Reaction of small olefins on zeolite H-ZSM-5. A thermogravimetric study at low and intermediate temperatures

Abstract

Oligomerization and cracking reactions of ethene, propene, and isobutene on zeolite H-ZSM-5 (300 ≤ T < 600K) were investigated using temperature-programmed adsorption and desorption experiments, high-resolution 13C-NMR spectroscopy, and gas Chromatographic product analysis. Evidence is gained that at 300K only the stronger part of the Brønsted-acid sites are active in ethene oligomerization, while at increased temperatures more sites become active. On the contrary, in propene and isobutene oligomerization all sites are already active at 300K. This results in completely analogous products formed upon oligomerization of ethene, propene, and isobutene above 373K. The rate of oligomerization increases sharply with increasing reaction temperatures, resulting in a hindered transport of reactant molecules through the pores due to pore mouth blocking. The reactions on the outer surface become more important, which results in an increased branching of the oligomers formed at higher reaction temperatures. At 400K cracking of the oligomers starts, and at 490K the rate of cracking equals the rate of oligomerization. At this temperature desorption products show considerable branching, while at 573K only highly branched products are desorbed. At temperatures above 500K zeolite H-ZSM-5 becomes a dynamically operating catalytic system in the conversion of small olefins.