Mechanistic insights into the catalytic transfer hydrogenation of furfural to furfuryl alcohol over a N-doped carbon-supported Ni single atom catalyst from first principles

The carbon-supported Ni-single atom catalysts provide a promising strategy for the selective hydrogenation of furfural to furfuryl alcohol, but the detailed mechanism is still unclear. To gain insights into the mechanism of the catalytic transfer hydrogenation of furfural, first-principles calculations have been performed. The results suggested that the reaction proceeds via a series of hydrogen-transfer steps rather than direct H transfer. The whole reaction can be divided into two stages: dehydrogenation of the H-donor (isopropanol) and hydrogenation of furfural. In the first stage, neither Ni nor N atoms of the NiN4 site can serve as the active site for the initial O-H/C-H bond activation of the H-donor. The most favorable pathway is initiated by the O-H bond activation of isopropanol via a H-transfer to the non-coordinated pyridinic N atom. The Ni atom is responsible for the subsequent C-H bond activation to generate a metal-hydride bond. The second stage involves a H atom transfer from the Ni site to the carbonyl carbon of furfural, followed by the H transfer from the non-coordinated pyridinic N to the carbonyl oxygen. As such, the hydrogen atoms in both O-H and C-H maintain their identity in the catalytic transfer hydrogenation process. These findings can expand our understanding of the catalytic transfer hydrogenation process and might guide the design of efficient carbon-supported single atom catalysts.