Flame-made platinum/alumina: structural properties and catalytic behaviour in enantioselective hydrogenation

Flame spray pyrolysis (FSP) has been used for the production of alumina-supported platinum catalysts (1 to 10 wt% Pt on alumina). Liquid precursors containing specific amounts of aluminium isopropoxide and platinum acetylacetonate dissolved in xylene/ethylacetate were dispersed by oxygen and combusted, resulting in nanostructured powders. The as-prepared powders were collected on a filter and characterised by high-resolution transmission electron microscopy, hydrogen chemisorption, nitrogen adsorption, X-ray diffraction, and laser ablation inductively coupled plasma mass spectrometry. The specific surface areas of the powders ranged from 70 to 140 m2/g depending on the oxygen and precursor flow rates. Platinum was well dispersed and confined to the alumina surface. Platinum dispersion strongly depended on the platinum loading, decreasing from ca. 77 to 24% when the loading was increased from 0.1 to 0.9 mgPt m−2. These FSP-made catalysts showed higher activity for the enantioselective hydrogenation of ethyl pyruvate than a standard commercial platinum/alumina catalyst (E4759) with about the same platinum loading. Enantiomeric excess of (R)-ethyl lactate formation reached 87% at 100% conversion for FSP-derived catalysts pretreated in hydrogen. Turnover frequency increased with higher platinum loading, i.e., lower dispersion, indicating structure sensitivity of the reaction. In contrast, enantiomeric excess was virtually independent of platinum loading in the range 0.2–0.9 mgPt m−2.