H2O-Functionalized Zeolitic Zn(2-methylimidazole)2 Framework (ZIF-8

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HO-functionalized Zeolitic Zn(2methylimidazole) Framework (ZIF-8) for H Storage 2

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Peifu Cheng, and Yun Hang Hu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp507030g • Publication Date (Web): 28 Aug 2014 Downloaded from http://pubs.acs.org on September 2, 2014

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The Journal of Physical Chemistry

H2O-functionalized Zeolitic Zn(2-methylimidazole)2 Framework (ZIF-8) for H2 Storage

Peifu Cheng and Yun Hang Hu*

Department of Materials Science and Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931-1295, USA.

Abstract It is generally recognized that H2O adsorption on a porous material would inhibit H2 adsorption. However, this paper reports that stable H2O-functionalized Zeolitic Zn(2methylimidazole)2 Framework (ZIF-8), which was obtained by the simple water treatment of ZIF-8 at ambient temperature, can increase its H2 adsorption heat from 5.2 to 10.1 kJ/mol. As a result, the reversible H2 capacity at ambient temperature increased by 77%. This indicates that H2O-functionalization provides a novel approach to enhance ambient-temperature hydrogen storage in ZIF-8 and other metal-organic frameworks (MOFs). KEYWORDS: Metal-organic framework, ZIF-8, H2O, Functionalization, Hydrogen storage.

*To whom correspondence [email protected]

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1. Introduction The utilization of hydrogen as an energy source for fuel-cell powered vehicles is limited by inefficient hydrogen storage technologies.1-6 Liquefaction, compression, adsorption, and hydrogenation are being considered for hydrogen storage.1-14 However, a large energy consumption and continuous hydrogen boiling-off limit the application of liquefaction for hydrogen storage.2 An ultrahigh pressure up to 700 bar, which is required to compress enough hydrogen for a reasonable driving cycle of 300 miles, would cause safety issues related to a tank rupture in an accident.1 Therefore, current attention is focused on hydrogen storage in solid materials via one of the three ways: (1) chemical reaction, in which hydrogen can be stored by hydrogenation of solid materials, (2) adsorption of hydrogen on solid materials with large surface areas, and (3) capturing H2 into a cage.1, 3, 6-14 However, so far, those technologies have not reached the DOE targets for a practical on-board hydrogen storage in fuel cell vehicles. As a new type of porous materials, metal-organic frameworks (MOFs) are being explored for their potential applications.1, 6, 7, 15-29 The tunable pore structures and ultrahigh surface areas of MOFs constitute their suitability for gas separation and storage.1, 6, 7, 15-25

Zeolitic imidazolate frameworks(ZIFs), which are an interesting type of MOFs, are

constructed with tetrahedral units of one bivalent metal M2+ cations (usually Zn2+) and four imidazolate anions (Im-) analogous to {SiO2} tetrahedra in zeolites.18-25 As a representative ZIF, Zn(2-methylimidazole)2 (ZIF-8) has a sodalite zeolite-type structure with large pores (11.6 Å in diameter), which are connected by apertures (3.4 Å) large enough for H2 (with kinetic diameter of 2.9 Å) to penetrate into the large pores. Furthermore, ZIF-8 has excellent thermal and chemical stability.18 Yaghi et al. reported


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that the structure of ZIF-8 could remain in nitrogen atmosphere at a high temperature up to 550 °C and in boiling solvents.18 Furthermore, our previous work showed that ZIF-8 was stable in aqueous univalent-metal salt solution at room temperature.29 Although H2 storage in MOFs has been widely investigated, only limited effort was made to explore ZIF-8 for hydrogen adsorption.18,

26-53

Yaghi and co-workers

obtained H2 uptake of 3.1 wt% with ZIF-8 at 77 K and 55 bar.18 Zhou et al. examined H2 adsorption on ZIF-8, showing 3.3 wt% H2 capacity at 77 K and 30 bar and 0.13 wt% at ambient temperature and 60 bar.28 Goddard and co-workers employed grand canonical Monte Carlo (GCMC) simulation for hydrogen adsorption on ZIFs, predicting that H2 uptake of ZIF-8 is 0.64 wt% at 100 bar and 300K.51 Furthermore, hydrogen adsorption on some modified ZIFs were also evaluated.28, 31-47 The effect of water on the structures and properties of ZIFs was investigated by several groups.18, 54-58 Yaghi and co-workers demonstrated high chemical and thermal stability for 12 ZIFs.18 Furthermore, water physisorption on ZIF-8 and other MOFs was examined by Kaskel and his co-workers.54 They found that HKUST-1 exhibited the highest affinity for water adsorption, but it was unstable in liquid water. In contrast, ZIF8, which is hydrophobic, was stable in water. Chance and his co-workers revealed the excellent suitability of ZIF-8 and ZIF-71 for the extraction of butyl/propyl alcohols from water due to their hydrophobic nature.56 Additionally, molecular simulation showed that water molecules were less hydrogen-bound in ZIF-8 membrane than in the water bulk phase.55 However, so far, no researches have been reported to explore the effect of water on H2 storage in ZIFs. This encouraged us to investigate hydrogen storage in H2Ofunctionalized ZIF-8. In contrast to the general recognition that water adsorption on a 3 ACS Paragon Plus Environment


