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 (Ga+/GaH2+) 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 GaH2+ 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.

The deoxygenation of benzaldehyde over gallium-modified ZSM-5 catalysts results in two main products benzene and toluene, depending on the reaction conditions and pretreatment. In the absence of hydrogen, Ga/HZSM-5 catalyzes benzaldehyde decarbonylation resulting in benzene with CO as a side product. By contrast, when hydrogen is present, toluene (plus water) is the dominant product produced over reduced gallium sites by hydrogenation/hydrogenolysis. The formation of benzene and methane via dealkylation of toluene represents only a small contribution compared to the production of benzene via decarbonylation.Figure optionsDownload high-quality image (38 K)Download as PowerPoint slide