Mechanistic Insight into Favorable Aldehyde Hydrogenation over Hydroxymethyl Hydrogenolysis in 5‑Hydroxymethylfurfural by Formic Acid over Single-Atom Co‑N₃/C

Although the Co–N–C catalyst exhibits good catalytic performance toward the hydrogenation of 5-hydroxymethylfurfural (HMF) with formic acid (FA) as the H source, its catalytic mechansim is still unclear at the molecular level, including competitive hydrogenation of the aldehyde (−CHO) group and hydrogenolysis of the hydroxymethyl (−CH2OH) group. Here, a single-atom Co-N3/C surface was modeled as a Co–N–C catalyst with an armchair model of activated carbon support. Over Co-N3/C, the catalytic mechanism for the hydrogenation/hydrogenolysis of HMF with FA has been theoretically investigated in 1,4-dioxane solution at the GGA-PBE/DNP level. The hydrogenation product, i.e., 2,5-dihydroxymethylfuran (DHMF), should be predominant, whereas the hydrogenolysis products, i.e., 5-methylfurfuryl alcohol (5-MFA), 5-methylfurfural (5-MF), and 2,5-dimethylfuran (DMF), should be minor. The rate-determining steps are concerned with the cleavage of the C–H bond in the FA moiety through an intermolecular H-shift. For the crucial transition states, compared with the Co–C group in the hydrogenolysis of the −CH2OH group, the Co–O–C group in the hydrogenation of the −CHO group can more efficiently promote the cleavage of the C–H bond in the FA moiety, which stems from the electrophilic properties of aldehyde oxygen. Kinetically, Co-N3/C boosts the hydrogenation of the −CHO group to a −CH2OH group and selectively hampers the hydrogenolysis of the −CH2OH group to a −CH3 group.