Abstract: Development of pore structures of activated carbon (AC) from activation of biomass with ZnCl₂ relies on content and structure of cellulose/hemicellulose in the feedstock. Thermal pretreatment of biomass could induce dehydration and/or aromatization to change the 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 ℃ and the subsequent activation with ZnCl₂ at 550 ℃. The results showed that thermal pretreatment at 360 ℃ (WB-360) could lead to substantial pyrolysis to form biochar, with a 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 the 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 infrared characterization of the activation showed that ZnCl₂ mainly catalyzed dehydration, dehydrogenation, condensation, and aromatization but not cracking, suppressing the formation of derivatives of cellulose and lignin in bio-oil. The thermal pretreatment formed phenolic -OH and CO with higher chemical innerness, which changed the reaction network in activation, shifting morphology of fibrous structures in AC-blank to “melting surface” in AC-200 or AC-280.

Key words: Thermal pretreatment, Activation with ZnCl₂, Willow branch, Activated carbon, Biochar

摘要： Development of pore structures of activated carbon (AC) from activation of biomass with ZnCl₂ relies on content and structure of cellulose/hemicellulose in the feedstock. Thermal pretreatment of biomass could induce dehydration and/or aromatization to change the 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 ℃ and the subsequent activation with ZnCl₂ at 550 ℃. The results showed that thermal pretreatment at 360 ℃ (WB-360) could lead to substantial pyrolysis to form biochar, with a 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 the 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 infrared characterization of the activation showed that ZnCl₂ mainly catalyzed dehydration, dehydrogenation, condensation, and aromatization but not cracking, suppressing the formation of derivatives of cellulose and lignin in bio-oil. The thermal pretreatment formed phenolic -OH and CO with higher chemical innerness, which changed the reaction network in activation, shifting morphology of fibrous structures in AC-blank to “melting surface” in AC-200 or AC-280.

关键词: Thermal pretreatment, Activation with ZnCl₂, Willow branch, Activated carbon, Biochar