(211g) Reaction Free Energies In Solutions for Glucose Conversion Into Biomass Intermediates

Sugar dehydration reactions are of great importance in the conversion of biomass into fuels and chemicals. In this study, we have computed reaction free energies in solution for glucose conversion into levulinic acid and levoglucosan (LG). We computed vapor phase energies using the G4 ab initio method in Gaussian and estimated solvation free energies using COSMO-SAC (conductor-like screening model-segment activity coefficient) model.  We used water and dimethyl sulfoxide (DMSO) as solvents. Our calculations predict glucose isomerization into fructose and mannose is reversible in both water and DMSO, whereas fructose dehydration into 5-(hydroxymethyl)furfural (HMF) is practically irreversible reaction. We predict that glucose dehydration into LG can be a reversible reaction depending on the reaction conditions as well as the solvent choice. In the vapor phase, at higher temperature, the entropic effect dominates and LG formation becomes favorable. It is found that glucose dehydration into levoglucosan is not thermodynamically feasible below 200^oC in aqueous media but can become feasible in organic solvents. These calculations provide guidance into how to selectively control sugar dehydration by changing the reaction conditions. Experiments conducted in our lab and from the literature are contrasted with model predictions and good overall agreement is found.