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Selective H2S/CO2 Separation by Metal−Organic Frameworks Based on Chemical-Physical Adsorption Jia Liu,†,‡ Yajuan Wei,‡ Peizhou Li,‡ Yanli Zhao,*,‡ and Ruqiang Zou*,† †

Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371

‡

S Supporting Information *

ABSTRACT: The removal of hydrogen sulfide (H2S) is essential in various industry applications such as purification of syngas for avoiding its corrosion and toxicity to catalysts. The design of adsorbents that can bear corrosion of H2S and overcome the competitive adsorption from carbon dioxide (CO2) is a challenge. To obtain insight into the stability and adsorption mechanism of metal−organic frameworks (MOFs) during the H2S separation process, 11 MOF-based materials were employed for H2S capture from CO2. Density functional theory, molecular dynamic studies, and dynamic separation experiments were used to investigate selective H2S/CO2 separation. Most of these MOFs showed one-off high capacity and selectivity to H2S. Complete reversible physical adsorption was proven on Mg-MOF-74, MIL-101(Cr), UiO-66, ZIF-8, and Ce-BTC. Incomplete reversible adsorption occurred on UiO-66(NH2). Disposable chemical reaction happened on HKUST-1, Cu-BDC(ted)0.5, Zn-MOF-74, MIL-100(Fe) gel, and MOF-5. Using breakthrough experiments, UiO-66, Mg-MOF-74, and MIL-101(Cr) were screened out to present promising performance on the H2S capture. The present study is useful to identify and design suitable MOF materials for high-performance H2S capture and separation.

1. INTRODUCTION To pave the way for downstream applications, capturing hydrogen sulfide (H2S) from mixture gases such as syngas is essential on account of its high toxicity and corrosion. Syngas is an important source of many chemical products, consisting a mixture of hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), and trace H2S.1 The removal of the H2S impurity is always an indispensable process at the first step, in which the adsorption separation is an ideal method with low energy cost in comparison with current chemical adsorption technique via the reaction with metal oxides at high temperature.2 Although another valid strategy to adsorb H2S in organic amine or amine modified porous materials3,4 is a simple and alternative method, the low selectivity to CO2 results in a simultaneous saturation sorption of both CO2 and H2S. The same consequence occurs on porous adsorbents such as molecular sieve,5 silica gel, and active carbon.6 Since both CO2 and H2S are electron acceptors as Lewis acids, they have the similar effect with amine. Therefore, simple acid−base reactions or physical adsorption cannot gratify demands for highly efficient separation to CO2/H2S. The key solution is to employ selective sorption for H2S. The principal quantum number of sulfur atom is 3, which is larger than that of carbon. The energy level of p orbital on sulfur is approachable by the d orbital of transition metal. Strong covalent bonds with feedback p−d π bonds could be formed by filling p electron of sulfur to d orbital of metal, while © 2017 American Chemical Society

no bond could be formed between CO2 and metal. Hence, transition metal can be the best choice for selective adsorption of H2S. In this regard, metal−organic frameworks (MOFs) incorporating metal cation/cluster and organic linker could bear such responsibility, resulting from their many promising advantages of high surface area, tunable porous structure, and unique pore environment.7−17 In the past, H2S adsorption on several MOFs has been reported by experimental and simulation methods but with controversial results.18−28 Most results are considered as physical adsorption such as those with HKUST-1,21 MIL-53(Cr, Al),22,29,30 and MIL-47(V),1 while MIL-100(Cr),29 MIL-101(Cr),29 MIL-53(Fe),29 IRMOF-1,31 and IRMOF-331 are considered chemisorption with partial destruction. No further studies have been performed to explain and confirm these observations. To be applied in CO2/H2S separation, the adsorption mechanism of H2S must be investigated clearly. Only feedback p−d π bond can avoid the competition from CO2 and meet the requirement of residual H2S adsorption. Thus, can we find MOFs suitable for highly efficient CO2/H2S separation? In this work, 11 MOFs including MOF-5, MOF-74 (Mg, Zn), ZIF-8, UiO-66, UiO-66(NH2), MIL-101(Cr), M-BTC(Cu, Fe, Ce), and Cu-BDC(ted)0.5 with various metal sites, ligands, surface areas, and porous structures Received: May 10, 2017 Revised: June 2, 2017 Published: June 5, 2017 13249

