Hydrogenation of carbon monoxide over vanadium oxide-promoted rhodium catalysts

Abstract

The effect of vanadium oxide as support and promoter on supported rhodium catalysts on the CO hydrogenation has been investigated at 0.15 and 4.0 MPa. Rh/V2O3 reduced at 723 K has a good selectivity toward oxygenated products, especially C2-oxygenates, but has a low activity and stability. Vanadium oxide added as a promoter to catalysts consisting of rhodium supported on silica and alumina showed a remarkable effect on the activity of these systems. For the silica-supported systems the activity increased by a factor of 40, the deactivation of these catalysts was low (2 % h−1) and the oxo-selectivity was very high (70 %). Although the vanadium oxide blocks part of the active metal surface, as became evident from a suppressed chemisorption capacity, it also enhances the rate of CO dissociation in those locations where reaction is still possible. The enhancement prevails over the blocking in the case of silica- and alumina-supported vanadium oxide-promoted catalysts, while blocking dominates for the vanadium oxide-supported catalyst after high temperature reduction. Experiments in which ethylene was added to a working catalyst, provided indications that the main promoter action of the vanadium oxide is to increase the CO dissociation, thereby increasing the activity of the catalyst. For the alumina-supported catalysts, most of the vanadium oxide is scavenged by the support and only at a high V/Rh ratio, the activity of the Rh/Al 2O3 catalyst is increased. The addition of vanadium oxide to the alumina-supported catalysts caused a suppression of the formation of ethers. The vanadium oxide probably covers the acidic ether-forming sites of the alumina support. During the first hours of reaction no acetic acid was observed for the alumina-supported catalysts due to a chromatographic effect. Initially, the acetic acid that was produced was adsorbed by basic sites of the support and only after prolonged reaction was acetic acid observed at the reactor outlet.