Adsorption of Gases in Microporous Organic Molecular Crystal, a

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Adsorption of Gases in Microporous Organic Molecular Crystal, a Multiscale Computational Investigation Wenliang Li,† Jingping Zhang,*,† Haichao Guo,‡ and Godefroid Gahungu†,§ † ‡

Faculty of Chemistry, Northeast Normal University, Changchun 130024, China State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Nanjing, 210009 China

bS Supporting Information ABSTRACT: The grand canonical Monte Carlo (GCMC) method and high-level first-principle calculations are performed to investigate the role of a constrained channel of microporous organic molecular crystal in separating H2 from binary mixtures containing N2, CH4, or CO2. GCMC simulations show that the selectivity of N2, CH4, or CO2 over H2 is in the order of N2/ H2 < CH4/H2 < CO2/H2, which is consistent with the order of isosteric heats of adsorption. Particularly at low pressure the selectivity is very high because CO2, CH4, or N2 initially occupies the preferential site in the channel with less sites left for H2. In addition, dispersion corrected density functional theory (DFT-D) is introduced to study the interaction energies and structural properties of the conjugated channel and gases. By comparing with the benchmark data of the coupled-cluster calculations with singles, doubles, and perturbative triple excitations [CCSD(T)] estimated at the complete basis set (CBS) limit, the proper functional is selected. The first-principle calculations confirm that the heterogeneous channel can hold CO2, CH4, or N2 much stronger than H2, suggesting the microporous organic molecular crystal is a good candidate for potential hydrogen purification.

’ INTRODUCTION The storage and separation of important gases, such as H2, N2, CH4, and CO2, is a challenging research field, due to the impending energy crisis and related global pollution that are dramatic issues today. A range of approaches have been explored to utilize materials of different property and construction. Metal organic frameworks (MOFs)1,2 and zeolitic imidazolate frameworks (ZIFs)3-5 are new classes of microporous materials with potential applications in adsorption separations, catalysis, and gas storage. Because the property of flexibility promises to be particularly important in gas adsorption, the preparation of microporous organic polymers with permanent microporosity and high surface areas, such as covalent organic frameworks (COFs),6,7 polymers of intrinsic microporosity (PIM),8,9 porous aromatic framework (PAF),10,11 hypercrosslinked polymers (HCPs),12,13 crystalline triazine-based organic frameworks (CTFs),14and conjugated microporous polymers (CMPs),15,16 is currently of intense interest. Additionally, study of microporous organic molecular crystals (MOMCs) is an emerging area of interest.17-22 The microporous crystal of tris(o-phenylenedioxy)cyclotriphosphazene (TPP) is one of the examples that has been discussed the most among the MOMCs.20-25 Very recently, McKeown and his co-workers26 discovered a MOMC, as displayed in Figure 1, formed by 3,30 ,4,40 -tetra(trimethylsilylethynyl)biphenyl (TTB). Figure 1 c and d showed the attractive structural feature of the TTB crystal of threedimensional interconnectivity of the void space. It allows multiple paths for the adsorbents to access each micropore and avoids potential reduction of adsorption because of pore blocking. The three-dimensional interconnectivity of microporosity is expected r 2011 American Chemical Society

to enhance the kinetics of adsorption in comparison with crystals of one-dimensional channels of a similar size. They found that the crystals of TTB can capture a significant amount of H2 or N2 at 77 K. In this work, a combination of classical and quantum calculations is performed to investigate the adsorption and separation of some critical gases (including H2, N2, CH4, and CO2) in crystal TTB. We first used grand canonical Monte Carlo (GCMC) to simulate pure-component adsorption of H2, N2, CH4, and CO2 in TTB crystal and verify the model with available experiments. Binary mixture simulations were then performed for adsorption of N2, CH4, or CO2 over H2 at 50% mixtures, and adsorption selectivities were calculated. Furthermore, first-principle calculations were performed to scan the potential energy surface of small molecules moving along the channel, and the most stable adsorption configurations were investigated.