DOI: 10.1021/acs.jpcc.7b04465 J. Phys. Chem. C 2017, 121, 13249−13255

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The Journal of Physical Chemistry C

24 h. After the solution was cooled down to room temperature, the solid was filtered. The particles obtained were denoted as “as-synthesized”. The precipitates were immersed in DMF (20 mL) for 5 h, then centrifuged and washed with ethanol (10 mL). After this procedure, the desired materials were obtained.14 For MOF-74(Zn, Mg), a mixture of 2,5-dihydroxy-1,4benzenedicarboxylic acid (H2-DHBDC, 0.096 mmol) and (Zn,Mg) (NO3)2·4(H2O) (0.203 mmol) was dissolved in a solvent consisting of DMF (2.0 mL), 2-propanol (0.1 mL), and water (0.1 mL) and then placed in a Pyrex tube frozen in a liquid nitrogen bath. After being evacuated and flame-sealed, it was heated to 105 °C for 20 h. After cooling down to room temperature, yellow crystals were produced, which were dried in air after washing with DMF and ethanol.15 MIL-100(Fe) gel was synthesized at room temperature. Fe(NO3)3·6H2O and H3BTC were dissolved in ethanol (30 mL) at room temperature, respectively. After mixing the solution, the system was stirred quickly until the gel solid was observed. After keeping it at room temperature for 1 h and drying by CO2 drying process, the product was obtained.16 Ce-BTC was synthesized by dissolving Ce(NO3)3·6H2O (3.677 g, 9.6 mmol) and 1,3,5-benzenetricarboxylic acid (1.681 g, 8.0 mmol) in a mixed solvent of DMF (50 mL) and deionized water (10 mL) in a volumetric flask at room temperature. The mixture was stirred by adding 8 drops of triethylamine and 3 drops of nitric acid successively. The resulting mixture was sealed and placed in an oven at 100 °C for 17 h. After the solution was cooled down to room temperature, the resulting solid was filtered and repeatedly washed with absolute ethanol three times. The white powder was filtered and then dried under vacuum at ambient temperature.17 DFT with the Perdew−Burke−Ernzerhof (PBE)32 functional was employed. The projector augmented wave method (PAW)33 was used to describe the interaction between the ions and the electrons with the frozen-core approximation. The kinetic energy cut off was set to 300 eV. The models for partial density of state (PDOS) calculation were periodic with a (1 × 1 × 1) unit cell except MIL-101 and MIL-100(Fe). Trimetric metal (Fe(III) and Cr(III)) octahedral clusters were used as the representation for PDOS calculation. MD simulations were used to simulate the H2S adsorption on Mg-MOF-74, Mil101(Cr), and UiO-66. DFT-D method was used to describe the van der Waals interaction. A plane-wave basis kinetic energy cutoff of 310 eV and a convergence criterion of 10−5 eV were used in all the calculations. In the simulations, the MOF structures were assumed to be rigid, because the experimental results showed that there was no change before and after the adsorption of H2S. In each simulation, the atom position was taken from reported MIL-101(Cr) and UiO-66. The box consisting of 1 × 1 × 1 unit cell represents UiO-66, and the 1 × 1 × 1 unit cell represents MIL-101(Cr).

were investigated by breakthrough experiments and density functional theory (DFT) study for H2S adsorption and separation, aiming at an achievement of pure syngas.

