390087 5-Hydroxymethyl Furfural Derived Humins: Growth Rates and Elucidation of Their Molecular Structure
5-hydroxymethylfurfural (HMF) is an extremely valuable biomass derived chemical, since it can be transformed to specialty chemicals, polymer precursors or fuels (e.g. 5-ethoxymethylfurfural (EMF), 2,5-dimethylfuran (DMF), p-xylene etc.). HMF is usually synthesized by hexose dehydration. However, the HMF yields are typically low due to side reactions such as further HMF 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 their structure and formation mechanisms. Furthermore, their formation rates are typically determined indirectly based on the concentration of water soluble products. The aim of this work is to address both of these issues.
HMF was dissolved in acidified aqueous solutions (CHCL=0.05-1M) and heated at different temperatures (50-120oC). The FTIR spectra of humin particles changed with HMF conversion indicating, that the functional groups present in the polymer also change with time. The most pronounced difference appears at a band at ~1650 cm-1 that can be ascribed to carbonyl groups. This band is more pronounced at the particles collected at low HMF conversions and its absorbance decreases with time indicating the disappearance of these groups. The effect of HCl concentration and temperature on apparent humin growth rates and activation energies were also determined. The apparent activation energies were in the range of 20 - 40 kJ/mol, which is less than what typically reported in the literature based on indirect estimations of humin formation rates. Our findings will be discussed considering the possible humin structures and formation/ growth mechanisms.
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