’ COMPUTATIONAL DETAILS We adopted a multiscale theoretical method to predict the gas adsorption in crystal TTB. In the classical simulation, the force field parameters for the interaction between H2 and the TTB were fitted by the calculated adsorption isotherms to the experimental data. Then, using the force fields, the GCMC simulations were subsequently implemented to predict the adsorption isotherms of N2, CH4, and CO2 in the microporous organic crystal. In the quantum simulation, by comparison with benchmark data Received: November 17, 2010 Revised: February 4, 2011 Published: February 28, 2011 4935

dx.doi.org/10.1021/jp110995u | J. Phys. Chem. C 2011, 115, 4935–4942

The Journal of Physical Chemistry C

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Figure 1. (a) Molecular structure of TTB. (b) Channel structure including four molecules of TTB. (c) The interconnectivity of the void space in threedimensional. (d) Unit cell of crystal TTB.

of coupled-cluster calculations with singles, doubles, and perturbative triple excitations [CCSD(T)] estimated at the complete basis set (CBS) limit, our high-level DFT augmented with dispersive interaction correction (DFT-D) method was applied to describe the van der Waals (vdW) interaction between gases and the organic host. Models. The model of crystal TTB used in this work was constructed from experimental XRD crystallographic data with the box size of a = b = c = 29.238 Å.26 The atomic partial charges of molecule TTB were calculated at the fine level with a PBE functional implemented by the module Dmol3 of Materials Studio package.27 The short-range van der Waals interactions are discribed by Lennard-Jones (LJ) potential due to its computational simplicity, and the LJ potential is used extensively in computer simulations even though more accurate potentials exist. H2 was treated as a united atom molecule, and its parameters were taken from Michels et al.28 CH4 was modeled as a single-center Lennard-Jones (LJ) molecule using the TraPPE force field developed by Martin and Siepmann.29 N2 was modeled as a three-site rigid molecule using the TraPPE force field developed by Potoff and Siepmann.30 In this model, point charges are centered on each LJ site, and electric neutrality is maintained by placing a point charge of þ0.964 e at the center of mass of the N2 molecule. CO2 was represented as a three-site rigid molecule. The intrinsic quadrupole moment is described by the partial charge model. The CO bond length is 1.18 Å, and the bond angle — OCO is 180.30 All of these parameters are listed in the Table S1 in the Supporting Information. Fitting of the Force Field. Lots of force-field-based GCMC simulation investigations of gas adsorption in MOF materials have already been performed. Generally standard force fields (Dreiding,31 universal force field (UFF),32 or consistent valence force field (CVFF)33) were used in these materials, and they

seem to reproduce the physical properties of MOF materials fairly well. However, for organic systems the standard force fields are rarely used because it is not yet feasible to evaluate their fitness to portray the adsorption isotherms. Additionally, there are no dedicated force fields developed in organic materials for the analysis of adsorption properties. We, therefore, adjusted the standard Dreiding parameters by scaling their intermolecular parameters as mentioned by Carlos Nieto-Draghi and his coworkers,34 and then our GCMC simulation results are used to compare with the experimental adsorption data for crystal TTB. GCMC Simulation. GCMC simulations were conducted to predict the adsorption isotherms and isosteric heats of adsorption (Qst) in microporous organic crystal TTB. In GCMC simulations, a chemical potential was imposed on a constantvolume system at constant temperature. The number of molecules was then allowed to fluctuate until equilibrium at the required chemical potential had been attained. All simulations were performed with the Monte Carlo simulation suite of the MUSIC35 code and included a 2 107 step equilibration period followed by a 2 107 step production run. LJ interactions were implored to describe the vdW interaction with a cutoff distance of 10.0 Å 2 !12 !6 3 σ σ ij ij 5 Vij ¼ 4εij 4 rij rij where i and j are atoms of gases and TTB; r is the distance between two atoms; and ε and σ are LJ well depth and diameter, respectively. Lorentz-Berthelot mixing rules were used to calculate the ε and σ between different atomic species. The Ewald sum method was used to compute the Coulombic interactions. For single-center LJ models (H2 and CH4), three 4936

dx.doi.org/10.1021/jp110995u |J. Phys. Chem. C 2011, 115, 4935–4942


Figure 2. Comparison of simulated and experimental adsorption isotherms for H2 in organic crystal TTB at 77 K (solid line, experimental data from ref 26 at 77 K; square, simulations during this work; cross, simulations by using standard Dreiding force fields; dotted line, fitting of Langmuir-Freudrich equation to simulation results.

