Carbon materials have received increasing attention due to their physicochemical properties and various application possibilities, especially in catalysis. Hydrothermal carbonization is one of the promising methods for obtaining carbonaceous materials, offering advantages such as low operating temperature and reduced generation of toxic waste. Carbon spheres applied in catalysis bring some interesting points, such as good thermal stability, high micropore volume, high surface area, and the possibility of incorporating functional groups onto their surface. The samples were synthesized from the hydrothermal carbonization of ascorbic acid at 180 °C, and the carbon functionalization was done with sulfuric acid, p-toluenesulfonic acid, and 4-hydroxybenzenesulfonic acid. The physicochemical characterizations of the materials include scanning electron microscopy, infrared spectroscopy, X-ray diffraction, thermodecomposition study through thermogravimetric analysis, determination of the density of acidic sites, and testing in the esterification reaction of oleic acid with methanol. The results showed success in the synthesis of spherical particles with sulfonic groups anchored on the surface. The sample functionalized with sulfuric acid had the best conversion result (91%), compared to those functionalized with p-toluenesulfonic acid and 4-hydroxybenzenesulfonic acid, which achieved 51% and 56%, respectively.
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