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.

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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Biodiesel is a less hazardous, environmentally friendly biofuel. Waste biomass derived heterogeneous sulfonated catalysts were a significant focus of the most advanced biodiesel processing techniques for simple and low-cost manufacturing processes.

The synthesis of biodiesel from renewable resources has immense potential as a sustainable and cost-effective energy alternative. In this work, a reusable –SO3H functionalized heterogeneous catalyst that has a total acid density of 2.06 mmol/g was prepared from walnut (Juglans regia) shell powder by low-temperature hydrothermal carbonization (WNS-SO3H). Walnut shell (WNS) contains more lignin (50.3%), which shows great resistance toward moisture. The prepared catalyst was employed for the effective conversion of oleic acid to methyl oleate by a microwave-assisted esterification reaction. The EDS analysis revealed the significant presence of sulfur (4.76 wt%), oxygen (51.24 wt%), and carbon (44 wt%) content. The results of the XPS analysis confirm the bonding of C–S, C–C, C=C, C–O, and C=O. Meanwhile, the presence of –SO3H (the responsible factor for the esterification of oleic acid) was confirmed by FTIR analysis. Under the optimized conditions (9 wt% catalyst loading, 1:16 oleic acid to methanol molar ratio, 60 min reaction time, and 85 °C temperature), the conversion of oleic acid to biodiesel was found to be 99.01 ± 0.3%. The obtained methyl oleate was characterized by employing 13C and 1H nuclear magnetic spectroscopy. The conversion yield and chemical composition of methyl oleate were confirmed by gas chromatography analysis. In conclusion, it can be a sustainable catalyst because the catalyst preparation controls the agro-waste, a great conversion is achieved due to the high lignin content, and the catalyst was reusable for five effective reaction cycles.

In this work, the author developed Ca4Fe9O17/biochar (CFB) via a green method through a facile co-precipitation procedure involving egg shells as calcium precursor and investigating its performance in single as well as binary solution of methylene blue (MB) and rhodamine B (RhB). The CFB nanocomposite was characterized by XRD, SEM, TEM, XPS, Raman, FTIR, BET, and VSM. ESR studies show the presence of hydroxyl (·OH) and superoxide (O2·¯) radicals, which are primary radical species for pollutant degradation. The average crystalline size of CFB nanocomposites was found to be 32.992 nm using XRD, whereas TEM analysis indicates a particle diameter of 35–36 nm. The degradation efficacy of MB and RhB dyes was achieved at 99.2% and 98.6%, respectively, in a single solution, whereas 99.4% and 99.2%, respectively, in a binary solution within 36 min. Additionally, an iron cluster was formed during the degradation process of MB dye. The degradation of organic contaminants and generation of iron clusters from the degraded dye products were both expedited by the remarkable extension effect of the Ca4Fe9O17 in the CFB nanocomposites. The three processes were achieved using CFB nanocomposite: (1) the advanced oxidation process; (2) degradation of MB and RhB dye in single as well as binary solution with enhanced efficiency, (3) the production of the iron cluster from degraded products. Thus, these three steps constitute a smart and sustainable way that leads to an effective effluent water treatment system and the generation of iron clusters preventing secondary pollution.

Metal ferrites like nickel ferrite, copper ferrite, zinc ferrite, cobalt ferrite, and other ferrites are highly abundant and used as catalysts in various transformations. However, CaFe2O4 is the most abundant alkali metal ferrite, which is eco-friendly and non-toxic. CaFe2O4 is a superparamagnetic material that can be easily recovered from the reaction media due to its magnetic properties. Therefore, it has been widely used in numerous applications, such as dye degradation, removal of heavy metal ions, transesterification reaction, gas sensing, photocatalytic water splitting, drug delivery, etc. In this review, (1) magnetic properties and crystal structure of spinal CaFe2O4 are discussed, (2) potential applications of CaFe2O4 are discussed, and (3) various synthesis techniques of CaFe2O4 are demonstrated. CaFe2O4 shows photocatalytic properties due to its narrow band gap (1.9 eV), abundant functional group, and high surface area. It is found that CaFe2O4 possesses a remarkable potential for energy and environmental remediation. In this review, we have added the photocatalytic mechanism of various pollutants. At last, future perspectives are given for developing novel, sustainable CaFe2O4 or CaFe2O4-based nanocomposite.