Transformation of butanes over ZSM-5 zeolites. Part 1.—Mechanism of cracking of butanes over H-ZSM-5

Abstract

The transformations of butane and isobutane over H-ZSM-5 have been studied at 773 K with a special emphasis on the mode of activation of butanes on the acidic sites. The change in the product selectivity with the total conversion of butane (and isobutane) was examined. At low conversion levels, butane is cracked exclusively by a carbonium-ion mechanism into three combinations of products, CH₄+ C₃H₆, C₂H₆+ C₂H₄, and H₂+ C₄H₈. The contributions of the three pathways are 42–44%, 38–36% and 20%, respectively. The ratio does not depend on the partial pressure of butane. As the degree of conversion increases, the formation of propane increases, indicating that the activation of butane by a hydride-transfer mechanism is involved. Thus, in the conversion range 20–40%, the contribution of the carbonium-ion mechanism is reduced to 50–60%, 40–50% of butane being activated by the hydride-transfer mechanism. The contribution of the hydride-transfer mechanism increases as the partial pressure of the reactant increases.

At very low conversion levels, isobutane undergoes cracking through a carbonium-ion mechanism to form (CH₄+ C₃H₆) and (H₂+ C₄H₈) with a ratio of 2:1. At higher conversion levels the activation by the hydride-transfer mechanism predominates. In the conversion range 13–24% this mechanism accounts for ca. 60% of isobutane activation.