Ti-doped γ-Al ₂ O ₃ versus ZSM5 zeolites for methanol to dimethyl ether conversion: In-situ DRIFTS investigation of surface interactions and reaction mechanism

Solid acids are promising catalytic materials for the dehydration of methanol vapor to dimethyl ether, which has recently been recognized as an ultraclean environmentally friendly fuel. In the present work, methanol conversion to dimethyl ether was studied on the surface of Ti(IV)-doped γ-Al ₂ O ₃ and over selected ZSM5 zeolites aiming at understanding the possible effect of the Ti ions, and to compare between the possible mechanisms over the studied catalyst surfaces. Doping γ-Al ₂ O ₃ with Ti resulted in modified textural properties and an enhanced overall surface acidity. In-situ DRIFTS study of adsorption showed that the presence of Ti resulted in the formation of more acidic hydroxyl groups on the surface leading to an enhanced adsorption of methanol and conversion to dimethyl ether. Comparison between the studied alumina-based solids and ZSM5 zeolites showed that the acidic character of their surface OH groups varied and therefore, different routes of methanol reaction were proposed for the two types of materials. Methanol adsorption was proposed to be associative on the surface of ZSM5 zeolites, where Brønsted acid sites played a key role in the adsorption and dehydration reaction. On the other hand, dissociative adsorption on Lewis acid-base pairs dominates the interactions with γ-Al ₂ O ₃ -based solids.