ACTIVATED CARBONS FROM YELLOW-POPLAR AND WHITE OAK BY H3PO4 ACTIVATION

Results are presented from continuing investigations of the phosphoric acid activation of hardwoods. Earlier work with white oak has been ex tended to include yellow poplar. II is found that the same general che mical and physical changes occur with both precursors. A discussion is presented on the possible mechanisms of phosphoric acid activation, d rawing upon extensive research on the use of phosphorous compounds as fire retardants for wood and cellulose. Phosphoric acid appears to fun ction both as an acid catalyst to promote bond cleavage reactions and the formation of crosslinks via processes such as cyclization, and con densation, and to combine with organic species to form phosphate and p olyphosphate bridges that connect and crosslink biopolymer fragments. The addition or insertion of phosphate groups drives a process of dila tion that, after removal of the acid, leaves the matrix in an expanded state with an accessible pore structure. It is considered that activa tion of the amorphous polymers produces mostly micropores, while activ ation of crystalline cellulose produces a mixture of pore sizes. The d ifferent response of crystalline cellulose is attributed to a much gre ater potential for structural expansion than is possible with the amor phous polymers due, among other factors, to its higher density and its chemical structure that allows for a more extensive degree of combina tion with phosphoric acid, and hence ''bulking'' of the cell walls. Th e pore size distribution obtained from crystalline cellulose can be al tered by increasing the HTT and/or the ratio of acid to precursor such that, eventually, the structure is dominantly mesoporous. At temperat ures above 450 degrees C, a secondary contraction of the structure occ urs when the phosphate linkages become thermally unstable. The reducti on in crosslink density allows the growth and alignment of polyaromati c clusters, producing a more densely packed and less porous structure. (C) 1998 Elsevier Science Ltd. All rights reserved.