Selective Adsorption of Manganese onto Rhodium for Optimized Mn/Rh/SiO2 Alcohol Synthesis Catalysts

Using supported rhodium-based catalysts to produce alcohols from syngas provides an alternative route to conventional fermentation methods. If left unpromoted, Rh catalysts have a strong selectivity towards methane. However, promotion with early transition metal elements has been shown to be effective to increase alcohol selectivity. Therefore, a key design objective is to increase the promoter-metal interaction to maximize their effectiveness. This can be achieved by the use of the strong electrostatic adsorption (SEA) method, which utilizes pH control to steer the promoter precursor (in this case MnO4-) onto Rh oxide supported on SiO2. Mn-promoted catalysts were synthesized by both SEA and traditional incipient wetness impregnation (IWI) and subsequently characterized by STEM and extended X-ray absorption fine structure methods. Using STEM-electron energy loss spectroscopy mapping, catalysts prepared by SEA were shown to have a higher degree of interaction between the promoter and the active metal. The reduction behavior of the catalysts obtained by X-ray absorption near-edge spectroscopy and temperature-programmed reduction demonstrated a minimal change in Rh if promoted by SEA. However, catalytic results for CO hydrogenation revealed that a significant improvement of ethanol selectivity is achieved if the promoter was prepared by SEA in comparison with the promoter prepared by IWI. These results suggest that intimate interaction between the promoter and the metal is a critical factor for improving selectivity to higher alcohols.