Activation of pine needles with zinc chloride: Evolution of functionalities and structures of activated carbon versus increasing temperature

Functionalities of activated carbon (AC) is one of the essential factors determining its performance for adsorption. In this study, the evolution of functionalities and structure of AC from activation of pine needles with ZnCl₂ versus temperature was explored. The results indicated that the ZnCl₂ activator could remarkably increase yields of solid product (AC) via catalyzing dehydration, condensation and aromatization of sugary structures, minimizing the formation of aliphatic and phenolic organics in bio-oil. The pore structures of AC generated by this process also exhibited high sensitivity to temperature. The specific surface area of resulting AC could increase from 699.2 m²g⁻¹ at 400 °C to 2030.2 m²g⁻¹ at 500 °C, which could also decrease to 907.9 m²g⁻¹ at 700 °C. Migration and aggregation of ZnO at 700 °C resulted in the loss of capability for supporting carbon structures, leading to shrinkage or collapse of the pore structures. The characterization with in-situ IR technique showed that ZnCl₂ catalyzed dehydration of hydroxyl group and conversion of carbonyls in aldehydes/ketones into larger π-conjugated structures but not worked on the C-O-C bonds for bridging benzene rings. However, the transformation of ZnCl₂ to ZnO above 500 °C lost the catalytic activity, inducing little change of elemental composition but re-structuring of AC through merge of pores.