Modification et interaction des fonctions oxygenees en surface des charbons actifs

Abstract

Four different, i.e., carboxylic, lactone, phenol, and carbonyl groups, are detected on the surface of carbons by neutralization with bases of increasing strengths. Also, basic groups of the pyrone type (Fig. 1) are often encountered. Table 1 indicates the origins and the activation methods of the studied samples, which were modified either by HCl washing (sample 1), heat treatment at 800°C (sample 2), heat treatment followed by HCl washing (sample 3), esterification with methanol (sample 4), extraction with toluene (sample 5), or oxidation with HNO 3 (sample 5). Figures 2, 3, and 4 exhibit the oxygenated surface group contents of the initial and modified active carbon samples, respectively, prepared from coconut shells (CO), beech wood (B), peat (T), or bituminous coal (CB). It is seen that the treatments cause significant changes in the chemical surface structure of the active carbons: changes which depend on the origin of these carbons. The reactivity toward neutralization chemicals of either acidic or basic surface groups are probably related, as shown schematically in Fig. 5, which indicates possible electron migrations between carboxylic acid and pyrane sites, which in turn influence the dissociation ability of, e.g., carboxylate groups. In order to check this hypothesis, various initial or oxidized samples were submitted to heat treatments, between 110° and 800°C in an N 2 atmosphere, so as to progressively eliminate the acidic groups. Figure 6 relates the contents of basic groups to the heat treatment temperature of initial (B) and oxidized beech active carbon and of various coconut shell carbons: initial (sample 1), oxidized with HNO 3 (65% to 68%) during 0.5 h (sample 2), oxidized with HNO 3 (32.5%) during 0.5 h (sample 3), and oxidized with HNO 3 (32.5%) during 1 h. It is seen that the total amount of pyrone-type groups, created by heat treatment, is proportional to the initial degree of oxidation of the active carbons and, that the variation of pyrone content, with temperature, follows similar trends as the temperature is increased.