Sustainable catalysts for esterification: Sulfonated carbon spheres from biomass waste using hydrothermal carbonization

Sulfonated catalysts derived from carbon spheres obtained from açai seeds (Euterpe oleracea) were synthesized using a hydrothermal carbonization method, followed by subsequent direct sulfonation. These solid acid catalysts, in the form of carbon spheres, were characterized through various techniques, including thermogravimetry, X-ray diffraction, infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy for elemental mapping (SEM-EDS), and Boehm titration. Two types of catalysts were produced through low-temperature hydrothermal carbonization. One catalyst underwent hydrothermal carbonization with the assistance of a catalyst, while the other was carbonized solely using water. The catalytic activity was assessed for up to the fifth reuse in the esterification reaction between oleic acid and methanol, employing 5% of the catalyst by mass, a 1-h reaction time at 100 °C, and a molar ratio of oleic acid to methanol at 1:12. The catalyst produced through hydrothermal carbonization in water demonstrated the best performance, with a conversion rate ranging from 91% to 88% over the five cycles, while the catalyst produced with the aid of a catalyst yielded conversion rates of 89%–82% over the same five cycles. Total acidity and sulfonic group acidity were calculated for each recycling of the aforementioned catalysts. As expected, catalytic activity was primarily influenced by the concentration of strong acid sites from the sulfonic groups, which decreased with each use of the catalyst. Scanning electron microscopy analysis confirmed that the removal of lignin from biomass residues increased the conversion of holocellulose into carbon spheres, and the functionalization with sulfuric acid did not disrupt the spherical structures of the catalysts. Thermogravimetric and differential thermogravimetric analyses showed that both hydrochars and catalysts possessed satisfactory thermal stability for use in sulfonation and esterification reactions, respectively. The TG-MS analysis allowed for the observation of the decomposition temperature of sulfur-containing gaseous emissions. FTIR and XPS confirmed the expected functional groups for hydrochars and biomass-derived catalysts. Moreover, XPS confirmed that the majority of sulfur present in the catalysts existed in the form of sulfonic groups. The optimization of reaction parameters, such as temperature, time, catalyst loading, and the molar ratio of oleic acid to methanol, was carried out using the catalyst with the best performance over the recycling cycles (91%–88%). By slightly increasing the values of these parameters in the esterification model reaction (5% catalyst by mass, 1-h reaction time at 100 °C, and a molar ratio of oleic acid to methanol of 1:12), oleic acid conversion between 89% and 93% was achieved. The utilization of açai seed-derived materials in this study showcases potential environmental benefits by reducing waste associated with açai seed commercialization. Additionally, these catalysts are considered green, as they are quick, cost-effective, and simple to produce, while exhibiting excellent catalytic activity and recyclability.