Lignin is one of the major components in woody-biomass and composed of aromatic and aliphatic carbohydrates with oxygen-containing functional groups. Conversion of lignin to important functional materials is important issue as plant-derived raw materials are increasingly used as a renewable source. Here, we present the unique strategy for the preparation of SO₃H-containing carbon catalysts from lignin and their catalytic properties for the acid-catalyzed reactions. Sulfonated carbon catalysts are obtained by imcomplete carbonization of lignin at around 673 K followed by sulfonation of the resulting carbon in concentrated sulfuric acid solution. The obtained material consists of mainly nanosized graphenes in size of ~ 1.5 nm. XRD, XPS, and solid-state NMR reveal that the resulting catalyst contains -SO₃H, COOH, and OH groups, which are attached to the graphene sheets. The amount of sulfonic acid groups, which are act as Brönsted acid sites in acid-catalyzed reaction, are estimated as 1.3 mmol·g^-1. Acid catalysis of this catalyst is demonstrated through esterification of acetic acid with ethanol. The resulting catalysts produced ethyl acetate in high yield: the activity is comparable to those of conventional ion-exchangable resins (Nafion and Amberlyst-15). Catalytic activity and S content remains unchanged after reuse of this catalyst for three times, indicating the introduced sulfonic acid groups are stabilized on the carbon framework. Because of the presence of benzene ring, sulfonic acid groups are introduced in the bare lignin and sulfonated lignin also catalyzes the corresponding reaction. However, severe leaching of sulfonic acid groups occurs when the sulfonated lignin is repeatedly used as an acid catalyst. The lignin-derived solid acid also shows higher catalytic activity for esterification of stearic acid with ethanol than those of Nafion and Amberlyst-15. Therefore, the resulting carbon catalyst is suitable for solid material in acid-catalyzed reactions.

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