Enhancing the Synergistic Effect of Bifunctional Pd-Based Catalyst by Phosphonic Acid for Cellobiose Conversion to Sorbitol

Catalytic conversion of cellulosic biomass into polyols is an effective approach for biomass upgrading. However, the conversion involves cascade reactions which require various catalytic sites cooperation for activating specific steps, resulting in low efficiency. Here, we propose a bifunctional catalyst containing acid sites and hydrogenation capacity for polyol production with cellobiose as the model compound of cellulose. Phosphonic acids (PAs) containing either methyl (MPA) or aminomethyl (NH2MPA) tails were used to tune the Pd/Al2O3 catalyst to form monolayers, thereby modulating the surface physiochemical properties and introducing multiple catalytic sites. An increasing ratio of PdO/Pd and a strong metal–support interaction were observed after PA modification, which could boost the hydrogenation activity for cellobiose. Furthermore, Pd/Al2O3-MPA exhibited excellent hydrolytic hydrogenation activity due to the increase of Lewis acid sites with good durability under hydrothermal conditions by MPA modification, achieving a 78.5% yield of sorbitol at 180 °C for 5 h. A plausible mechanism for enhancing the synergistic effect of Pd and support over Pd/Al2O3-MPA was also proposed. This investigation provides a novel strategy for rationally designing hydrothermally stable bifunctional catalysts to utilize biomass in a more effective way.