Selective hydrogenation of cinnamaldehyde over Raney cobalt catalysts modified with salts of heteropolyacids

The hydrogenation of cinnamaldehyde to cinnamyl alcohol was carried out over Raney cobalt catalysts modified by Cu_3/2PMo₁₂O₄₀. It was found that the conversion of cinnamaldehyde decreases substantially with increases in the amount of Cu_3/2PMo₁₂O₄₀ deposited on Raney cobalt catalysts. The maximum yield of cinnamyl alcohol varies with the amount of deposited Cu_3/2PMo₁₂O₄₀. The more the deposited modifier, the higher the attainable maximum yield of cinnamyl alcohol. With 2.8% Cu_3/2PMo₁₂O₄₀ modified Raney cobalt, the selectivity for cinnamyl alcohol does not change with reaction temperature and reaction time. Thus, it is likely that the selectivity of ca. 80% for cinnamyl alcohol represents a specific value which is related to the nature of the surface modifier, Cu_3/2PMo₁₂O₄₀. The influences of the heteropolyanions and the competitive cations on the conversion and the selectivity were also investigated. It was found that copper salts of different heteropolyacids acting as the modifier of the catalyst bring about different enhancements in the selectivity for cinnamyl alcohol. The order of enhancement of selectivity in terms of heteropolyacid anion is PMo₁₂O^3?₄₀>SiMo₁₂O^4?₄₀>PW₁₂O^3?₄₀. There is little difference in selectivity by modification with (NH₄)₆Mo₇O₂₄ and Cu₂SiMo₁₂O₄₀. Raney cobalt catalyst modified by different 12-molybdophosphate containing cations of alkali, alkaline earth and transition metals gives a similar selectivity of about 75%. Among all of the 12-molybdophosphates, that containing the cation Cu²⁺ is the best in improving the selectivity of the catalysts. A selectivity as high as 83% was attained when an amount of 2.8% Cu_3/2PMo₁₂O₄₀ was deposited on the catalyst.