Zeolite-supported metals by design: organometallic-based tin-promoted rhodium NaY catalysts

Rhodium-tin bimetallic particles entrapped in NaY cages were used to study the mechanism of tin-promotion in the selective hydrogenation of alpha,beta -unsaturated aldehydes. These model materials were obtained by chemical vap our deposition (CVD) and subsequent H-2 reduction of Sn(R)(4) (R=C2H5; C6H5 ) Onto reduced Rh/NaY samples that were prepared by ion-exchange (IE) or by chemical vapour deposition (CVD). In the former case, we have catalysts co ntaining appreciable amounts of proton, while non-acidic metal-in-zeolite s amples are obtained with CVD. TPRD studies indicate that the decomposition of tin precursors takes place on the surface of the rhodium particles only if the monolayer capacity is not exceeded. In addition, the mechanism of de composition is influenced by protons and by the tin precursor used. Carbony l DRIFT spectra reveal clear evidences of a surface tin-enriched Rh-Sn phas e only for proton-free CVD-based samples. In this respect, Sn(C6H5)(4) lead s to the formation of a higher tin coverage than that obtained from Sn(C2H5 )(4). In the selective hydrogenation of citral (3,7-dimethyl-2,6-octadienal ), the presence of protons was highly detrimental leading to the acetal for med by reaction with the solvent (ethanol). With proton-free catalysts, the formation of the saturated aldehyde and of the two unsaturated alcohols is observed. Selectivities could be influenced by both monolayer and multilay er deposits of tin on Rh/NaY, The promotion effect under running catalytic conditions is ascribed to the presence of non-ionic oxidised SnOx phases. ( C) 1999 Elsevier Science B.V. All rights reserved.