Textural Properties of a Large Collection of Computationally Constructed MOFs and Zeolites

Metal\u2013organic frameworks (MOFs) are porous crystals with the potential to improve many industrial gas adsorption and separations processes. Because MOFs are synthesized in a \u2018\u2018building block\u2019\u2019 fashion and can incorporate a wide range of organic linkers, an almost unlimited number of different MOFs are possible. Here, we applied high-throughput computational analysis methods to 137,000 hypothetical MOFs and calculated their geometric and adsorption properties. For every structure and its energy minimized counterpart, we calculated the underlying net (framework topology), pore limiting diameter, largest cavity diameter, accessible void volume, accessible surface area, as well as the Henry\u2019s constant and equilibrium loading of methane at 35 bar and 298 K. The analysis showed that these hypothetical MOFs have a wide range of geometric properties but lack topological diversity. The analysis also provides insights into the geometric method used to generate the hypothetical MOFs. Finally, we compared with hypothetical zeolites, finding that for the materials analyzed here, the MOFs tend to be more texturally diverse than the zeolites

Method for analyzing structural changes of flexible metal-organic frameworks induced by adsorbates

Metal−organic frameworks (MOFs) have crystal structures that exhibit unusual flexibility. An extreme example is that of the "breathing MOF" MIL-53 that expands or shrinks to admit guest molecules like CO2 and water. We present a powerful simulation tool to quickly calculate unit cell shape and size at 0 K for structures loaded with adsorbates. The method can be applied to unit cell minimization of periodic systems such as metal−organic frameworks and zeolites for vibrational analysis (IR spectra and mode analysis), force field development, and computation of elastic constants at 0 K. The expressions for first- and second-derivatives for rigid guest molecules that are missing in the literature are described in this paper. In addition, two case studies about determination of the structure of IRMOF-1 at 0 K and about the influence of water on the structure of MIL-53 showed that the simulation results correspond well with experimental results and other computational results. Our analysis scheme has significant advantages over other schemes, and the IRMOF-1 case study shows how these methods could potentially fail

RASPA: molecular simulation software for adsorption and diffusion in flexible nanoporous materials

A new software package, RASPA, for simulating adsorption and diffusion of molecules in flexible nanoporous materials is presented. The code implements the latest state-of-the-art algorithms for molecular dynamics and Monte Carlo (MC) in various ensembles including symplectic/measure-preserving integrators, Ewald summation, configurational-bias MC, continuous fractional component MC, reactive MC and Baker's minimisation. We show example applications of RASPA in computing coexistence properties, adsorption isotherms for single and multiple components, self- and collective diffusivities, reaction systems and visualisation. The software is released under the GNU General Public License

Elucidating steric effects on enantioselective epoxidation catalyzed by (salen)Mn in metal-organic frameworks

The steric effects of a metal-organic framework (MOF) on the enantioselectivity of a (salen)Mn were studied using classical atomistic modeling. Rotational energy profiles for the approach of 2,2-dimethyl-2H-chromene to the active site of (salen)Mn were mapped for the homogeneous catalyst and the catalyst immobilized as a linker in a MOF. The model corroborated that the Re enantioface is favored by the homogeneous catalyst. It was shown that the predicted enantioselectivity when chromene approaches the more accessible (salen)Mn in the MOF is highly sensitive to the distance between the (salen)Mn linkers of the interpenetrated frameworks. Calculated rotational energy profiles for a hypothetical non-interpenetrated MOF revealed that this MOF catalyst should exhibit enantioselectivity intermediate to the interpenetrated MOF and the homogeneous catalyst. Finally, the continuous chirality measure of the catalyst was found to correlate well with the energy of the homogeneous catalyst-reactant complex, suggesting that high chirality content is related to high enantioselectivity. This correlation, however, did not apply for some of the MOF catalysts. For heterogeneous catalysts, the mechanism of asymmetric induction depends on the steric environment in addition to the chirality of the catalyst

Self-diffusion studies in CuBTC by PFG NMR and MD simulations

Self-diffusion and relaxation time studies of C-3 to C-6 hydrocarbons adsorbed in the microporous metal-organic framework CuBTC were performed by nuclear magnetic resonance (NMR) in the temperature range of 193-373 K. The presence of paramagnetic copper species in the solid CuBTC framework leads to short longitudinal (T-1) and transverse (T-2) relaxation times of the hydrocarbons with typical values of T-1 less than or similar to 10 ms and T-2 less than or similar to 3 ms. Under these conditions, pulsed field gradient (PFG) NMR self-diffusion studies could only be performed at short observation times using the primary spin echo sequence with high-intensity pulsed magnetic field gradients. The obtained temperature dependent self-diffusion coefficients were analyzed using an Arrhenius approach. The activation energies of the alkanes are in the range of 6.5-8.5 kJ/mol, increasing slightly with increasing number of carbon atoms. Significantly higher values were found for propene (13.2 kJ/mol) and 1-butene (15.0 kJ/mol). These tendencies are consistent with corresponding measurements of heats of adsorption and with data obtained in molecular dynamics (MD) simulations. The MD simulations show a strong dependence of the heat of adsorption and diffusion on loading and temperature. This is caused by the preferential adsorption of small alkanes such as propane and butane in the side pockets of the CuBTC structure at low loading and temperature

Molecular Simulations of Adsorption and Diffusion in Crystalline Nanoporous Materials

The following sections are included: Introduction Benefits of computational experiments Nanoporous materials Pore system of a material Structure models The unit cell Structure file formats Where can you find zeolite and MOF structural information? Periodic boundary conditions and the minimum-image convention Force fields Running a simulation Units and conversions Outline Structural Properties Energy landscapes Characterization of the pore system of a material Surface areas, pore volume and pore-size distributions Molecular Mechanics (MM) Monte Carlo (MC) and Adsorption Basics NVT ensemble Adsorption: Gibbs and grand-canonical ensemble Enthalpy of adsorption Mixture adsorption simulations Ideal Adsorbed Solution Theory (IAST) Molecular Dynamics (MD) and Diffusion Newton’s equations of motion Thermo- and barostats Radial distribution functions Dynamic correlation functions Mean-squared displacement and single component diffusion Activation energy Free energy analysis and dc-TST Selectivity Breakthrough Simulation of Fixed-bed Separation Devices Introduction Modeling the fixed-bed Conclusion Acknowledgements References Appendix: RASPA Input for Computing Adsorption and Diffusion Properties Introduction Force field files for CO2 and CH4 in zeolites pseudo atoms.def atom definition file CO2.def molecule file methane.def molecule file nitrogen.def molecule file ITQ-29.cif framework definition file force field mixing rules.def force field definition file Blocked pocket definitions Structural properties of ITQ-29 Surface area Void-fraction Pore-size distribution Minimization of CO2 in ITQ-29 Gibbs single component adsorption of CO2 in ITQ-29 using CBMC CO2 single component adsorption using CBMC CH4 single component adsorption using CBMC Mixture adsorption of CO2/CH4 in ITQ-29 Mixture adsorption of CO2/CH4 in ITQ-29 using CBMC Dynamical properties of CO2 and CH4 in ITQ-29 MSD, VACF, and RDF of CH4 in ITQ-29 using MD Free energy profiles of CH4 in ITQ-29 at 300 K and 4 molecules per cavit