A recent study published in Scientific Reports explored the use of metal-organic frameworks (MOFs)-mediated synthesis (MOFMS) for producing stable heterogeneous CaO/ZnO composite catalysts for biodiesel production from soybean oil at room temperature. The synthesized CaO@ZnO and ZnO@CaO nanocomposites achieved biodiesel conversion rates of 99 % and 92 %, respectively, within 25 minutes.
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Background
The rising levels of greenhouse gases due to excessive fossil fuel consumption have become a significant global concern. Biodiesel presents a viable alternative, produced through the transesterification of renewable oils such as palm, soybean, and vegetable oils with methanol or ethanol in the presence of a catalyst.
Catalysts for transesterification can be homogeneous or heterogeneous. While homogeneous acidic and basic catalysts offer high selectivity and activity, they are difficult to recover and separate from the final product. In contrast, heterogeneous catalysts, such as alkali and mixed metal oxides, are easily recoverable and reusable.
However, most heterogeneous catalysts require external heating via oil/water baths, microwaves, or ultrasound. This study introduced an alternative approach, utilizing MOFs—porous hybrid materials composed of metal clusters and organic ligands—to synthesize effective and reusable catalysts under mild reaction conditions.
Methods
CaO@ZnO and ZnO@CaO nanocomposites were synthesized using an impregnation method. The MOFMS of CaO@ZnO was achieved by stabilizing calcium acetate inside the zeolitic imidazolate framework-8 (ZIF-8), while ZnO@CaO was synthesized by incorporating zinc nitrate into calcium-fumarate-based MOFs.
X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and proton nuclear magnetic resonance spectroscopy (1H NMR) confirmed the catalysts' successful synthesis. Surface area, pore volume, and pore size were measured using N2 adsorption/desorption at 77 K. Additional characterization techniques included field emission scanning electron microscopy (FESEM) and inductively coupled plasma optical emission spectrometry (ICP-OES).
The basicity of the catalysts was determined using CO2-temperature programmed desorption (CO2-TPD), while potentiometric titration measured their acid number. Biodiesel production was conducted at room temperature via transesterification catalyzed by CaO@ZnO and ZnO@CaO. After the reaction, catalysts were filtered out, and the crude biodiesel was analyzed using 1H NMR spectroscopy. Biodiesel yield was quantified from the integration values of glyceridic and methyl ester protons in the spectra.
The catalysts’ reusability in the transesterification of soybean oil with methanol was assessed over multiple cycles, with XRD monitoring the recovered catalysts’ structure. The catalytic performance of CaO@ZnO and ZnO@CaO was also compared to nano CaO and bulk CaO.
Results and Discussion
XRD confirmed the presence of CaO and ZnO phases in the nanocomposites, while FESEM images revealed a homogeneous agglomerated structure with sintered nanoparticles. The uniform distribution of Ca, Zn, and O in the nanocomposites demonstrated the effectiveness of MOFs in facilitating even elemental dispersion, enhancing transesterification efficiency through improved reactant interaction with active sites.
Increasing the transesterification time improved reaction efficiency, with CaO@ZnO reaching 99 % conversion in 25 minutes, leading to its selection as the optimal reaction time. A methanol-to-oil ratio of 40:1 yielded the highest conversion under mild conditions. The 1H NMR spectrum of the resulting biodiesel confirmed a 99 % yield, consistent with results obtained using anhydrous methanol.
Among the tested catalysts, CaO@ZnO demonstrated the highest biodiesel yield in 25 minutes. Both CaO@ZnO and ZnO@CaO exhibited similar efficiency when reaction time was extended to 60 minutes. Catalyst concentration also played a role, with maximum biodiesel yield achieved using 20 mg of catalyst (5.4 wt%).
Varying the molar ratios of Ca/Zn (1:0.5, 1:1, 1:2, and 1:3) in CaO@ZnO affected efficiency, with the highest yield observed at a 1:2 ratio. The encapsulation of the CaO precursor within ZIF-8 enhanced catalytic activity by improving Zn-MOF dispersion while reducing CaO agglomeration.
Conclusion
The study successfully synthesized efficient heterogeneous CaO@ZnO and ZnO@CaO catalysts for biodiesel production from soybean oil. The catalysts were easily prepared via one-pot impregnation of calcium and zinc salts in cost-effective MOFs, followed by calcination.
The synthesized CaO/ZnO nanocomposites demonstrated high catalytic efficiency and reusability. CaO@ZnO achieved a maximum biodiesel conversion of 99 % at room temperature within 25 minutes, maintaining 70 % conversion after six cycles with minimal Ca2+ leaching (2 %). The presence of ZnO played a crucial role in maintaining catalyst stability and dispersion.
These findings suggest that CaO@ZnO is a promising catalyst for transesterification reactions, offering a sustainable and scalable solution for biodiesel production.
Journal Reference
Sharifi, M., et al. (2025). Metal-organic frameworks-derived CaO/ZnO composites as stable catalysts for biodiesel production from soybean oil at room temperature. Scientific Reports. DOI: 10.1038/s41598-025-87393-x, https://www.nature.com/articles/s41598-025-87393-x
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