Preparation and application of a novel Brönsted-Lewis acid catalyst LaPW₁₂O₄₀/SiO₂ for the synthesis of biodiesel via esterification reaction

In this study, H₃PW₁₂O₄₀ (Tungstophosphoric acid) was applied as matrix, and which was modified by La³⁺ through conventional impregnation method, ultrasonic impregnation method and sol-gel method, obtained three solid acid catalysts: A-LaPW₁₂O₄₀, B-LaPW₁₂O₄₀/SiO₂ and C-LaPW₁₂O₄₀. These above catalysts were characterized by X-ray fluorescence spectrometer, specific surface area and porosity analyzer, X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectoscopy, thermogravimetric analysis, N₂/adsorption-desorption, NH₃ temperature programmed desorption, pyridine adsorption IR spectra and X-ray photoelectron spectroscopy. The catalytic activities and stabilities of them were compared when they were used for the catalytic synthesis of biodiesel from the esterification reaction of oleic acid and methanol. Results shown that the B-LaPW₁₂O₄₀/SiO₂ has highest catalytic activity and stability: the conversion of oleic acid can be high to 93% when the molar ratio of methanol to oleic acid was 8:1, mass ratio of catalyst to reactants was 2%, reaction temperature was 65 ℃ and reaction time was 1 h; the conversion of oleic acid maintained 86.4% after B-LaPW₁₂O₄₀/SiO₂ had been cycled six times. The high catalytic activity and stability of B-LaPW₁₂O₄₀/SiO₂ can be explained as follows: a SiO₂ network was formed from the hydrolytic action of Si(OC₂H₅)₄ (TEOS) under acidic conditions via Sol-Gel process. The H⁺ of H₃PW₁₂O₄₀ will bond with Si-OH in SiO₂ network to form a (≡Si-OH²⁺)(H₂PW₁₂O₄₀^-) complex with strong electrostatic adsorption force, thus promoting the adsorption of La³⁺ on the surface of SiO₂, greatly. As a result, the pore structure of H₃PW₁₂O₄₀ will be blockaged, the grow up of H₃PW₁₂O₄₀ particles in the roasting process also will be inhibited. In addition, SiO₂ may be existed in the form of dry gel in the B-LaPW₁₂O₄₀/SiO₂ catalyst and acted as carrier. It will be favorable for the improvent of the surface area of B-LaPW₁₂O₄₀/SiO₂ since SiO₂ has high surface area, so the surface area of B-LaPW₁₂O₄₀/SiO₂ has increased from the 1.4 m²/g of H₃PW₁₂O₄₀ to the 31.3 m²/g. And more, LaPW₁₂O₄₀/SiO₂ has been determined from the Py-FTIR spectra of pyridine adsorption analysis, which is a Brönsted-Lewis solid acid. The formation of Lewis acid sites can help to reduce the deactivation of a solid acid catalyst: some H₂O will be generated from the esterification reaction, and hydration will occur between Brönsted acid site and H₂O, so the deactivation will occur. The formation of Lewis acid sites can be ascribed to the strong electrophilic action of La³⁺ after it has been bonded with (≡Si-OH²⁺)(H₂PW₁₂O₄₀^-) to form LaPW₁₂O₄₀/SiO₂.