Benzene, benzophenone, toluene and benzylalcohol are byproducts in the selective deoxygenation of benzoic acid to benzaldehyde on ZnO and ZrO2. In this paper, the pathways to the byproducts are discussed and a complete overview of the reaction network is presented. Benzene and benzophenone are products of the decomposition of the strongly adsorbed benzoic acid at elevated temperature. Toluene is formed by the deoxygenation of benzaldehyde at oxygen vacancy sites. Direct hydrogenation of benzaldehyde yields benzylalcohol. Selectivity to byproducts is not only determined by the reaction temperature, but also by the degree of surface reduction and the difference in adsorption strength between reactant and product. At temperatures <360° benzoic acid forms a strongly benzoate layer which is stable and prevents readsorption of benzaldehyde. Reduction of the catalyst by hydrogen leads to a small number of oxygen vacancy sites which deoxygenate benzoic acid to benzaldehyde and as a result selectivity to benzaldehyde is very high (>95%). Higher temperatures induce decomposition of the benzoate layer into benzene and more importantly enhance the degree of reduction of the catalyst, which results in the formation of toluene from benzaldehyde.
de Lange, M. W., van Ommen, J. G., & Lefferts, L. (2002). Deoxygenation of benzoic acid on metal oxides. 2. Formation of byproducts. Applied catalysis A: general, 231, 17-26. https://doi.org/10.1016/S0926-860X(01)00702-5