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solid surface would decrease H2 adsorption, this work demonstrates that reversible H2 adsorption capacity on ZIF-8 at ambient temperature can be highly enhanced by its H2Ofunctionalization.

2. Experimental Section 2.1 Water treatment of ZIF-8 ZIF-8 sample (from Sigma-Aldrich) was immersed in deionized water (with sonication for 40 min) in a closed bottle at ambient temperature for a selected time (1, 1.5, 2, or 3 days), followed by drying in atmosphere at ambient temperature. The as-obtained samples were denoted as ZIF-8-1d, ZIF-8-1.5d, ZIF-8-2d, and ZIF-8-3d (for H2Otreatments of 1, 1.5, 2, and 3 days, respectively).

2.2 H2 adsorption measurements H2 adsorption on ZIF-8 samples with and without water treatment was measured by volumetric method at a selected temperature (77 K, ambient temperature, or 323 K) as follows (Figure 1): 200 mg of ZIF-8 sample were loaded into a reactor and degassed under vacuum at 100 oC for 12 h. Then, H2 was introduced into the reactor for adsorption at a selected temperature. The H2 pressure change during adsorption was measured with a digital pressure gauge (Accu-Cal Plus 75514-29B55 with a precision of 1 Torr), generating the adsorption isotherm of pressure vs. amount of adsorbed H2. After the adsorption, H2 desorption was carried out by decreasing pressure.

2.3 Powder X-ray diffraction (XRD) 4 ACS Paragon Plus Environment

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The crystal structures of ZIF-8 samples with and without water-treatment were measured by X-ray diffraction with a Scintag XDS2000 Powder Diffractometer at 45 kV and 35 mA for Cu Kα (λ = 1.54062 Å) radiation at a scan speed of 1 °/min and a step size of 0.03 ° in 2θ.

2.4 Surface area Micromeritics ASAP 2000 sorptometer was exploited to measure the surface areas of ZIF-8 and H2O-treated ZIF-8 samples by nitrogen adsorption at liquid-nitrogen temperature (77 K). The samples were degassed at 100 oC for 12 hours before the surface measurement. The surface areas were calculated by Brunauer-Emmett-Teller (BET) model.59

2.5 Fourier transform infrared spectroscopy (FT-IR) FT-IR spectra were obtained by using a Shimadzu Corporation’s IRAffinity-1 spectrometer combined with a Pike Technologies’ DiffusIR (diffuse reflectance accessory) in-situ cell. Before the IR measurement, the sample in the in-situ cell was degassed by a diffusion pump at 100 oC for 12h.

3. Calculation methods The B3LYP is one of the most popular hybrid density functional theory methods.60 In this research, the B3LYP hybrid DFT method with orbital basis set 6-31G(d) 5 ACS Paragon Plus Environment


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was selected for full geometry optimization, energy calculation, and prediction of the harmonic vibrational frequencies. The cluster model of ZIF-8 for the calculations consists of a 2-Methylimidazole and 2 Zn saturated with H atoms (Figure 2). All calculations were carried out with the Gaussian 03 program.61 Furthermore, the vibrational modes were assigned by means of visual inspection using the GAUSSVIEW program, in which all vibrations due to saturating H atoms were excluded.

