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Effect of Framework Flexibility on C Aromatic Adsorption at High Loadings in Metal-Organic Frameworks Jason A. Gee, and David S. Sholl J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10260 • Publication Date (Web): 08 Dec 2015 Downloaded from http://pubs.acs.org on December 9, 2015

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The Journal of Physical Chemistry

Effect of Framework Flexibility on C8 Aromatic Adsorption at High Loadings in Metal-Organic Frameworks

Jason A. Gee and David S. Sholl* School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA, 30332-0100

*Corresponding author: [email protected]

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ABSTRACT Many simulations of adsorption in nanoporous materials such as metal-organic frameworks (MOFs) treat the adsorbing materials as rigid. We show that the use of this approximation to describe the multicomponent adsorption of C8 aromatics in MOFs under industrial conditions gives results that differ dramatically from descriptions that include local framework flexibility. To address this issue, we develop an efficient method for capturing the effect of framework flexibility on adsorption in nanoporous materials. Our “flexible snapshot” method uses GCMC simulations to model adsorption in snapshots collected using fully-flexible MD simulations and can be applied to any framework-adsorbate system for which reliable force fields are available. Our method gives considerably better agreement with experiments for multicomponent C8 aromatic selectivities in multiple MOFs than more traditional calculations using a single rigid framework. The rotation of organic linkers in the MOFs has a strong influence on selectivities in these systems. Because many MOFs contain this structural feature, we expect that using simulations that incorporate this kind of internal flexibility will be important in obtaining accurate adsorption predictions in a range of circumstances. This is especially true for many industrially-relevant separations in MOFs, in particular, those that exploit high pore loadings of adsorbed species.

KEYWORDS: MOF, Flexibility, Molecular Dynamics, GCMC, Hybrid GCMC, Aromatics, Adsorption


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1. INTRODUCTION The separation of para-xylene (pX) from a stream of mixed C8 aromatic hydrocarbons is critical in the petrochemical industry. The industrial UOP PAREX process uses simulated moving bed (SMB) chromatography to achieve this separation.1 The traditional adsorbent used in this process is a Ba-exchanged form of zeolite X (BaX). This zeolite shows an enhanced affinity towards pX under saturation conditions that has been attributed to entropic effects that allow pX to stack more efficiently than the other C8 aromatics at high pore loadings.2 A comprehensive review on stacking mechanisms associated with adsorption in zeolites and metal-organic frameworks (MOFs) can be found in Torres-Knoop et al.3 In any separation that uses these effects, it is advantageous (and often necessary) to operate at conditions that correspond to high adsorbate densities. It is therefore useful to understand the physical phenomena that control adsorption selectivity in MOFs and similar materials under these conditions. Recent simulation work has calculated the adsorption properties of C8 aromatics in MOFs using conventional simulation methods. Castillo et al.4 and Granato et al.5 used generic force fields to describe single-component adsorption in MIL-47 and UiO-66, respectively, and found good agreement with experimental data. However, Castillo et al.4 found only qualitative agreement with experiments when modeling binary mixtures of C8 aromatics. Peralta et al.6 examined the adsorption of C8 aromatics in Cu-BTC and CPO-27 and determined that the adsorption mechanism for enhanced ortho-xylene (oX) selectivity in these systems was due to a combination of framework topology and electrostatic effects. Lahoz-Martín et al.7 investigated the separation of C8 aromatics in several MOFs and found that the size of the cavities and topology of the structure dictated their separation performance. More recently, simulation studies have used computational screening to identify MOFs and zeolites that possess high pX3 ACS Paragon Plus Environment


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selectivity and capacity from a large library of materials for industrial applications. For example, Torres-Knoop et al.8 used generic force fields to screen MOFs and zeolites for pX-selectivity at high loadings and identified a MOF with a predicted performance significantly better than BaX. In addition, Gee et al.9 screened the CoRE MOF database10 using generic force fields and found two MOFs that ultimately showed similar performance to BaX using breakthrough experiments. In both of these works, the enhanced pX separation ability of these materials was attributed to the preferential stacking of pX in the pores of these MOFs at high loadings. In all of the simulation studies just mentioned, calculations were performed by holding the adsorbing material rigid, typically in its reported crystal structure. This is a widely adopted approximation while simulating adsorption in nanoporous materials that has enabled screening calculations that have examined large libraries of materials for a variety of applications.10-13 There have been hints, however, that this approximation is not always appropriate, particularly for conditions involving high adsorbate densities. For example, Vlugt et al. found that the effect of framework flexibility significantly influences the adsorption properties of molecules such as isobutane and heptane at high loadings in silicalite.14 More recently, Wang et al.15 used experiments to examine the crystal structure of MIL-47 during single-component adsorption of a number of hydrocarbons from a bulk liquid phase and found subtle but distinct adsorbateinduced variations in the crystal structures of the framework. We subsequently performed GCMC simulation of multicomponent C8 adsorption in a number of these crystal structures, and found that the calculated adsorption selectivity varied strongly between them.16 These observations raise the possibility than including the local framework flexibility that controls the differences between these crystal structures may be necessary to make accurate predictions of adsorption selectivity in materials of this kind. 4 ACS Paragon Plus Environment

