Catalytic deoxygenation of benzaldehyde over gallium-modified ZSM-5 zeolite

The deoxygenation of benzaldehyde has been investigated over gallium-modified ZSM-5 catalysts. In the absence of H2, Ga/HZSM-5 catalyzes benzaldehyde decarbonylation resulting in benzene and CO. The active sites for this reaction are the strong Brønsted acid sites. In the presence of H2, the main product is toluene. It is believed that Ga cationic species image generated during H2 reduction can promote the hydrogenation/hydrogenolysis reactions that give toluene and water. In the absence of H2, toluene can only be observed in transient experiments when the Ga/HZSM-5 catalysts are reduced. It is suggested that the image species generated under H2 play an important role in the hydrogenation/hydrogenolysis. However, they readily decompose to Ga+ in the absence of H2. The addition of water to the feed modifies the catalytic activity and selectivity of Ga/HZSM-5 catalysts. On the one hand, water generates additional Brønsted acid sites from the reaction of extra-framework Ga with chemisorbed water (GaO(OH)) and with defect hydroxyls of the zeolite framework (GaOHSi). These additional sites enhance the production of benzene but decrease the production of toluene, due to a decrease in the density of reduced Ga cationic species.