Thermal pretreatment of willow branches impacts yield and pore development of activated carbon in subsequent activation with ZnCl2 via modifying cellulose structure

Development of pore structures of activated carbon (AC) from activation of biomass with ZnCl2 relies on content and structure of cellulose/hemicellulose in feedstock. Thermal pretreatment of biomass could induce dehydration and/or aromatization to change structure of cellulose/hemicellulose. This might interfere with evolution of structures of AC, which was investigated herein via thermal pretreatment of willow branch (WB) from 200 to 360 °C and the subsequent activation with ZnCl2 at 550 °C. The results showed that thermal pretreatment at 360 °C (WB-360) could lead to substantial pyrolysis to form biochar with yield of 31.9%, accompanying with nearly complete destruction of cellulose crystals and remarkably enhanced aromatic degree. However, cellulose residual in WB-360 could still be activated to form AC-360 with specific surface area of 1837.9 m2·g−1, which was lower than that in AC from activation of untreated WB (AC-blank, 2077.8 m2·g−1). Nonetheless, the AC-200 from activation of WB-200 had more developed pores (2113.9 m2·g−1) and superior capability for adsorption of phenol, due to increased permeability of ZnCl2 to the largely intact cellulose structure in WB-200. The thermal pretreatment did increase diameters of micropores of AC but reduced overall yield of AC (26.8% for AC-blank versus 18.0% for AC-360), resulting from accelerated cracking but reduced intensity of condensation. In-situ IR characterization of the activation showed that ZnCl2 mainly catalyzed dehydration, dehydrogenation, condensation and aromatization but not cracking, suppressing formation of derivatives of cellulose and lignin in bio-oil. The thermal pretreatment formed phenolic —OH and C=O with higher chemical innerness, which changed reaction network in activation, shifting morphology of fibrous structures in AC-blank to "melting surface" in AC-200 or AC-280.