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The effect of framework flexibility on adsorption is typically not considered in simulations because there are numerous examples where using the rigid framework approximation have generally given good agreement with experimental data.17-20 In addition, there is typically a very significant increase in computational cost to model this effect in GCMC simulations. There are of course examples where chemical bonds form between adsorbates and a framework or where crystals can undergo dramatic structural transitions upon adsorption.21-24 The former situation can be identified simply by considering the nature of the chemistry available in the system of interest. In the latter, structural transitions are often indicated by the presence of steps in experimentally measured isotherms. In both of these cases, including framework deformation22,23,25 or considering adsorption in a set of possible metastable crystal structures21,24,26 is necessary to accurately describe adsorption using simulations. In this paper, we are concerned with systems where adsorbate-framework interactions occur by physisorption only and where single-component isotherms follow a simple shape with no obvious evidence of steps or other complicating effects. These two factors would often be taken to support that validity of simulating adsorption using a rigid crystal structure. We show below by focusing on multicomponent adsorption of C8 aromatics in MOFs that this is not always the case. If framework flexibility is important in describing adsorption in MOFs or similar materials, there is a great need for simulation methods that address these effects in an efficient way. Although hybrid Monte Carlo can be used in this context,27,28 this approach is often computationally inefficient to a level that prevents its routine use in calculations aimed at materials discovery. The main goal of this work is to develop an efficient methodology for describing the effect of framework flexibility on MOFs at conditions of high pore loadings. Throughout the paper, we focus on the adsorption of C8 aromatics in MOFs, although the 5 ACS Paragon Plus Environment


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methods we introduce will be broadly applicable. In section 2, we describe the development of our “flexible snapshot” method for characterizing framework flexibility effects on adsorption. In the next section, we evaluate the magnitude of this effect by comparing to conventional simulations using a rigid crystal structure. Finally, we show that including framework flexibility makes a marked difference in the ability of simulations to quantitatively predict the adsorption selectivity of C8 aromatics in MOFs. 2. SIMULATION DETAILS Most details of our simulations were adapted from our earlier work, which reported GCMC calculations of a variety of C8 aromatics in MIL-47, DMOF-1, UiO-66, and IRMOF-1.16 As mentioned above, we found that small changes to the positions of the framework atoms in MIL-47 can significantly affect the multicomponent adsorption properties of C8 aromatic molecules at high loadings. In principle, hybrid Monte Carlo27,28 can be used to describe the effect of framework flexibility on adsorption for this system, provided that a suitable force field to describe the framework is available. This technique includes a molecular dynamics (MD) move that updates the atomic positions of the system along with conventional GCMC moves. This method has previously been used to describe structural transitions due to temperature and CO2 adsorption in MIL-53(Cr)22 and N2 and small alcohol adsorption in ZIF-8.23,25,29 We found, however, that this method was challenging to use for high loadings of C8 aromatics in MOFs because of the significant computational expense of the GCMC simulations for this system. In this paper, we explore a simplified method to account for framework flexibility in which GCMC is performed independently in a series of snapshots taken from flexible MD simulations of the empty MOF in a constant pressure and temperature ensemble (i.e. NPT MD). A related method has recently been used to efficiently calculate the diffusivity of small 6 ACS Paragon Plus Environment