2. EXPERIMENTAL SECTION All of MOFs were synthesized and activated according to reported methods. N2 isotherm curves of 11 samples were collected by surface area analyzer micromeritics (ASiQwin 2.0) purchased from Quantachrome Instrument (USA) for pore size distribution and Brumauer−Emmett−Teller (BET) surface area calculations. Powder X-ray diffraction (PXRD) patterns of all the samples were collected on Rigaku D/max 2250 VB/ PC diffract meter (Rigaku, Japan) with Cu Kα radiation (λ = 1.5406 Å) before and after adsorbing H2S for comparison. The mixed gas for breakthrough experiments was composed of 1% H2S, 10% CO2, and 89% He in volumetric ratios. All the breakthrough curves for CO2/H2S sorption were recorded by a homemade instrument illustrated in Scheme S1. H2S uptake and selectivity were calculated from the breakthrough curves as described in the Supporting Information. Briefly, the synthesis of MIL-101 was conducted by the hydrothermal reaction of H2BDC (166 mg at 1 mmol) with Cr(NO3)3·9H2O (400 mg at 1 mmol), fluorhydric acid (0.2 mL at 1 mmol), and H2O (4.8 mL at 265 mmol) at 220 °C for 8 h.9 For MOF-5, Zn(NO3)2·6H2O (7.18 g) and H2BDC (1.33 g) were dissolved into DMF solution (200 mL). The mixture was reacted under 60 °C for 3 days. After filtration, the obtained solution was added with hexadecyl trimethylammonium bromide (CTAB, 10 g) under stirring for 30 min. Adding triethylamine (7.5 mL) into the mixture yielded a colorless fine powder that was formulated as Zn(BDC)(DMF)(H2O).10 Cu-BTC (HKUST-1) was prepared by a solvothermal method. Benzene-1,3,5-tricarboxylic acid (5.0 g, 24 mmol) and copper(II) nitrate hemipentahydrate (10.0 g, 43 mmol) were stirred in solvent (250 mL) consisting of DMF, ethanol, and deionized water (1:1:1) in a 1 L glass jar for 15 min. The jar was tightly capped and placed in oven at 85 °C for 20 h. After rinsing with DMF, the product was soaked in dichloromethane for 3 days with replenishing three times. The product was obtained after the removal of solvent under vacuum at 170 °C for 8 h.11 ZIF-8 was synthesized in a pure aqueous system. First, Zn(NO3)2·6H2O (1.17 g) was dissolved in deionized water (8 g). Second, 2-methylimidazole (22.7 g) was dissolved in another aliquot of deionized water (80 g). The two solutions were mixed under stirring at room temperature. The synthesis solution turned milky almost instantly after the mixing. After stirring for 5 min, the product was collected by centrifugation and then washed with deionized water several times. The product was dried at 65 °C in vacuum overnight.12 For [Cu-BDC(ted)0.5]·2DMF, a mixture of copper(II) nitrate trihydrate (740 mg), terephthalic acid (680 mg), and triethylenediamine (480 mg) in DMF (150 mL) was heated at 100 °C for 36 h. The blue powder was obtained after filtration, rinsed with DMF, and dried under vacuum.13 To synthesize UiO-66 and UiO66-NH2, ZrCl4 (0.08 g, 0.343 mmol for UiO66; 0.062 g, 0.343 mmol for UiO66-NH2) and 30 equiv of modulator (0.588 mL, 10.29 mmol of acetic acid) were dissolved in DMF (20 mL) and treated with ultrasound for 10 min. The organic linker was added to the clear solution in an equimolar ratio with regard to ZrCl4 and dispersed by ultrasound for about 10 min until completely dissolved. The resulting mixture was placed in a preheated oven at 120 °C for

3. RESULTS AND DISCUSSION According to the principle of whether a chemical reaction occurs during the adsorption process, chemical adsorption or physical adsorption can be distinguished. In physical adsorption, the adsorptive interaction is mainly attributed by noncovalent interactions, which is highly replated to the distance between adsorbents and adsorbates. When the electrons are obviously transferred between adsorbents and 13250

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Mg. The formation of ion bond rather than coordinative bond through electron overlapping by Mg and S atoms is preferred, since there is a large gap between p orbitals of Mg and S. In contrast, the oxygen atom with stronger electronegativity limits the Mg−S bond formation. Therefore, Mg-MOF-74 presents a good stability for H2S. The peak of pMg PDOS appeared at Fermi level near zero, implying that electrons from psulfur orbital can shift to pMg orbital to form weak intermolecular interaction. By molecular dynamic (MD) simulation, sulfur atom orients to Mg atom with an SH2S−Mg distance of about 2.8 A (Figure 2a), indicating a physical adsorption, which matches well with the experimental results.

adsorbates durign the adsorption process, it is considered to be chemical adsorption. The N2 adsorption isotherms of these MOFs were first measured (Figures S1−S11). The saturated H2S uptake over 11 MOF materials was obtained from breakthrough experiments at 298 K under 1 atm with H2S partial pressure of 1000 Pa (Figures S12−S23). As shown in Figure 1, most MOFs showed

