Activated carbons from yellow poplar and white oak by H 3PO 4 activation

Abstract

Results are presented from continuing investigations of the phosphoric acid activation of hardwoods. Earlier work with white oak has been extended to include yellow poplar. It is found that the same general chemical and physical changes occur with both precursors. A discussion is presented on the possible mechanisms of phosphoric acid activation, drawing upon extensive research on the use of phosphorous compounds as fire retardants for wood and cellulose. Phosphoric acid appears to function both as an acid catalyst to promote bond cleavage reactions and the formation of crosslinks via processes such as cyclization, and condensation, and to combine with organic species to form phosphate and polyphosphate bridges that connect and crosslink biopolymer fragments. The addition or insertion of phosphate groups drives a process of dilation that, after removal of the acid, leaves the matrix in an expanded state with an accessible pore structure. It is considered that activation of the amorphous polymers produces mostly micropores, while activation of crystalline cellulose produces a mixture of pore sizes. The different response of crystalline cellulose is attributed to a much greater potential for structural expansion than is possible with the amorphous polymers due, among other factors, to its higher density and its chemical structure that allows for a more extensive degree of combination with phosphoric acid, and hence “bulking” of the cell walls. The pore size distribution obtained from crystalline cellulose can be altered by increasing the HTT and/or the ratio of acid to precursor such that, eventually, the structure is dominantly mesoporous. At temperatures above 450 °C, a secondary contraction of the structure occurs when the phosphate linkages become thermally unstable. The reduction in crosslink density allows the growth and alignment of polyaromatic clusters, producing a more densely packed and less porous structure.