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molecules in MOFs and zeolites.30-32 This method assumes that interactions between the adsorbates and framework do not have a significant effect on the framework dynamics and the resulting ensemble of framework structures that must be sampled. We investigate the strength of this effect in the Results and Discussion section. Our Grand Canonical Monte Carlo (GCMC) simulations were performed using the RASPA simulation code.33,34 The continuous fractional component Monte Carlo method of Dubbeldam et al. was used to improve the computational efficiency of these calculations.35 The GCMC simulations used an equilibration and production period of 5×105 Monte Carlo (MC) cycles. Each MC cycle consisted of a trial insertion, deletion, reinsertion, translation or rotation with equal probability. For our binary mixture adsorption simulations, the GCMC simulations included an additional MC move for exchanging adsorbed molecules to improve convergence. The adsorption selectivity for mixtures of C8 aromatics was calculated based on the conventional definition of selectivity provided in our previous works.9,16 The uncertainty was determined by dividing the simulation run into five blocks and calculating the standard error of the selectivity. The Lennard-Jones (L-J) potentials were shifted and cut at 13 Å in all calculations. The electrostatics were handled using the Ewald summation method with a relative precision of 10-6. The L-J and Coulombic interaction parameters for the adsorbate atoms were taken from the OPLS36 force field. The adsorbate-framework cross-terms were obtained using the LorentzBerthelot mixing rules combined with the DREIDING37 force field for the framework atoms. The bonded and non-bonded interactions of the framework were described using the force field of Yot et al.38 This force field has been shown to describe the structural properties of MIL-47 under a variety of different pressures with reasonable accuracy. The MIL-47 force field was extended to MOF-48 using parameters from the CVFF force field.39 The force field parameters 7 ACS Paragon Plus Environment


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for the framework atoms of both MOFs are provided in the Supporting Information. These force field parameters have been previously found to give good agreement with experiments for single component adsorption properties of the C8 aromatics in MIL-47.16 We will show below that selectivity in multi-component adsorption of these species is more sensitive to the underlying structure of adsorbing framework than single component adsorption. Our NPT MD simulations were performed using LAMMPS.40 All MD simulations were performed at T = 300 K and P = 1 atm. The temperature and pressure was controlled using a Nosé-Hoover thermostat and barostat with a decay period of 0.1 and 1.0 ps, respectively. All MD simulations used a timestep of 1.0 fs, equilibration period of 200 ps, and production period of 1 ns. MD simulations at finite loadings were initialized using the coordinates of the adsorbate atoms at the end of our GCMC simulations. Snapshots from our flexible MD simulations were taken every 200 ps from the production period of MD trajectories for a total of 5 snapshots. In the Supporting Information, we show that temperature has a minimal impact on the structural properties of the MOFs. Therefore, we used structures obtained from the T = 300 K trajectory in all of our adsorption calculations. The adsorption properties of the C8 aromatics were calculated using GCMC simulations in snapshots taken from a flexible MD trajectory. The convergence of these properties was tested by performing calculations in many different snapshots. We show later that C8 mixture selectivities are well-converged using only a small number of snapshots. Therefore, the selectivity data we present using the flexible snapshot method was averaged over five distinct snapshots. The uncertainty that we report for this data also includes the error in selectivity for the individual snapshots.


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3. RESULTS AND DISCUSSION To examine the effect of framework flexibility on C8 aromatic adsorption in MOFs using our flexible snapshot method, we began by analyzing binary C8 selectivities in a system for which experimental data was available for comparison. Figure 1 shows the simulated selectivities for ortho-xylene over ethylbenzene (oX/eb) in MIL-47 using flexible MD snapshots of the empty framework compared to the Density Functional Theory (DFT) energy-minimized rigid crystal structure taken from our previous work.16 We found similar results using the reported crystal structure from the literature41 in calculations for the rigid crystal structure. For convenience, we will label the DFT energy-minimized rigid crystal structure as “rigid crystal” for the remainder of this work. As shown in the figure, the oX/eb selectivities are relatively constant across the five snapshots. More importantly, the selectivities in the snapshots are approximately an order of magnitude lower than simulations with the rigid crystal structure. The leading factor behind this trend is a large increase in eb loading (0.92 mmol/g vs. 0.15 mmol/g) compared to a moderate decrease of oX loading (2.45 mmol/g vs. 4.05 mmol/g) in the flexible snapshots compared to the rigid crystal structure. This result indicates that the flexibility of the structure allows eb to stack more effectively than oX in the flexible MIL-47 structure compared to the rigid case. A similar effect was also observed for a mixture of oX/mX (see Supporting Information). The effect of framework flexibility is less significant for the separation of oX/pX and results in an increase in selectivity by a factor of two relative to the rigid crystal result. The structural properties of the MIL-47 framework that cause these changes in selectivities are discussed in detail later in this work



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Figure 1. GCMC-simulated binary selectivities for bulk equimolar mixtures of oX/eb in MIL-47 at T = 343 K and P = 5 kPa using our flexible snapshot method. The GCMC simulation results using the DFT energy-minimized rigid crystal structure are shown with a dashed line for comparison. Figure 2 compares our simulated binary C8 selectivities in MIL-47 to binary experimental breakthrough measurements from Finsy et al.42 Two important observations can be made from Figure 2. First, the selectivities predicted by our calculations that incorporate flexibility are qualitatively different than the results from the more conventional rigid crystal calculations. Second, the simulated selectivities in the flexible snapshots are in significantly better agreement with the experimental data than those from the rigid crystal structure. Although our simulations now capture the experimentally observed selectivities with reasonable accuracy, the simulations still systematically overestimate the binary selectivities. We expect that some of this deviation is associated with imprecision in the generic force fields that have been used in our calculations, although it would also be useful to repeat the experimental measurements to allow quantification of the experimental uncertainties.


