Unifying Mechanistic Analysis of Factors Controlling Selectivity in Fructose Dehydration to 5-Hydroxymethylfurfural by Homogeneous Acid Catalysts in Aprotic Solvents

The need for liquid fuels from inexpensive and abundant biomass sources continues to increase in light of the growing environmental and strategic consequences of relying on depletable petroleum. Fructose, a monomeric sugar derived from biomass, can be dehydrated to 5-hydroxymethylfurfural (HMF), an intermediate to furans that are high-energy-content compounds miscible with petroleum. To be economically competitive, production of HMF from fructose must achieve high selectivities and yields, but the design of catalysts that achieve high-yield HMF production is made difficult by the lack of understanding of the mechanistic aspects of fructose conversion to HMF. Various studies examining fructose conversion to HMF by homogeneous acids have proposed that different factors control selectivity, including the acid type, fructose tautomer distribution, and solvent type. A high-throughput system was utilized to develop detailed insights into mechanisms and factors controlling HMF selectivity from fructose dehydration by homogeneous acid catalysts. The high-throughput system utilized a 96-well Hastelloy plate reactor to facilitate the development of extensive data over a range of aqueous solvent systems, temperature, time, acid types, and acid concentrations. In situ NMR was also employed to analyze the relative distribution of fructose tautomers as a function of solvent type and temperature. HMF selectivity was directly correlated with the distribution of furanose and open-chain tautomers of fructose as a function of reaction temperature, time, and solvent composition. The observed correlation supported the hypothesis that selectivity in fructose conversion to HMF is primarily controlled by the equilibrium between the tautomeric forms of fructose in solution. Further, it was identified that difructose anhydrides act as slow-converting, protective intermediates that increase HMF production during longer reaction times to a selectivity higher than that which would be predicted by the fructose tautomeric distribution alone.