CO Hydrogenation to C2 Oxygenates over SiO2 Supported Rh-Based Catalyst: The Effect of pH Value of Impregnation Solution

Abstract

The synthesis of C2 oxygenates including ethanol directly from coal-derived syngas is significant from both academic and practical points of view and SiO2 supported Rh-based catalysts are very effective for this conversion. However, the high price of Rh requires the improvement of its dispersion to maximize Rh efficiency. The adjustment of impregnation solution pH value can modify effectively the metal dispersion over the support. Herein, we reported the pH effect on catalytic performance of Rh-Mn-Li/SiO2 for CO hydrogenation to C2 oxygenates for the first time. A series of catalysts were prepared from different pH values of impregnation solutions, and were characterized by various techniques. With the increasing of solution pH value above zero point of charge (ZPC) of silica, Rh particle sizes increased with much wider size distribution and reduction of Rh species was restrained over the prepared catalysts owing to the stronger interaction between Rh and support. As a result, the active sites for CO insertion, especially for CO or H2 dissociation was lowered, leading to the depletion of the activity for the formation of C2 oxygenates from CO hydrogenation, and larger Rh particle size with wider size distribution favors the production of long chain hydrocarbons. On the contrary, when the catalyst was prepared using solution with pH below ZPC of silica, the dispersion and the reduction of Rh were promoted due to suitable Rh-support interaction, and in the CO hydrogenation reaction, the space time yield and selectivity of C2 oxygenates reached 679.4 g/kg-cat/h and 73.3%, respectively. The pH value of impregnating solution regulate the metal-support interaction and thus Rh particle sizes and its reduction, which effects strongly CO hydrogenation activity and selectivity.