-74(Ni); (a) as-synthesized, (b) calcined at 150, (c) 200, (d) 250, (e) 300, and (f) 350 °C.

some positive or negative shifts have been observed between two samples. The strong stretching vibration of carboxylate anions at 1664 and 1667 cm1 for MOF-74(Ni) and MOF-74(Co), respectively, proving the existence of the reaction of –COOH groups in the DHTA with metal ions [58]. These shifts may attribute to the different van der Waals forces between the hydrogen molecules. The vibration bands in the range of 700–1700 cm1 are assigned to the existence of organic groups. The other strong bands at 838, 896 in MOF-74(Ni) and 829, 898 cm1 in MOF-74(Co) are assigned to the m(OH) and d(OH) modes, respectively, corresponding to the l2-hydroxo groups which are presents at the corner-sharing hexagonal units M2(C8H2O6) (M represents the corresponding metal ions). The shifts at 1026 and 1208 (MOF-74(Ni)) and 1032 and 1207 (MOF-74(Co)) are also corresponds to the d (OH) bands of the hydroxyl groups. Both spectra show a very broad band at around 2700–3600 cm1 indicated the presence of water and the –OH groups in the both materials. 3.5. Pore textural properties The N2 adsorption isotherms of as-synthesized and modified MOF-74(Ni) and MOF-74(Co) samples are presented in Fig. 8. The

BET and Langmuir surface area are pore volume were measured using Barret–Joyner–Halenda (BJH) model and the average pore diameter was calculated by density functional theory (DFT) method using ASAP 2020 analyzer’s built-in software. The pore textural properties of the samples are shown in Table 1. The N2 adsorptions follow the Type-1 isotherms with virtually no hysteresis loop corresponding to the microporous material. Different groups have reported different specific surface areas for MOF-74 family frameworks. Glover et al. [59], Botas et al. [60] and Srinivas et al. [61] have reported different surface areas for MOF-74(Zn), such as; 496, 850 and 1000 m2 g1, respectively. In case of MOF-74(Mg) this value have been reported as 1206, 1525, 1400, 1174, and 1495 m2 g1 by Glover et al. [59], Yang et al. [62], Schoenecker et al. [63], Bao et al. [64], and Caskey et al. [65], respectively. However, Golver et al. [59] has reported the surface are of MOF-74 (Ni) and MOF-74(Co) as 599 and 835 m2 g1, respectively. Peng et al. [29] has reported the surface are of MOF-74(Ni) as 1350 m2 g1 and Cho et al. [66] has reported 1327 m2 g1 for MOF-74(Co). Interestingly, these values are much lower than our obtained results, such as; 1418 (MOF-74(Ni)) and 1404 m2 g1 (MOF-74(Co)), respectively. The reason of obtaining dissimilar results also explained in different ways by some groups. Botas

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Fig. 3. FE-SEM micrographs of MOF-74(Co); (a) as-synthesized, (b) calcined at 150, (c) 200, (d) 250, (e) 270, and (f) 300 °C.

(a)

20 nm

(b)

20 nm

Fig. 4. HR-TEM images of modified (a) MOF-74(Ni)-Pd and (b) MOF-74(Co)-Pd.

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508

Absorbance (a.u.)

Relative intensity (a.u.)

(a)

350ºC 300ºC 250ºC 200ºC 150ºC As-synthesized 5

10

15

20

25

30

35

40

45

838 896 1026 1208 1367 1461 1664

1354

(a) OH group as-synthesized 150ºC 200ºC 250ºC 300ºC

500

50

1000 1500 2000 2500 3000 3500 4000 Wave number (cm-1)

300ºC 270ºC 250ºC 200ºC

10

15

20

25

30

35

40

45

1456 1667

250ºC

500

80

5

70

4 MOF-74(Ni) TG MOF-74(Co) TG MOF-74(Ni) DTA MOF-74(Co) DTA

3 2

40

1

30

0 100

600

200 300 Temperature (ºC)

400

Derivative weight (wt% min-1)

6 344ºC

1000 1500 2000 2500 3000 3500 4000 Wave number (cm-1)

Fig. 7. FTIR spectra of (a) MOF-74(Ni), (b) MOF-74(Co); as-synthesized and calcined at different temperatures.

7

258ºC

90 Weight (%)

200ºC

50

Fig. 5. XRD patterns of (a) MOF-74(Ni), and (b) MOF-74(Co); as-synthesized and calcined at different temperatures.