Figure 1. H2S uptake on 11 MOF materials at 298 K under 1 atm with H2S partial pressure of 1000 Pa. The filled column refers to the uptake on fresh MOF adsorbents, and the striped column indicates that on the refreshed adsorbents. Figure 2. H2S adsorption sites on various MOFs. (a) Mg-MOF-74; (b) MIL-101(Cr); (c) UiO-66; (d) Cu-BDC(ted)0.5; (e) ZIF-8; (f) Ce-BTC.

high uptake of H2S at the first cycle except ZIF-8 and Ce-BTC. While at the second cycle, most of them lost part of their capacity for H2S except Mg-MOF-74, MIL-101(Cr), UIO-66, and UIO-66-NH2. Experimental results indicate that Mg-MOF74 (SBET = 1244 m2/g) is more stable when exposed to H2S, on which H2S adsorption is reversible with the uptake as high as 0.24 mmol/g (Figure S12). PXRD patterns of these MOFs before and after H2S adsorption were also measured (Figures S24−S35). PXRD patterns show that there was no obvious peak change before and after the H2S adsorption by Mg-MOF74 (Figure S25), implying its crystal structure integrity upon exposure to H2S. After regeneration at 200 °C, the H2S uptake of Mg-MOF-74 can remain at a high level under H2S partial pressure of 1000 Pa. The regeneration temperature we chose is based on two reasons. First, it is generally sufficient to disperse the physically adsorbed gas at 200 °C. For most of the MOF materials, the outgas temperature is below 200 °C when carrying out surface area measurement experiments. Higher temperature may lead to the structure collapse of MOFs. Second, when H2S molecule is chemically adsorbed or chemically reacted with MOFs, higher temperature could promote the reactions, finally leading to structure changes of MOF materials. With increasing the temperature from 25 to 100 °C, the H2S uptake decreased obviously (Figure S13). All above phenomena agree with physical adsorptive mechanism. The stability of Mg-MOF-74 for H2S is attributed to strong Mg−Ocarboxylate bond and large gap between S and Mg atoms. The Ohydroxyl−Mg−Ocarboxylate bond in Mg-MOF-74 is formed by sp−p overlap, which is stronger than that formed by sp−d overlap in transition metal. A large overlap between sp orbital of O and p orbital of Mg, as well as low p orbital energy level, were observed (Figure S36b), and obvious electron transfer from pMg to poxygen atoms appeared in PDOS, indicating the strong Ohydroxyl−Mg−Ocarboxylate bond. Moreover, the energy of p orbital in sulfur atom is much higher than that of p orbital of

Octahedral coordinated Cr(III) atoms in MIL-101(Cr) (SBET = 3203 m2/g) are d2sp3 hybridized. The d orbital is divided into 2-fold degenerate dε orbital and triplet degenerate dγ orbital. Above Fermi level, d orbital can accept electrons from H2S, leading to a strong interaction between H2S and MIL-101(Cr) and reversible high H2S uptake of approaching 0.4 mmol/g. Both regeneration experiments (Figures S14 and S15) and PXRD patterns (Figure S26) prove that none of structure exchange happened when exposing MIL-101(Cr) to H2S. There are two reasons. First, the Cr−O bond is very strong on account of low d orbital energy level of Cr and large electron transfer from Cr to O as shown in Figure S36c. Second, the LUMO energy is so high as to allow elections to completely transfer from S 2p orbital to Cr 3d orbital, leading to nonoxidation reduction reaction. By the MD simulation, p orbital electrons of sulfur tend to approach unsaturated Cr atom. H2S prefers to adsorb at open Cr sites with a S−Cr distance of 2.58 Å, implying a strong physical binding interaction (Figure 2b). Another MOF available for reversible adsorption of H2S is UiO-66 (SBET = 1322 m2/g), which was proven by PXRD patterns (Figure S27) and cyclic experiments (Figure S16). The stability of UiO-66 for H2S is attributed to strong Zr−O bonds. A large overlap of p−d PDOS and an obvious electron transfer from dZr to poxygen were observed in Figure S36d, indicating the presence of strong Zr−O bonds. Octuple-coordinated O atom also has strong shield effect to H2S, leading to moderate uptake of H2S (0.234 mmol/g). By simulation, H2S adsorptive sites occurred at the center of tetrahedron cage constructed by aromatic fragments rather than near Zr metal ion (Figure 2c). In UiO-66, the main molecular force interacting with H2S 13251

