Influence of ammonium salts and temperature on the yield, morphology and chemical structure of hydrothermally carbonized saccharides

Abstract

Abstract: In this work, the influence of (NH4)2SO4 and (NH4)2HPO4 as well as temperature is examined on the hydrothermal carbonization of glucose, fructose and sucrose. Increasing the temperature from 160 to 220 °C increased the yield of hydrothermal carbon for each saccharide for the (NH4)2SO4 solution, whereas (NH4)2HPO4 produced a yield that was independent of temperature. The addition of (NH4)2SO4 increased the yield obtained at 220 °C by 4.27, 7.03 and 2.01 wt% for glucose, fructose and sucrose over the baseline salt free solution, respectively. (NH4)2SO4 also increased the quantity of acid produced and the average size of the hydrothermal carbon spheres. Conversely, (NH4)2HPO4 produced carbon structures consisting of interlocked spherical shapes and produced almost no acidic products. XPS analysis revealed that (NH4)2SO4 incorporated nitrogen and sulfur into the hydrothermal structure, while (NH4)2HPO4 only allowed nitrogen to be incorporated. It was assessed that NH4+ enhances the production of hydrothermal carbon, except in the presence of PO43−, which prevents the reaction from effectively forming hydrothermal carbon and organic acids. Graphical abstract: [Figure not available: see fulltext.]