50

150ºC

300ºC

2θ (degree)

100

OH group

270ºC

500

Fig. 6. TG/DTA curves of as-synthesized MOF-74(Ni) and MOF-74(Co).

et al. [60] has explained that the activation process of the MOF samples was not similarly effective for each sample. Caskey et al. [65] has mentioned that it might be due to the different atomic weight of the central atoms and they have different affinity to adsorbed N2 molecules per unit cell. The DFT pore size distribution of MOF-74(Ni) and MOF-74(Co) samples are shown in Fig. 9 and the values of average pore size and volume are presented in Table 1. These results also suggest that these samples are microporous materials. The pore volume of Pd modified samples (0.78 and 0.75 cm3 g1) are smaller than that of pristine MOFs (0.86 and 0.82 cm3 g1).The Pd dopant may occupy the micropores of the frameworks, thus leading to the reduction of BET specific surface area.

Volume adsorbed (cm3·g-1)

5

(b)

as-synthesized

150ºC As-synthesized

60

445

Absorbance (a.u.)

Relative intensity (a.u.)

(b)

829 898 1032 1207

2θ (degree)

500 400 300 MOF-74(Ni) MOF-74(Co) MOF-74(Ni)-Pd MOF-74(Co)-Pd

200 100 0 0.0

0.2

0.4

0.6

0.8

1.0

P/P0 Fig. 8. N2 adsorption/desorption isotherms of as-synthesized MOF-74(Ni) and MOF-74(Co) measure at 77 K. Solid and open symbols represent the adsorption and desorption of nitrogen gases, respectively.

3.6. XANES study XANES studies are performed to reveal the information on the energy and electronic distribution of the molecular orbitals by probing the unoccupied density of states of the sample. It is highly sensitive for transition metal complexes on X-ray absorption edges and (pre)edge features to their chemical environment. Analyzing of XANES spectra with the theoretical methods [67] can obtain the detail electronic and structural information of the probing samples. Normalized Ni K-edge spectra for different MOF-74(Ni) samples are plotted in Fig. 10 with Ni(0) standard. The pre-edge features

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A.K. Adhikari, K.-S. Lin / Chemical Engineering Journal 284 (2016) 1348–1360 Table 1 Pore textural properties of MOF-74(Ni) and MOF-74(Co) samples.

a

Sample name

BET surface area (m2 g1)

Langmuir surface area (m2 g1)

Pore volume (cm3 g1)a

Average pore size (Å)a

Micropore (%)

Mesopore (%)

MOF-74(Ni) MOF-74(Ni)-Pd MOF-74(Co) MOF-74(Co)-Pd

1418 1115 1404 1088

2148 1756 2120 1727

0.86 0.78 0.82 0.75

8.54 8.23 8.32 8.15

71 73 63 65

18 17 32 31

Average pore size and pore volume were measured by BJH desorption method.

2.0 1.6

Absorbance (a.u.)

Pore volume (cm3g-1)

(a) 20.01 Å

1.2 0.8

0.0

10

100

1000

8325

Pore width (Å)

1.6

19.98 Å

(b)

8340

8355 8370 Photon energy (eV)

250oC 300oC 350oC

8385

8400

Fig. 10. Ni K-edge XANES spectra of MOF-74(Ni) calcined at different temperatures with Ni(0) standard.

1.2 0.8 0.4 0.0

10

100

1000

Pore width (Å)

Absorbance (a.u.)

Pore volume (cm3g-1)

as-synthesized 150οC 200oC Ni Powder

0.4

Fig. 9. DFT pore size distribution of (a) MOF-74(Ni) and (b) MOF-74(Co).

originate from the partial hybridization of orbitals allowing dipole forbidden transition [68]. The peak positions were determined by taking the second derivative of the normalized XANES data in order to easily recognize the exact positions of the different hybridization, which were directly correlate to the positions of the individual empty orbitals probed. The main edge corresponds to the 1s to 4p dipole allowed transition for the complex which is the last edge contribution, before the first absorption maximum of the absorption edge is reached. The edge position of synthesized MOF-74 (Ni) samples is very close to the edge position of Ni(II). However, the offset is about 0.4 eV compared to the standard Ni(II) which correlates the formation of hydroxides (OH) with covalent hybridization and central Ni atoms are not entirely surrounded by the oxygen atoms. In case of MOF-74(Co), the offset is about 0.3 eV compared to Co(II) standard edge position (Fig. 11). The pre-edge XANES spectra of Ni(II) and Co(II) exhibit an absorbance feature at 8333 and 7709 eV, respectively which are responsible for the 1s to 3d transition. Notably, the 1s > 3d transition is dipole-forbidden in a rigorously centrosymmetric environment and thus, it becomes partially allowed when a lowering of the symmetry induces a mixing of 3d and 4p orbitals. Because the transition 1s > 4p is dipole-allowed and normally the area of the pre-edge peak increases when the occupancy of d-orbital and the degree of symmetry decreases. The intense peak at around

As-synthesized 150oC 200oC Co Powder

250oC 270oC 300oC

7700 7710 7720 7730 7740 7750 7760 7770 Photon energy (eV) Fig. 11. Co K-edge XANES spectra of MOF-74(Co) calcined at different temperatures with CuO, Cu2O and Cu(0) standards.

