Effect of Water on Cumene Dealkylation over H-ZSM-5 Zeolites

Abstract

The influence of water on Brønsted acid chemistry has been studied extensively recently for the conversion of oxygen-containing reactants. The effect of water on hydrocarbon transformations is far less explored and understood. Here, we show that cumene dealkylation over H-ZSM-5 zeolites is strongly influenced by the location of Brønsted acid sites and the presence of water molecules inside zeolite pores. While increasing acid site density leads to enthalpic stabilization of both adsorbed states and transition states due to a greater fraction of acid sites within more confined voids, the presence of water enhances rates via entropic stabilization of transition states at the expense of an increased intrinsic activation enthalpy. Both effects of water and the acid site location on the improving reaction rate are discussed. This direct participation of water is contrasted with water-induced modifications to the catalyst surface, such as the generation and mobility of extra-lattice species. These extra-lattice species have been shown to influence reaction rates for more demanding chemical transformations such as alkane cracking but are less effective at promoting the facile dealkylation chemistry studied here. We further reveal how the addition of moisture also induces positive consequences on catalyst stability by limiting subsequent oligomerization reactions with formed products.

Water structures on acidic zeolites and their roles in catalysis

“…Gešvandtnerová et al 151 observed that water molecules generated from catalytic reaction could mediate proton transfer processes and strongly stabilize the π-complexes of alkenes when performing DFT calculations to explore the monomolecular mechanism of isobutanol conversion to butenes on acidic chabazite zeolite. In another typical work of Chau et al , 152 enhanced rates were observed for cumene dealkylation on H-ZSM-5 zeolites upon water co-feeding. Upon water adsorption, protons from the framework can be delocalized in the form of hydronium ion clusters and the interactions between cumene and hydronium ion clusters were proposed to result in a reaction transition state with more degrees of freedom and thus a less negative activation entropy, comparing to the condition without the presence of water.…”

“…Above cases show that the active participation of water in catalytic reactions can be reflected by the formation/stabilization of surface intermediates and transition states, thus influencing the catalytic rates on zeolitic materials. 96,116,148–152…”