The Alpha-Bet(a) of Salty Glucose Pyrolysis: Computational Investigations Reveal Carbohydrate Pyrolysis Catalytic Action by Sodium Ions

Biomass pyrolysis is a promising technology for the production of renewable fuels and chemicals from nonfood biomass. Given the potential of pyrolysis as a viable, cost-effective biomass deconstruction method, there is active interest in understanding the chemical transformations at the heart of the technology. It has long been known that the presence of alkali- and alkaline-earth-metal ions in biomass, such as Na⁺, significantly alters product yields of biomass pyrolysis, but the mechanism behind this effect has not been elucidated. In this work, we employ density functional theory (DFT) to reveal the stereoelectronic basis of the effect of sodium ions on several key glucose thermal decomposition reactions, such as the formation of levoglucosan and 5-hydroxymethylfurfural (5-HMF). β-D-Glucose is of interest for pyrolysis, as it is the monomer of cellulose and a key intermediate in cellulose pyrolysis. α-D-Glucose is included in this study, as the two anomers can readily interconvert under pyrolysis conditions. The computational results are consistent with the experimental results for α- and β-D-glucose pyrolysis with NaCl, which demonstrate that the products are the same as those produced in neat pyrolysis, but with differing relative yields. We find that the sodium ion changes the reaction rate coefficients to varying degrees, with approximately 70% of the reactions in this study catalyzed by Na⁺, approximately 25% inhibited by Na⁺, and the remainder showing virtually no effect on the rate coefficient. The variations in how the ion modifies the rate coefficient reflect how the particular stereochemistry of the transition state interacts with the ion. The sodium ions have a more subtle effect on reactant electronic structure. The results of this study provide a molecular-level understanding of how naturally occurring salts act as catalysts in biomass pyrolysis. (Chemical Equation Presented).