Low temperature sulfonation of acai stone biomass derived carbons as acid catalysts for esterification reactions

Abstract

Heterogeneous acid catalysts derived from acai stone (Euterpe oleracea Mart.), a large-scale residue biomass, were synthesized by partial carbonization followed by sulfonation within a short preparation time under controllable conditions. Their catalytic activity was tested in the esterification reaction of oleic acid with alcohols, with varying chain-lengths, and with a modified edible oil with high fatty acid content. The sulfonated carbon materials were characterized by X-ray diffraction (XRD), scanning electron microscopy coupled with an EDS system for elemental mapping (SEM-EDS), thermogravimetry (TG), infrared spectroscopy (FT-IR), low-temperature N2 sorption, and X-ray photoelectron spectroscopy (XPS). The reaction conditions and catalyst preparation were systematically investigated with respect to carbonization and sulfonation temperatures, reaction temperature and time, reactants’ molar ratio, catalyst loading, alcohol type, and recyclability. A low carbonization temperature created carbon materials with surface features that favored the anchoring of sulfonic groups, in turn providing desirable sulfonation under mild temperatures. After optimization of the reaction conditions, a yield of 93% was obtained when 5% catalyst loading was used to convert a 1:12 oleic acid to methanol molar ratio at 100 °C for 1 h. Comparatively, the uncatalyzed reaction yielded only 11% conversion. The sulfonated carbon catalyst was tested in the esterification of soybean oil that was modified with 20% oleic acid to reduce the quality of feedstock and simulate harsh conditions; the catalyzed reaction showed yields up to 80%. Moreover, the synthesized catalyst was stable for up to three reaction cycles, displaying a decrease in efficiency of only 8%. This study shows promising results for obtaining sulfonated carbon catalysts from acai stone biomass, thereby potentially solving one of the major environmental problems in the Amazon region and providing a sustainable alternative for fuel production.