Adsorption of C1–C4 Alcohols in Zeolitic Imidazolate Framework-8

Article pubs.acs.org/JPCC

Adsorption of C1−C4 Alcohols in Zeolitic Imidazolate Framework-8: Effects of Force Fields, Atomic Charges, and Framework Flexibility Kang Zhang,† Liling Zhang,‡,§ and Jianwen Jiang*,† †

Department of Chemical and Biomolecular Engineering, National University of Singapore, 117576, Singapore Institute of High Performance Computing, 1 Fusionopolis Way, Connexis 138632, Singapore § Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials, Soochow University, Suzhou, Jiangsu 215123, China ‡

S Supporting Information *

ABSTRACT: A molecular simulation study is reported for the adsorption of normal alcohols (methanol, ethanol, propanol, and butanol) in zeolitic imidazolate framework-8 (ZIF-8). The effects of force fields, atomic charges, and framework flexibility are systematically examined and compared with experimental data. Among three force fields (UFF, AMBER, and DREIDING), DREIDING has the best agreement with experiment. The atomic charges and framework flexibility are found to have negligible effects. The four alcohols exhibit S-shaped isotherms without hysteresis loop, as attributed to adsorption at different preferential sites. At a low pressure, cluster formation is observed near the organic linker (2-methylimidazolate) in ZIF-8; with increasing pressure, cage-filling occurs in the large sodalite cage. The interaction between alcohol and ZIF-8 framework is enhanced as the chain length of alcohol increases; thus, the isosteric heat of adsorption rises with chain length. The simulation study provides microscopic insight into alcohol adsorption in ZIF-8, which is useful for quantitative understanding of adsorption mechanism in other ZIFs and nanoporous materials.

1. INTRODUCTION Emerging as a new family of nanoporous materials, metal− organic frameworks (MOFs) are produced by the assembly of metal clusters and organic linkers.1 In principle, there exist unlimited number of MOFs with a wide range of surface area and pore size. Intriguingly, the structure and functionality of MOFs can be readily tunable by the judicious selection of building blocks. Therefore, MOFs have been considered as versatile materials for many potential applications such as storage, separation, catalysis, etc.2 Most experimental and simulation studies for MOFs have been focused on gas storage and separation, particularly the storage of low-carbon footprint energy carriers (e.g., H2 and CH4) and the separation of CO2-containing gas mixtures for CO2 capture.3−9 For instance, H2 storage capacities in different MOFs were determined and compared with those in other nanoporous materials.10 By combining various clusters and linkers, a large number of hypothetical MOFs were computationally generated and screened for CH4 storage.11 Molecular simulation was conducted for CO2/N2 mixture in Cu-BTC and for CO2/H2 mixture in catenated IRMOFs.12 Separation performance of CO2/N2 mixture was compared in a series of MOFs and zeolites.13 High selectivity was predicted by simulation for gas mixtures including CO2/CH4, CO2/N2, and CO2/H2 in rho zeolite-like MOF.14 With increasing interest in biofuel production, a number of studies have examined alcohol adsorption in MOFs toward © XXXX American Chemical Society

biofuel purification. Experimentally, Li and co-workers determined methanol adsorption in a highly stable microporous MOF, namely Zn(tbip) (tbip = 5-tert-butyl isophthalate).15 Subsequently, they observed high uptake of methanol and ethanol in paddel-wheel M(bdc)(ted)0.5 (M = Zn, bdc = 1,4benzenedicarboxylate, and ted = triethylenediamine).16 Denayer and co-workers measured the adsorption of C1−C5 alcohols in ZIF-8.17 Koros and co-workers examined ethanol adsorption in ZIF-7118 and reported experimental data for the adsorption of C1−C4 alcohols in ZIF-8, ZIF-71, and ZIF-90.19 In addition, they also investigated the adsorption and diffusion of ethanol in ZIF-8 with different crystal sizes.20 Currently, only few simulation studies have been conducted for alcohol adsorption in MOFs. Specifically, we simulated the adsorption of methanol and ethanol in two types of MOFs (hydrophobic ZIF-71 and hydrophilic rho-ZMOF),21 and ethanol adsorption predicted in ZIF-71 matches favorably well with experiment.18 The adsorption isotherms of methanol simulated in Zn(bdc)(ted)0.5 and Zn4O(bdc)(bpz)2 (bpz = tetramethyl bipyrazolate) were found to be consistent with measured data.22,23 From experiment and modeling, Gee et al. determined the adsorption and diffusion of methanol and ethanol in ZIF-8 and ZIF-90.24 Received: October 4, 2013 Revised: November 12, 2013

