Chemists have reported that a lanthanum molybdenum perovskite catalyst, LaMoO3, can convert waste cooking oil into biodiesel with a conversion of 88.7% when hexane is added as a co-solvent to the methanol-based transesterification reaction. The study, published open access in Discover Catalysis on 1 April 2026, examined both the role of the co-solvent in overcoming mass transfer limitations and the surface thermodynamics of the reaction near the boiling point of methanol. The work addresses one of the persistent practical obstacles in biodiesel production: the fact that oil and methanol do not mix readily, which slows the reaction when a solid catalyst is used.
The research team, led by Ketshepile Baabua with Rapelang Patala, Hui Li and Ndzondelelo Bingwa, synthesized the LaMoO3 perovskite by a sol-gel method. Molybdenum trioxide was dissolved in deionized water with citric acid, heated to 80 degrees Celsius, and ethylene glycol was added dropwise to condense the solution into a gel. The gel was then calcined at temperatures up to 650 degrees Celsius to produce the lanthanum-based perovskite. Powder X-ray diffraction indexed the material as an ABO3-type perovskite with a trigonal crystal system and R3c space group, matched to the JCPDS reference 96-100-0501 with lattice parameters a = 17.0051 Å and b = 6.8607 Å. Diffraction peaks near 28 and 32 degrees 2θ were assigned to the MoO and La components respectively.
Microscopy and spectroscopy confirmed that the catalyst formed as a single perovskite phase rather than a mixture of less active monometallic oxides. High-resolution transmission electron microscopy showed a structurally uniform material with no evidence of separate MoOx or LaOx phases, and elemental mapping revealed a uniform distribution of the components. Energy-dispersive X-ray spectroscopy verified the expected elements, with weight percentages of 66.4% La, 26.7% Mo and 6.9% O, and detected no impurities. Scanning electron micrographs showed irregular agglomerates with sharp edges, a morphology the authors note is desirable for catalysis because it exposes surface area. Thermogravimetric analysis between 25 and 1000 degrees Celsius indicated a thermally stable material.
The catalytic reactions were carried out in a three-neck round-bottom flask fitted with a reflux condenser and thermocouple, using 11 mL of waste cooking oil, 10 mL of methanol and, where applicable, 15 mL of hexane. The mixture was heated between 40 and 70 degrees Celsius before 0.06 g of catalyst was added to start the reaction with constant stirring. The fatty acid methyl ester layer was separated from glycerol and excess methanol in a separating funnel, washed with hot water until neutral, filtered and dried over sodium sulfate. Product formation was quantified by proton nuclear magnetic resonance spectroscopy on a 500 MHz instrument, using Knothe’s formula based on the integrated areas of the methyl ester and methylene proton signals, and complemented by infrared spectroscopy showing ester C–O and C=O bands.
Temperature had a decisive influence on conversion. At 40 degrees Celsius with a 20:1 alcohol-to-oil molar ratio, conversion reached 43.6% after two hours. Raising the temperature increased conversion, with 60 degrees Celsius emerging as the optimum, consistent with an earlier literature report on waste cooking oil transesterification. At 70 degrees Celsius, however, conversion declined because the temperature exceeds methanol’s boiling point: most of the methanol becomes gaseous, and the authors also note that accumulating bubbles at the catalyst surface block active sites. The finding reinforces the principle that, to avoid alcohol evaporation while attaining good yields, the reaction temperature should sit close to the alcohol’s boiling point without exceeding it.
Thermodynamic analysis between 40 and 60 degrees Celsius, with 70 degrees excluded because methanol is gaseous there, yielded activation energies of 29.68 kJ/mol without hexane and 30.62 kJ/mol with the co-solvent. The near-identical values led the authors to conclude that the reaction is governed primarily by the catalyst surface rather than by solvent properties, with the solvent’s role confined to controlling mass transfer and reactant solubility. Using Eyring-type analysis, the enthalpy and entropy values were computed as 646.60 kJ/mol and 47.3 kJ/mol without hexane, and 687.46 kJ/mol and 48.4 kJ/mol with hexane. The positive enthalpy and entropy values indicate an endothermic process with increased disorder at the solid-liquid interface during adsorption, and the slightly higher enthalpy with hexane suggests that the less polar medium stabilizes the transition state somewhat less effectively.
