Molecular structure, morphology and growth mechanisms and rates of 5-hydroxymethyl furfural (HMF) derived humins

We apply ATR-FTIR spectroscopy, Scanning Electron Microscopy (SEM) and Dynamic Light Scattering (DLS) experiments to investigate the molecular structure, morphology and growth mechanism of 5-hydroxymethyl furfural (HMF) derived humins as a function of HMF conversion. Our FTIR data support a reaction pathway in which humins form either through a ring opening mechanism and/or through substitution at the α or β position via nucleophilic attack. The addition of DMSO as a co-solvent leads to significant changes in the FTIR spectra of humins. We find that the nucleophilic attack pathway is suppressed in the presence of DMSO co-solvent and rationalizes the very small humin particles (∼100 nm) observed in SEM images contrary to the large particles (with multimodal size distribution and largest particles of up to 3-4 μm) observed in neat water. DLS experiments under several reaction conditions further confirm the particle size distribution observed via SEM. A plausible reaction network for humin formation, which rationalizes qualitatively our experimental results as well as those reported in the literature, is also postulated.