4. Results and Discussion 4.1 Structure change and functionalization of ZIF-8 by H2O treatment XRD measurements were exploited to evaluate the effect of water on the crystal structure of ZIF-8. As shown in Figure 3, the sample without water treatment exhibited a typical cubic crystal structure of ZIF-8, in which all diffraction peaks are consistent with those reported by Yaghi et al.18 However, after water treatment at room temperature, some new diffraction peaks were observed at 2θ of 11.5°, 17.2°, 19.4°, and 24.4°, which were also showed by Kaskel et al.54 and Matzger et al.57 Furthermore, as the treatment time increased, the intensities of those new peaks increased, whereas the original diffraction peaks of ZIF-8 decreased. This indicates that the water treatment at room temperature caused a gradual change in ZIF-8 crystal structure, leading to some defects. This was supported by BET surface area measurements, namely, the surface area of ZIF8 decreased from 1504 to 961 m2/g with increasing water treatment time from 0 to 3 days (Figure 4). IR spectrum of ZIF-8 was evaluated by both DFT calculation and experimental measurement. Although a simple cluster model was used for the B3LYP hybrid DFT


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calculation, the predicted IR bands are consistent with those obtained from experimental FTIR measurement (note: Overtones in range of 1700 to 2800cm-1 should not be accounted) (Figure 5). This allowed us to correlate the IR bands of ZIF-8 with its structure and to summarize them in Table 1. Furthermore, IR bands at 995 cm-1 (C=C-N twisting), 1146 cm-1 (=C-H bending and C-N bending), 2932-2957cm-1 (C-H stretching of CH3), and 3134 cm-1 (=C-H asymmetric stretching) were employed to examine the structures of ZIF-8 samples with and without water treatment (Figure 6). All those feature IR bands decreased with increasing water treatment time, indicating the partial damage of ZIF-8 structure due to water treatment. This supports the conclusions from XRD and BET results. It has been demonstrated that the bond between an inorganic cluster and an organic ligand in a MOF molecule is the weakest.7, 15-17 When ZIF-8 was treated by H2O, some of Zn-N bonds between its inorganic cluster (Zn) and organic ligand (2Methylimidazole) would be destroyed, leading to unsaturated Zn-sites and N-sites. As a result, those unsaturated sites would be re-saturated by H2O molecules, generating H2Ofunctionalized ZIF-8 molecules. To confirm this, DFT calculations were carried out for H2O interaction with the unsaturated N-site and Zn-site of the defected ZIF-8. The DFToptimized geometry showed that H2O was bound to the unsaturated N-site of ZIF-8 via its hydrogen atom with binding energy of 31.7 kJ/mol (Figure 7). In contrast, the oxygen atom of H2O was bound to the unsaturated Zn-site of ZIF-8 with binding energy of 80.5 kJ/mol (Figure 7). This indicates that the bonds of H2O to unsaturated Zn and N sites are very strong, leading to H2O-functionalized ZIF-8. This was further supported by FTIR measurements. The subtraction FTIR spectra were recorded for ZIF-8 samples, which



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were previously treated by H2O at room temperature for a selected time and then vacuumed at 100 oC (Figure 8). The IR bands of H2O bending vibrations were observed at 1605 cm-1. This clearly indicates the presence of H2O functional group that cannot be desorbed at 100 oC. The content of the H2O functional group, which was calculated from the FTIR spectra, is 6.8, 11.7, 12.5, and 13.1 wt% for ZIF-8 samples treated by water at room temperature for 1, 1.5, 2, and 3 days, respectively. This indicates that H2O functional groups increased with increasing H2O treatment time. Therefore, the H2O treatment at room temperature provides a simple and effective approach to prepare H2Ofunctionalized ZIF-8, which is stable even at temperature of 100 oC.

4.2 Hydrogen storage in H2O-functionalized ZIF-8 Hydrogen storage in ZIF-8 with and without H2O treatment was evaluated. As shown in Figure 9, hydrogen adsorption on pristine ZIF-8 increased with increasing pressure to reach the maximum capacity of 3.1 wt% at 20 bar and then remained unchanged. Furthermore, the desorption curve coincides with adsorption one, indicating the excellent reversibility of H2 adsorption on ZIF-8. Those are in good agreement with the results reported by Yaghi et al.18 and Zhou et al.28 However, the adsorption capacity of H2 on ZIF-8 decreased with increasing water treatment time (Figure 9). This happened because H2 adsorption at 77 K is mainly depended on the BET surface area of an adsorbent.7 In contrast to H2 adsorption at 77 K, H2 adsorption capacity at room temperature was lower on pristine ZIF-8 than on H2O-treated ZIF-8 (Figure 10). The H2 capacity on ZIF-8 without H2O-treatment was around 0.13 wt% at 50 bar, which was in good


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agreement with the result (0.13 wt% at room temperature and 60 bar) reported by Zhou et al.28 The H2 capacity was increased by 77% to 0.23 wt% at 50 bar after ZIF-8 was pretreated with water for 1.5 days (Figure 10). Furthermore, the H2 capacity on ZIF-8 at ambient temperature increased and then decreased with increasing H2O-treatment time. The similar trend was also observed for H2 adsorption at 323 K (Figure 11). It is well-known that H2 adsorption at 77 K is essentially dependent on BET surface area, whereas H2 adsorption at ambient temperature or higher is determined by both BET surface area and H2 adsorption strength.7,

