(560w) Evaluation of Al, Ti, and Nb Oxides As Support for H3PMo12O40 to be Used As Catalyst in Biodiesel Production from Low-Grade Oils

Heteropolyacids (HPA) with Keggin structures, such as H₃PW₁₂O₄₀, H₄SiW₁₂O₄₀, H₃PMo₁₂O₄₀, and H₄SiMo₁₂O₄₀, have been described as efficient catalysts in trans/esterification reactions due to their tolerance to water and free fatty acids contents, with particularly well-suited characteristics of high proton mobility and stability due to their strong Brønsted-profile. Despite the numerous advances over the past years, a few challenges remain as focus of study, particularly in increasing the thermal stability, specific area, and solubility in polar media. The versatility array of HPA is considerably increased when such catalysts are supported onto solid matrices, such as activated carbon, and on Zr-, Ti-, Nb-, and Al-oxides. Nonetheless, the characteristics of the support have some direct implications on the performance of the HPA catalytic properties. While zirconia has been demonstrated as one of the best supports for HPA, the use of a catalyst prepared as HPA supported on Zr-oxides is hindered by the costs associated with the matrices, which often affect the economics of bulk-production chemicals, as biodiesel. In this sense, this study evaluated the synthesis of a 12-phosphomolybdic acid catalyst supported on Al₂O₃, TiO₂, and Nb₂O₅ and its application on simultaneous esterification and transesterification reaction to produce biodiesel using low-grade lipid feedstock, i.e., acid-rich macaw palm oil (Acrocomia aculeata). Results show that under optimized conditions, the Al-oxide supported-catalyst presented the highest surface area (31.2 m² g^-1), and a considerable acidity value of 7.83 mmol H⁺ g^-1. While the catalysts supported on Ti-oxide presented the highest acidity (8.21 mmol H⁺ g^-1), the surface area was the lowest observed on all three matrices (13.2 m² g^-1). Considering the reaction conditions tested on a pressurized reactor to produce standard-grade ethyl esters, the HPA catalyst supported on Ti-oxide required the least amount on a weight basis in regards to the oil used, i.e., 5 wt.% of HPA/TiO₂ was enough to produce a mixture with 98.22% of ethyl esters and viscosity of 4.81 mm² s^-1, which fits within the standard values for using as fuel. The HPA/Al₂O₃ and HPA/Nb₂O₅ catalysts, on the other hand, were used at higher concentrations (13 and 20 wt.%, respectively) to produce ethyl esters of biodiesel grade. Nonetheless, the use of the catalysts supported on Al-oxide required lower reaction temperature (190 °C, compared to 210 °C for HPA/Nb₂O₅ and 200 °C for HPA/TiO₂) and also a lower ethanol-to-oil molar ratio (50:1 for HPA/Al₂O₃ and 90:1 for HPA/Nb₂O₅ and 60:1 for HPA/TiO₂) when compared to the others. Thus, the Al₂O₃ support demonstrated to be feasible under the reaction conditions to convert low-grade oil to ethyl esters of fuel grade. These results may, for instance, provide a novel alternative to the utilization of HPA with a special look on the production of bulk chemicals, as biodiesel, since the costs associated with the use of Al₂O₃ as support can potentially lower the costs associated with the production of the catalyst.