Understanding Acidity of Molten Salt Hydrate Media for Cellulose Hydrolysis by Combining Kinetic Studies, Electrolyte Solution Modeling, Molecular Dynamics Simulations, and 13C NMR Experiments

Depolymerization of lignocellulosic biomass in concentrated metal salts and more specifically in acidified LiBr molten salt hydrate (AMSH) results in high glucose yields at low acid concentrations, low temperatures, and very short times with potentially considerable economic benefits. However, our understanding of this promising medium is limited. Here, we study the effect of different LiBr concentrations on acidity and hydrolysis of cellobiose, a cellulose surrogate molecule, in dilute H2SO4 solutions. We use thermodynamic modeling to predict the H+(hydron) activity and the speciation arid correlate these with the experimentally measured reaction rates. We find that the main contribution of the salt to the reactivity stems from the dramatic increase in H+ activity and secondary to an interaction of salt with the acid species that effectively renders the inorganic acid very strong. We perform molecular dynamics simulations and reveal that the increased hydron activity can be attributed to the decrease in the number of water molecules in the hydron solvation shell upon salt addition. Additionally, we extend the analysis to other salts and acids, concluding that the effects of different cations, anions, and acids in cellobiose hydrolysis likewise can be attributed to primarily changes in acidity. A key physicochemical descriptor of various salts is their enthalpy of dissolution. Finally, we explore the use of C-13 NMR spectroscopy to estimate the pH of AMSH solutions.