(109b) Molecular Modeling of Nanoporous Materials for Gas Storage and Separations

Molecular modeling has become a powerful tool for predicting the adsorption of gases and liquids in nanoporous materials such as zeolites and metal-organic frameworks.  This talk will give an overview of some recent examples.  The thermodynamics of adsorption from the liquid phase tends to be complex and highly nonideal.  In collaboration with Gino Baron and Joeri Denayer, we have investigated the adsorption of n-alkane mixtures in the zeolite LTA-5A under liquid-phase conditions.  Advanced simulation techniques, such as parallel tempering, can capture the complex selectivity behavior observed in experiments such as selectivity inversion and azeotrope formation.  We have also combined simulation and experiment to determine the adsorption selectivities for propene/propane mixtures in a series of isostructural M-MOF-74 materials (M=Co, Mn, and Mg) with high concentrations of open metal sites. Adsorption experiments, ideal adsorbed solution theory, breakthrough experiments, first-principles calculations, and grand canonical Monte Carlo (GCMC) simulations reveal that Co-MOF-74 exhibits a high thermodynamic propene/propane selectivity (ca. 46), due to strong pi-complexation between the open Co2+ sites and the propene molecules. These high selectivities occur at ambient pressure and temperature, conditions favorable for industrial adsorptive separations. An unusual increase of the propene/propane selectivity with increasing pressure is observed due to emergent behavior of the system that arises from the appropriate pore size relative to the size of the propene molecules.  In the context of natural gas storage, we have recently developed a computational method for generating all possible MOF structures (subject to certain constraints) from a given library of MOF building blocks (nodes and linkers).  From a library of 102 building blocks, for example, we created over 137,000 hypothetical MOFs on the computer and screened them for methane storage using short atomistic GCMC simulations.  This method has the potential to greatly speed up the development of new MOFs for adsorption applications.