Methanol production through combined carbon capture and hydrogenation: unravelling the benefits of two-step hydrogenation

The combined carbon capture and hydrogenation towards methanol was demonstrated using dipropylamine as carbon capture agent. Previous works highlighted the reaction to follow a complex mechanism, in which the carbamate species formed during carbon capture is hydrogenated into N-formamide species, which are subsequently hydrogenated into methanol. The rate limiting step was observed to be the second step. Therefore, this work aimed to improve the conversion of N-formamide species into methanol. Comparing Au/ZnO, Au/MgO, Au/CeO 2 , Au/ZrO 2 and Au/TiO 2 catalysts using N,N-dipropylformamide as starting material, it was concluded that the methanol productivity strongly correlates with the surface basicity of the catalyst support. This was rationalized in relation to the reaction mechanism, in which the first hydrogenation step of the formamide species is proposed to occur via a hemiaminal intermediate. While basic catalysts are required for the hydrogenation of N-formamides into methanol, the formation of N-formamides from N-carbamates requires acidic sites. Therefore, we proposed a two-step strategy within this work, using first an acidic catalyst bed (Pd/Al 2 O 3 ), followed by a basic catalyst bed (Au/ZnO). The novel process configuration was compared to the state-of-the-art single step process showing the complete reaction from CO 2 captured in dipropylamine in the form of carbamate to methanol. Here, the methanol productivity was observed to increase by ∼100× compared to the previously proposed single step configuration. • Co and Au-based catalysts were screened under a wide variety of reaction conditions • For the first time, no catalyst deactivation is observed for the studied reaction • Au supported on basic and amphoteric supports was demonstrated to enhance the hydrogenation of N-formamides to methanol • The mechanistic reason for slow N-formamide hydrogenation was unraveled • Acidic catalysts facilitate the hydrogenation of N-formamides into N-carbamates • Basic catalysts facilitate the hydrogenation of N-formamides into methanol • A novel process configuration was proposed and demonstrated based on mechanistic insights, increasing methanol production