62-64

As shown above, the H2O-

treatment created stable H2O functional groups on ZIF-8. Furthermore, our DFT calculations showed that the H2 could be adsorbed on H2O bound to unsaturated N-site of ZIF-8 (Figure 12b), leading to adsorption energy of 7.3 kJ/mol, which is larger than that (1.2 kJ/mol) for H2 adsorption on pristine ZIF-8 without H2O-functional groups (Figure 12a). This demonstrates that the H2 adsorption is much stronger on H2O-functionalized ZIF-8 than on pristine one. This happened because polar coordinated-H2O molecules on the defective ZIF-8 induced the polarization of H2 molecules, leading to a stronger interaction between the dipole of H2O and the induced-dipole of H2. This was further supported by the adsorption heats that were obtained from isotherms with ClausiusClapeyron equation. At zero loading, the adsorption heat for pristine ZIF-8 was 5.2 kJ/mol, while the maximum adsorption heat of H2 on H2O-functionalized ZIF-8 was 10.1 kJ/mol. Therefore, the increase of H2 capacity on ZIF-8 with increasing H2Opretreatment time would be due to the enhancement of H2 adsorption energy, whereas the further increase of H2O-pretreatment time caused the tremendous reduction of ZIF-8 surface area and thus a decrease of H2 capacity.



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Finally, we found that isotherms of H2 on ZIF-8 (with and without H2O functional groups) at ambient temperature could be fitted well with Freundlich model (Eq.1):

V = KP1/ n

(1)

where, V was the weight of adsorbed H2 per unit weight of adsorbent, and K and n were Freundlich model constants. Those constants were determined and summarized in Table 2.

5. Conclusion In conclusion, H2O-treatment at ambient temperature provides a simple and effective approach to prepare H2O-functionalized ZIF-8, which is stable even at 100 oC. Furthermore, the H2O-functionalization can increase the adsorption heat of H2 on ZIF-8 from 5.2 to 10.1 kJ/mol. As a result, the reversible H2 capacity at ambient temperature increased by 77% from 0.13 to 0.23 wt%. Therefore, H2O-functionalization provides a novel approach to increase reversible ambient-temperature hydrogen storage capacity for ZIF-8 and other MOFs.

Acknowledgement This work was supported by the U.S. National Science Foundation (NSF-CBET0929207).


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(43) Chen, E. Y.; Liu, Y. C.; Sun, T. Y.; Wang, Q.; Liang, L. J. Effects of Substituent Groups and Central Metal Ion on Hydrogen Adsorption in Zeolitic Imidazolate Frameworks. Chem. Eng. Sci. 2013, 97, 60-66. (44) Yang, T.; Chung, T. S. High Performance ZIF-8/PBI Nano-Composite Membranes for High Temperature Hydrogen Separation Consisting of Carbon Monoxide and Water Vapor. Int J Hydrogen Energy 2013, 38, 229-239. (45) Chen, H.; Wang, L.; Yang, J.; Yang, R. T. Investigation on Hydrogenation of Metal-Organic Frameworks HKUST-1, MIL-53, and ZIF‑8 by Hydrogen Spillover. J. Phys. Chem. C 2013, 117, 7565-7576. (46) Fischer, M.; Bell, R. G. Interaction of Hydrogen and Carbon Dioxide with Sod-Type Zeolitic Imidazolate Frameworks: A Periodic DFT-D Study. CrystEngComm. 2014, 16, 1934-1949. (47) Cacho-Bailo, F.; Seoane, B.; Téllez, C.; Coronas, J. ZIF-8 Continuous Membrane on Porous Polysulfone for Hydrogen Separation. J. Membr. Sci. 2014, 464, 119-126. (48) Liu, Y.; Liu, H.; Hu, Y.; Jiang, J. Development of a Density Functional Theory in ThreeDimensional Nanoconfined Space: H2 Storage in Metal-Organic Frameworks. J. Phys. Chem. B 2009, 113,12326-12331. (49) Fischer, M.; Hoffmann, F.; Fröba, M. Preferred Hydrogen Adsorption Sites in Various MOFs-A Comparative Computational Study. ChemPhysChem 2009, 10, 2647-2657. (50) Rankin, R. B.; Liu, J.; Kulkarni, A. D.; Johnson, J. K. Adsorption and Diffusion of Light Gases in ZIF-68 and ZIF-70: A Simulation Study. J. Phys. Chem. C 2009, 113, 16906-16914. (51) Han, S. S.; Choi, S. H.; Goddard, W. A. Zeolitic Imidazolate Frameworks as H2 Adsorbents: Ab Initio Based Grand Canonical Monte Carlo Simulation. J. Phys. Chem. C 2010, 114, 1203912047. (52) Assfour, B.; Leoni, S.; Yurchenko, S.; Seifert, G. Hydrogen Storage in Zeolite Imidazolate Frameworks. A Multiscale Theoretical Investigation. Int J Hydrogen Energy 2011, 36, 6005-6013.



