(476f) Free Energy Landscape for Glucose Isomerization to Fructose

Cellulosic biomass represents an abundant renewable resource for the production of bio-based products and biofuels. 5-hydroxymethylfurfural (HMF) is a critical and versatile intermediate for converting biomass to liquid alkanes and many other value-added products. High HMF yields from fructose have been achieved. However, cost effective conversion of glucose, the dominant biomass monomer sugar, to HMF remains a critical issue. Here Car-Parrinallo ab initio molecular dynamics simulations (CPMD) coupled with metadynamics (MTD) simulations were conducted to investigate glucose to fructose isomerization reaction in acidic aqueous solution. The mechanisms, energetics and the rate-limiting steps were determined. Glucose to fructose isomerization is initiated by protonation of the C2-OH and the formation of a furanose aldehyde intermediate. This furanose aldehyde is also the intermediate to produce HMF, a critical and versatile intermediate for biomass conversion to biofuel and bio-based products. Fructose is produced via a hydride transfer from C2-H to C1-H on the furanose aldehyde followed by the rehydration of the C2 carbocation. This hydride 1,2 shift to form a C2 carbocation is an energetically favorable process but the barrier is relatively high at around 35 kcal/mol.  Fructose is produced by the rehydration of the C2 carbocation with an estimated barrier of 25 kcal /mol from our CPMD-MTD simulations.