Atomistic simulation of hydrocarbon diffusion in silicalite

Molecular dynamics simulation using a burchart-DREIDING force field has been employed to determine the selfdiffusion coefficients of methane, ethane, propane, n-butane, isobutane, benzene, and pyridine in silicalite at 300 K. In the case of the alkanes, Einstein diffusion was observed early in a 500-ps simulation. Values of the diffusion coefficient for the alkanes ranged from about 9 x 10(-4) cm(2) s(-1) for methane to 2 x 10(-6) cm(2) s(-1) for isobutane at a loading of 0.5 molecule per unit cell (mpuc) or 2 molecules per simulation box. For the alkanes, there is good agreement between the results of simulation and experimental values of the self-diffusion coefficient obtained by microscopic techniques such as pulsed-field gradient NMR measurements. In the case of benzene and pyridine, anomalous diffusion was observed for time scales up to 9 ns and apparent diffusion coefficients (range of 10(-11) to 10(-8) cm(2) s(-1)) calculated form the Einstein relation increased with increased loading (up to 3.75 mpuc) and, as shown for pyridine, with increasing temperature. In the case of benzene where experimental data was available, the apparent diffusion coefficients calculated at high loadings were within an order of magnitude of experimental values. Heats of adsorption were calculated by a canonical Monte Carlo method. For the alkanes (methane, ethane, propane, and n-butane), the magnitude of the heats of adsorption linearly increased with the number of alkane carbons in qualitative and quantitative agreement with experimental data. Heats of adsorption calculated for isobutane and benzene also agreed well with experimental values. No experimental values were available for pyridine for comparison. (C) 1998 Elsevier Science B.V. All rights reserved.