Structural and Catalytic Assessment of Clay-Spinel-TPA Nanocatalysts for Biodiesel Synthesis from Oleic Acid

A series of clay–spinel nanocomposites reinforced by tungstophosphoric acid (TPA) were prepared and examined for the esterification of oleic acid. The type of spinel (ZnAl2O4 and CuAl2O4) and the weight ratio of clay-to-spinel were evaluated. The characterization results revealed that the clay–ZnAl2O4 nanocomposite formed better than the clay–CuAl2O4, with fewer other phases, such as ZnO or CuO. Moreover, clay–ZnAl2O4 showed higher pore volume and pore size, which led to higher conversion of oleic acid to biodiesel. The nanocomposite exhibited a good interaction between the spinel phase and clay, preventing the agglomeration of TPA. Assessing the weight ratio of clay-to-ZnAl2O4 (0.5, 1, and 1.5) showed that the same ratio of clay-to-spinel provided higher activity. It can be attributed to its rough surface, which facilitates vortex flow on the catalyst surface, its high pore volume (0.122 cc/g), and pore size (24.6 nm), enabling the diffusion of reactants and the egression of products, as well as its high acidic (453.9 μmol/g) and basic (731.6 μmol/g) properties. The clay–ZnAl2O4(1)–TPA with the largest particle size in the range of 10–30 nm converted 81.9% of oleic acid under the conditions of 120 °C, a 12 molar ratio of methanol-to-oleic acid, 4 wt.% of catalyst, and 4 h of reaction time. Due to both acidic and basic properties, along with its good reusability, the clay–ZnAl2O4(1)–TPA nanocatalyst can be a suitable catalyst for industrial biodiesel production via esterification and transesterification reactions.