In our previous work, amorphous carbon with high density of SO₃H groups was demonstrated to function as an efficient replacement for sulfuric acid catalyst. The carbon material can incorporate large amounts of hydrophilic molecules into the carbon bulk, due to the high density of the hydrophilic functional groups bound to the flexible carbon sheets. This incorporation provides good access by reactants in solution to the SO₃H groups in the carbon material, giving rise to high catalytic performance, despite the small surface area (2 m² g^-1). However, the hydrophilic functional groups prevent incorporation of hydrophobic molecules into the bulk, and thereby hydrophobic acid-catalyzed reactions proceed only on the surface, resulting in poor or no catalytic activity for the reactions. In this study, sulfonated activated carbon materials prepared at low temperatures were studied as solid acid catalysts. Activated carbon catalysts were prepared by sulfonation of activated carbon obtained by carbonization of wood powder with ZnCl₂ at 523-873 K. The surface areas of the activated carbon catalysts increased with increasing carbonization temperature, reaching a plateau at 773 K. The maximum surface area of the samples was 1560 m² g^-1. Structural analyses revealed that the activated carbon catalysts have sulfonic acid, carboxylic acid, and hydroxyl groups and these functional groups decreased with increasing carbonization temperature. The catalytic activity of the activated carbon catalysts was demonstrated through the esterification of acetic acid (343 K) and the alkylation of toluene (373 K). In the case of the esterification of acetic acid, the catalytic activity depended only on the acid amount. On the other hand, the alkylation of toluene on the carbon material depended on the surface area in addition to the acid amount. These results suggest that the activated carbon catalyst with high surface area act as a solid acid even for hydrophobic acid-catalyzed reactions.

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