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Diffusion in High Occupancy Zeolites

Published online by Cambridge University Press:  28 February 2011

James Wei
James Wei*
Affiliation:
Department of Chemical Engineering, MIT
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Abstract

Diffusion of small molecules in zeolites most often takes place in the configurational diffusion region, where the molecular diameter is approximately the same or slightly greater than the channel diameter. Since two molecules may not pass each other in a pore, the random walk based diffusion equation does not apply under high occupancy conditions in zeolites with one-dimensional pores.

For zeolites with multi-dimensional pores, such as ZSM-5 and A, one sometimes encounters the counter-intuitive result that diffusivity dramatically rises with occupancy. There are two explanations for this behavior: one from irreversible thermodynamics and the Darken equation, which predicts that diffusivity will always rise with occupancy; the other from a Markov model of random activated jumps between low energy positions, such as pore crossings, which predicts that diffusivity will increase with occupancy if the activation energy of diffusion decreases with occupancy– such as due to swelling.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 111: Symposium O – Microstructure and Properties of Catalysts , 1987 , 413
Copyright
Copyright © Materials Research Society 1988

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