This work investigates the catalytic activity of geopolymers produced using two different alkali components (sodium or potassium) and four treatment temperatures (110 to 700 °C) for the methyl transesterification of soybean oil. The geopolymers were prepared with metakaolin as an aluminosilicate source and alkaline activating solutions containing either sodium or potassium in the same molar oxide proportions. The potassium-based formulation displayed a higher specific surface area and lower average pore size (28.64–62.54 m²/g; 9 nm) than the sodium formulation (6.34–32.62 m²/g; 17 nm). The reduction in specific surface area (SSA) after the heat treatment was more severe for the sodium formulation due to the higher thermal shrinkage. The catalytic activity of the geopolymer powders was compared under the same reactional conditions (70–75 °C, 150% methanol excess, 4 h reaction) and same weight amounts (3% to oil). The differences in performance were attributed to the influences of sodium and potassium on the geopolymerization process and to the accessibility of the reactants to the catalytic sites. The Na-based geopolymers performed better, with FAME contents in the biodiesel phase of 85.1% and 89.9% for samples treated at 500 and 300 °C, respectively. These results are competitive in comparison with most heterogeneous base catalysts reported in the literature, considering the very mild conditions of temperature, excess methanol and catalyst amount and the short time spent in reactions.
Geopolymer powders and 3D‐printed lattices have shown promising preliminary results as heterogeneous catalysts for the transesterification of vegetable oils to produce biodiesel. However, questions about the basicity of catalytic sites and the leaching characteristics of metals (K, Na) and hydroxyl groups in the reactional mixtures remained. The leaching of alkaline ions in methanol and biodiesel for powder and printed geopolymer formulations based on K, Na, or Na+K activators and treated at 110 to 700°C was investigated, as well as the physiochemical modifications of the materials. The Hammett indicators were used to determine base strength, and both leachable and total basicities were quantified. The amount of Na and K leached into the biodiesel phase was negligible (<1% wt.%). Methanol leaching reached a maximum of 29.3%. The base strength ranged between 11.0 and 18.4. Potassium‐based geopolymer lattices presented the highest basicity, followed by sodium and sodium‐potassium geopolymer catalysts. The basicity of all formulations decreased gradually as the calcination temperature increased. When compared to the homogeneous catalysts NaOH and KOH, the level of biodiesel contamination with Na and K is 81–93% lower. The findings support the heterogeneous nature of geopolymers as biodiesel catalysts and further validates their use for this application.
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