8350 eV and well defined shoulders at 8344 eV in XENES spectra of MOF-74(Ni) were due to the dipole-allowed of 1s to 4pxy electron transition due to the Ni(II). In case of MOF-74(Co) these values were 7725 and 7718 eV and indicate the existence of Co(II) in MOF-74(Co) samples.

3.7. EXAFS study EXAFS spectroscopy was analyzed to obtain the local structure information (coordination number, neighbor atoms and bond distances) of central Ni and Co atoms in MOF-74(Ni) and MOF-74 (Co), respectively. EXAFS primarily probes the 1s to 4p transition performed in R space. The Athena and Artemis programs based on the IFEFFIT library of numerical XAFS algorithms were applied to perform the study [49]. The EXAFS oscillations v(k) were performed from the experimental data using standard procedures [69]. The Fourier transformation was calculated using the Hanning

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‒Ni Ni‒ FT magnitude (a.u.)

(a)

Table 2 Fine structural parameters of different MOF-74(Ni) and MOF-74(Co) samples analyzed by using Ni and Co K-edge EXAFS, respectively.

(b)

Sample

Shell

CNa (± 0.5)

Rb (±0.02 Å)

Dr2 (Å2)c

(c)

Ni(0) MOF-74(Ni) as-synthesized MOF-74(Ni) calcined at 200 °C MOF-74(Ni) calcined at 250 °C MOF-74(Ni) calcined at 300 °C MOF-74(Ni) calcined at 350 °C Co(0) MOF-74(Co) as-synthesized MOF-74(Co) calcined at 200 °C MOF-74(Co) calcined at 250 °C MOF-74(Co) calcined at 270 °C MOF-74(Co) calcined at 300 °C

Ni–Ni Ni–O Ni–O Ni–O Ni–O Ni–O Co–Co Co–O Co–O Co–O Co–O Co–O

5.7 5.4 4.7 4.5 4.3 5.2 5.6 5.3 4.8 4.7 4.9 5.1

2.41 1.96 1.97 1.98 1.98 1.98 2.39 1.97 1.97 1.98 1.98 1.98

0.00073 0.00513 0.00308 0.00245 0.00335 0.00434 0.00132 0.00074 0.00086 0.00078 0.0076 0.0075

(d) (e) (f) (g)

Ni‒ ‒O 0

2

4

6

8

10

Bond distance (Å) a

Fig. 12. Ni K-edge K3 weighted Fourier transform (FT) spectra of MOF-74(Ni); (a) Ni (0), (b) as-synthesized, (c) calcined at 150, (d) 200, (e) 250, (f) 300, and (g) 350 °C. The best fitting of the EXAFS spectra are expressed by the circled lines.

‒ Co Co‒ FT magnitude (a.u.)

(a) (b) (c) (d) (e) (f) (g)

Co‒ ‒O 0

2

4

6

8

10

Bond distance (Å) Fig. 13. Co K-edge K3 weighted Fourier transform (FT) spectra of MOF-74(Co); (a) Co(0), (b) as-synthesized, (c) calcined at 150, (d) 200, (e) 250, (f) 270, and (g) 300 °C. The best fitting of the EXAFS spectra are expressed by the circled lines.

filtering function. The k3 weighted v(k) data were Fourier transformed to enhance the oscillations at higher k. The Fourier transformed fitting data of MOF-74(Ni) and MOF-74(Co) are plotted in Figs. 12 and 13, respectively and the obtained structural parameters from the best fit to the EXAFS data are shown in Table 2. The Ni–O bond distance in as-synthesized MOF-74(Ni) is 1.96 Å with the coordination number of 5.4. In case of as-synthesized MOF-74(Co), Co–O bond distance is 1.97 Å with the coordination number of 5.3. While the calcination temperature is increasing, there is a trend to decrease in coordination number. It may be due to elimination of some oxygen or hydroxyl (OH) groups from the structure. Hence, the bond distances of Ni–O or Co–O are also faintly increased. However, the overall structure remains same though some of oxygen atoms removed from the structure until the structure were destroyed at very high calcination temperature. 3.8. CO2 adsorption Adsorption and desorption equilibrium isotherms of low pressure CO2 on as-synthesized and modified MOF-74(Ni) and MOF-74(Co) at 298 K are plotted in Fig. 14. Virtually identical isotherms obtained for both MOFs, though CO2 adsorption capacities are not identical. It can be seen that MOF-74(Ni) has comparatively higher CO2 adsorption capacity than MOF-74(Co). Their adsorption

b c

Coordination number. Bond distance. Debye–Waller factor.