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The Journal of Physical Chemistry C comes from the potential field constructed by the sp2 carbon of aromatic fragments. Zr atom was fully bonded with oxygen atoms, which has strong repulsive force to electronegative H2S molecule to prevent the adsorption of nearby Zr atom. The H2S uptake by UiO-66-NH2 (SBET = 1097 m2/g) was also investigated, looking at enhancing the reaction with NH2 group (Figure S17). The PXRD patterns (Figure S28) did not have an obvious change after the H2S adsorption. However, it was found that the H2S uptake of 0.909 mmol g−1 was obtained in the first cycle and then reduced to 0.38 mmol g−1 after regeneration. The reduced uptake of H2S is most possibly caused by the inactivation of NH2 group. NH2 group can react with H2S to form NH3HS, which is difficult to decompose even with heating of 200 °C. Compared with as-synthesized UiO-66NH 2 , the surface area of the regenerated UiO-66-NH2 decreased a little due to binding with H2S (Figure S4). Therefore, the H2S uptake contributed by NH2 is not reversible, and the uptake contributed from adsorptive sites at the center of tetrahedron cage is recyclable. It is interesting that the CO2 uptake of UiO-66-NH2 was not evidently reduced after exposure to H2S. It was well-known that the CO2 uptake of UiO-66-NH2 is higher than that of UiO-66, which is also attributed to the NH2 group. After the adsorption of H2S, NH2 group connected with H2S should loose the impaction with CO2. The effect to CO2 may come from the S atom. Connected with NH2, lone pairs on p orbital of sulfur atom can form p−p π bonds with overlapping electron clouds of p orbital in carbon atom. The other two MOFs that can avoid corrosion by H2S are ZIF-8 and Ce-BTC. However, their H 2 S uptake was considerably low. Zeolitic porous ZIF-8 composed of zinc and 2-methylimidazolate with surface area of 1602 m2/g shows saturated four-coordinated Zn−N bonds. Its H2S uptake of 0.05 mmol g−1 (Figure S18) is lower than most of other samples due to a lack of open active sites, in which the tetrahedralcoordinated Zn atoms are fully bonded with N atoms. In the region of the Zn−N cluster, the Zn−N cluster with high symmetry does not have dipole and quadrupole. Only weak Coulomb force of Zn−N clusters with H2S molecule is produced just by the dispersion force and octupole moment. During the experiment, no obvious changes of PXRD patterns (Figure S29) were observed on ZIF-8 after exposure to H2S, implying the structural integrity. By the PDOS calculation, d orbital of tetrahedral-coordinated Zn divides into 2-fold degenerate dε orbital and triplet degenerate dγ orbital. A large overlap of electrons between sp2 orbital of N atom and dε of Zn comprising dx2y2 and dz2 was observed in PDOS of ZIF8, which indicates that a strong Zn−N bond was formed in ZIF-8. Compared with the other two PDOS as shown in Figure S36e, the Zn−N bond of ZIF-8 is much stronger than those of Zn−O bonds of MOF-5 or Zn-MOF-74. Except for strong Zn−N bonds, there are still two reasons leading to the H2S adsorption by ZIF-8. First, Zn adopts tetrahedral coordination resulting in splitting energy of d orbital, which is not as large as octahedral coordination. The dγ orbital energy level is lower than that in octahedral coordination. Therefore, electrons of atom SH2S cannot overlap or transit to dγ orbital to break Zn−N bond, thus reserving the original topological structure of ZIF-8. Second, the Zn atom is saturated by the coordination of N atom in imidazole, which prevents approaching of H2S to zinc atom. ZIF-8 can bear H2S corrosion even at 200 °C (Figure S18). As compared to the Zn−N cluster, imidazole ring with delocalized electrons has