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tially unexplored how different force fields perform for alcohol adsorption in ZIFs. Consequently, alcohol adsorption in ZIF-8 is examined here using three typical force fields including UFF, DREIDING, and AMBER.37 Tables S1−S3 list the parameters of these force fields for ZIF-8. For the Coulombic potential, the atomic charges of ZIF-8 framework were calculated by density functional theory (DFT) on a fragmental cluster (see Figure S1).38 The DFT calculation used the Becke exchange plus the Lee−Yang−Parr functional (B3LYP) and was carried out by Gaussian 03.39 The accuracy of DFT-derived atomic charges depends on the choice of functional and basis set. Expressed as both local and gradient electron densities, the B3LYP has been widely used in the field of solid materials including MOFs.40,41 For small basis sets, the atomic charges fluctuate appreciably but tend to converge beyond the 6-31G(d) basis set.42 Therefore, 6-31G(d) was used for all the atoms in ZIF-8 except Zn atoms, for which the LANL2DZ basis set was used. By fitting the electrostatic potentials produced from the cluster model, the atomic charges were estimated as listed in Table S4. To evaluate the effect of atomic charges, simulation was also performed for methanol adsorption by turning off the atomic charges. To assess the effect of framework flexibility, the bonded interactions in ZIF-8 framework were incorporated, including stretching, bending, and torsional potentials

For the development of novel MOFs for biofuel purification by adsorption technology, first, it is critical to better understand alcohol adsorption in MOFs. In this study, we simulate the adsorption of a series of normal alcohols (C1−C4) in ZIF-8. The predicted adsorption isotherms are compared with experimental data, and the adsorption mechanism is elucidated from a microscopic level. In particular, we systematically assess the effects of force fields, atomic charges, and framework flexibility on alcohol adsorption in ZIF-8. While these effects have been, to a certain extent, examined for the adsorption of small gas molecules (e.g., CO2, N2, and CH4) in ZIF-8,25−27 it is unknown or very limited on how they would affect alcohol adsorption. Thus, quantitative understanding will be provided by this study. In section 2, the molecular models are described, followed by simulation methods in section 3. In section 4, the simulated isotherms of C1−C4 alcohols in ZIF-8 are presented and compared with experimental data. The effects of force fields, atomic charges, and framework flexibility are discussed in detail. Concluding remarks are summarized in section 5.

2. MOLECULAR MODELS ZIF-8 has a sodalite zeolite-like topology with a cubic space group I-43m.28 Figure 1a illustrates that each Zn metal in ZIF-8

Ustretching =

Figure 1. Structures of (a) ZIF-8 and (b) C1−C4 normal alcohols. Color code: Zn, red; N, blue; C, cyan; H, white; and O, orange.

Unonbonded

⎡⎛ ⎞12 ⎛ ⎞6 ⎤ σij σij = ∑ 4εij⎢⎢⎜⎜ ⎟⎟ − ⎜⎜ ⎟⎟ ⎥⎥ + r ⎝ rij ⎠ ⎦ ⎣⎝ ij ⎠

∑

(2)

1 kθ(θijk − θijk0 )2 2

(3)

∑

Utorsional =

∑ kϕ[1 + cos(mϕijkl − ϕijkl0 )]

∑ kξ[1 + cos(mξijkl − ξijkl0 )]

(4)

where kr, kθ, kϕ, and kξ are the force constants; rij, θijk, ϕijkl, and ξijkl are bond lengths and angles, proper and improper dihedrals, respectively; m is the multiplicity and was set to two for most dihedrals; r0ij, θ0ijk, ϕ0ijkl, and ξ0ijkl are the equilibrium values. All these parameters have been optimized in our recent study to mimic the structural transition of ZIF-8.43 Four normal alcohols (methanol, ethanol, propanol, and butanol) considered under this study are shown in Figure 1b. The alcohol molecules were represented by a united-atom model with each CHx as a single interaction site. The potential parameters were adopted from the transferable potentials for phase equilibria (TraPPE) force field, which was fitted to the critical properties and vapor−liquid equilibria of alcohols.44 The bond lengths of alcohols were fixed, while the nonbonded and bending potentials were described by eqs 1 and 3, respectively. In addition, the torsional potential was represented by