capacities are about 4.06 and 3.67 mmol g1, respectively measured at 298 K and 1.1 bar pressure. The fully reversible desorption confirms that there are no chemical adsorption occurs during the adsorption process which is very common for microporous materials. Similar behavior was also found during N2 adsorption/ desorption process as mentioned in earlier. The adsorption slope is steady uphill that may indicate the possibility of higher adsorption capacity at higher pressure. It should be noted that the comparatively higher CO2 adsorption capacity of MOF-74(Ni) may attribute to its higher surface area and the strong affinity to CO2 molecules of Ni atoms. MOF-74 materials generate coordinatively unsaturated (open) metal sites having Lewis acidity [70]. To understand the Lewis acid characteristics we performed the NH3-TPD (data are not shown here) and found MOF-74(Ni) has higher Lewis acidity compared to MOF-74(Co). This is one of the significant causes that MOF-74(Ni) adsorbed higher amount of CO2. In case of Pd doped modified materials, the similar trend were also found for CO2 adsorption. Interestingly, the adsorption capacities were much higher than that of pristine MOF-74 materials. For MOF-74(Ni)-Pd and MOF-74(Co)-Pd the CO2 adsorption capacities were recorded to 4.38 and 3.96 mmol g1, respectively as we expected. Mishra et al. [42] has reported this extra CO2 adsorption was possible due to the strong interaction of polarized CO2 molecule with Pd metal nanoparticles, i.e. the interaction between partially negative charge oxygen atoms of polarized CO2 molecule and partially positive Pd metal due to its low electronegativity. Another possibility is the incorporation of AC in the MOF materials, which may form composite with MOFs and facilitate the gas adsorption process through spillover [71]. High pressure CO2 adsorption isotherms were also performed for the modified and pristine MOF-74 materials shown in Fig. 15. The CO2 adsorption capacity were found as 11.06 and 10.28 mmol g1 for MOF-74(Ni) and MOF-74(Co), respectively measured at 298 K and 32 bar. However, these value were increased to 12.24 and 11.42 mmol g1, respectively, measured at the same condition. We have analyzed the pore structure of these materials from the N2 sorption isotherms to understand the pore structure effect on CO2 adsorption capacity. Interestingly, we realized that the percentage (based on total pore volume per gram) of micropore (>2 nm) is higher in case of MOF-74(Ni) and Pd-modified MOF-74(Ni) compared to MOF-74(Co) and MOF-74 (Co)-Pd. The percentage of micro and mesopore of these materials are shown in Table 1. It can be seen that the micropore percentage of MOF-74(Ni) is 71%; while in case of MOF-74(Co) this value is 63% which is also obvious from pore size distribution (Fig 8). Thus, we believe that it might be an important issue in case of higher CO2

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Table 3 Fitted values of Langmuir–Freundlich parameters for low and high pressure gravimetric CO2 adsorption on MOF-74 samples.

CO2 adsorption (mmol g-1, STP)

5.0 4.5 4.0

Sample

3.5

Low pressure CO2 adsorption

3.0 2.5 2.0 1.5

MOF-74(Ni) MOF-74(Co) MOF-74(Ni)-Pd MOF-74(Co)-Pd

1.0 0.5 0.0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

High pressure CO2 adsorption

KL

R2

n

MOF-74(Ni) 0.0146 ± 0.006 0.498 ± 0.047 0.9966 MOF-74(Co) 0.0160 ± 0.003 0.681 ± 0.015 0.9997 MOF-74(Ni)-Pd 0.0161 ± 0.003 0.680 ± 0.015 0.9997 MOF-74(Co)-Pd 0.0612 ± 0.014 0.498 ± 0.047 0.9966 MOF-74 (Ni) 0.1908 ± 0.005 1.102 ± 0.030 0.9985 MOF-74(Co) 0.1908 ± 0.005 1.102 ± 0.030 0.9985 MOF-74(Ni)-Pd 0.2122 ± 0.006 1.030 ± 0.038 0.9978 MOF-74(Co)-Pd 0.1908 ± 0.005 1.102 ± 0.030 0.9985

KL, n, and R are Langmuir equilibrium constant, Langmuir parameter, and correlation value, respectively.

Pressure (bar)

14 CO2 adsorption (mmol g-1)

12 10 8 6 MOF-74(Ni) MOF-74(Co) MOF-74(Ni)-Pd MOF-74(Co)-Pd

4 2 0

0

5

10

15 20 Pressure (bar)

25

30

Fig. 15. High pressure CO2 adsorption isotherms of MOF-74(Ni) and MOF-74(Co) measured at 298 K and up to 31 bar. Solid symbols and dashed lines represent the experimental data and fitted curves respectively.

