Sulfonated palm kernel shell biochar as a pore-blocking-driven selective solid acid catalyst for free fatty acid esterification in high-acid crude palm oil

Abstract

This study investigates the catalytic specificity of sulfonated palm kernel shell (PKS)-derived biochar for the esterification of free fatty acids (FFA) in high-acid crude palm oil (HACPO). Biochar was prepared by pyrolysis (500 °C, 5 h, N₂), activated with KOH (2 M, 1:4 w/v, 90 °C, 5 h) and HCl (1 M, 1:4 w/v), and sulfonated with concentrated H₂SO₄ (1:4 mass ratio, 100 °C, 5 h). Comprehensive characterization using FTIR, XPS, XRD, and N₂ physisorption revealed that KOH activation generated substantial microporosity (BET surface area 251.85 m²/g), which was subsequently blocked by the grafted –SO₃H groups after sulfonation, causing the surface area to collapse to 4.95 m²/g. XPS confirmed the exclusive presence of covalently bound aromatic –SO₃H groups (S 2p at 168.7 eV) with no residual sulfate impurities. The resulting catalyst exhibited a competitive acid site density of 2.80 mmol g⁻¹ and achieved an 84.44% FFA conversion at a 4 wt % catalyst loading. Notably, the near-complete loss of internal porosity confined the active acid sites predominantly to the external particle surface, creating a catalyst that is highly specific toward FFA esterification while showing negligible activity for triglyceride transesterification. This pore-blocking-driven substrate selectivity establishes a clear structure–activity relationship and highlights the potential of PKS-derived sulfonated biochar as a selective solid-acid catalyst for high-FFA feedstock pretreatment.