Crystal size of MFI-type zeolites for catalytic cracking of n-hexane under reaction-control conditions

Abstract

A kinetic study of n-hexane cracking over MFI-type zeolite was conducted in the temperature range from 748 to 923K using a fixed-bed flow reactor under atmospheric pressure. The n-hexane cracking reaction exhibited first-order kinetics with respect to n-hexane concentration. In contrast, the activation energy for n-hexane cracking at high temperatures (823–923K) was greater than that at low temperatures (748–823K) because the major reaction mechanism shifted from monomolecular cracking to bimolecular cracking as reaction temperature decreased. Crystal size of the MFI-type zeolite needed to achieve reaction-control conditions, in which the active sites within zeolite are fully utilized during n-hexane cracking, was investigated using the kinetic data. The effect of crystal size and Si/Al ratio of the zeolite on the rate-limiting step of the n-hexane cracking could be expressed using the Thiele modulus. Results indicated that the size of the zeolite must be less than several hundred nanometers for the reaction to proceed under reaction-control conditions.