adsorption of MOF-74(Ni) at high pressure compared to MOF-74 (Co). Unfortunately, we are unable to measure the exact amount of CO2 adsorption by micro or mesopores separately due to the limitation of the high pressure TGA analyzer. However, if we consider the CO2 adsorption capacities of modified and unmodified samples, it would be clear that beside the pore structure, there are some other parameters that affect the CO2 adsorption capacity. Because the micropore percentage of these two samples are similar (71% and 73% for MOF-74(Ni) and MOF-74(Ni)-Pd, respectively; 63% and 65% for MOF-74(Co) and MOF-74(Co)-Pd, respectively). Thus, it is confirmed that the enhanced CO2 uptakes of MOF-74 (Ni)-Pd and MOF-74(Co)-Pd are attributed to the strong affinity of the doped Pd towards CO2 molecules. The experimentally described CO2 sorption data were fitted using the most commonly used isotherm models (Langmuir–Freundlich models) and the model parameters are listed in Table 3. The fits having a high correlation value (R2) indicate that Langmuir isotherm may describe the absorption of CO2 on MOF-74 samples. 3.9. Breakthrough curves of CO2/N2 binary gas mixtures The dynamic separation performances of as-synthesized and Pd modified MOF-74 samples were carried out with separate tests using breakthrough experiments using CO2/N2 mixture as shown in Fig. 16. The samples were degassed at 150 °C and atmospheric

1.8 Relative concentration (C/C0)

Fig. 14. Low pressure CO2 adsorption isotherms of pristine and modified MOF-74 (Ni) and MOF-74(Co) measured at 298 K. Solid and open circles represent the adsorption and desorption, respectively.

1.6

N2 on MOF-74(Ni) N2 on MOF-74(Co) CO2 on MOF-74(Ni) CO2 on MOF-74(Co)

1.4 1.2 1.0 0.8

N2 on MOF-74(Ni)-Pd N2 on MOF-74(Co)-Pd CO2 on MOF-74(Ni)-Pd CO2 on MOF-74(Co)-Pd CO2 on quartz sand

0.6 0.4 0.2 0.0

0

30

60

90

120

150

180

210

Retention time (min) Fig. 16. Breakthrough curves of binary mixture of CO2/N2 gases (0.8/0.2, v/v) in the fixed bed packed with as-synthesized and Pd modified MOF-74(Ni) and MOF-74 (Co). Pure CO2 gas was used in case of quarts sand.

pressure for 6 h and cooled down to 298 K. Then the CO2/N2 gas mixture was passed through the adsorption bed at 298 K and ambient pressure. It can be seen that N2 elutes out first because the samples have high affinity for CO2 than N2 (Fig. 16) or It can be illustrated as MOF-74 samples have low N2 adsorption capacity than CO2 which was expected. In contrast, the samples showed significant CO2 adsorption capacity which can be verified by the rollup in the N2 breakthrough curves. The breakthrough curves exhibited roll-up due to N2 desorption because initially adsorbed N2 was replaced by incoming CO2 until breakthrough occurred. After the saturation of MOF-74 samples CO2, the N2 effluent composition decreases and matches the feed composition [72]. The in-situ FTIR system was conducted to collect the FTIR data for continuous monitoring of the CO2 in effluent. The collected data in certain intervals for CO2 in effluent for MOF-74(Ni)-Pd and MOF-74(Co)-Pd are presented in Fig. 17(a) and (b), respectively. These data clearly reveal the gradual saturation of the effluent with CO2 which is consistent with the CO2 breakthrough experiments. The apparent CO2 adsorption capacity in breakthrough experiments includes the excess adsorption, amounts of CO2 accumulated in the dead space of the adsorption system, in the gap space between the adsorbent pellets and the pores space of adsorbent pellets. Thus, to determine the excess adsorption capacity we have to calculated the each part of overall CO2 adsorption. Dubinin-Radushkevitch (D-R) equation has been widely used to study CO2 adsorption on porous materials which can be written as shown in Eq. (3) [73], where V is the excess adsorption volume of CO2 adsorbed in cm3 per gram, E0 the characteristic adsorption energy of MOF in kJ/mol, V0 the volume of pores smaller than

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using the density data of MOF samples and quartz sand following the procedure described by Yin et al. [73]. We have used the 200 mesh quartz sand for this method as it was similar to MOF-74 particle size and assuming the gap space for the same volume of quartz and MOF samples to be the same. The CO2 adsorption for quartz sand is shown in Fig. 16 and in this case we have used pure CO2 gas instead of mixture of CO2 and N2 gases. As quartz sand is non-porous, Vpore for quarts sand is zero. Thus, the sum of Vpore and Vgap is equal to Vgap only in case of quarts sand. Replacing the Vgap value in case of MOF-74 samples we can easily calculate the Vpore values which are shown in Table 4. It should be noted that in case of MOF-samples Vpore and Vgap value was the sum of volume occupied by CO2 and N2 gases. The apparent adsorption (q) of CO2 and N2, in MOF samples which includes the excess adsorption (qexcess), adsorption in adsorbent pores (qpore), between the pellets (qgap), and in dead space (qds) can be calculated by the following equations [73]:

