Promoting effect of SnOx on selective conversion of cellulose to polyols over bimetallic Pt–SnOx/Al2O3 catalysts

Abstract

Cellulose is the most abundant source of biomass in nature, and its selective conversion into polyols provides a viable route towards the sustainable synthesis of fuels and chemicals. Here, we report the marked change in the distribution of polyols in the cellulose reaction with the Sn/Pt atomic ratios in a wide range of 0.1–3.8 on the SnO_x-modified Pt/Al₂O₃ catalysts. Such a change was found to be closely related to the effects of the Sn/Pt ratios on the activity for the hydrogenation of glucose and other C₆ sugar intermediates involved in the cellulose reaction as well as to the notable activity of the segregated SnO_x species for the selective degradation of the sugar intermediates on the Pt–SnO_x/Al₂O₃ catalysts. At lower Sn/Pt ratios of 0.1–1.0, there existed electron transfer from the SnO_x species to the Pt sites and strong interaction between the catalysts, as characterized by temperature-programmed reduction in H₂ and infrared spectroscopy for CO adsorption, which led to their superior hydrogenation activity (per exposed Pt atom), and in-parallel higher selectivity to hexitols (e.g. sorbitol) in the cellulose reaction, as compared to Pt/Al₂O₃. The hexitol selectivity reached the greatest value of 82.7% at the Sn/Pt ratio of 0.5, nearly two times that of Pt/Al₂O₃ at similar cellulose conversions (∼20%). As the Sn/Pt ratios exceeded 1.5, the Pt–SnO_x/Al₂O₃ catalysts exhibited inferior hydrogenation activity (per exposed Pt atom), due to the formation of the crystalline Pt–Sn alloy, which led to the preferential conversion of cellulose to C₂ and especially C₃ products (e.g. acetol) over hexitols, most likely involving the isomerization of glucose to fructose and retro-aldol condensation of these sugars on the segregated SnO_x species, apparently in the form of Sn(OH)₂. These findings clearly demonstrate the feasibility for rational control of the cellulose conversion into the target polyols (e.g. acetol or propylene glycol), for example, by the design of efficient catalysts based on the catalytic functions of the SnO_x species with tunable hydrogenation activity.