Some considerations of the origins of porosity in carbons from chemically activated wood

Abstract

Investigations have been made of the conversion of white oak to activated carbons by reaction with phosphoric acid at temperatures up to 650°C. At low temperatures, reaction with the acid promotes dehydration reactions, and the redistribution of biopolymers. It is considered that the subsequent formation of crosslinks leads to an increase in carbon yield above about 300°C. Detailed observations by optical microscopy, and measurements of dimensional changes, have established for the first time a link between the origins of porosity and reaction mechanisms. Following an initial contraction, the structure undergoes considerable expansion between 250 and 450°C, which corresponds to the development of high surface area. Above 450°C, there is secondary contraction and an accompanying loss of accessible porosity. It is concluded that porosity development is directly related to the retention and dilation of cellular material, which creates an extensive surface accessible to adsorbent molecules. The results may be relevant to understanding equivalent processes with other biomass and coal precursors that contain biopolymers or altered biopolymers.