Theoretical Insight into the Mechanism for the Cellobiose-to-Sorbitol Hydrogenation Over Diatomic Ru₂/NC Catalyst

Abstract

Ru/NC shows a good catalytic performance in cellobiose-to-sorbitol hydrogenation. However, the molecular origins of the selective orientation of the reaction pathway remain unclear. Here, we rationally designed the Ru 2 /NC catalyst, for which Ru2@N8 V4 is preferred as the model. The hydrogenation mechanisms for the hydrogenation of β-cellobiose to sorbitol employing H 2 as the H-source in aqueous solution have been investigated over Ru2@N8 V4 at the GGA-PBE/DNP level. For the hydrogenation of β-cellobiose to sorbitol, the optimal reaction pathway involves the ring-opening of cellobiose with H 2 O as a promoter and then the hydroreduction of aldehyde group, followed by the β-1,4-glycosidic bond hydrolysis. The selective orientation of the optimal reaction pathway originates from the dissociation of H 2 O on Ru-sites of Ru2@N8 V4 to form Brønsted acid (Ru−H + ) and Brønsted base (Ru−OH − ), which collaboratively promote the ring-opening. The rate-determining steps are relative to the β-1,4glycosidic bond cleavage, where an applicable π−π interaction between reactant molecule and Ru2@N8 V4 is of critical importance. Kinetically, the β-1,4-glycosidic bond cleavage from cellubitol is more favorable than that from β-cellobiose. For the hydrogenation of β-cellobiose to cellubitol, the first ring-opening with H 2 O as promoter and then hydrogenation are kinetically superior to the direct hydrogenation and ring opening. This derives from its dissociation over Ru-sites to Ru−H and Ru−OH groups. Predictably, protic solvents (HOR) are readily dissociated into Ru−H and Ru−OR at Ru-sites, which can promote the ring-opening of pyranring. The present research outcomes should contribute to the theoretical understanding necessary for the development of novel supported noble metal N-doped carbon catalysts for the hydrogenation of cellulose.