Activation of cotton with transition metal-based chlorides: Correlation of generation of pore structures with evolution of oxygen-containing species

Abstract

Lewis acidity of chlorides is known to significantly impact fragmentation/condensation reactions in activation of biomass. Herein, activation of cellulose-rich cotton with CuCl2, ZnCl2 or FeCl3, the chlorides with low to strong Lewis acidity, was performed and the influences of varied chlorides on evolution of the pyrolytic products and pore structures of activated carbon (AC) were studied. The results indicated that ZnCl2 and FeCl3 with strong Lewis acidic sites catalyzed condensation of reaction intermediates of light or heavier ones to form AC with yield higher than that from cotton pyrolysis by 6.7 % and 53.3 %, respectively. Increasing AC production was mainly due to retainment of more oxygen (23.8 % of oxygen content for AC-FeCl3 and 20.7 % for AC-ZnCl2 versus 11.5 % from blank experiment). CuCl2 with weak Lewis acidity promoted dehydration of sugary structures, forming more sugar-derivatives in bio-oil but diminished production of AC. However, higher Lewis acidity does not meant higher capability for creating pores. Specific surface area for AC-ZnCl2-500 was much bigger than that of AC-FeCl3-500 (1783.9 versus 22.8 m2/g). Characterization of the activation with in-situ IR indicated that ZnCl2 catalyzed rapid dehydration of -OH and condensation of carbonyls below 500°C, transforming oxygen in stable C-O-C form and carbon in olefinic CC form for maintaining generated pore structures.