Evolution of carbon structure in chemically activated wood

Abstract

13C NMR and FTIR analyses have been employed to follow the evolution of chemical structure in relation to porosity development, as a function of heat treatment temperature (HTT), for activated carbons produced from white oak by phosphoric acid activation. The chemical changes effected by acid treatment at low HTT are: by 50 °C there is significant alteration of the lignin structure; by 100 °C a significant portion of the cellulose has reacted, with the formation of ketones and esters; the formation of phosphate esters becomes apparent around 150 °C; crosslinking reactions are initiated below 150 °C, consistent with the higher carbon yield obtained in chemical activation; and generally there is an increase in aromaticity and loss of aliphatic, carboxyl, and carbonyl groups. The low temperature phenomena precede, and relate to, the development of porosity and structural dilation that commences around 250 °C, and attains a maximum between 350 and 450 °C. Up to 450 °C, pore volume is found to correlate with crosslink density. Above 450 °C, there is a dimensional contraction and a reduction in porosity. Among the accompanying phenomena are: the elimination of cellulose phosphates and oxygen functionalities; and a dramatic increase in the estimated aromatic cluster size. The latter would require a reduction in crosslink density to facilitate cluster growth, and the resulting structural rearrangement and increased alignment of clusters would produce a more densely packed structure with reduced porosity.