qiqj 4πε0rij

1 kr(rij − rij0)2 2

Ubending =

+

is tetrahedrally coordinated by four N atoms of 2methylimidazolate. Because of the presence of long linkers rather than bridging O atoms, the sodalite cage in ZIF-8 possesses a diameter of 11.6 Å and is almost twice as large as that in zeolitic counterpart. The sodalite cages are connected via small apertures with a diameter of 3.4 Å. However, the framework of ZIF-8 is not completely rigid and may undergo structural transition; thus, guest molecules can pass through the apertures and be adsorbed in the cages.29,30 If the ZIF-8 framework is considered as rigid, the framework atoms can be simply represented by nonbonded interactions including Lennard-Jones (LJ) and Coulombic potentials

∑

(1)

where εij and σij are the well depth and collision diameter, rij is the distance between the atoms i and j, qi is the atomic charge of atom i, and ε0 = 8.8542 × 10−12 C2 N−1 m−2 is the permittivity of vacuum. In the literature, universal force field (UFF)31 and DREIDING32 are most widely used to mimic the LJ potential in MOFs.33−36 It is well recognized, however, they overestimate the adsorption of small gas molecule (e.g., CO2, N2, and CH4) in ZIFs.25−27 Although our previous study demonstrated that UFF and DREIDING can fairly well describe the adsorption of methanol or ethanol in ZIF-71, Zn(bdc)(ted)0.5 and Zn4O(bdc)(bpz)2,21−23 it remains essen-

Utorsional(ϕ) = c0 + c1[1 + cos ϕ] + c 2[1 − cos(2ϕ)] + c3[1 + cos(3ϕ)]

(5)

where ci (i = 0, 1, 2, and 3) are force constants. Table S5 gives the TraPPE parameters for the normal alcohols.44 The cross interaction parameters between ZIF-8 and alcohols were estimated by the Lorentz−Berthelot combining rules. B

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Finally, the effect of force fields is examined on the adsorption of ethanol, propanol, and butanol. 4.1. Methanol. Figure 2 shows the adsorption isotherms of methanol in ZIF-8 predicted by three force fields (UFF,

3. SIMULATION METHODS To simulate alcohol adsorption in ZIF-8 with a rigid framework, grand canonical Monte Carlo (GCMC) method was used. The chemical potentials of an adsorbate in adsorbed and bulk phases are identical at thermodynamic equilibrium, and the GCMC method allows one to directly relate the chemical potentials in both phases and has been widely used to simulate adsorption. The simulation box contained eight (2 × 2 × 2) unit cells of ZIF-8, and the periodic boundary conditions were exerted in three dimensions. A spherical cutoff of 15 Å was used to evaluate the LJ interactions, and the long-range corrections were added. For the Coulombic interactions, the Ewald sum with a tinfoil boundary condition was used. The real/reciprocal space partition parameter and the cutoff for reciprocal lattice vectors were chosen to be 0.2 Å−1 and 8, respectively, to ensure the convergence of the Ewald sum. The number of trial moves in a typical simulation was 2 × 107, in which the first half moves were used for equilibration and the second half moves for ensemble averages. Five types of trial moves were randomly attempted in the GCMC simulation, namely, displacement, rotation, and partial regrowth at a neighboring position, complete regrowth at a new position, and swap between reservoir including creation and deletion with equal probability. To improve sampling efficiency, the configurational-bias technique was adopted in which alcohol molecules were grown atom-by-atom biasing toward energetically favorable configurations while avoiding overlap with other atoms.45−47 For the first and subsequent atoms, 15 and 10 trial positions were generated, respectively, with a probability proportional to exp(−βUiintra), where β = 1/kBT and Uiintra is the intramolecular interaction energy at a position i. Then, one of the trial positions was chosen with a probability proportional to exp(−βUiinter)/∑iexp(−βUiintra), where Uiinter is the intermolecular interaction energy. A modified version of BIGMAC code48 was used for the GCMC simulation. In the above-mentioned GCMC method, the framework of ZIF-8 was kept rigid. As pointed out earlier, however, ZIF-8 is not completely rigid. To incorporate framework flexibility, we used a hybrid MC/MD simulation method developed recently to mimic the structural transition of ZIF-8.43 Specifically, configurational-bias GCMC simulation was first used to calculate alcohol adsorption in rigid ZIF-8 at a given pressure, then MD simulation in NVT ensemble was performed to relax ZIF-8 framework as well as adsorbed alcohol molecules, and the relaxed framework was used in the subsequent GCMC simulation. The MC/MD simulations were repeated until adsorption capacity converged. In each cycle, the number of trial moves in the GCMC simulation was 107, and the MD simulation was run for 600 ps. The MD simulations were conducted in DL_POLY.49 The equations of motion were integrated by the velocity Verlet algorithm with a time step of 1 fs. A relaxation time of 0.8 ps was to maintain the constant temperature.

