In this study, we have synthesized a solid acid catalyst by areca nut husk using low temperature hydrothermal carbonization method. The fabricated catalyst has enhanced sulfonic actives sites (3.12%) and high acid density (1.88 mmol g−1) due to –SO3H, which are used significantly for effective biodiesel synthesis at low temperatures. The chemical composition and morphology of the catalyst is determined by various techniques, such as Fourier transform infrared (FTIR), powder X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), Scanning electron microscope (SEM), Energy disruptive spectroscopy (EDS), Mapping, Thermogravimetric analysis (TGA), CHNS analyzer, Transmission electron microscopy (TEM), particle size analyzer, and X-ray photoelectron spectroscopy (XPS). Acid–base back titration method was used to determine the acid density of the synthesized material. In the presence of the as-fabricated catalyst, the conversion of oleic acid (OA) to methyl oleate reached 96.4% in 60 min under optimized conditions (1:25 Oleic acid: methanol ratio, 80 °C, 60 min, 9 wt% catalyst dosage) and observed low activation energy of 45.377 kJ mol−1. The presence of the porous structure and sulfonic groups of the catalyst contributes to the high activity of the catalyst. The biodiesel synthesis was confirmed by gas-chromatography mass spectrometer (GC–MS) and Nuclear magnetic resonance (NMR). The reusability of the catalyst was examined up to four consecutive cycles, yielding a high 85% transformation of OA to methyl oleate on the fourth catalytic cycle.

To produce biodiesel from oleic acid (OA), the effectiveness of sweet lemon ( Citrus limetta ) waste peels as an acidic catalyst in an esterification process is examined in the current work. A biowaste-derived sulfonated carbon-based catalyst is fabricated without high temperatures via a simple one-pot process. Several techniques are used to investigate the chemical components and morphology of the catalyst, including Fourier transform infrared spectroscopy (FTIR), powder X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), Brunauer–Emmett–Teller (BET), and N 2 adsorption–desorption. The biodiesel conversion is observed by gas chromatography–mass spectrometry (GC–MS), proton nuclear magnetic resonance 1 H NMR, and carbon nuclear magnetic resonance 13 C NMR. The excellent biodiesel conversion of 96% was obtained using optimized conditions, i.e., 1:20 of OA/MeOH, 5 wt % catalyst loading, 70 °C temperature, and 3 h. The catalyst shows 87% conversion in just 1 h, and the maximum conversion was found to be ≈96%. This high activity of the catalyst can be attributed to the presence of sulfonic groups and its porous nature. The formed catalyst shows excellent catalytic activity up to three cycles.

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Sulfonated polycyclic aromatic carbon (SPAC) catalysts have specific surface characteristics owing to the presence of two types of functional groups: SO3H (sulfonic) and oxygen‐containing functional groups (COOH and OH groups). Oxygen‐containing functional groups provide a synergistic effect to the sulfonic group, resulting in a high total acid density and enhanced catalytic activity of SPAC. Owing to the high acid density, SPAC catalysts are used for various applications such as organic transformations, electrocatalysis and water treatment. The natural abundance of raw materials, easy processing for catalyst fabrication, excellent catalytic performance and high thermal and chemical stability make SPAC cost‐effective and environmentally benign. Biomass‐derived heterogeneous catalysts possess unique surface characteristics, high reusability and low cost owing to their easy accessibility, along with various environmental benefits. This review demonstrates biodiesel production using oleic acid as a feedstock material catalyzed by biomass‐derived SPAC catalysts. Additionally, various parameters such as catalyst synthetic parameters, esterification reaction parameters, the esterification reaction mechanism and catalytic stability, which affect the catalytic performance and esterification efficacy, are discussed. Finally, the future scope for developing novel SPAC catalysts has been demonstrated for young researchers in this field.

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