Hydrogenation versus isomerization in a,b–unsaturated alcohols reactions over Pd/TiO2 catalysts

Liquid phase hydrogenation and isomerization of some ,-unsaturated primary and secondary alcohols have been investigated in tetrahydrofuran over a 2.5% TiO2 supported palladium catalyst at 303K and 0.01MPa partial hydrogen pressure. The double bond isomerization reaction of these substrates leads also to formation of the corresponding saturated aldehydes or ketones. Catalytic activity and selectivity were found to depend strongly on the steric and electronic effects of the substituents on the double bond of the alcohol. The less crowded is the olefinic bond of the unsaturated alcohol, the higher is the activity. Formation of two -alkyl palladium bonded intermediates was postulated to explain the different selectivity towards hydrogenated and isomerized products observed for ,-unsaturated alcohols used. 2-Propen-1-ol exhibited the highest activity and selectivity for double bond migration leading to propanal up to 80% yield.