different types of Monte Carlo trials were used in the simulations: insertion of a new adsorbate molecule at a random position in the adsorbent, removal of a randomly chosen adsorbed molecule from the adsorbent, and translation of a randomly chosen adsorbate molecule within the adsorbent. For the models of N2 and CO2, an additional Monte Carlo trial was needed: rotation of a randomly chosen adsorbate molecule within the adsorbent. The nonideal behavior of the bulk pure gas and gas mixtures was described by the Peng-Robinson equation of state (PREOS).36 For all of the binary mixtures, the binary interaction parameter in the PREOS was assumed to be zero. GCMC simulation gives the absolute number of molecules adsorbed. However, experimental data are typically reported as the excess amount adsorbed. Thus, all absolute loadings were converted to excess loadings to allow direct comparison of the results with experimental data.37 Qst values were also calculated for each component from simulations as the difference of the partial molar enthalpy of the sorbate in the bulk phase and the partial molar internal energy in the adsorbed phase. This is given by the following equation DU Qst ¼ RT DN T , V where N is the adsorption loading and U is the internal energy of the sorbate in the adsorbed phase that includes contributions from both adsorbate-adsorbent and adsorbate-adsorbate interactions.38 Adsorption selectivities were calculated from binary adsorption isotherms to examine the competitive adsorption between two components. To examine the results from molecular simulation, the Langmuir-Freundlich (LF) adsorption model is adopted39 N ¼

Mkf β 1 þ kf β

where f is the fugacity of the bulk gas at equilibrium with the adsorbed phase, and M, k, and β are model parameters of maximum adsorption amount, the affinity constant, and the deviation from the simple Langmuir equation, respectively. First-Principles Calculations. First-principle calculations were introduced to investigate the vdW interaction between TTB and the small gas molecules to predict some detailed information (including configurations of gases adsorbed in TTB and energy surface of the constraint channel) and verify

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Figure 3. Comparison of simulated and experimental adsorption isotherms for N2 in organic crystal TTB at the high-pressure and lowpressure region of the isotherm at 77 K (solid line, experimental data from ref 26 at 77 K; square, simulations during this work; dotted line, fitting of Langmuir-Freudrich equation to simulation results).

our GCMC results. However, a standard DFT functional, in which the dispersion energy is not correctly included for vdW complexes, either gives sporadic results or fails completely. Many techniques have been recommended for DFT to treat weak interactions accurately.40-45 Grimme and Antony reported numerous encouraging results by using DFT-D.46,47 Our current calculations are performed with the TPSS-D,48 BP86-D,49,50 and BLYP-D51,52 methods and the Dunning correlation-consistent basis sets Aug-cc-pVTZ, to predict the interaction of the vdW complexes. The DFT-D calculations were performed using the modern program ORCA.53 Basis set superposition error (BSSE) was not taken into account during the DFT-D calculations, as suggested by Grimme.42 The Gaussian 03 programs were used for CCSD(T)/CBS calculations to benchmark the DFT-D results. The details of CCSD(T)/CBS calculations were described in the Supporting Information. The corresponding interaction energies obtained from single-point energy calculations were corrected with the BSSE by means of the counterpoise scheme of Boys and Bernardi.54

’ RESULTS AND DISCUSSION Validation of Force Field. We optimized the force field by adjusting the standard Dreiding force field parameters by scaling their intermolecular parameters as mentioned by Carlos Nieto.34 Figure 2 demonstrates the good agreement between experimentally measured adsorption isotherms of hydrogen on crystal TTB at 77 K and those obtained from simulations. The factor σ is kept still, and the factor ε is adjusted as described in the following equation

