PREDICTION OF LOW OCCUPANCY SORPTION OF ALKANES IN SILICALITE

Alkane sorption in a pentasil zeolite has been studied through molecular simulations. A series of alkanes, including methane, n-butane, and three hexane isomers, were studied in silicalite by using a detailed atomistic representation that allows for torsion around skeletal bonds. Statistical mechanical principles have been employed to predict sorption equilibria at low occupancy. Henry's constants and isosteric heats of sorption were calculated through the evaluation of configurational integrals with a Monte Carlo integration scheme. Results are in good agreement with experiment. The spatial distribution of sorbate molecules within the pore network, as well as perturbations to their conformation due to confinement in the pores, were determined via Metropolis Monte Carlo algorithm. Simulations of sorbate spatial distributions show that linear alkanes, such as n-butane and n-hexane, prefer to reside in the channels and avoid the channel intersections; on the contrary, bulky side groups in branched alkanes, such as 2- and 3-methylpentane, force these molecules toward the more spacious channel intersections. An analysis of sorbate conformations indicates that molecules are perturbed from the ideal gas dihedral angle distribution toward the more linear trans states when confined in silicalite. © 1990 American Chemical Society.