Molecular dynamics simulation of propane and methane in silicalite

The authors have investigated the diffusion of propane and methane in the molecular sieve silicalite by computer simulation using energy minimization and molecular dynamics techniques. They present heats of adsorption and self-diffusion constants, calculated using four sets of nonbonded interactions, and compare them to experimental values. Extensive simulation results for large ensembles of methanes in a rigid-molecule approximation are presented. Methane is studied at infinite dilution and at loadings of 2, 4, 8, 12, and 16 molecules per unit cell. Theoretical self-diffusion constants range from 11.5 [times] 10[sup [minus]5] to 2.0 [times] 10[sup [minus]5] cm[sup 2]/s at 300 K, in excellent agreement with pulsed field-gradient spin-echo nuclear magnetic resonance measurements. Simulations of propane allowed free movement of all the internal coordinates of the molecule and incorporated large ensembles to achieve accurate representations of bulk properties. Propane is studied at infinite dilution and loadings of 4 and 12 molecules per unit cell. The corresponding theoretical self-diffusion constants are 2.3 [times] 10[sup [minus]5] and 6.0 [times] 10[sup [minus]7] cm[sup 2]/s at 300 K. These simulated diffusion rates are also in excellent agreement with NMR measurements. Center-of-mass time distributions were calculated and energy minimizations of the molecules within the zeolite lattice were done. 60 refs., 19 figs., 3 tabs.