(a)

150 120 90 80 70 60 in) 50 (m 40 e 30 20 Tim 10

1000 1500 2000 2500 3000 3500 4000

Wave number (cm-1)

(b)

qCO2MOF ¼

v c44

Rt 

22:4

1

0

C MOF ðtÞ c



dt

qpMOF V MOF

¼

C CO 44

mAreaMOF  22:4 V MOF

2

qpMOF

¼ qexcess þ qpore þ qgap þ qds ðmg CO2 =g MOFÞ 150 120 90 80 70 in) 60 (m 50 e 40 30 Tim 20 10

qN2MOF ¼

v c14

qCO2qz ¼

-1



C MOF ðtÞ c

dt

¼

C N 14 2

qpMOF

mAreaMOF  22:4 V MOF

v c44

Rt

22:4

0

C qz ðtÞ c

1



dt

qpqz V qz

¼

C CO 44 2

qppz

mArea

 22:4V qzqz

1.6 nm, and b the affinity coefficient of CO2 and MOF which is nearly constant for microporous carbonaceous material 0.35 [74]. Eq. (3) is a straight line equation with the slope of 1/E20 and intercept lnV0 from where V0 and E0 can be calculated at given temperature T. The density of CO2 at the adsorbed temperature can be obtained using Eq. (4) [75], where qb is the density of CO2 at boiling temperature Tb, Tc the critical temperature of CO2, 304.1 K, M the molecular weight of CO2, 44 g/mol, and b the van der Waals constant, 42.67 ml/mol


2 RT P0 ln ln V ¼ ln V 0  2 P E0 b 1

ð3Þ

ðT  T b Þ

ð4Þ

To measure the packing density of MOF samples (qp) a glass tube with 18 mm inner diameter was filled with MOF-74 sample and hot air was passed through for 4 h. Then calculated the volume of the samples and weighted. The pore volumes of MOF samples (Vpore) and the gap space (Vgap) between pellets were determined

ðqgap þ qds ÞCO þ ðqgap þ qds ÞN ¼ ðqgap þ qds Þqz 2

ð6Þ

ð7Þ

¼ qgap þ qds ðmg CO2 =g MOFÞ

Fig. 17. The in-situ FTIR spectra of (a) MOF-74(Ni)-Pd and (b) MOF-74(Co)-Pd during CO2/N2 breakthrough experiments.

Tc  Tb

1

qpMOF V MOF

Wave number (cm )

qb  M=b

0

¼ qexcess þ qpore þ qgap þ qds ðmg N2 =g MOFÞ

1000 1500 2000 2500 3000 3500 4000

q ¼ qb 

Rt

22:4

ð5Þ

ð8Þ

2

where, v is the volumetric flow rate of the feed gas, c the concentration of the respective gas in feed (CO2 or N2), qp the packing density, AreaMOF and Areaqz are the integrated area of adsorption curves. The values of qpare and qgap at pressure P and temperature T can be calculated by the Eqs. (9)–(12), where V total pore is the total pore volume of MOF and V total gap the total volume of the gap space in MOF

qpore ¼

C  44 

PV total pore RT

qpMOF  V MOF

C  44  PV total pore

¼

qpMOF  RT  V MOF

ð9Þ

C  44  PV pore ¼ ¼ ðmg CO2 =g MOFÞ RT qpore ¼

qgap ¼

C  14  PV pore ðmg N2 =g MOFÞ RT C  44 

PV total gap RT

qpMOF  V MOF

¼

ð10Þ

C  44  PV total gap

qpMOF  RT  V MOF

ð11Þ

C  44  PV pore ¼ ðmg CO2 =g MOFÞ RT

Table 4 Various kinds of adsorption capacities of CO2 and N2 gases and selectivity of CO2 over N2 for MOF-74 samples. Sample

Vpore (cm3 g1)

Gas

qexcess (mmol g1)

Capacity in various void spaces (mmol g1) qds

qgap

qpore

MOF-74(Ni)

0.34

MOF-74(Co)

