Influence of synthesis pH and oxidative strength of the catalyzing acid on the morphology and chemical structure of hydrothermal carbon.

A specific control of the morphology and chemical structure of hydrothermal carbon (HTC) is of crucial importance for its application, both in catalyst supports or electrochemical devices. Here we show how the morphology, that is, particles size and homogeneity, and the distribution of functional groups can be controlled by the control of the synthesis pH of the hydrothermal carbonization. A complementary analysis of liquid byproducts by HPLC provides useful information on the nature of the polymeric species produced during the poly-condensation in the hydrothermal process and reveals the potential implementation of the process into the biorefinery concept. The acidic byproducts levulinic acid and formic acid determine the hydrothermal carbonization autocatalytically by additional supply of protons to the reaction medium. Thus, for a starting pH>3, only minor structural differences can be detected for HTC. The use of oxidizing acids favors higher yields of HTC and improves carbonization towards higher condensed carbon domains. Scaling up the process in a stirred 2 L batch reactor favors carbonization leading to higher condensed carbonaceous products. The relative trends of pH variation are maintained.