Microporous and Mesoporous Activated Carbons from Tea Stalk and Tea Stalk Pulps: Effect of Lignin Removal by One-Step and Two-Step Organosolv Treatment

Abstract

Delignification is a crucial pretreatment in the production of diverse value-added products from lignocellulosics. While modifying the surface functional groups, delignification also increases the specific surface area by providing a porous structure to the lignocellulosic biomass. Hydrothermal pretreatment can be used prior to delignification, to recover hemicellulose and boost delignification. By removing lignin and hemicellulose, cellulose-rich pulp becomes more accessible for activation. In the present study, three different activated carbons were prepared: activated carbon from tea stalk itself (ATS), activated carbon from tea stalk pulp obtained by using glycerol organosolv pretreatment (ATP), activated carbon from tea stalk hydrochar pulp obtained by using sequential hydrothermal pretreatment-organosolv delignification (AHTP). Each precursor was carbonized (at 800 °C) in the presence of KOH (KOH/precursor: 2/1). Activated carbons were characterized for their elemental content, surface functional groups, thermal stability, crystallinity, surface morphology, surface area and porous structure using elemental analysis (C-H-N-S), FTIR, TGA, XRD, SEM and, BET analysis, respectively. While hydrothermal pretreatment prior to organosolv pulping reduced the delignification yield, it also altered the pore structure of activated carbon. Among the activated carbons, only ATS had microporous structure with an average pore radius of 1 nm. ATP had the highest surface area (2056.72 m2/g) and micropore volume (0.81 cm3/g). Having mesopores (with an average pore radius of 5.74 nm) in its structure, AHTP had the least micropore volume (0.464 cm3/g) and surface area (1179.71 m2/g). The presence of micro and mesopores broadens the potential applications of activated carbon ranging from environmental applications to energy storage.