Modifications of ZSM-5 zeolite catalyst for dimethyl ether conversion to olefins

Abstract

The influence of modifications of the ZSM-5 catalyst on the conversion of dimethyl ether to olefins is studied at low hydrocarbon conversion in order to explore the possibility of selective formation of C₄⁼-C₆⁼. Three modifications designed in order to reduce catalyst acidity without increasing steric hindrance are compared, namely, dealumination, partial

neutralization with ammonia and ion-exchange with lithium. The unmodified H-ZSM-5 produces 60% of ethene and propene, 30% of olefins C₄⁼-C₆⁼and 10% of heavy olefins plus aromatics. In the same conversion range, the three modified catalysts promote the formation of C₄⁼-C₆⁼and even more the formation of C₇⁼-C₈⁼, and they lower the formation of light

olefins and aromatics. There is only a slight difference between modified catalysts:lithium exchanged ZSM-5 leads to higher ratio of C₄⁼-C₆⁼ to heavy by-products. An enhanced diffusivity in modified catalysts probably plays a role in the increase in the average molecular weight of olefins produced.