Growth Kinetics of Humins Studied via X-Ray Scattering

We use ultra-Small Angle X-ray Scattering (USAXS) to investigate the evolution of size, morphology, volume fraction and number concentration of humins formed during dehydration of fructose to 5-hydroxymethylfurfural (HMF), and the further polymerization of HMF between 80-95 °C in 1 M hydrochloric acidic solutions. The radius of gyration (Rg) of suspended humins particles grows linearly with time and is accompanied by an increase in polydispersity (PD), before possible precipitation occurs. The scattering patterns indicate fractal-like structures. The apparent activation energy of humins growth is 102±0.4 kJ/mol. The trends in the number of particles over time reveal competing processes entailing the continuous formation of new particles (nucleation) leading to increasing polydispersity with time and the aggregation of existing particles and possible precipitation. The direct observation of growth of humins indicates that humins form primarily from HMF rather than fructose.