Effect of pyrolysis on basic functional groups of hydrochars

Hydrothermal carbonization (HTC) is a thermochemical process, where biomass is treated in the presence of water at subcritical condition to produce functionalized carbon-rich solid materials known as hydrochar. Hydrochar has a core and shell structure which contains both acidic and basic functional groups but has very small surface area. Pyrolysis is often performed to expose pores and corresponding functional groups of amorphous carbon materials like hydrochar. The aim of this work was to quantify the basic functional groups on hydrochars and the effect of pyrolysis temperature on these basic functional groups. Hydrochars were produced from cellulose and wood at 220 and 260 °C followed by pyrolysis on the corresponding hydrochars at 400, 500, and 600 °C under N₂ environment. Afterward, FTIR and Boehm titration (direct method) were performed to identify and quantify the basic functional groups, respectively. HTC treatment reduced the basic groups on the cellulose from 89.3 ± 3.0 to 13.6 ± 6.0 μmol/g and the wood surface from 160.0 ± 10.0 to 64.7 ± 6.5 μmol/g mainly due to the dehydration reaction and formation of a condensation layer. Pyrolysis resulted in a continuous increase of basic groups on cellulose hydrochar produced at 220 °C, while a significant increase from 13.6 ± 6.0 to 55.3 ± 9.8 μmol/g was observed during low temperature pyrolysis and remains similar at high temperature for cellulose hydrochar produced at 260 °C. In contrast, the basic functional groups increased from 59.6 ± 10.8 to 214.9 ± 8.8 μmol/g and from 64.7 ± 6.5 to 154.0 ± 8.5 μmol/g with pyrolysis temperature of wood hydrochars produced at 220 and 260 °C, respectively. The significant difference between wood and cellulose-derived pyrolyzed hydrochars could be due to the presence of lignin in wood.