Ammonium salts modified silica supported Rh–Mn–Li catalyst for CO hydrogenation to C2 oxygenates

Abstract

The silica (S0) was pretreated with slightly acidic (S1) or basic (S2) aqueous solutions of ammonium salts before using as a support to prepare Rh–Mn–Li catalyst for CO hydrogenation to C2 oxygenates. The results showed that the weak acidity of the modified silica stem from silanol groups where the number of free silanol groups increased as S0 < S1 < S2. This evolution could tune the interactions among support, Rh, and Mn components, and results in the variation of dispersion and chemical states of Rh species. Therefore, Rh/S1 catalyst with smaller Rh particle sizes and more Rh0 sites for CO dissociation exhibited higher CO hydrogenation activity, giving higher space time yield of C2 oxygenates of 766.1 g/kg/h than the untreated Rh/S0 catalyst (674.3 g/kg/h). Meanwhile, Rh/S1 catalyst has improved ethanol selectivity (19.3%) comparing with Rh/S0 (15.7%). Though Rh/S2 catalyst showed similar catalytic performance to Rh/S0 catalyst, it further increases the ethanol selectivity to 19.9%. The H2–TPD results suggested that the catalyst with high fraction of free silanol groups improved the migration of activated hydrogen spillover from metal Rh centers, where C2 oxygenates precursors adsorbed on the support could be hydrogenated into ethanol.