0.31

MOF-74(Ni)-Pd

0.32

MOF-74(Co)-Pd

0.32

CO2 N2 CO2 N2 CO2 N2 CO2 N2

1.97 0.65 1.88 0.61 2.14 0.76 2.05 0.73

0.55 0.18 0.53 0.17 0.60 0.21 0.58 0.20

0.017 0.005 0.016 0.005 0.018 0.006 0.017 0.006

0.0002 0.0001 0.0002 0.0001 0.0002 0.0001 0.0002 0.0001

Selectivity of CO2/N2

11.3 9.7 14.6 12.4

A.K. Adhikari, K.-S. Lin / Chemical Engineering Journal 284 (2016) 1348–1360

qpore ¼

C  14  PV pore ðmg N2 =g MOFÞ RT

ð12Þ

Based on the Eqs. (9)–(12), qexcess for CO2 and N2 gases can be determined by subtracting qpore, qgap, and qds from qCO2MOF and qN2MOF , respectively and the obtained results are presented in Table 4. It can be seen from Fig. 16 that the breakthrough time of CO2 on MOF-74(Ni) was longer than that on MOF-74(Co). This suggests that MOF-74(Ni) has higher CO2 apparent adsorption capacity. Moreover, the Pd modified MOF-74 samples showed much longer CO2 breakthrough time than their corresponding pristine MOFs. These results suggest that incorporated Pd metal has positive effect on CO2 adsorption due to its partial positive charges. It should be noted that the CO2 adsorption capacities of Pd modified MOF-74 samples are higher than that of as-synthesized MOF-74 samples. This result is in good agreement with the static measurement of CO2 adsorption. Eventually, MOF-74(Ni) has higher CO2 adsorption capacity than MOF-74(Co) as we have seen the similar results on static CO2 adsorption measurement. The selectivity for CO2 gas relative to N2 gas can be defined by the following equation:

SCO2 =N2 ¼

xCO2 =yCO2 xN2 =yN2

ð13Þ

where, x and y represent the mole fractions in the adsorbed phase and in the bulk phase, respectively [76]. The excess CO2 adsorption capacity for the MOF-74(Ni)-Pd and MOF-74(Co)-pd at 298 K and ambient pressure were determined as 2.14 and 2.05 mmol g1, respectively, which are consistent with the static CO2 adsorption in similar condition. However, as seen from the Fig. 14, these values are somewhat less than the capacity obtained in static condition. It should be noted that in CO2 breakthrough experiment Pd modified MOF-74 samples also adsorbed higher amount of CO2 than as-synthesized samples. The CO2/N2 (0.2/0.8) selectivity for MOF-74(Ni)-Pd and MOF-74(Co)-Pd were calculated as 14.6 and 12.4, respectively, which are much higher than those experimental results reported for some MOFs [77]. The higher selectivity may be due to the unsaturated metal sites of MOF-74 samples and the strong affinity of Pd metals towards CO2. 4. Conclusions The MOF-74 (Co and Ni based) materials were synthesized and modified with Pd loaded AC successfully. All samples were characterized with SEM, TGA, powder XRD, FTIR, N2 sorption and pore size distribution including their fine structural properties by XANES/EXAFS. Finally, these materials were evaluated for CO2 adsorption and CO2/N2 separation. CO2 adsorption capacities of modified MOF-74(Ni)-Pd and MOF-74(Co)-Pd were12.24 and 11.42 mmol g1 respectively, measured at 298 K and 32 bar. The bond distances of NiO, and CoO in synthesized MOF-74(Ni) and MOF-74(Co) are 1.96 and 1.97 Å with the coordination numbers of 5.4 and 5.3, respectively. In summary, we have shown that incorporation of Pd containing AC in MOFs can enhance the uptakes of CO2 and CO2/N2 separation efficiencies. The Pd modified MOF-74 samples are capable to store higher amount of CO2 compared to the unmodified MOF-74 samples. These materials are capable to store significant amount of CO2 at relatively lower pressure and can completely separate CO2 from the mixture of CO2/N2 gases. The adsorption and separation of CO2 were facilitated due the interaction between partially negative charge oxygen atoms of polarized CO2 molecule and partially positive Pd metal due to its low electronegativity. The reported enhanced CO2 adsorption mechanism is interesting, and is worthy of further exploiting for obtaining novel adsorbents. The superior properties of modified