stronger impact on H2S molecule. By the simulation, H2S prefers adsorption near imidazole ring rather than near the Zn− N cluster (Figure 2d). HH2S atom presents a positive charge and prefers to approach N atom with delocalized π electrons in the imidazole ring. S has a weak interaction with the H atom on the imidazole ring, resulting from the interaction between electrons from p orbital of sulfur and antibonding orbital of H2S. It is noteworthy that the uptake of H2S on ZiF-8 is very low. A possible reason is the prevention effect of cage windows of ZIF8. In ZIF-8, the fragment of imidazole unit may cover a part of windows to prevent gas molecules passing through. Under the current experimental condition, H2S molecules cannot enable imidazole units to change positions for their entering. Similarly, H2S uptake of 0.126 mmol g−1 (Figure S19) on Ce-BTC (SBTE = 930 m2/g) with the 1D channels and open metal active sites assembled by Ce(III) and μ2-carboxylate groups is a bit higher than that of ZIF-8. As shown in Figure S36f, f and d orbitals of Ce are divided into several parts with low energy overlapping to sp orbital of Ocarboxylates, implying strong Ce−Ocarboxylate bonds to prevent the H2S corrosion. The d PDOS density of Ce is much lower than p PDOS of oxygen, indicating that the electron transfer from d orbital to p orbital results in a strong binding of Ce−O bonds. The f PDOS was divided into two parts. One peak of f PDOS was just located at Fermi energy level corresponding to Ce(III) 4f1. The energy of this orbital is nearly zero due to the shield effect of inner electrons from d orbital. The LUMO energy of f orbital is higher than the HOMO energy of H2S at about 1.16 eV. It is hard to transfer an electron from H2S LUMO to HOMO of Ce atom. By simulation, H2S prefers to locate near aromatic fragments (Figure 2e) instead of open metal sites due to the large gap of HOMO−LOMO energy level. Even having open metal sites in Ce-BTC, H2S does not have obvious interaction with Ce and cannot break Ce−Ocarboxylate bond even at a temperature as high as 200 °C due to shielding effect and strong Ce−O bond. The H2S uptake of Ce-BTC is about zero, and no changes were observed from PXRD patterns after exposure to H2S at 200 °C (Figure S30), both of which agree with the simulation results. The remaining MOFs including Zn-MOF-74, MOF-5, CuBTC, and MIL-100(Fe) have one-off high uptake to H2S. However, their H2S adsorption was irreversible. Different from MIL-101(Cr) and Mg-MOF-74, Zn-MOF-74 and Cu-BTC (or HKUST-1) having open metal sites cannot reserve the original topological structures after exposure to H2S. Diffraction peaks at low angles disappeared or became wide (Figures S31−S34), implying the decomposition of original crystal structures. For Zn-MOF-74 (SBET = 920 m2/g) constructed from infinite helical rods of Zn3[O3(CO2)3], its H2S uptake reached 1.64 mol/g in the first cycle and then dramatically decreased to 0.043 mmol/g. The Ohydroxyl−Zn−Ocarboxylate bond is easily broken by H2S because of matched d orbital energy level of Zn and p orbital energy level of S atom for the electron transfer and open Zn sites for easy H2S interaction. PDOS presents higher overlapping between p PDOS of O atom and d PDOS of Zn atom (Figure S36g), implying weak Zn−O bonds in ZnMOF-74. The LUMO energy of Zn(II) in Zn-MOF-74 is low; therefore, the electron transition from p orbital of S to LOMO of metal is easy. Zn-MOF-74 has open metal sites without repulsive force of ligands, which can react with H2S via the electrons transfer from S to Zn. When exposing Zn-MOF-74 to H2S, H2S molecule prefers to adsorb at Zn open sites. The 13252

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have higher energy, preferring to transfer to Fe 3d orbital. Due to a low 3d energy level, the electrons transfer from S to Fe completely. Fe(III) is reduced by H2S and converted to Fe(II) with a production of S8 molecule when the temperature is high enough to overcome the activity energy. The high temperature causes an irreversible conversion. At high temperature of 200 °C, sorption of H2S increases to 1.0 mmol g−1 (Figure S24), just slightly higher than the uptake at low temperature of 25 °C, demonstrating that the uptake of H2S on MIL-100(Fe) gel is attributed to the reaction. Since physical adsorptive interaction can be negligible at such a high temperature, we proposed that uptake of H2S comes from the reaction caused by overlapping of electron cloud between p orbital of S and d orbital of Fe(III). After overcoming energy barriers by heating to 200 °C, electron transit from S to Fe results in a reduction of Fe(III) and production of atom S to form molecular S8 located in the pore of MIL-100(Fe) gel. Thereby, different from other MOFs, H2S adsorption on MIL-100(Fe) gel is oxidative−reductive reaction with a high uptake. H2S molecule is first adsorbed at Fe(III) site, with overlapping electron cloud between d orbital. At high temperature, as the electron of S atom overcomes the energy barrier and tranfers to d orbital of Fe(III), Fe−O coordinated bonds are elongated and weakened, and low spin Fe(III) is reduced to high-spin Fe(II). Proton H decomposed from H2S combines with carboxylate oxygen, and S forms S8 molecule. An interesting phenomenon occurs when exposing MIL-100(Fe) gel to air, which adsorbs H2S at high temperature. Self-ignition and emission of gas SO2 were observed. Nano S8 dispersion in the pore can be oxidized by oxygen easily, attributed to the catalysis of Fe(II). The H2S/CO2 selectivity based on breakthrough experiments on 11 MOF materials is shown in Figure 3. The representative