εnew ¼ 0:5εDreiding This modification reflects a reduction of the vdW cohesive energy of H2 with respect to the original Dreiding force field. Figure 2 also shows the fitting of the LF equation to GCMC simulation results for the adsorption of H2 in TTB. On the whole, the fitting is quite good with the R-square value of 0.998. From Figure 3 we can observe acceptable agreement between experimental data and simulation results over the whole range of pressures for the N2 isotherm without any extra modification to the force-field parameters. The fittings of the LF equation to GCMC simulation results are good with the R-square values of 0.957 and 0.850 for low and high 4937


The Journal of Physical Chemistry C pressure, respectively. As a result, we believe the adjusted parameters could also be suitable for CH4 and CO2 adsorption.

Figure 4. Adsorption isotherms calculated for CO2 and CH4 in organic crystal TTB at 273 K. The dotted lines are fits of the LangmuirFreundlich equation.

Figure 5. Qst as a function of loading for H2, N2, CO2, and CH4 at 273

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Adsorption of Pure Components. The isotherms computed from single-component GCMC simulations for H2 and N2 at 77 K and for CH4 and CO2 at 273 K are shown with the available experimental results in Figures 2-4. Figure 2 reveals that the adsorbed amount increases gradually with rising pressure for H2. Compared with adsorption behavior of H2, the loading of N2 reaches saturation at the pressure less than 0.1 bar (Figure 3). The adsorption isotherm of CO2 and CH4 at 273 K is depicted in Figure 4. As in most MOFs, CO2 is more strongly adsorbed than CH4 because of the strong quadrupole moment of CO2. The fitting of the LF equation is pretty good in view of the large number (20) of data points evaluated with both R-square values being 0.999. Besides the adsorption isotherm, the Qst is another important parameter reflecting interactions between the adsorbate and the adsorbent. The Qst values for pure H2, N2, CO2, and CH4 at 273 K in organic crystal TTB are clarified in Figure 5. The values of

Figure 7. Adsorption selectivities of N2, CH4, and CO2 over H2 in a 1:1 binary mixtures at 273 K.

Figure 6. Equilibrium snapshots of gases (including H2, N2, CH4, and CO2, represented in cyan, black, magenta, and red, respectively) adsorbed at 100 kPa (in the middle column) and at 3000 kPa (in the bottom column) on organic crystal TTB. The molecular structure was extracted from the snapshots to the top column of the figure for clarity. K.

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Figure 8. Comparison of calculated IEs of (left column) ethyne-gas (H2 (A), N2 (C), CH4 (E), CO2 (G)) complexes and benzene-gas (H2 (B), N2 (D), CH4 (F), CO2 (H)) complexes at the DFT-D/Aug-cc-pVTZ levels with the ones estimated at the MP2/CBS and CCSD(T)/CBS levels. The green lines between the two molecules denote the direction of scanning, and the x-axis donates the distance between center of mass of each molecule. Apart from these complexes, another two cuts of PES for ethyne- or benzene-gas (H2, N2, CH4, and CO2) complexes are listed in the Supporting Information.

Qst are increasing in the order of H2 < N2 < CH4 < CO2, suggesting the organic crystal is a good candidate for potential hydrogen purification, fuel storage, and carbon dioxide removal from the air. CO2 exhibits the highest Qst over the pressure range. The initial decrease of Qst is a consequence of the heterogeneous character of the organic crystal surface, in which the more energetically favorable sites of conjugated surroundings in the channel for adsorption are occupied first, and then the less favorable sites of the void space interconnected by the channel are occupied as the loading increases. Figure 6 illustrates the

equilibrium snapshots of the four gases at 100 and 3000 kPa, respectively. For H2, the snapshots at low and high pressure are almost the same, which means the adsorption capacity of channel and void space is similar. Different from H2, the loading of the other three gases at 100 kPa is concentrated in the channel due to the large affinity of conjugated conditions to the gases. This finding is in accordance with the fact that the Qst of H2 is the lowest among the gases taken into consideration. Actually, these characteristics are the key points in separating H2 from other gases. 4939



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Figure 10. Stable structures of channel-gas (a, H2; b, N2; c, CH4; d, CO2) complexes with channel frozen at the TPSS-D/cc-pVTZ level.