Figure 2. Effect of force fields on methanol adsorption.

AMBER, and DREIDING) as well as experimentally measured.19 The isotherms belong to S-shaped type V, which signifies the adsorption of weakly interacting adsorbate in a microporous framework. Similar types of isotherms were observed in our previous studies for methanol adsorption in ZIF-71, Zn(bdc)(ted)0.5, and Zn4O(bdc)(bpz)2.21−23 With increasing pressure, the isotherm can be characterized into three regimes. In the low-pressure regime, adsorption extent is small, and as discussed below, this corresponds to cluster formation at preferential adsorption sites. With increasing pressure, cage-filling occurs with sharp increase in uptake. Finally, saturation is gradually approached in high-pressure regime. Among the three force fields, the uptake predicted decreases in the order of UFF > AMBER > DREIDING. The reason for such a trend will be discussed below. Apparently, DREIDING gives the best agreement with experiment, though it overestimates at low pressures and underestimates at high pressures. The effects of atomic charges and framework flexibility are shown in Figure 3, in which DREIDING was used for the LJ potential. Despite marginal differences at intermediate and high pressures, the predictions with and without atomic chargers are very close. This implies that the interactions between methanol and ZIF-8 framework are predominated by the LJ potential, and the inclusion of atomic charges is insignificant in this case. Such behavior was also observed for CO2 adsorption in IRMOFs36

4. RESULTS AND DISCUSSION All the simulation and experimental results presented are at 308 K (35 °C). The pressure range examined for each alcohol is below its saturation pressure (see Table S6).50 First, methanol adsorption in ZIF-8 is discussed focusing on the effects of force fields, atomic charges, and framework flexibility. Then, the microscopic mechanism for methanol adsorption is elucidated by analyzing favorable adsorption sites and isosteric heats.

Figure 3. Effects of atomic charges and framework flexibility on methanol adsorption. C

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where r is the distance between atoms i and j, Nij(r, r + Δr) is the number of atom j around i within a shell from r to r + Δr, V is the system volume, and Ni and Nj are the numbers of atoms i and j, respectively. Figure 5 shows the g(r) of methanol around the heavy atoms (Zn, N, C1, C2, and C3) in ZIF-8. At 1 kPa, a pronounced peak at r = 3.6 Å exists in the g(r) around the C2 atom. This confirms the preferential location of methanol proximal to the CC bond observed in Figure 4. A slightly less pronounced peak at r = 4.0 Å is seen around the C3 atoms, which implies the methyl (−CH3) group is less favorable. The g(r) around the Zn and C1 atoms exhibit pronounced peaks at long distances r = 6.2 and 5.4 Å, respectively; thus, the Zn and C1 atoms are also favorable for methanol adsorption. With increasing pressure from 1 to 5 kPa, the peak heights around all the atoms drop. This is attributed to the occurrence of cagefilling at a high pressure; consequently, more methanol molecules are located in the sodalite cage and move away from the framework atoms. The organic linker is the most favorable adsorption site, particularly the C2 and C3 atoms. Therefore, the interactions of methanol with the C2 and C3 atoms largely govern adsorption particularly at low pressures. As listed in Tables S1−S3, the overall ε/kB for the C2 and C3 atoms in the three force fields decreases approximately in the order of UFF > AMBER > DREIDING. Therefore, among the three force fields, the uptake decreases following UFF > AMBER > DREIDING, as seen in Figure 2. To quantitatively examine adsorption energy, the isosteric heat of adsorption Qst was calculated by54

and a few ZIFs.51 By taking into account the framework flexibility, the predicted isotherm remains nearly the same. Thus, the framework flexibility appears to have a negligible effect on methanol adsorption. A similar effect was recently observed in the simulation of methanol adsorption in ZIF-90.24 Unless otherwise stated, the simulation results discussed below for methanol are based on DREIDING, with atomic charges and rigid framework. Figure 4 illustrates the density contours of methanol in ZIF-8 at 1 and 5 kPa. At 1 kPa, clusters (indicated by the dotted

