Flame spray synthesis of Pd/Al2O3 catalysts and their behavior in enantioselective hydrogenation

Flame spray pyrolysis (FSP) was used for the synthesis of alumina-supported palladium catalysts containing 1-7.5 wt% Pd. Precursor solutions of aluminum sec-butoxide and palladium acetylacetonate were sprayed and combusted, resulting in nanostructured materials that were characterized by high-resolution transmission electron microscopy, CO-pulse chemisorption, nitrogen adsorption, and X-ray diffraction. Well-dispersed palladium particles (1-5 nm) were confined to the alumina surface. Palladium dispersion depended on the metal loading and decreased for higher amounts of Pd. The FSP-derived alumina-supported palladium catalysts were chirally modified with cinchonine and tested for the enantioselective hydrogenation of 4-methoxy-6-methyl-2-pyrone. Rate and enantioselectivity decreased with higher Pd dispersion. A comparison of these catalysts to a corresponding commercial Pd/Al2O3 catalyst revealed lower reaction rate and enantioselectivity for the FSP-derived catalysts. However, hydrogen pretreatment of the flame-made catalysts at 500 degreesC improved rate and enantioselectivity strongly, whereas only a comparatively weak beneficial effect was observed with the corresponding wet-phase-derived catalysts. Enantiomeric excess in the formation of (R)-4-methoxy-6-methyl-5,6-dihydro-2-pyrone reached 80% for flame-made catalysts after pretreatment in hydrogen. Hydrogen pretreatment temperatures above 600 degreesC led to sintering of the Pd particles and drastic loss in activity and enantioselectivity. (C) 2003 Elsevier Inc.