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Transesterification of sunflower oil was carried out over CaO loaded onto γ-Al 2 O 3 as heterogeneous base catalysts. The solid base catalysts were synthesized by the modified wet impregnation of calcium acetate monohydrate onto neutral γ-Al 2 O 3 , calcined at different temperatures in an inert atmosphere and characterized by FESEM, XRPD and FTIR techniques. The catalyst containing 5.5 wt. % CaO supported on γ-Al 2 O 3 calcined at 700 °C exhibited the highest FAME yield at mild reaction conditions (60 °C, atmospheric pressure, methanol-to-oil molar ratio of 9:1 and 0.51% of active species-CaO by weight of oil). The best CaO/γ-Al 2 O 3 catalyst showed a similar or lower catalytic activity than some other CaO-based catalysts at a smaller amount of the catalyst (at least two times less than in other studies).

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The statistical experimental design (DoE) and optimization (Response Surface Methodology combined with Box–Behnken design) of sunflower oil transesterification catalyzed by waste chicken eggshell-based catalyst were conducted in a custom-made microreactor at 60 °C. The catalyst was synthesized by the hydration–dehydration method and subsequent calcination at 600 °C. Comprehensive characterization of the obtained catalyst was conducted using: X-ray powder diffractometry (XRD), X-ray fluorescence (XRF), Fourier-transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), N2 physisorption, and Hg-porosimetry. Structural, morphological, and textural results showed that the obtained catalyst exhibited high porosity and regular dispersity of plate-like CaO as an active species. The obtained optimal residence time, catalyst concentration, and methanol/oil volume ratio for the continuous reaction in microreactor were 10 min, 0.1 g g−1, and 3:1, respectively. The analysis of variance (ANOVA) showed that the obtained reduced quadratic model was adequate for experimental results fitting. The reaction in the microreactor was significantly intensified compared to a conventional batch reactor, as seen through the fatty acid methyl esters (FAMEs) content after 10 min, which was 51.2% and 18.6%, respectively.

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Coal fly ash (CFA) is generated during the combustion of coal for energy production. Many studies are based on its utilization as the most abundant, cheap aluminosilicate industrial residue, which is recognized as a risk for the environment and human health. The present review is focused on CFA origin, chemical properties, and its catalytic application for biodiesel production. The aluminosilicate nature and the presence of rare earth elements make CFA suitable for different adsorption, catalytic, and extraction processes for obtaining valuable products including alternative fuels and pure elements. However, the presence of toxic elements is a potential environmental problem, which should be solved in order to avoid soil, water, and air pollution. The most used modification methods are alkali activation, hydrothermal, and thermal treatment that improve the structural, morphological, and textural properties. The active catalytic form could be obtained by impregnation or ion exchange method. It was found that such synthesized materials have significant catalytic potential in the biofuel chemistry. In the case of biodiesel production, the high values of conversion or yield can be achieved under mild low-energy reaction conditions in the presence of low-cost waste-based catalysts.