Understanding Humins: Molecular Characterization and Growth Rates
University of Delaware
Dynamic light scattering (DLS) and ex situ Fourier transform infrared spectroscopy (FTIR) experimental techniques were utilized to investigate growth rates and chemical structure of humins formed during biomass conversion. We show experimental proof that a rigorous procedure is needed to remove adsorbed products from the humin structure. These products consist mostly of 5-hydroxymethylfurfural (HMF), levulinic acid (LA), and formic acid (FA). We also demonstrate the difference in humin structure observed when humins are formed from HMF, fructose, or glucose. This difference is mainly observed in a new peak that appears at 1650 cm-1 that is ascribed to HMF incorporated in the humins. This peak is consumed over the course of the reaction, and at 100% conversion of HMF the peak disappears entirely. Lastly, we fit humin structures proposed in the literature to the FTIR data we collected, and find that the structures proposed by Lund et al. and by Weckhuysen et al. appear to be the most consistent with our data. A novel experimental technique measuring humin formation was developed by using in situ DLS. We have been able to measure the activation energy of humin formation directly with this technique, and found activation energies between 18 – 55 kJ/mol. We attribute the differences between our measured values and previously reported values mostly to transport limitations in the DLS cell. The results from these two studies need to be synthesized further via increased physical and chemical mechanistic understanding.