Hydrogenation of diacetyl over composite-supported egg-shell noble metal catalysts

Two composite supports with a mixed inorganic-organic structure were synthesized: BTAl and UTAl. Hydrophilic-hydrophobic dual properties of the supports were suitable for preparing egg-shell-supported metal catalysts for selective hydrogenation reactions. The catalysts were characterized by ICP, XRD, OM, TEM, EPMA, XPS and TGA. Their mechanical resistance was assessed. Activity and selectivity were tested with the hydrogenation of 2,3-butanedione (diacetyl) to 3-hydroxy-2-butanoneacetoin (acetoin). The same order of increasing metal particle size was found for the two tested supports: Pt < Ru < Pd. The XPS analysis showed that the metal/composite catalysts reduced in H2 at 503 K had two kinds of active sites: reduced (MeA degrees) and electron-deficient (Me+). It was rationalized that the hydrogen bond cleavage was performed on the MeA degrees active sites, while reactant adsorption occurred on the Me+ sites. The differences in activity and selectivity between the composite catalysts were attributed to electronic effects on the different metals and to different adsorptive properties of the different polymers. The high selectivity to acetoin was attributed to the preferential adsorption of diacetyl as compared to the adsorption of acetoin. The BTAl catalysts were slightly more active and selective than the UTAl ones. This was attributed to electronic effects caused by remnant organic groups on the composite supports (urethane or biphenyl on UTAl or BTAl, respectively). Pd-BTAl was the most active and selective catalyst, a fact related to electronic effects of both palladium and the support.