strong covalent bonds between Zn and S are formed by breaking the Zn−O bonds, resulting in structural decomposition of Zn-MOF-74. The PXRD patterns further indicate the structural change (Figure S31). The same situation happened in MOF-5 (SBET = 2250 m2/g) derived from the basic zinc cluster unit Zn4O(CO2)6, with linear ditopic carboxylates. Zn adopts tetrahedral coordination similar to that in ZIF-8. However, PDOS (Figure S36h) shows a less overlapping of d orbital of Zn and sp orbital of O as compared with that of Zn−N in ZIF-8, implying weaker bond energy of Zn−O than Zn−Nimidazole. Partial overlapping between p and d PDOS is at the Fermi energy level, resulting in low bond energy of Zn−O bond. Allowing the acceptance of electrons from H2S, Zn−O bonds in MOF-5 could easily convert to Zn−S bonds. Therefore, the H2S uptake on MOF-5 presents a relatively high value of 1.11 mmol/g and dramatically decreases to 0.04 mmol/g after regeneration. PXRD patterns (Figure S32) indicate the structure collapse of MOF-5 after exposure to H2S.21 As for microporous paddlewheel framework Cu-BTC (SBET = 1590 m2/g) consisting of copper paddlewheel and benzene1,3,5-tricarboxylate, similar one-off high H2S uptake (1.1 mmol/g) was observed with a color change from deep blue to dark. Cu atom prefers to combine with S atom to form CuS that is firmly stable even in strong acid solution. As shown in Figure S36i, d PDOS of Cu was separated into several parts. One peak at Fermi level corresponds to Cu 3d orbital. The energy level of this orbital is lower than that of p orbital of S atom, resulting in a trend of electron transfer. The matched d orbital of Cu and p orbital of sulfur can form strong covalent bonds with lower energy level. Thus, Cu−Ocarboxylate bonds are easily broken by H2S to form Cu−S bonds. As shown in PXRD patterns (Figure S33), a shark peak at low angles disappeared after the H2S adsorption, indicating porous structure collapse of Cu-BTC after exposure to H2S. The sorption reaction also occurred on another microporous framework Cu-BDC(ted)0.5 (SBET = 1045 m2/g) consisting of copper paddlewheel and 1,4 benzenedicarboxylate with a layered structure supported by triethylenediamine (ted). After the adsorption of H2S, it was found that the sample’s color changed from blue to dark brown, but no evident changes of PXRD patterns were observed (Figure S34). Color change of Cu-BDC(ted)0.5 implies that the d−d transition energy is reduced because dCu orbital can obtain electrons from H2S more easily than from Ohydroxyl. No changes on PXRD patterns after H2S adsorption indicate structural integrity. Copper in Cu-BDC(ted)0.5 is fully coordinated by Ocarboxylate and Nted. PDOS (Figure S36j) showed that the overlap of electrons appears on dcopper, pnitrogen, and poxygen. As an electron donor, Nted prefers to bind with H2S via Nted−HH2S bond. H2S was fixed near the ted molecule, losing the capacity to break Cu−O bonds. After first breaking Cu−Nted bonds to form amine hydrosulfide, it then binds with Cu atom as shown in Figure 2f. Supported by sulfur amine salt, the layered structure of CuBDC(ted)0.5 can be reserved. The last MOF material we investigated is MIL-100(Fe) gel. Compared with MIL-101(Cr), MIL-100(Fe) gel (SBET = 816 m2/g) owns a similar metal cluster but shows a completely different result. Uptake of H2S on MIL-100(Fe) gel is about 0.9 mmol g−1 at 25 °C. However, the structure change happened after H2S adsorption, which was proven by PXRD patterns as shown in Figure S35. PDOS (Figure S36k) presents a low Fermi level. When approached by H2S, electrons from p orbital

