Cellulose, a major component of renewable biomass, is a polymer of glucose. Abundant and cheap iron salts promote the conversion of glucose to 5-hydroxymethylfurfural (HMF) and levulinic acid (LA). In this study, glucose transformations catalyzed by iron (III) chloride (FeCl3) in aqueous and in biphasic media (water and 2-methyltetrahydrofuran (MeTHF)) were investigated. Until now, FeIII was thought to be the main active sites. We show compelling evidence that FeIII is reduced to FeII (over 95%) readily in the early stage of carbohydrate conversion and FeII is the active catalytic species in the reaction. The speciation of iron under the relevant reaction conditions was elucidated by mass spectrometry (MS), UV-Vis, and operando X-ray absorption spectroscopy (XAS). The reaction time profiles of reactants (glucose and fructose) as well as products (HMF and LA) were modeled using MATLAB to obtain reaction rate constants. The contributions of iron and the intrinsic Brønsted acidity of iron salts in the sugar conversion are discussed. The kinetic study of sugar conversion indicated that the water-MeTHF biphasic system can achieve higher yields and selectivity of products (HMF and LA) as a result of slower kinetics for HMF reactions, but more importantly due to the previously unrealized suppression of fructose degradation to humins in the active aqueous phase as a result of having MeTHF present. This work helps identify avenues that favor HMF or LA production. By adjusting concentrations of FeII and Brønsted acidity, yields of 88% LA (FeCl3, pH=1) or 56% HMF (FeSO4, pH=2) from glucose in a water-MeTHF biphasic system are achieved. The optimized reaction conditions proved effective in the conversion of milled poplar biomass to LA (53% yield based on glucose content) and furfural (64% yield based on xylan content) using iron salt, outperforming aluminum and chromium salts.

Speciation and kinetic study of iron promoted sugar conversion to 5-hydroxymethylfurfural (HMF) and levulinic acid (LA)

251st National Meeting (2016)

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