ROLE OF SILVER PROMOTER IN CARBON-MONOXIDE HYDROGENATION AND ETHYLENE HYDROFORMYLATION OVER RH/SIO2 CATALYSTS

The effect of silver promotion on CO hydrogenation and ethylene hydroformylation over Rh/SiO2 has been studied. Catalyst characterization before reaction reveals that Ag and Rh form separate crystallites; part of the Ag is atomically spread on the surface of Rh crystallite and blocks the Rh ensembles required for the bridge CO. Although both reactions restructure the catalyst surface and increase the ratio of bridge to linear CO sites on AgRh/SiO2 catalysts, the ratio of the bridge to the linear CO sites remains lower on Ag-Rh/SiO2 than on Rh/SiO2 under reaction conditions. Ag is found to increase the selectivity and the rate of formation for acetaldehyde (C2 oxygenate) during CO hydrogenation and for propionaldehyde (C3 oxygenate) during ethylene hydroformylation on Rh/SiO2. The increased activity and selectivity for C2 and C3 oxygenates is attributed to a high ratio of the linear CO to bridge CO sites and a possible presence of isolated Rh+ sites under reaction conditions. The results are consistent with a previous report that the single Rh atom site that chemisorbs linear CO is active for CO insertion. © 1992.