DIFFUSION OF SORBATES IN ZEOLITE-Y AND ZEOLITE-A - NOVEL DEPENDENCE ON SORBATE SIZE AND STRENGTH OF SORBATE-ZEOLITE INTERACTION

A systematic investigation of monatomic spherical sorbates in the supe rcages of zeolites Y and A by molecular dynamics technique is presente d. Rates of intercage diffusion, rates of cage visits, and the diffusi on coefficients have been calculated as a function of the sorbate-zeol ite interaction strength. These properties exhibit markedly different dependences on interaction strength for the two zeolites. The observed behavior is shown to be a consequence of the two principal mechanisms of intercage diffusion and the energetic barrier associated with them . The diffusion coefficient and other properties associated with inter cage diffusion are found to be directly proportional to the reciprocal of the square of the sorbate diameter when the sorbate diameter is si gnificantly smaller than the window diameter. As the sorbate diameter increases, a peak is observed in all the transport properties investig ated including the diffusion coefficient. We call this surprising effe ct as the ring or levitation effect and it explains several anomalous results reported in the literature and suggests a breakdown of the geo metrical criterion for diffusion of sorbates. It shows that under cert ain conditions nongeometrical factors play a major role and geometrica l factors become secondary in the determination of the molecular sieve property. A generalized parameter has been proposed which suggests co nditions under which one can expect the ring or levitation effect in a ny porous medium. Inverse size selectivity becomes operative under the se conditions.