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

The influence of water on Bronsted acid chemistryhas beenstudied extensively recently for the conversion of oxygen-containingreactants. The effect of water on hydrocarbon transformations is farless explored and understood. Here, we show that cumene dealkylationover H-ZSM-5 zeolites is strongly influenced by the location of Bronstedacid sites and the presence of water molecules inside zeolite pores.While increasing acid site density leads to enthalpic stabilizationof both adsorbed states and transition states due to a greater fractionof acid sites within more confined voids, the presence of water enhancesrates via entropic stabilization of transition states at the expenseof an increased intrinsic activation enthalpy. Both effects of waterand the acid site location on the improving reaction rate are discussed.This direct participation of water is contrasted with water-inducedmodifications to the catalyst surface, such as the generation andmobility of extra-lattice species. These extra-lattice species havebeen shown to influence reaction rates for more demanding chemicaltransformations such as alkane cracking but are less effective atpromoting the facile dealkylation chemistry studied here. We furtherreveal how the addition of moisture also induces positive consequenceson catalyst stability by limiting subsequent oligomerization reactionswith formed products.