The Water-Mediated Reaction Pathway for Catalytic Opening of the Furanic Ring on Platinum Catalysts

Abstract

The fundamental understanding of C-O bond activation in bioheterogeneous catalysts is essential for the lignocellulosic upgrading reaction in the liquid phase. Yet, multifaceted solvent effects complicate the analysis of the atomistic reaction mechanism. The use of protic solvents in the conversion of biomass-derived furanics into chain alcohols, carboxylic acids, and amines can lead to high rates, but the origin of the solvent-mediated rate enhancements remains largely unknown. Here, we consider 2,5-bis(hydroxymethyl)furan (BHMF) as a model substrate and elucidate the significant role of water-mediated protonation in selectively cleaving particular C-O-C bonds through insight from quantum mechanics (QM) theory combined with solid experimental kinetic evidence. Depending on the solvent, we observe that the initial product formation rate in water is about 2 orders of magnitude larger than that in dioxane. We show that water participates directly in reductive C-O-C bond scission via assisted proton transfer, activating a low-energy barrier path. We find that a water molecule acting as a nucleophile then subsequently attacks the C(2) carbon atom to initiate a hydroxyl shift process in the intermediate, which then undergoes stepwise hydrogenation to produce a chain alcohol. Throughout this catalytic cycle, hydronium ions are generated spontaneously at the metal/water interface to indirectly impact the mechanism and kinetics of the reactions. Furthermore, we reveal how substituent groups (e.g., hydroxymethyl) affect the direct nucleophilic attack of the furan ring by water, a finding that rationalizes long-standing selectivity challenges in biomass conversion. Our QM calculations provide new insights into C-O bond activation in the liquid phase, highlighting the influence of the microsolvation environment on controlling the reaction path.