The Gibbs free energy, calculated from the enthalpy and entropy values, came out negative in both systems: −15,111.39 kJ/mol without hexane and −15,437.00 kJ/mol with hexane, indicating a spontaneous reaction in the presence of the catalyst. On the time dependence of the reaction, conversions rose with longer reaction times, from 47.3% to 70.0% with methanol alone and from 49.7% to 89.6% with the hexane co-solvent at a hexane-to-oil molar ratio of 1:1, all at 60 degrees Celsius over two hours with 50 mg of catalyst. Varying the catalyst amount between 20 and 60 mg produced conversions of 32.5% to 59.9% without hexane and 48.1% to 93.9% with hexane, consistent with the expected increase in active surface area.
The co-solvent comparison was central to the study. Hexane outperformed iso-octane and tert-butanol, which showed no activity in the reactions performed in this work. The authors attribute hexane’s effectiveness to the nonpolar character of waste cooking oil meeting a polar methanol phase: hexane disturbs this biphasic system, improves the mutual solubility of methanol and oil, and reduces mass transfer limitations, allowing faster interactions at the perovskite surface. They also argue against alternatives: ethanol can promote saponification and lower yields, tetrahydrofuran complicates product separation by forming a uniform mixture, and toluene’s high boiling point makes removal energy-intensive and costly. Variations in methanol-to-oil molar ratios raised conversions from 43.6% to 77.8% without the co-solvent, but addition of hexane pushed the yield up to 88.7%.
Recyclability testing over five catalytic cycles showed gradual deactivation attributed to loss of catalyst mass during recycling. Without hexane, conversion fell from 84.4% to 60.6%; with the co-solvent, it fell from 90.6% to 65.0%, reflecting a reduction in available active sites. The authors propose a reaction mechanism in which the methoxy oxygen of methanol adsorbs on a lanthanum cation with the proton on lattice oxygen, the triglyceride carbonyl oxygen adsorbs on a molybdenum site, the adsorbed methoxy then bonds with the adsorbed triglyceride to form fatty acid methyl ester and a diglyceride anion, and the diglyceride finally reacts with reactive hydrogen species and desorbs, closing the catalytic cycle.
In a techno-economic discussion, the team acknowledges that molybdenum is costly compared with lime-based CaO catalysts, but argues that the Mo-based lanthanum perovskite offers better catalytic activity, greater tolerance of the free fatty acids and water found in complex waste feedstocks, and thermal stability, whereas CaO-based catalysts lack structural stability and zeolites are expensive and typically require sophisticated packed-bed reactors. Because waste cooking oil composition is far from that of a single-substrate feed, the authors conclude that the advantages of the Mo-based system outweigh its cost in complex waste scenarios. The study was corrected after publication, with a revision to the reference list in July 2026 and a published correction in August 2026. The overall conclusion is that manipulating solvent properties, rather than the intrinsic activation energetics, is the practical lever for improving biodiesel yields from waste cooking oil over this perovskite catalyst.
Subject of Research: Transesterification of waste cooking oil to biodiesel over a LaMoO3 perovskite catalyst with hexane co-solvent and surface thermodynamics analysis
Article Title: Biodiesel synthesis using waste cooking oil via transesterification over heterogeneous inorganic perovskites catalysts: a study of co-solvent effect and surface thermodynamics
Article References: Baabua, K., Patala, R., Li, H., & Bingwa, N. (2026). Biodiesel synthesis using waste cooking oil via transesterification over heterogeneous inorganic perovskites catalysts: a study of co-solvent effect and surface thermodynamics. Discover Catalysis, 3(1), Article 8. https://doi.org/10.1007/s44344-026-00037-8
Image Credits: AI Generated
DOI: 10.1007/s44344-026-00037-8
Keywords: biodiesel, waste cooking oil, transesterification, LaMoO3 perovskite, heterogeneous catalysis, hexane co-solvent, fatty acid methyl esters, surface thermodynamics, mass transfer, sol-gel synthesis, methanol, catalyst recyclability
News Source: Bethany Barker. (October 29, 2026). Hexane co-solvent lifts waste cooking oil biodiesel conversion over LaMoO3 perovskite catalyst. Scienmag.
