Synergistic promotion of oxygen vacancy and Lewis acidity of Nb2O5 on the preferential hydroxymethyl hydrogenolysis of 5-hydroxymethylfurfural catalyzed by single atom Pt

Abstract

Pt₁/Nb₂O₅ exhibits good catalytic performance towards the hydrogenolysis/hydrogenation of HMF. However, the chemical nature that affects its activity and selectivity is not yet clear at the molecular level. For Pt₁/Nb₂O₅, two kinds of Pt-containing active sites are modelled, i.e., [–(NbO)PtNb(ONb)₅–] ([OPtNb]) in the absence of oxygen vacancy, and [–(NbO)PtNb(ONb)₄–] ([OPtNb-O_v]) in the presence of oxygen vacancy. Over both [OPtNb-O_v] and [OPtNb], the catalytic mechanism for hydrogenolysis/hydrogenation of 5-hydroxymethylfurfural (HMF) with H₂ as an H-source has been theoretically investigated in tetrahydrofuran solution at the GGA-PBE/DNP level. The hydrogenolysis of –CH₂OH (hydroxymethyl) groups to –CH₃ groups is predominated with the cleavage of –CH₂–OH bonds as the rate-determining step, whereas the hydrogenation of –CHO (aldehyde) groups to –CH₂OH groups is very minor with the addition of –CHO groups as the rate-determining step. Here, 5-methylfurfural (5-MF) is predominant, whereas 2,5-dihydroxymethylfuran (DHMF) is very minor. The strong Lewis acidity of Nb₂O₅ promotes the Pt-site to accept the lone pair electrons of the oxygen atom, in which the oxygen atom of the –CH₂OH group is more prone than that of the –CHO group to donate its lone pair electrons to the Pt-site. Thus, Pt₁/Nb₂O₅ facilitates the hydrogenolysis of the –CH₂OH group and relatively inhibits the hydrogenation of the –CHO group. Compared with [OPtNb], [OPtNb-O_v] displays higher catalytic activity. This stems from the promoting effect of oxygen vacancy on the capacity of the Pt-site to receive lone pair electrons of the oxygen atom in the –CH₂OH group.