Figure 4. Density contours of methanol in ZIF-8 at 1 and 5 kPa (from left to right). The unit of density scale is the number of molecules per Å3.

circles) are observed to form around the organic linkers (2methylimidazolate), particularly proximal to the CC bonds at the aperture. Meanwhile, methanol is also adsorbed onto the cage surface. With increasing pressure to 5 kPa, the clusters grow and cage-filling occurs in the sodalite cage. Because of strong surface interaction, the density at cage surface is higher than at the cage center. At even a higher pressure (not shown), the whole sodalite cage is almost filled including the cage center. Therefore, the organic linker in ZIF-8 is the most favorable adsorption site, rather than the metal cluster. This phenomenon is similar to methanol adsorption in ZIF-71,21 as also observed in the experimental and simulation studies of gas adsorption in ZIF-8.25−27,52,53 It should be noted, however, metal clusters in many other MOFs appear to be the most favorable sites, e.g., for CO2 adsorption in IRMOFs.36 To quantify the structural properties, radial distribution functions of methanol around the framework atoms in ZIF-8 were calculated by gij(r ) =

⎛ ∂U ⎞ Q st = R gT − ⎜ ad ⎟ ⎝ ∂Nad ⎠T , V

where Rg is the gas constant, T is temperature, and Nad and Uad are the number of adsorbates and adsorption energy, respectively. The partial derivative in eq 7 was evaluated by the fluctuation theory ⎛ ∂Uad ⎞ ⟨NadUad⟩ − ⟨Nad⟩⟨Uad⟩ = ⎜ ⎟ ⟨Nad 2⟩ − ⟨Nad⟩2 ⎝ ∂Nad ⎠T , V

(8)

where the brackets ⟨···⟩ denote ensemble average. As plotted in Figure 6, the predicted Qst exhibits an increasing trend, which is similar to the experimental data of ethanol adsorption in ZIF8.20 At zero loading, Qst is approximately 20 kJ/mol, reflecting

Nij(r , r + Δr )V 4πr 2ΔrNN i j

(7)

(6)

Figure 5. Radial distribution functions of methanol around framework atoms at 1 and 5 kPa. D

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ethanol is longer than methanol and has more interaction sites and thus interacts more strongly with ZIF-8. However, the saturation loading of ethanol (6.1 mmol/g) is lower than that of methanol (10 mmol/g). This is because adsorption near saturation is primarily governed by entropic (size) effect, and fewer ethanol molecules can be adsorbed compared to methanol. The same behavior is also observed with increasing the chain length of alcohol (or the number of carbon atoms). Specifically, the saturation loadings of propanol and butanol are 4.9 and 4.3 mmol/g, lower than 6.1 mmol/g of ethanol and 10 mmol/g of methanol. Regarding the effect of framework flexibility, Figure S3 also reveals the effect is negligible on ethanol adsorption in ZIF-8. To quantitatively evaluate the adsorption of C1−C4 alcohols as a function of the number of carbon atoms, the isosteric heats at infinite dilution were estimated by

Figure 6. Isosteric heat of methanol adsorption.

the interaction between a single methanol molecule and ZIF-8 framework. At low loadings (9 mmol/g), methanol molecules are closely packed and hydrogen bonding is strong, leading to the increase of Qst. Because of the confinement effect, apparently, the Qst at high loadings is higher than the enthalpy of condensation (36.96 kJ/mol)50 for methanol at 35 °C. 4.2. Ethanol, Propanol, and Butanol. Figure 7 shows the adsorption isotherms of ethanol, propanol, and butanol in ZIF8. All the isotherms are S-shaped, similar to methanol. Among the three force fields, the predictions follow the same trend as in Figure 2; i.e., UFF > AMBER > DREIDING. DREIDING exhibits the best agreement with experimental data.19 The desorption isotherms simulated using DREIDING are plotted in Figure S2. There is no hysteresis loop for C1−C4 alcohols, consistent with the experimental measurement of ethanol.20 This also confirms that the S-shaped isotherms observed here are not caused by gate-opening effect, which is associated with a distinct hysteresis loop. As discussed above, the S-shaped isotherms are attributed to the cluster formation and cage-filling at different adsorption sites. Compared to methanol adsorption, the loading of ethanol at a low pressure is higher (see Figure S2). The reason is that

