Catalytic and spectroscopic study of the allylic alcohol synthesis by gas-phase hydrogen transfer reduction of unsaturated ketones on acid-base catalysts

The hydrogen transfer reduction of an alpha,beta-unsaturated ketone, mesityl oxide, was studied on several single oxides in the gas-phase using 2-propanol as a hydrogen source. Selectivity is essentially determined by oxide electronegativity because the reaction proceeds via surface intermediates formed by coordination of 2-propanol and also of C=O and C=C groups of the reactant ketone on the Lewis acid sites provided by the metal cations. Oxides that combine weak Lewis acid cations and strongly basic oxygens such as MgO or Y2O3 yield allylic alcohols as the main reduction products of the gas-phase reaction. Adsorption experiments on MgO monitored by FTIR spectroscopy show that competitive 2-propanol and mesityl oxide adsorption on Mg2+ cations favors the hydrogen transfer process by a concerted Meerwein-Ponndorf-Verley mechanism whereas strong mesityl oxide adsorption causes simultaneous reduction of C=O and C=C bonds with formation of the saturated alcohol. High reaction temperatures strongly affect the stability of the complex reaction intermediates postulated for saturated and allylic alcohol formation, thereby favoring the simpler double bond migration reaction over the reduction reactions. More electronegative oxides such as ZrO2 or Al2O3 tend to promote the C=C bond reduction giving the saturated ketone. On these oxides, 2-propanol decomposes into acetone and molecular hydrogen at high, rates, which is detrimental to carbonyl reduction. (C) 2006 Elsevier B.V. All rights reserved.