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Figure 2. Binary selectivities for equimolar mixtures of C8 aromatics in MIL-47 calculated using the rigid crystal structure and flexible MD snapshots compared to experiments.42 Simulations were performed under the same conditions as the corresponding experiments (T = 343 K and P = 5 kPa). The black line indicates equality between the experimental and simulated values. To understand the effect described above in more detail, we studied structural features of the MOF that have an effect on the adsorption properties in this system. We began this analysis by selecting an order parameter representing a significant mode of flexibility of the framework. We observed substantial rotation of the organic linkers of the MOF based on visual inspection of the MD trajectory. We selected the O2-C3-C2-C1 linker torsion angle depicted schematically in Figure 3 as the order parameter for the MOF structure. As shown in Figure 3, this torsion angle has a high degree of flexibility and rotates up to ~±60° during the course of an MD simulation of the empty framework. In the rigid crystal structure and reported experimental crystal structure this value is fixed at approximately 0° and ±180°.



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Figure 3. (a) Schematic of the rotation of the O-C3-C2-C1 linker torsion angle in MIL-47 and (b) distribution of this torsion angle over the range ±180° from classical MD at T = 300 K and P = 1 atm. The torsion angles observed in the rigid crystal structure are shown in dashed lines for reference. Once we found a suitable order parameter governing the flexibility of the framework, we were able to test our assumption that the dynamics of the framework and adsorbate are decoupled. We compared the lattice constants and torsion angle distributions by performing MD simulations of the framework for different loadings of C8 aromatics. Figure 4 shows the distribution of the linker torsion angle in the empty MIL-47 structure and two different 12 ACS Paragon Plus Environment

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compositions of a mixture of oX and eb. These two compositions correspond to the absorbed mixtures observed in Figure 1 using GCMC simulations for both the rigid crystal structure and flexible MD snapshots. As shown in Figure 4, the distribution of linker torsion angle is not significantly affected by the presence of C8 aromatics for both cases, although the presence of aromatics makes torsion angles close to zero slightly more probable than in the empty structure. We also found that the lattice constants were similar in the empty and loaded frameworks (see Supporting Information). These results suggest that methods such as the one we used above that assume that the interactions of the framework and adsorbates are decoupled give a reasonable description of the system.

Figure 4. Distribution of the linker torsion angle for the empty and C8 aromatic-loaded MIL-47 structures over the range 0° to 60° at multiple adsorbed phase compositions. To elucidate the effect of the linker torsion angle on binary selectivities, we generated several MIL-47 structures with different fixed torsion angles. In each of these structures, the torsion angle is identical for all of the linkers in the unit cell of the structure to probe a single degree of freedom of the system. We then performed GCMC calculations with binary equimolar



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mixtures of C8 aromatics in each of these rigid structures. Figure 5 shows that the oX/eb selectivity decreases dramatically for structures with torsion angles between ϕ = 0° and 60°. By comparing these selectivities with the overall probability distribution of torsion angles in the MOF, we can see that this effect plays a significant role in the selectivity that is observed when including the framework dynamics. We weighted these selectivities based on the probability of observing a given torsion angle from on the flexible MD snapshots and determined oX/eb selectivity of 8.3. This value is ~4 times lower than the value observed in the rigid crystal structure. Although this analysis does not entirely explain the ~10 fold decrease in oX/eb selectivity in the rigid versus flexible MD snapshots, it clearly shows that the linker torsion angle plays a critical role in the selectivities observed in this system.

