新型Brnsted-Lewis酸性催化剂LaPW_(12)O_(40)/SiO_2制备及其在催化酯化反应合成生物柴油中的应用

以十二磷钨杂多酸(Tungstophosphoric acid,H_3PW_(12)O_(40))为基体,分别通过普通浸渍法、溶胶凝胶法和超声浸渍法进行了La3+改性作用,合成了三种固体酸催化剂A-LaPW_(12)O_(40)、B-LaPW_(12)O_(40)/Si O2和C-LaPW_(12)O_(40)。采用X射线荧光光谱(XRF)、孔径比表面积测定、X射线粉末衍射(XRD)、透射电镜(TEM)、红外光谱(FT-IR)、热重(TG)、N2吸附-脱附、NH3程序升温脱附(NH3-TPD)、吡啶吸附红外光谱(Py-FTIR)、X射线光电子能谱(XPS)等方法对合成的催化剂进行了表征,并比较了以上催化剂在用于催化以油酸和甲醇为反应物经酯化反应合成生物柴油时的活性和稳定性。结果表明,B-LaPW_(12)O_(40)/Si O2具有最高催化活性,当甲醇与油酸的物质的量比为8∶1,催化剂用量为反应物总质量的2%,反应温度为65℃,反应1 h后,油酸的转化率即高达93%。循环使用B-LaPW_(12)O_(40)/Si O2催化剂六次后,油酸的转化率仍高达86.4%。B-LaPW_(12)O_(40)/Si O2的高催化活性和稳定性可归因于在溶胶凝胶的转化过程中,作为硅源材料的四乙氧基硅(TEOS)易在酸性条件下发生水解反应形成Si O2网络,进而Si O2网络中的硅醇键与H_3PW_(12)O_(40)中的H+发生配位作用,生成具有强静电吸附力的(≡Si-OH2+)(H2PW12O-40)络合物。随着该络合物的形成,促进了La3+在Si O2表面的吸附而堵塞了H_3PW_(12)O_(40)的孔道结构,抑制了H_3PW_(12)O_(40)颗粒在焙烧过程中进一步聚集长大。Si O2将作为载体并以干凝胶状态存在于B-LaPW_(12)O_(40)/Si O2催化剂中,由于Si O2凝胶的高比表面积而使B-LaPW_(12)O_(40)/Si O2具有了较大的比表面积,从H_3PW_(12)O_(40)的1.4 m2/g增加至31.3 m2/g。并且,通过吡啶吸附红外光谱确定B-LaPW_(12)O_(40)/Si O2为Br9nsted-Lewis酸型固体酸,由于Br9nsted酸位易与酯化反应过程中生成的水发生水合反应而失活,因而Lewis酸位的形成有助于减少催化剂的失活现象发生。Lewis酸位的出现可归因于(≡Si-OH2+)(H2PW12O-40)与吸附在其表面的具有强吸电子作用的La3+发生键合作用后生成了LaPW_(12)O_(40)/Si O2。

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.