Catalytic hydrothermal conversion of CO2 captured by ammonia into formate using aluminum-sourced hydrogen at mild reaction conditions

Abstract

The catalytic conversion of CO2 captured in aqueous media into formate was studied using aluminum-sourced hydrogen in a batch reaction system. To do so, the main ammonia-based CO2 absorption derivatives: ammonium carbamate, carbonate and bicarbonate and sodium bicarbonate were selected as CO2 source. The performance of the different species was determined under mild hydrothermal reaction conditions (120 °C), using Pd/C 5 wt% catalyst. In these conditions, the formate yield and selectivity increase in the order ammonium bicarbonate < sodium bicarbonate < ammonium carbonate < ammonium carbamate. Ammonium bicarbonate and sodium bicarbonate reagents needed higher temperature (250 °C) for an increased yield. Results with ammonium carbamate as starting material indicate a significant effect of time and catalyst content on formate yield, which ranged between 4 and 38%. Experiments with gaseous H2 showed that a comparable yield with Al can be obtained at a similar level of pressure. The reutilization and characterization of the reaction solid, comprising exhausted aluminum and Pd/C catalyst, showed that the aluminum was not completely oxidized up to the 5th re-use, and Pd can play a reducing role through the formation of palladium hydride species. The process can be improved by operating at higher pressure and lower temperature, to avoid loss of yield by dehydration of formate.