Understanding the mechanism of enhanced alcoholysis of biomass carbohydrates to alkyl levulinates over bifunctional catalysts: does it resemble that in water?

The catalytic hydrolysis/alcoholysis of biomass carbohydrates into levulinic acid/levulinate esters (alkyl levulinates) as green biofuels and value-added chemicals holds great promise for achieving the goals of sustainable chemistry. Bifunctional catalysts with Bronsted- and Lewis-acidic sites have been widely reported to boost the efficiency of both processes. However, current understanding of the origin of the enhancement in alcoholic media is still in its infancy compared to that in aqueous media. Herein, we investigate the reaction process of glucose in methanol using a representative bifunctional catalyst (aluminum phosphotungstate) and its Bronsted-acid form (phosphotungstic acid), respectively. Isotopic labeling experiments were first applied to monitor the potential intermediates in glucose methanolysis. Coupling with mass spectrometric analysis and theoretical calculations, it has been revealed that Al3+ ions may accelerate the rate-limiting step by inducing ring opening of the key intermediate, methyl-d-glucopyranoside (MDGP), to d-glucose dimethyl acetal, rather than the prevalent speculation of facilitating the isomerization of MDGP to methyl fructoside. These findings refresh and reinforce our knowledge of the mechanism behind catalytically producing alkyl levulinates from biomass carbohydrates and provide a basis for the rational design of superior catalysts for biomass valorization.