Figure 9. (Upper) Modified model used to compute the gas-channel interaction energies. The gas molecule translates along z with respect to the principal axis of the channel. (Lower) IE of the host and the guest at each z.

Selectivity of Hydrogen from Binary Mixtures. Adsorptive separation in a binary mixture of components i and j is characterized by selectivity S = (xi/xj)(yj/yi), where xi and yi are the mole fractions of component i in the adsorbed and bulk phases, respectively. The simulated selectivities of the N2, CH4, and CO2 over H2 mixtures were demonstrated in Figure 7. The selectivity of N2/H2 remains nearly constant over the whole pressure range. However, the selectivity of CH4/H2 initially decreases obviously with increasing pressure and then approaches a platform because of the counterbalance between energetic and packing effects. At low pressure, CH4 molecules in organic crystal TTB are preferentially occupied in the constricted channels, and the small volume of these tight channels gets saturated rapidly. Consequently, CH4 molecules tend to adsorb in less favorable sites with increasing pressure, and the selectivity decreases accordingly. The trend of selectivity of CO2 over H2 is similar to the one of CH4. At low pressure, the selectivity of CO2/H2 reaches 400, which is much higher than the others. When the pressure increases, the value decreases gradually and saturates at around 30. As expected, the order of the selectivity is consistent to the Qst. First-Principle Calculations. The configurations of gases adsorbed in TTB and the energy surface of the seamless channel are investigated by first-principle calculations. Our DFT-D methods are benchmarked by the CCSD(T)/CBS results as displayed in Figure 8. The CCSD(T) method represents the most robust tool for the evaluation of correlation energy of molecular clusters and should preferentially be used.55,56 We chose the complexes ethyne-gas and benzene-gas as the models to select the suitable functional, since the conjugated channel mainly contains phenyl and alkynyl groups. By comparing with CCSD(T)/CBS results, TPSS-D is chosen and utilized for the investigation on the adsorption of small molecules into the constraint channel of TTB. On the other hand, the TPSS-D functional has also been applied successfully for many similar systems,25,57,58 implying the advanced nature of the TPSS-D functional in treating van der Waals interaction of dimers.

To increase the computational efficiency, the channel structure was modified properly as represented on the top of Figure 9. Our channel model is efficient and adequate because the key parts of the channel are included. The interaction energy (IE) values between the channel and a single gas molecule moving along z are predicted in the increasing order of channel-H2 , channel-N2 < channel-CH4 < channel-CO2 as described in the bottom of Figure 9. This order is coincide with the Qst results calculated using the GCMC method. Another feature can also be figured out: the curve of the IE for H2 is relatively flat, while the curves for the other three gases fluctuate a lot. This suggests that the heterogeneous characters of the conjugated channel have little effect on H2 adsorption but obviously influence the adsorption of the other gases. The phenomenon can also be observed in equilibrium snapshots of gases adsorbed in organic crystal TTB as shown in Figure 6, where H2 distributed homogeneously in the channel, while the snapshots of the other gases have break points along the channel even at high pressure. In addition, the preferential adsorption configurations of gases in the channel are optimized at the TPSS-D/cc-pVTZ level with several special points as the initial configurations. The most stable points are displayed in Figure 10, and the IEs are 1.88, 6.29, 6.28, and 7.30 kcal/mol for H2, N2, CH4, and CO2 at the TPSSD/aug-cc-pVTZ level, respectively. Due to the limitation of the channel size, all the centers of mass of the gases are stable on the principal line of the channel. The H2 molecule is perpendicular to the principal line of the channel, while the N2 molecule is parallel to the line. The configuration of the host-CO2 complex is found to be similar to that of host-N2 with the carbon atom of CO2 upon the conjugated rings. For the host-CH4 complex, the C2 axis of CH4 overlaps the principal line of the channel.

