Rh/NaY: A selective catalyst for direct synthesis of acetic acid from syngas

摘要

Marked differences in catalytic selectivity are observed for Rh catalysts on two different supports, NaY or SiO2, for the conversion of synthesis gas to hydrocarbons or C-2-oxygenates. High selectivity towards acetic acid is specific for the zeolite-supported catalysts; acetaldehyde is formed over Rh/SiO2. For Rh/NaY, the selectivity varies strongly during the first hours on-stream, indicating some reconstruction of the catalyst particles while the zeolite framework remains fully intact. Also the response to changes in pressure is unique: at 1 MPa the conversion is one order of magnitude lower than at 0.1 MPa, but the selectivity for acetic acid is two orders higher at 1 MPa. However, when the pressure is lower ed, the catalyst still "remembers" its high pressure activity and selectivity. Adding water vapor to the feed lowers the activity of Rh/NaY and increases the selectivity for methane and acetaldehyde. A qualitative rationalization of these findings is proposed, based on the assumption that Rh carbonyl clusters are formed in the zeolite cavities at high pressure of CO. Rh clusters, providing the metal ensembles required for CO dissociation, operate in concert with carbonyl clusters, which are instrumental for the CO insertion into metal alkyl bonds and the formation of acetate groups. (C) 1998 Academic Press. [References: 52]