Kinetics of n-Hexane Cracking over Mesoporous HY Zeolites Based on Catalyst Descriptors

Abstract

A simple kinetic model based on the zeolite acid strength, the number of Brønsted acid sites, and the catalyst efficiency was developed for the cracking of n-hexane. A series of HY zeolites with a mesopore volume from 0.04 to 0.32 cm3/g was synthesized and characterized by various physical-chemical methods and tested for n-hexane cracking. The generation of mesoporosity influenced several other important parameters, such as acidity and extra-framework aluminum. Zero-length column diffusion measurements for mesitylene showed a large decrease in the characteristic diffusion time upon the introduction of mesoporosity, which changed only slightly with a further increase in mesoporosity. Similar n-hexane physisorption enthalpies were measured for all samples. The highest initial activity for n-hexane cracking per catalyst volume was observed for the sample with an intermediate mesopore volume of 0.15 cm3/g. The three mesoporous H-USY zeolites showed the same value of the intrinsic rate constant and the same activation energy. The difference in initial activity of the mesoporous zeolites was caused by the difference in the number of Brønsted acid sites. The increase in initial activity for the mesoporous zeolites compared to a microporous zeolite was caused by an increase in the acid strength.