Figure 3. (a) Breakthrough curves of H2S/CO2 mixture gas on MOFs at 298 K under 1 atm. (b) Selectivity on various MOF materials at 298 K.

breakthrough curves of three MOFs present reversible H2S adsorption. It was evident that except Mg-MOF-74, MIL101(Cr), UiO-66, ZIF-8, and Ce-BTC, the rest presents one-off high selectivity due to the reaction. However, the materials with physical adsorption of H2S did not exhibit ultrahigh selectivity. For example, the highest selectivity is from Ce-BTC, but this has considerably low H2S uptake, which limits its further 13253

DOI: 10.1021/acs.jpcc.7b04465 J. Phys. Chem. C 2017, 121, 13249−13255

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The Journal of Physical Chemistry C Author Contributions

application. Considering the trade-off of selectivity and uptake (Figure 3b), only UiO-66, Mg-MOF-74, and MIL-101(Cr) are promising in the H2S/CO2 separation. The high H2S selectivity of UiO-66 lacking open metal sites is mainly resulted from large difference of dispersion force. In contrast, both Mg-MOF-74 and MIL-101(Cr) having unsaturated coordinative metal sites showed strong interactions with H2S, giving relatively high H2S uptake. Especially in MIL-101(Cr), its LUMO energy level approximates the HOMO energy level of H2S, leading to an easy transfer of electrons with a strong interaction between MIL-101(Cr) and H2S. The HOMO energy level of CO2 is lower than that of H2S. Electrons from O 2p orbital or π electrons from π(3−4) delocalized orbital are stable in their own orbitals, showing a relatively weak interaction of CO2 with the open metal sites. Among these three MOF materials with large surface areas and open metal sites, MIL-101(Cr) showing the H2S uptake of 0.4 mmol/g and relatively high selectivity of 5 is most significant in the H2S/CO2 separation.

J.L., Y.J., and P.L.contributed equally to this research. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 21371014 and 21406004), National Program for Support of Top-Notch Young Professionals, Singapore Academic Research Fund (RG112/15 and RG19/16), and Singapore Agency for Science, Technology and Research (A*STAR), AME IRG grant (No. A1783c0007).

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4. CONCLUSIONS Eleven classic MOF-based materials have been screened as adsorbents for the CO2/H2S separation. The stability of these MOFs when exposed to H2S has been studied via DFT simulations and PDOS analysis. On the basis of breakthrough experimental results, these materials were clarified into three families based on their different adsorptive mechanisms. Reversible physical sorption occurs on Mg-MOF-74, MIL101(Cr), UiO-66, ZIF-8, and Ce-BTC, of which high uptake is present on the MOFs with open metal sites and high surface area such as MIL-101(Cr) with a selectivity about 5. MOFs composed of Cu or Zn such as Zn-MOF-74, Cu-BTC, and MOF-5 could easily react with H2S to form metal sulfide, leading to the structure damage. These kinds of MOFs usually exhibit one-off high uptake and selectivity. However, their disabled performance restricts their future applications. Similar situations were faced by UiO-66(NH2) and Cu-BDC(ted)0.5, although they can keep their original structures. Oxidation− reduction reaction happened on MIL-100(Fe) gel, which was elucidated by experiments. It was interesting that nano S8 was observed during the adsorption process on MIL-100(Fe) gel, of which Fe(III) was reduced to Fe(II) by H2S. The present work provides a new perspective for functional MOF-based materials for gas separation and purification, especially in toxic gas removal.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b04465. N2 isotherm curves, PXRD patterns, breakthrough curves, and details of measurements and DFT calculations (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yanli Zhao: 0000-0002-9231-8360 Ruqiang Zou: 0000-0003-0456-4615 13254

DOI: 10.1021/acs.jpcc.7b04465 J. Phys. Chem. C 2017, 121, 13249−13255

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DOI: 10.1021/acs.jpcc.7b04465 J. Phys. Chem. C 2017, 121, 13249−13255