Researchers in Nigeria have developed a reusable calcium oxide and aluminum oxide catalyst from calcined snail shells that enables direct transesterification of high-FFA jatropha oil, simplifying the process by avoiding the acid pretreatment that such oils normally require.
The study addresses a known technical challenge: biodiesel production from non-edible jatropha oil with high free fatty acid content is complicated by soap formation during conventional base-catalyzed transesterification, which typically necessitates a costly and time-consuming acid pretreatment. Jatropha curcas oil is attractive as a non-edible feedstock, but the oil analyzed in this study contained 3.09 percent free fatty acids and an acid value of 37.365 milligrams of potassium hydroxide per gram. When such oil meets a basic catalyst like calcium oxide, the free fatty acids react with basic sites to form soaps that block active surfaces, complicate separation and depress yield. The usual remedy is a costly and time-consuming acid pretreatment to lower the acidity before transesterification. A bifunctional catalyst carrying both acid and base sites can instead perform esterification and transesterification simultaneously, collapsing the process into one step and cutting equipment, chemical and energy costs.
To make their catalyst, the team cleaned snail shells collected from Gurore in Adamawa State, dried them at 105 degrees Celsius, and calcined them at 900 degrees Celsius for four hours. X-ray diffraction of the raw shells showed aragonite, a crystalline form of calcium carbonate, made up 86.6 percent of the material, with quartz and minor silicate impurities making up the rest. The calcined powder was then impregnated with aluminum oxide at various ratios, dried and recalcined to produce the final CaO/Al2O3 composite.
Microscopy revealed how the impregnation reshaped the catalyst. Scanning electron images of the bare calcium oxide showed a highly porous, rough surface built from fine, loosely aggregated particles, a morphology favorable for catalysis because it maximizes surface area and active sites. After aluminum oxide was added, the structure became denser and more compact, with larger clusters and agglomerates where the alumina had coated or filled the pores. Infrared spectroscopy reinforced the picture of a genuine composite: the carbonate band shifted slightly, several bands weakened or vanished, and a new absorption near 712.73 inverse centimeters appeared, which the authors attribute to metal-oxygen lattice vibrations of Ca-O-Al or Al-O-Al linkages, evidence for a mixed oxide structure rather than a simple physical mixture.
The heart of the study is a statistical optimization of the transesterification conditions using a central composite design. Four variables were explored: reaction temperature, reaction time, catalyst loading and the methanol-to-oil molar ratio. Analysis of variance showed the model was highly significant, explaining 97.78 percent of the variation in yield, with the interaction between temperature and methanol ratio the single most influential factor. The optimum landed at 64.3 degrees Celsius, 2.65 hours of reaction, 2.29 percent catalyst by weight of oil, and a methanol-to-oil molar ratio of 0.396, producing the 93.4 percent peak yield. Notably, the authors caution that this optimum is a model-derived operating condition within the investigated domain, not a general stoichiometric requirement; the theoretical methanol-to-triglyceride ratio remains 3:1.
The design experiments also exposed how unforgiving the process is to off-optimum settings. Runs held at the lowest tested temperature of 63 degrees Celsius with only 2 percent catalyst dropped to yields between 80.6 and 85.2 percent, while a run at 70 degrees Celsius with 2 percent catalyst fell as low as 63.3 percent. Excess catalyst proved actively harmful: a run with 5 percent catalyst at 63 degrees Celsius produced the worst yield in the design, just 45.5 percent, which the authors attribute to hindered mass transfer or side reactions at high loading. Pushing reaction time to four hours at 70 degrees helped, lifting yield to 83.5 percent, but still fell short of the optimum because the other variables were misaligned. The desirability score of 0.97 for the optimum versus 0.53 for the least favorable condition confirmed the robustness of the optimization.