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Table 1. FT-IR Bands of ZIF-8 from Experimental Measurement and B3LYP/631G(d) DFT Calculation Experiment Frequency (cm-1) 685 , 694 758 953, 995

DFT calculation Frequency (cm-1) 665 , 681 739 920 982, 1034 1135 1015, 1038, 1090, 1377 1310, 1383, 1454 1397 1435, 1450 1471 1019, 1113 1146, 1180 1157, 1246 1512, 1584 1507 2932 2920 2957, 2999 2976, 2999 3134 3164

Assignment Out of plane bending of organic ring =C-H wagging (out of plane bending) C=C-N twisting (out of plane bending) C-H twisting (out of plane bending) of CH3 C-H scissoring (in plane bending) C-H bending of CH3 C-H bending of CH3 and organic ring C-H bending of CH3 and C-C stretching C-H twisting (out of plane bending) =C-H bending and C-N bending =C-H bending and C-N stretching =C-H bending and C=C stretching C-H symmetric stretching of CH3 C-H asymmetric stretching of CH3 =C-H asymmetric stretching

Table 2. Parameters of Freundlich Model for H2 Adsorption on H2O-Functionalized ZIF-8 at 298 K and 323 K Adsorbent ZIF-8 ZIF-8-1d ZIF-8-1.5d ZIF-8-2d ZIF-8-3d

298 K K 0.003 0.005 0.006 0.005 0.004

n 1.050 1.043 1.040 1.027 1.010

323 K K 0.003 0.004 0.004 0.004 0.003


n 1.049 1.042 1.039 1.026 1.010

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Figure 1. Schematic diagram of volumetric measurement system (a: cylinder of H2 gas, b: regulator, c: valve, d: digital pressure gauge, e: furnace, f: mechanical pump, g: sample, h: H2 reservoir container).

Figure 2. Cluster model of ZIF-8 for DFT calculations.



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Figure 3. XRD patterns of ZIF-8 and H2O-treated ZIF-8 (Note: ZIF-8-1d, ZIF-8-1.5d, ZIF-8-2d, and ZIF-8-3d are ZIF-8 samples treated with H2O for 1, 1.5, 2 and 3 days at ambient temperature, respectively).


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Figure 4. BET surface areas of ZIF-8 and H2O-treated ZIF-8.



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Figure 5. FT-IR spectrum of ZIF-8 (red for DFT simulated IR and black for experimental IR).


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(a)

(b)

(c)

(d)

Figure 6. FT-IR spectra of ZIF-8 and H2O-treated ZIF-8.



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Figure 7. Binding of H2O to unsaturated sites of defected ZIF-8 (a) H2O to N-site and (b) H2O to Zn-site.


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Figure 8. FT-IR subtraction spectra of H2O-treated ZIF-8 (Note: Before recording FT-IR spectra at ambient temperature, H2O-treated ZIF-8 was vacuumed at 100 oC for 12h. The FT-IR subtraction spectra were obtained by subtracting the IR spectrum of pristine ZIF-8 from those of H2O-treated ZIF-8).



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Figure 9. H2 adsorption and desorption on ZIF-8 and H2O-treated ZIF-8 at 77 K (closed and open symbols represent adsorption and desorption, respectively).


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Figure 10. H2 adsorption and desorption on ZIF-8 and H2O-treated ZIF-8 at 298K (closed symbols, open symbols, and lines represent adsorption, desorption, and Freundlich fitting lines, respectively).



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Figure 11. H2 adsorption and desorption on ZIF-8 and H2O-treated ZIF-8 at 323 K (closed symbols, open symbols, and lines represent adsorption, desorption, and Freundlich fitting lines, respectively).


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Figure 12. Adsorption of H2 on (a) ZIF-8 and (b) H2O-functionalized ZIF-8.



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H2O-Functionalized Zeolitic Zn(2-methylimidazole)2 Framework (ZIF-8

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