Vapor-phase hydrogenation of crotonaldehyde on titania-supported Pt and PtSn SMSI catalysts

Vapor-phase hydrogenation of toluene and selective hydrogenation of crotonaldehyde have been performed after reduction at low (523 K) and high (773 K) temperatures over Pt/TiO₂ and PtSn/TiO₂ catalysts prepared from the metal precursors tin(II) oxalate and H₂PtCl₆. A sequential impregnation method was used to prepare the bimetallic catalyst, wherein the tin precursor was loaded first. The surface characteristics of both catalysts were evaluated with X-ray photoelectron spectroscopy (XPS) after the reduction treatments. It was shown in this analysis that tin was mainly in an oxidized state in the bimetallic catalyst (about 80%) even after reduction at 773 K. The presence of tin and the increase in the reduction temperature strongly decreased the catalytic activity for toluene hydrogenation, but the opposite effect was observed for crotonaldehyde hydrogenation. After reduction at low temperature, the bimetallic catalyst was more active than the monometallic catalyst, but it was less selective toward hydrogenation of the carbonyl bond to yield the unsaturated alcohol. However, reduction at 773 K yielded a bimetallic catalyst that showed an initial activity five times higher than that of the Pt/TiO₂ catalyst reduced at the same temperature and a high selectivity (around 50%) toward the unsaturated alcohol, which was stable with time on reaction. Furthermore, the catalytic behavior of the bimetallic catalysts was also evaluated after reduction at even higher temperatures. Reduction at 873 K increased the catalytic activity in crotonaldehyde hydrogenation while the selectivity to crotyl alcohol remained at about 50% molar fraction; however, reduction at 973 and 1023 K caused a loss of catalytic performance in terms of both activity and selectivity.