A computer simulation investigation was made of the diffusion of propane and methane in this molecular sieve by using energy minimization and molecular dynamics techniques. Heats of adsorption and self-diffusion constants, were calculated using four sets of non-bonded interactions. Simulation results for large ensembles of methane atoms in a rigid-molecule approximation were presented. Methane was studied at infinite dilution and at loadings of 2, 4, 8, 12 or 16mol/unit-cell. The theoretical self-diffusion constants ranged from 1.15 x 10-4 to 2.0 x 10-5cm2/s at 300K, with an activation energy of 4.3kJ/mol; in excellent agreement with pulsed-field gradient spin-echo nuclear magnetic resonance measurements. The simulations of propane permitted free movement of all of the internal coordinates of the molecule and incorporated large ensembles in order to achieve accurate representations of bulk properties. Propane was studied at infinite dilution and loadings of 4 or 12mol/unit-cell. The corresponding theoretical self-diffusion constants were 2.3 x 10-5 and 6.0 x 10-7cm2/s at 300K. These simulated diffusion rates were 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 performed. This showed that the zig-zag channels were favored residence sites for both methane and propane. The calculated isosteric heats of adsorption of methane and propane were -5.8 and -10.3kcal/mol, respectively, in good agreement with experimental values.

Molecular Dynamics Simulation of Propane and Methane in Silicalite. J.B.Nicholas, F.R.Trouw, J.E.Mertz, L.E.Iton, A.J.Hopfinger: Journal of Physical Chemistry, 1993, 97[16], 4149-63

Table 45

Diffusion of Methyl Tertiary Butyl

Ether in Silicalite-1