Spectroscopic Characterization of Acid Catalyzed Hexose Derived Humins

In recent years, there has been a strong desire to replace petrochemical feedstocks with ones derived from non-edible biomass (i.e. cellulose). One approach for achieving this goal is converting biomass to intermediate platform molecules which can then be converted to a wide range of fuels or chemicals. 5-hydroxymethylfurfural(HMF) is an example of one such “top value-added chemical” because it can further be transformed to specialty chemicals, polymer precursors or fuels (e.g. 5-ethoxymethylfurfural (EMF), 2,5-dimethylfuran (DMF) and p-xylene etc.). HMF can be synthesized by the dehydration of carbohydrates, e.g. fructose. However, during fructose dehydration the yield to HMF is hindered by side reactions such as further rehydration to levulinic and formic acids as well as polymerization to insoluble low value humins. Despite their importance, very little attention has been devoted to understanding the structure of humins and the mechanism of their formation.

This work focused on characterizing the polymeric humin side products of the fructose to HMF reaction utilizing ATR/FTIR spectroscopy. We have investigated humins formed with different starting feedstocks (e.g. fructose vs. HMF). It was determined that in all cases a significant washing procedure including Soxhlet extraction was necessary to remove all adsorbed species on the humin structure, which included small concentrations of HMF, formic acid, and levulinic acid. It has been determined that there are slight differences in the infrared spectra due to the starting material. No structural differences were observed by changing the mineral acid or pH of the reaction solution. The analysis of the FTIR spectra will be presented and the tentative humin structures will be presented and discussed.