The Synergic Effect of h-MoO₃, α-MoO₃, and β-MoO₃ Phase Mixture as a Solid Catalyst to Obtain Methyl Oleate

Extensive research in the last few decades has conclusively demonstrated the significant influence of experimental conditions, surfactants, and synthesis methods on semiconductors' properties in technological applications. Therefore, in this study, the synthesis of molybdenum oxide (MoO₃) was reported by the addition of 2.5 (MoO₃_2.5), 5 (MoO₃_5), 7.5 (MoO₃_7.5), and 10 mL (MoO₃_10) of nitric acid, obtaining the respective concentrations of 0.6, 1.10, 1.6, and 0.6 mol L^-1. In this study, all samples were synthesized by the hydrothermal method at 160 °C for 6 h. The materials obtained were structurally characterized by X-ray diffraction (XRD) and structural Rietveld refinement, Raman spectroscopy, and infrared spectroscopy (FTIR), confirming the presence of all crystallographic planes and bands associated with active modes for the pure hexagonal phase (h-MoO₃) when the solution's concentration was 0.6 mol L^-1 of nitric acid. For concentrations of 1.10, 1.60, and 2.10 mol L^-1, the presence of crystallographic planes and active modes associated with the formation of mixtures of molybdenum oxide polymorphs was confirmed, in this case, the orthorhombic, monoclinic, and hexagonal phases. X-ray photoelectron spectroscopy reveals the occurrence of the states Mo⁴⁺, Mo⁵⁺, and Mo⁶⁺, which confirm the predominance of the acid Lewis sites, corroborating the analysis by adsorption of pyridine followed by characterization by infrared spectroscopy. The images collected by scanning electron microscopy confirmed the information presented in the structural characterization, where microcrystals with hexagonal morphology were obtained for the MoO₃_2.5 sample. In contrast, the MoO₃_5, MoO₃_7.5, and MoO₃_10 samples exhibited hexagonal and rod-shaped microcrystals, where the latter morphology is characteristic of the orthorhombic phase. The catalytic tests carried out in the conversion of oleic acid into methyl oleate, using the synthesized samples as a heterogeneous catalyst, resulted in conversion percentages of 52.5, 58.6, 69.1, and 97.2% applying the samples MoO₃_2.5, MoO₃_5, MoO₃_7.5, and MoO₃_10, respectively. The optimization of the catalytic tests with the MoO₃_10 sample revealed that the conversion of oleic acid into methyl oleate is a thermodynamically favorable process, with a variation in the Gibbs free energy between -67.3 kJ mol^-1 and 83.4 kJ mol^-1 as also, the energy value of activation of 24.6 kJ mol^-1, for the temperature range from 80 to 140 °C, that is, from 353.15 to 413.15 K, respectively. Meanwhile, the catalyst reuse tests resulted in percentages greater than 85%, even after the ninth catalytic cycle. Therefore, the expressive catalytic performance of the mixture of h-MoO₃ and α-MoO₃ (MoO₃_10) phases is confirmed, associated with the synergistic effect, mainly due to the increase in the surface area and available Lewis sites of these phases.