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MOF-74 materials such as; high CO2 adsorption capacity, high CO2 selectivity over N2 are interesting. These materials can be promising candidate for CO2 capture and separation of CO2/N2 in the cases of realistic situations. Acknowledgements The financial support of the Ministry of Science and Technology (MOST), Taiwan, R.O.C. (MOST 103-3113-E-008-001) is gratefully acknowledged. The authors are also grateful to Dr. Jyh-Fu Lee and Dr. Jeng-Lung Chen of Taiwan NSRRC for their helps on XANES/EXAFS measurement. References [1] Z. Chen, S. Deng, H. Wei, B. Wang, J. Huang, G. Yu, Polyethylenimineimpregnated resin for high CO2 adsorption: an efficient adsorbent for CO2 capture from simulated flue gas and ambient air, ACS Appl. Mater. Interfaces 5 (2013) 6937–6945. [2] C. Stewart, M.A. Hessami, A study of methods of carbon dioxide capture and sequestration-the sustainability of a photosynthetic bioreactor approach, Energy Convers. Manage. 46 (2005) 403–420. [3] X. Wang, L. Chen, Q. Guo, Development of hybrid amine-functionalized MCM41 sorbents for CO2 capture, Chem. Eng. J. 260 (2014) 573–581. [4] Y. Li, H. Yi, X. Tang, F. Li, Q. Yuan, Adsorption separation of CO2/CH4 gas mixture on the commercial zeolites at atmospheric pressure, Chem. Eng. J. 229 (2013) 50–56. [5] H. Deng, H. Yi, X. Tang, Q. Yu, P. Ning, L. Yang, Adsorption equilibrium for sulfur dioxide, nitric oxide, carbon dioxide, nitrogen on 13X and 5A zeolites, Chem. Eng. J. 188 (2012) 77–85. [6] L. Liu, D. Nicholson, S.K. Bhatia, Impact of H2O on CO2 separation from natural gas: comparison of carbon nanotubes and disordered carbon, J. Phys. Chem. C 119 (2015) 407–419. [7] Q. Liu, J. Shi, S. Zheng, M. Tao, Y. He, Y. Shi, Kinetics studies of CO2 adsorption/ desorption on amine-functionalized multiwalled carbon nanotubes, Ind. Eng. Chem. Res. 53 (2014) 11677–11683. [8] D.G.A. Smith, K. Patkowski, Benchmarking the CO2 adsorption energy on carbon nanotubes, J. Phys. Chem. C 119 (2015) 4934–4948. [9] Z. Zhang, M. Xu, H. Wang, Z. Li, Enhancement of CO2 adsorption on high surface area activated carbon modified by N2, H2 and ammonia, Chem. Eng. J. 160 (2010) 571–577. [10] C. Shen, C.A. Grande, P. Li, J. Yu, A.E. Rodrigues, Adsorption equilibria and kinetics of CO2 and N2 on activated carbon beads, Chem. Eng. J. 160 (2010) 398–407. [11] H. Yi, F. Li, P. Ning, X. Tang, J. Peng, Y. Li, H. Deng, Adsorption separation of CO2, CH4, and N2 on microwave activated carbon, Chem. Eng. J. 215–216 (2013) 635–642. [12] T. Ben, H. Ren, S. Ma, D. Cao, J. Lan, X. Jing, W. Wang, J. Xu, F. Deng, J.M. Simmons, S. Qiu, G. Zhu, Targeted synthesis of a porous aromatic framework with high stability and exceptionally high surface area, Angew. Chem. 121 (2009) 9621–9624. [13] B. Lukose, M. Wahiduzzaman, A. Kuc, T. Heine, Mechanical, electronic, and adsorption properties of porous aromatic frameworks, J. Phys. Chem. C 116 (2012) 22878–22884. [14] H. Ma, H. Ren, X. Zou, F. Sun, Z. Yan, K. Cai, D. Wang, G. Zhu, Novel lithiumloaded porous aromatic framework for efficient CO2 and H2 uptake, J. Mater. Chem. A 1 (2013) 752–758. [15] H. Furukawa, O.M. Yaghi, Storage of hydrogen, methane, and carbon dioxide in highly porous covalent organic frameworks for clean energy applications, J. Am. Chem. Soc. 131 (2009) 8875–8883. [16] Y.J. Choi, J.H. Choi, K.M. Choi, J.K. Kang, Covalent organic frameworks for extremely high reversible CO2 uptake capacity: a theoretical approach, J. Mater. Chem. 21 (2011) 1073–1078. [17] N. Huang, X. Chen, R. Krishna, D. Jiang, Two-dimensional covalent organic frameworks for carbon dioxide capture through channel-wall functionalization, Angew. Chem. Int. Ed. 54 (2015) 2986–2990. [18] F. Raganati, V. Gargiulo, P. Ammendola, M. Alfe, R. Chirone, CO2 capture performance of HKUST-1 in a sound assisted fluidized bed, Chem. Eng. J. 239 (2014) 75–86. [19] H.R. Abid, H. Tian, H.M. Ang, M.O. Tade, C.E. Buckley, S. Wang, Nanosize Zrmetal organic framework (UiO-66) for hydrogen and carbon dioxide storage, Chem. Eng. J. 187 (2012) 415–420. [20] K. Munusamy, G. Sethia, D.V. Patil, P.B. Somayajulu Rallapalli, R.S. Somani, H.C. Bajaj, Sorption of carbon dioxide, methane, nitrogen and carbon monoxide on MIL-101(Cr): volumetric measurements and dynamic adsorption studies, Chem. Eng. J. 195–196 (2012) 359–368. [21] A. Akgsornpeak, T. Witoon, T. Mungcharoen, J. Limtrakul, Development of synthetic CaO sorbents via CTAB-assisted sol-gel method for CO2 capture at high temperature, Chem. Eng. J. 237 (2014) 189–198. [22] Y. Wang, Y. Zhu, S. Wu, A new nano CaO-based CO2 adsorbent prepared using an adsorption phase technique, Chem. Eng. J. 218 (2013) 39–45.

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