Figure 5. Distribution of the linker torsion angle in MIL-47 over the range 0° to 60° calculated using flexible MD simulations (left axis, blue curve) and oX/eb selectivities in subsets of structures with all torsion angles fixed at the same degree of rotation (right axis, red curve). Next, we applied our flexible snapshot method to study the separation of pX from a stream of mixed C8 aromatics under representative industrial conditions. The simulated pX selectivities are compared to experimental breakthrough data from our previous work.9 The 14 ACS Paragon Plus Environment

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simulations and experiments were performed using a bulk liquid phase of 1:2:1:0.33 oX/mX/pX/eb at T = 50 °C and P = 9 bar. In Figure 6, we compare the pX selectivities for an oX- (MIL-47) and a pX-selective (MOF-48) MOF. MOF-48 can be viewed as a dimethyl functionalized version of MIL-47. We previously showed that the functionalization of MIL-47 with dimethyl groups causes a change in pore topology and a resulting shift in selectivity in MOF-48.9 As shown in Figure 6, the simulated pX selectivities for both MOFs are ~30% lower in the flexible snapshots than in the rigid crystal structure. The selectivity difference between the rigid and flexible cases is less significant for quaternary compared to binary selectivities because the change in packing of one isomer tends to be canceled by another component. For example, the binary selectivities for pX/mX and pX/eb using the bulk quaternary mixture of C8 aromatics are ~70% higher and ~50% lower, respectively, in the rigid crystal structure compared to the flexible snapshots. The figure also shows that the results computed using the flexible MD snapshots are in better agreement with experiments compared to the rigid crystal structure. This result shows that framework flexibility has a substantial effect on the simulated pX selectivities in these MOFs and therefore should be included in simulations to accurately describe adsorption in these systems.



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Figure 6. Ternary selectivities for a bulk mixture of 0.33:1:2:1 ethylbenzene/o-xylene/mxylene/p-xylene at 50°C and 9 bar in MIL-47 and MOF-48. The simulated selectivities were calculated using the rigid crystal structure and flexible MD snapshots and compared to breakthrough experiments. Simulations were performed under the same conditions as the corresponding experiments. To examine the effect of framework flexibility on C8 aromatic adsorption in MOF-48, we performed a similar flexibility analysis as described above for MIL-47. Figure 7 shows that the linker torsion angle distribution for MOF-48. The maxima in this distribution exist because the presence of dimethyl groups in MOF-48 causes the linkers to rotate to avoid steric hindrance with neighboring linkers along the channel axis. The width of the distributions for MOF-48 and MIL-47 (Figure 7b) are similar, which suggests that the degree of framework flexibility is similar for these two MOFs. It seems likely from these two examples that this effect may be a common feature of BDC-containing MOFs and perhaps to other types of MOFs as well. The extension of this analysis to a more diverse set of MOFs would be an interesting topic for future study.


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Figure 7. (a) Distribution of the linker torsion angle in MOF-48 over the range -180° to 180° and (b) comparison of the widths of the linker torsion angle distributions for MIL-47 and MOF-48 over the range 0° to 60° using classical MD at T = 300K and P = 1 atm. The torsion angles observed in the DFT energy-minimized rigid crystal structure are shown in (a) as dashed lines. In (b), the torsion angles in MOF-48 shifted by ~-30° so that the equilibrium angle is ~0° to allow easy comparison to MIL-47.



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4. CONCLUSION In this work, we used a simple but potentially powerful methodology to efficiently incorporate the effect of framework flexibility in molecular simulations of adsorption in nanoporous materials. We demonstrated our flexible snapshot method by calculating multicomponent C8 aromatic adsorption selectivities in two MOFs at high loadings for which experimental data was available for comparison. We found that the addition of framework flexibility in these simulations significantly affects C8 aromatic adsorption in MIL-47 and MOF48. In addition, we showed that this effect helps to resolve much of the discrepancy observed previously between simulations using rigid frameworks and experimental data for ortho-xylene over ethylbenzene. In both examples, including framework flexibility reduces the predicted adsorption selectivity. There does not appear to be any intrinsic reason why this trend should be universal; there may be examples where similar inclusion of framework flexibility will increase predicted selectivities. We also used our method to address the structural properties that lead to changes in multicomponent C8 aromatic adsorption in flexible MOF frameworks at high loadings. We found that the rotation of the organic linker is a significant mode of flexibility in MIL-47 and MOF-48. We showed that adsorption in these systems is significantly impacted by this single degree of freedom. Because this is a common mode of flexibility of many MOFs, we suspect that this may be a general feature of adsorption in these systems. The extension of our method to other MOFs to determine the generality of this effect would be an interesting topic for future study. As used in this paper, our approach assumes that the presence of adsorbed molecules does

not strongly affect the ensemble of structures available in a flexible MOF. We provided evidence supporting this assumption for high loading xylene adsorption in MIL-47 and MOF-48. It seems 18 ACS Paragon Plus Environment

