Application of sulfonated passion fruit seeds as a heterogeneous catalyst in the esterification of oleic and levulinic acids: Optimization of reaction parameters

Passion fruit pulp production generates substantial waste, particularly seeds. This study explores a new application for these seeds by sulfonating them in a gradient (25, 50, 75, and 100 % v/v of concentrated sulfuric acid) and thermally activating them (3h at 200 °C). The catalysts were characterized using FTIR, XRD, SEM-EDS, acidity measurements, and TGA, showing significant acid density (0.493–1.35 mmol H+/g), with the -SO₃H group verified by other analyses. SEM-EDX data for non-thermally activated catalysts indicated that more intense sulfonation conditions enhanced sulfonic group incorporation, reaching a sulfur content of 3.26 % in SPFS. Thermally activated catalysts showed a slight sulfur decrease, reaching 2.14 %. SPFS, the most promising catalyst in terms of acid density, sulfur content, and catalytic activity, was applied in the esterification of oleic (AO) and levulinic acids (LA) with optimized reaction conditions. For AO, optimal conditions yielded 97.1 % ester conversion, while LA achieved 95.9 % conversion. Different alcohols were tested for LA, all reaching over 95 % conversion under optimized methanol conditions. Kinetic analysis showed pseudo-first-order kinetics, with activation energies of 30.16 and 25.06 kJ/mol for AO and LA, respectively. This study shows that effective, eco-friendly heterogeneous catalysts can be derived from abundant waste for biofuel production.