OBTENTION AND SURFACE CHARACTERIZATION OF SEVERAL ASH-FREE CHARS

Four ash-free cellulose chars were obtained at temperatures of 400, 50 0, 600 and 700 degrees C, respectively, at which the thermogravimetric plots approach a final constant mass, avoiding further high-temperatu re vapour activations to prevent micropore formation. Stable thermal b ehaviour was obtained, however, under further heat treatment at temper atures equal to or lower than those of the respective preparations. Th e surface structures were characterized using thermogravimetry, BET su rface area, porosimetry, FTIR, ammonia adsorption and surface Bronsted acidity measurements. FTIR shows two main organic functional groups, i.e. aromatic lactones and carboxylic acid functions. Ammonia adsorpti on-desorption and cation exchange experiments show ammonia physisorpti on into the micropore system that is about three orders of magnitude h igher than the adsorption by the Bronsted carboxylic acid functionalit y itself The former increases and the latter decreases with increasing preparation temperature of the chars. These chars exhibit apparent BE T surface areas between 133-337 m(2) g(-1) and relatively low micropor e contents, especially those obtained at temperatures lower than 500 d egrees C. (C) 1998 Elsevier Science Ltd. All rights reserved.