° Q st° = R gT − Uad

° Uad = Utotal − (Uframework + Ualcohol)

(9) (10)

where U°ad is the adsorption energy at infinite dilution. Utotal, Uframework, and Ualchol are the potential energies of framework− alcohol, ZIF-8 framework, and a single alcohol molecule, respectively. The conformational change of alcohol upon adsorption was taken into account. As shown in Figure 8, Q°st

Figure 8. Heats of adsorption at infinite dilution for C1−C4 alcohols.

rises linearly with increasing number of carbon atoms in alcohol. As the number of carbon atoms increases, the alcohol molecule contains more nonpolar aliphatic sites and possesses a stronger affinity with hydrophobic ZIF-8. In a recent computational study, Heine and co-workers showed that

Figure 7. Effect of force fields on the adsorption of ethanol, propanol, and butanol. E

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and Dr. Ke Zhang (Georgia Institute of Technology) for providing experimental data.

dispersion interaction counts approximately 50% of total interaction energy for ethanol adsorption in Zn(bdc)(ted)0.5, which is stronger compared to methanol adsorption (around 40%).55 This suggests that the dispersion interaction increases with the number of carbon atoms in alcohol, leading to higher interaction energy. Among the four alcohols, butanol has the strongest interaction with ZIF-8 and hence the highest Qst° . Similar behavior was observed for the adsorption of normal alkanes in silicalite,56 alumina and 13X molecular sieve,57 and carbon nanotubes.58

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5. CONCLUSIONS Adsorption of C1−C4 alcohols in ZIF-8 has been investigated by molecular simulation. The isotherms of all the four alcohols are S-shaped type V, indicating relatively weak adsorption in a microporous framework. With increasing pressure, three adsorption regimes are observed. At a low pressure, clusters are formed proximal to organic linker (2-methylimidazolate). The CC bond of organic linker is identified to be the most favorable site for adsorption. At an intermediate pressure, cagefilling occurs in sodalite cage with sharp increase in adsorption. Finally, saturation is approached at a high pressure. The isosteric heat of adsorption at infinite dilution rises linearly with the chain length of alcohol, as attributed to the enhanced interaction between aliphatic tail and hydrophobic ZIF-8. At a low pressure, a longer alcohol has a greater uptake; however, its saturation loading is smaller. With regard to the effect of force fields, the simulated uptakes of the four alcohols decrease in the order of UFF > AMBER > DREIDING. Upon comparison with experimental data, DREIDING gives the best prediction. In addition, the atomic charges and framework flexibility have negligible effects on adsorption. This simulation study systematically examines the effects of force fields, atomic charges, and framework flexibility on alcohol adsorption in ZIF-8. The microscopic insight provided is helpful to better elucidate the adsorption behavior of alcohols in other ZIFs and MOFs and would facilitate the development of new nanoporous materials for biofuel purification.

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ASSOCIATED CONTENT

S Supporting Information *

A fragmental cluster of ZIF-8; UFF, DREIDING, and AMBER parameters for ZIF-8; atomic charges in ZIF-8; TraPPE parameters for normal alcohols; saturation pressures of normal alcohols at 308 K; simulated adsorption and desorption isotherms of C1−C4 alcohols in ZIF-8; effect of framework flexibility on ethanol adsorption. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.J.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National University of Singapore, the Ministry of Education of Singapore, the National Natural Science Foundation of China (21303112), and the Natural Science Foundation of Jiangsu Province (BK20130291). We also gratefully acknowledge Prof. J. Ilja Siepmann (University of Minnesota) for helpful discussions F

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The Journal of Physical Chemistry C

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(58) Jiang, J. W.; Sandler, S. I.; Schenk, M.; Smit, B. Adsorption and Separation of Linear and Branched Alkanes on Carbon Nanotube Bundles. Phys. Rev. B 2005, 72, 045447.

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dx.doi.org/10.1021/jp409869c | J. Phys. Chem. C XXXX, XXX, XXX−XXX