’ CONCLUSION We have performed multiscale computational calculations to investigate the selectivity of N2, CH4, or CO2 over H2 in microporous organic crystal TTB. Due to the lack of accuracy of standard force fields in describing the weak vdW interactions, a new force field, based on a modified version of the Dreiding force field, has been proposed. The available experimental data for N2 over the whole pressure range confirmed the transferability of the new force field. Our GCMC simulations based on the modified force field indicate that the selectivity of N2, CH4, or CO2 over H2 is in the increasing order of N2/H2 < CH4/H2 < CO2/H2, 4940


The Journal of Physical Chemistry C which is consistent with the order of isosteric heats of adsorption. Very interestingly, at low pressure the selectivity is very high because CO2, CH4, or N2 occupy the preference site in the channel, and the small volume of these tight channels gets saturated rapidly. In the first-principle calculations, the results at the TPSS-D/ Aug-cc-pVTZ level are in excellent agreement with those obtained from CCSD(T)/CBS for the interaction energies. So the TPSS-D/Aug-cc-pVTZ method was used to investigate the vdW interaction between gases and the constrained channel. The IEs between the channel and the gas molecules were predicted in the increasing order of channel-H2 , channel-N2 < channelCH4 < channel-CO2, which corresponds to the GCMC results. Additionally, the heterogeneous energy surface in the channel has little effect on the distribution of H2 but influences the adsorption of the other gases obviously.

’ ASSOCIATED CONTENT

bS

Supporting Information. The details of CCSD(T)/CBS calculations, charge of TTB molecule, another two cuts of PES for ethyne- or benzene-gas (H2, N2, CH4, and CO2) complexes, and the fitting parameters of the LF equation to GCMC simulation results. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Fax: (þ) 86-431-85099521. Phone: (þ) 86-431-85098652. E-mail: [email protected]. Permanent Address §

Departement de Chimie, Faculte des Sciences, UniVersite du Burundi, BP 2700 Bujumbura, Burundi.

’ ACKNOWLEDGMENT Financial support from the NSFC (Nos. 50873020, 20773022), the JLSDP (20082212), and the Fundamental Research Funds for the Central Universities (09ZDQD06) is gratefully acknowledged. ’ REFERENCES (1) Furukawa, H.; Ko, N.; Go, Y. B.; Aratani, N.; Choi, S. B.; Choi, E.; Yazaydin, A. O.; Snurr, R. Q.; O’Keeffe, M.; Kim, J.; Yaghi, O. M. Science 2010, 239, 424. (2) Tranchemontagne, D. J.; Mendoza-Cortes, J. L.; O’Keeffe, M.; Yaghi, O. M. Chem. Soc. Rev. 2009, 38, 1257. (3) Hayashi, H.; Côte, A. P.; Furukawa, H.; O’Keeffe, M.; Yaghi, O. M. Nat. Mater. 2007, 6, 501. (4) Park, K. S.; Côte, A. P.; Choi, J. Y.; Huang, R.; Uribe-Romo, F. J.; Chae, H. K.; O’Keeffe, M.; Yaghi, O. M. Proc. Natl. Acad. Sci. 2006, 103, 10186. (5) Guo, H.; Shi, F.; Ma, Z.; Liu, X. J. Phys. Chem. C 2010, 114, 12158. (6) Doonan, C. J.; Tranchemontagne, D. J.; Glover, T. G.; Hunt, J. R.; Yaghi, O. M. Nature Chem. 2010, 2, 235. (7) El-Kaderi, H. M.; Hunt, J. R.; Mendoza-Cortes, J. L.; C^ ote, A. P.; Taylor, R. E.; O’Keeffe, M.; Yaghi, O. M. Science 2007, 316, 268. (8) McKeown, N. B.; Budd, P. M. Chem. Soc. Rev. 2006, 35, 675. (9) McKeown, N. B.; Gahnem, B.; Msayib, K. J.; Budd, P. M.; Tattershall, C. E.; Mahmood, K.; Tan, S.; Book, D.; Langmi, H. W.; Walton, A. Angew. Chem., Int. Ed. 2006, 45, 1804.

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Adsorption of Gases in Microporous Organic Molecular Crystal, a

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