Integrated Humin Formation and Separation Studied In Situ by Centrifugation.

We present a novel method for studying the integrated formation and separation of humins formed during the Brønsted acid-catalyzed conversion of fructose (here, at 90 °C with 20 wt % fructose and 5 wt % sulfuric acid). For the first time, we report the reaction carried out in situ during systematic centrifugation experiments, which allows combining humin formation and separation along with investigation of the phase behavior of humins. Analysis of the formed humin deposits employing scanning electron microscopy reveals deposits that are formed from a layer of monodisperse microspheres with a narrow diameter range of 0.9-1.9 μm. In the centrifugal force field, the microspheres partially coalesce, which increases with time and relative centrifugal force up to the formation of a thin and uniform layer of microspheres covering a continuous humin bulk phase with 80-90 μm thickness. These findings give evidence that humin spheres are highly viscous droplets rather than solid particles during formation. Our result is in line with the often-reported spherical and planar deposits formed during acidic carbohydrate conversion in technical systems and supports the development of strategies for deposit prevention, on the one hand, and humin preparation for material utilization, on the other hand.