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unlikely that this assumption will be valid in all cases, but our results suggest a useful staged simulation approach to assist in estimating the impact of framework flexibility on adsorption properties. First, calculations can be performed using our flexible snapshot approach assuming no coupling between adsorbates and the framework degrees of freedom, and the results from these calculations can be compared to simulations with a rigid crystal structure. If the differences in the properties of interest (e.g. adsorbed amounts, adsorption selectivities) are negligible, then it is reasonable to conclude that framework flexibility can be neglected. If the effects of flexibility are pronounced, however, then it is possible to generate additional framework snapshots at adsorbate loadings of interest at loadings determined from GCMC simulations and to assess structural properties of the adsorbent under these conditions. In the calculations we reported, little change was seen in the adsorbent when this approach was used, supporting the validity of neglecting adsorbate-framework interactions in characterizing the impact of flexibility on adsorption. If substantial changes in the framework were detected in the presence of adsorbates, the flexible snapshot method could be reapplied using snapshots obtained in the presence of relevant adsorbate loadings to improve the precision of predictions of adsorption properties. The MOFs we have examined in this paper do not undergo crystallographic transformations during adsorption, unlike materials such as MIL-47 and ELM-11.43 Even in materials in which adsorption-induced phase transitions between crystal structures are observed, however, the effect of local framework motions on adsorption may have an impact on the details of adsorption. The methods we have discussed here should therefore be useful when used in combination with existing simulation approaches to provide a comprehensive description of adsorption in MOFs and related materials.



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ASSOCIATED CONTENT SUPPORTING INFORMATION AVAILABLE Additional comparison of C8 selectivities using our flexible snapshot method, comparison of simulated and experimental lattice constants, force field parameters, and analysis of effect of temperature on torsion angle distribution. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES (1) Sorbex: Industrial-Scale Adsorptive Separation, Advances in Separation Processes; Johnson, J. A.; Kabza, R. G., Eds.; Taylor & Francis, 1990. (2) Moïse, J.-C.; Bellat, J.-P. Effect of Preadsorbed Water on the Adsorption of p-xylene and m-xylene Mixtures on BaX and BaY Zeolites. J. Phys. Chem. B 2005, 109, 17239-17244. (3) Torres-Knoop, A.; Dubbeldam, D. Exploiting Large-Pore Metal–Organic Frameworks for Separations through Entropic Molecular Mechanisms. ChemPhysChem 2015. (4) Castillo, J. M.; Vlugt, T. J. H.; Calero, S. Molecular Simulation Study on the Separation of Xylene Isomers in MIL-47 Metal−Organic Frameworks. J. Phys. Chem. C 2009, 113, 2086920874. (5) Granato, M. A.; Martins, V. D.; Ferreira, A. F. P.; Rodrigues, A. E. Adsorption of Xylene Isomers in MOF UiO-66 by Molecular Simulation. Microporous Mesoporous Mater. 2014, 190, 165-170. (6) Peralta, D.; Barthelet, K.; Pérez-Pellitero, J.; Chizallet, C.; Chaplais, G.; SimonMasseron, A.; Pirngruber, G. D. Adsorption and Separation of Xylene Isomers: CPO-27-Ni vs HKUST-1 vs NaY. J. Phys. Chem. C 2012, 116, 21844-21855. (7) Lahoz-Martín, F. D.; Martín-Calvo, A.; Calero, S. Selective Separation of BTEX Mixtures Using Metal–Organic Frameworks. J. Phys. Chem. C 2014, 118, 13126-13136. (8) Torres-Knoop, A.; Krishna, R.; Dubbeldam, D. Separating Xylene Isomers by Commensurate Stacking of p-xylene within Channels of MAF-X8. Angew. Chem. 2014, 126, 7908-7912. (9) Gee, J. A.; Zhang, K.; Bhattacharyya, S.; Bentley, J.; Rungta, M.; Abichandani, J. S.; Sholl, D. S.; Nair, S. Computational Identification and Experimental Evaluation of MetalOrganic Frameworks for Xylene Enrichment. 2015. (10) Chung, Y. G.; Camp, J.; Haranczyk, M.; Sikora, B. J.; Bury, W.; Krungleviciute, V.; Yildirim, T.; Farha, O. K.; Sholl, D. S.; Snurr, R. Q. Computation-Ready, Experimental Metal– Organic Frameworks: A Tool to Enable High-Throughput Screening of Nanoporous Crystals. Chem. Mater. 2014, 26, 6185-6192. (11) Simon, C. M.; Kim, J.; Gomez-Gualdron, D. A.; Camp, J. S.; Chung, Y. G.; Martin, R. L.; Mercado, R.; Deem, M. W.; Gunter, D.; Haranczyk, M., et al. The Materials Genome in 20 ACS Paragon Plus Environment

