Mechanistic Insights into the Hydrogenolysis of Levoglucosanol over Bifunctional Platinum Silica–Alumina Catalysts

Herein, we report on the hydrogenolysis of the biorenewable intermediate levoglucosanol (Lgol) over bifunctional platinum catalysts supported on silica–alumina in tetrahydrofuran solvent. 13C radiolabeling is used to confirm the ring rearrangement forming tetrahydrofurandimethanol. The reaction rate and product selectivity are comparable between 1.1 and 5.3 wt % Pt loadings, indicating that, at these metal loadings, the rate-limiting step is acid catalyzed. The measured zero-order dependence in hydrogen indicates that a non-rate-determining hydrogenation step follows an acid-catalyzed irreversible rate-determining step. The measured first-order dependence in Lgol indicates that the acid sites are not highly covered by Lgol. A physical mixture of Pt/SiO2 and SiAl catalysts displayed product selectivity similar to that of the Pt/SiAl catalyst, indicating that nanoscale proximity of metal and acid sites is not required to carry out Lgol hydrogenolysis selectively. As the Pt loading in Pt/SiAl catalysts is decreased, or when the bare SiAl support is separated from a Pt/SiO2 catalyst in a dual-layer configuration, the selectivity toward identified products decreases. These results suggest that degradation reactions are avoided when the reactive intermediates formed over acid sites are rapidly hydrogenated over metal sites. First-principles simulations are performed to investigate the energetics of the proposed reaction pathway. A detailed reaction mechanism for Lgol hydrogenolysis is proposed on the basis of a combination of the experimental and computational results. These findings provide a fundamental understanding of the catalytic conversion of levoglucosanol over bifunctional metal–acid catalysts, facilitating rationally designed processes to produce renewable chemicals from biomass-derived levoglucosenone.