Obtention and surface characterisation of several ash-free chars Original Research Article

Four ash-free cellulose chars were obtained at temperatures of 400, 500, 600 and 700 °C, respectively, at which the thermogravimetric plots approach a final constant mass, avoiding further high-temperature vapour activations to prevent micropore formation. Stable thermal behaviour was obtained, however, under further heat treatment at temperatures equal to or lower than those of the respective preparations. The surface structures were characterized using thermogravimetry, BET surface area, porosimetry, FTIR, ammonia adsorption and surface Brönsted acidity measurements. FTIR shows two main organic functional groups, i.e. aromatic lactones and carboxylic acid functions. Ammonia adsorption-desorption and cation exchange experiments show ammonia physisorption into the micropore system that is about three orders of magnitude higher than the adsorption by the Brönsted carboxylic acid functionality itself. The former increases and the latter decreases with increasing preparation temperature of the chars. These chars exhibit apparent BET surface areas between 133–337 m2g−1 and relatively low micropore contents, especially those obtained at temperatures lower than 500 °C.