Page 21 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Action: Identifying the Performance Limits for Methane Storage. Energy Environ. Sci. 2015, 8, 1190-1199. (12) Watanabe, T.; Sholl, D. S. Accelerating Applications of Metal–Organic Frameworks for Gas Adsorption and Separation by Computational Screening of Materials. Langmuir 2012, 28, 14114-14128. (13) Wilmer, C. E.; Leaf, M.; Lee, C. Y.; Farha, O. K.; Hauser, B. G.; Hupp, J. T.; Snurr, R. Q. Large-Scale Screening of Hypothetical Metal–Organic Frameworks. Nat. Chem. 2012, 4, 8389. (14) Vlugt, T. J. H.; Schenk, M. Influence of Framework Flexibility on the Adsorption Properties of Hydrocarbons in the Zeolite Silicalite. J. Phys. Chem. B 2002, 106, 12757-12763. (15) Wang, X.; Eckert, J.; Liu, L.; Jacobson, A. J. Breathing and Twisting: An Investigation of Framework Deformation and Guest Packing in Single Crystals of a Microporous Vanadium Benzenedicarboxylate. Inorg. Chem. 2011, 50, 2028-2036. (16) Gee, J. A.; Sholl, D. S. Prediction of Adsorption Properties of Cyclic Hydrocarbons in MOFs Using DFT-Derived Force Fields. J. Phys. Chem. C 2015. (17) Dubbeldam, D.; Calero, S.; Vlugt, T.; Krishna, R.; Maesen, T. L.; Smit, B. United Atom Force Field for Alkanes in Nanoporous Materials. J. Phys. Chem. B 2004, 108, 12301-12313. (18) Keskin, S.; Liu, J.; Rankin, R. B.; Johnson, J. K.; Sholl, D. S. Progress, Opportunities, and Challenges for Applying Atomically Detailed Modeling to Molecular Adsorption and Transport in Metal− Organic Framework Materials. Ind. Eng. Chem. Res. 2008, 48, 2355-2371. (19) García-Sánchez, A.; Dubbeldam, D.; Calero, S. Modeling Adsorption and Self-Diffusion of Methane in LTA Zeolites: The Influence of Framework Flexibility. J. Phys. Chem. C 2010, 114, 15068-15074. (20) Babarao, R.; Hu, Z.; Jiang, J.; Chempath, S.; Sandler, S. I. Storage and Separation of CO2 and CH4 in Silicalite, C168 Schwarzite, and IRMOF-1: A Comparative Study from Monte Carlo Simulation. Langmuir 2007, 23, 659-666. (21) Zang, J.; Chempath, S.; Konduri, S.; Nair, S.; Sholl, D. S. Flexibility of Ordered Surface Hydroxyls Influences the Adsorption of Molecules in Single-Walled Aluminosilicate Nanotubes. J. Phys. Chem. Lett. 2010, 1, 1235-1240. (22) Salles, F.; Ghoufi, A.; Maurin, G.; Bell, R. G.; Mellot-Draznieks, C.; Férey, G. Molecular Dynamics Simulations of Breathing MOFs: Structural Transformations of MIL-53(Cr) Upon Thermal Activation and CO2 Adsorption. Angew. Chem. Int. Ed. 2008, 47, 8487-8491. (23) Zhang, L.; Hu, Z.; Jiang, J. Sorption-Induced Structural Transition of Zeolitic Imidazolate Framework-8: A Hybrid Molecular Simulation Study. J. Am. Chem. Soc. 2013, 135, 3722-3728. (24) Zhang, C.; Gee, J. A.; Sholl, D. S.; Lively, R. P. Crystal-Size-Dependent Structural Transitions in Nanoporous Crystals: Adsorption-Induced Transitions in ZIF-8. J. Phys. Chem. C 2014, 118, 20727-20733. (25) Gee, J. A.; Chung, J.; Nair, S.; Sholl, D. S. Adsorption and Diffusion of Small Alcohols in Zeolitic Imidazolate Frameworks ZIF-8 and ZIF-90. J. Phys. Chem. C 2013, 117, 3169-3176. (26) Gee, J. A.; Sholl, D. S. Characterization of the Thermodynamic Stability of Solvated Metal–Organic Framework Polymorphs Using Molecular Simulations. J. Phys. Chem. C 2013, 117, 20636-20642. (27) Duane, S.; Kennedy, A. D.; Pendleton, B. J.; Roweth, D. Hybrid Monte Carlo. Phys. Lett. B 1987, 195, 216-222.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 23