Kinetic analysis added a second layer of insight. Sampling the reaction at intervals from 2 to 180 minutes across four temperatures, the team found free fatty acid conversion rising fastest in the opening minutes and reaching 87 percent at 70 degrees Celsius after three hours, compared with 79 percent at 55 degrees. The data fit a second-order kinetic model well, with regression coefficients between 0.9254 and 0.9562, and the rate constant climbed from 7.6 times ten to the minus third to 1.37 times ten to the minus second as temperature rose from 55 to 70 degrees. An Arrhenius plot yielded an activation energy of 37.54 kilojoules per mole and a pre-exponential factor of 7.12 times ten cubed, figures the authors say fall within the range reported for heterogeneous bifunctional biodiesel catalysts and indicate a moderate energy barrier compatible with mild operating conditions.
Reusability testing over six cycles showed the catalyst degrading gracefully rather than collapsing. Yield slipped from about 93 percent with fresh catalyst to 92.3 percent in the second cycle, 90 percent in the third, and 87 percent in the fourth, staying above 85 percent for the first four cycles before a steeper decline to roughly 75 percent by the sixth. The authors attribute the later losses to pore blockage and partial leaching of calcium ions into the reaction medium.
Screening different CaO to Al2O3 ratios showed yield climbing from 73.0 percent at a 1:0.2 ratio to a maximum of 87.1 percent at 1:0.6, then falling to 70.5 percent at 1:1.6, a pattern the researchers link to dilution of the active calcium oxide phase and masking of basic sites when alumina dominates. A second series, varying calcium oxide from 0.2:1 to 2:1, rose from 64.5 to 82.5 percent but never matched the 1:0.6 optimum, underscoring that neither component alone suffices. The biodiesel itself passed spectroscopic scrutiny: infrared spectra showed the ester carbonyl peak at 1742.22 inverse centimeters and the ester C-O stretch at 1168.25, the defining fingerprints of fatty acid methyl esters, while gas chromatography-mass spectrometry revealed a dominant peak at about 12.4 minutes and a distribution of peaks consistent with the C16 to C18 methyl esters typical of jatropha biodiesel, though the authors note definitive compound identification requires the full mass spectral library matches.
The broader significance lies in what the catalyst is made of. The authors argue that Nigeria’s substantial snail shell resources, favorable climate for jatropha cultivation and growing renewable energy interest give the country the technical and resource potential to adopt such strategies, provided supportive policies, commercial investment and sustainable feedstock supply chains are established. They also position the work against a gap in the literature: most published jatropha biodiesel studies with bifunctional catalysts have focused on CaO-La2O3, CaO-ZnO or other mixed oxides, and comprehensive kinetic modeling of CaO/Al2O3 systems has been rare. By integrating catalyst development, characterization, response surface optimization, reusability assessment and kinetic modeling in a single investigation, the study offers a template for scaling waste-derived catalysts to industrial biodiesel production from difficult, high-acid feedstocks.
Subject of Research: Development and optimization of a snail-shell-derived CaO/Al2O3 bifunctional catalyst for single-step biodiesel production from high-FFA jatropha oil
Article Title: Development of CaO/Al2O3 bifunctional catalyst for biodiesel production from jatropha oil: optimization and kinetic study
Article References: Wali, S. A., Chiroma, T. M., Abubakar, A. M., Shamsuddin, R., & Arowo, M. N. (2026). Development of CaO/Al2O3 bifunctional catalyst for biodiesel production from jatropha oil: optimization and kinetic study. Discover Catalysis, 3(1), Article 22. https://doi.org/10.1007/s44344-026-00052-9
Image Credits: AI Generated
DOI: 10.1007/s44344-026-00052-9
Keywords: biodiesel, jatropha oil, CaO/Al2O3 catalyst, bifunctional catalyst, snail shells, transesterification, response surface methodology, kinetics, activation energy, catalyst reusability, free fatty acids, Nigeria
News Source: Bethany Barker. (November 6, 2026). Snail-shell catalyst turns high-acid jatropha oil into biodiesel in one step. Scienmag.
