Selective Hydrogenation of Aromatic Aldehydes Using Precious Metal Catalysts on New High Surface-Area TiO2 Supports

Abstract

New high surface-area TiO 2 extrudates, based on pyrogenic and precipitated TiO 2 , were used as supports for precious metal (Pd, Pt, Rh) catalysts. The precious metal catalysts were employed in the selective hydrogenation of aromatic aldehydes which have different substituents 1 (R = COOH, CH 3 , CI). Depending on the choice of catalyst and substrate, the selectivity of the hydrogenation reaction can be directed either toward a benzyl alcohol derivative 2 or toward a methyl-substituted compound 3. The benzyl alcohol is the reaction product obtained using catalysts which have weak or no acidic properties. If acidic catalysts are used, hydrogenolysis to the methyl-compound 3 dominates. Selectivity changes in the order from la to lc. If an alcohol is used as solvent, side reactions, e.g., ether formation or aromatic ring saturation, can be observed. In the case of catalysts on precipitated TiO 2 , the benzyl ether is obtained at high selectivity in almost quantitative yield. The rate of hydrogenation is determined by the precious metal profile as well as by the electronic properties of the substituents which are effective in the following order: COOH>CI>CH 3 .