(28) Chempath, S.; Clark, L. A.; Snurr, R. Q. Two General Methods for Grand Canonical Ensemble Simulation of Molecules with Internal Flexibility. J. Chem. Phys. 2003, 118, 76357643. (29) Zhang, K.; Zhang, L.; Jiang, J. Adsorption of C1–C4 Alcohols in Zeolitic Imidazolate Framework-8: Effects of Force Fields, Atomic Charges, and Framework Flexibility. J. Phys. Chem. C 2013, 117, 25628-25635. (30) Haldoupis, E.; Watanabe, T.; Nair, S.; Sholl, D. S. Quantifying Large Effects of Framework Flexibility on Diffusion in MOFs: CH4 and CO2 in ZIF-8. ChemPhysChem 2012, 13, 3449-3452. (31) Awati, R. V.; Ravikovitch, P. I.; Sholl, D. S. Efficient and Accurate Methods for Characterizing Effects of Framework Flexibility on Molecular Diffusion in Zeolites: CH4 Diffusion in Eight Member Ring Zeolites. J. Phys. Chem. C 2013, 117, 13462-13473. (32) Awati, R. V.; Ravikovitch, P. I.; Sholl, D. S. Efficient Calculation of Gas Diffusivity in Single-Component and Binary Mixtures of Spherical Adsorbates in Flexible 8MR Zeolites. J. Phys. Chem. C 2015. (33) Dubbeldam, D.; Calero, S.; Ellis, D. E.; Snurr, R. Q. RASPA: Molecular Simulation Software for Adsorption and Diffusion in Flexible Nanoporous Materials. Mol. Simul. 2015, 121. (34) Dubbeldam, D.; Torres-Knoop, A.; Walton, K. S. On the Inner Workings of Monte Carlo Codes. Mol. Simul. 2013, 39, 1253-1292. (35) Torres-Knoop, A.; Balaji, S. P.; Vlugt, T. J. H.; Dubbeldam, D. A Comparison of Advanced Monte Carlo Methods for Open Systems: CFCMC vs CBMC. J. Chem. Theory Comput. 2014, 10, 942-952. (36) Jorgensen, W. L.; Laird, E. R.; Nguyen, T. B.; Tirado-Rives, J. Monte Carlo Simulations of Pure Liquid Substituted Benzenes with OPLS Potential Functions. J. Comput. Chem. 1993, 14, 206-215. (37) Mayo, S. L.; Olafson, B. D.; Goddard, W. A. DREIDING: A Generic Force Field for Molecular Simulations. J. Phys. Chem. 1990, 94, 8897-8909. (38) Yot, P. G.; Ma, Q.; Haines, J.; Yang, Q.; Ghoufi, A.; Devic, T.; Serre, C.; Dmitriev, V.; Ferey, G.; Zhong, C., et al. Large Breathing of the MOF MIL-47(VIV) under Mechanical Pressure: A Joint Experimental-Modelling Exploration. Chem. Sci. 2012, 3, 1100-1104. (39) Dauber-Osguthorpe, P.; Roberts, V. A.; Osguthorpe, D. J.; Wolff, J.; Genest, M.; Hagler, A. T. Structure and Energetics of Ligand Binding to Proteins: Escherichia Coli Dihydrofolate Reductase-Trimethoprim, a Drug-Receptor System. Proteins: Struct., Funct., Bioinf. 1988, 4, 3147. (40) Plimpton, S. Fast Parallel Algorithms for Short-Range Molecular Dynamics. J. Comput. Phys. 1995, 117, 1-19. (41) Barthelet, K.; Marrot, J.; Riou, D.; Férey, G. A Breathing Hybrid Organic–Inorganic Solid with Very Large Pores and High Magnetic Characteristics. Angew. Chem. 2002, 114, 291294. (42) Finsy, V.; Verelst, H.; Alaerts, L.; De Vos, D.; Jacobs, P. A.; Baron, G. V.; Denayer, J. F. M. Pore-Filling-Dependent Selectivity Effects in the Vapor-Phase Separation of Xylene Isomers on the Metal−Organic Framework MIL-47. J. Am. Chem. Soc. 2008, 130, 7110-7118. (43) Tanaka, H.; Hiraide, S.; Kondo, A.; Miyahara, M. T. Modeling and visualization of CO2 adsorption on elastic layer-structured metal-organic framework-11: Toward a better understanding of gate adsorption behavior. J. Phys. Chem. C 2015, 119, 11533-11543. 22 ACS Paragon Plus Environment

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