摘要
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。
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-La PW12O40,B-La PW12O40/SiO2 and C-La PW12O40. 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-La PW12O40/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-La PW12O40/Si O2 had been cycled six times. The high catalytic activity and stability of B-La PW12O40/SiO2 can be explained as follows： a Si O2 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 Si O2 network to form a（ ≡Si-OH2＋）（ H2PW12O-40） 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 growup of H3PW12O40 particles in the roasting process also will be inhibited. In addition,Si O2 may be existed in the form of dry gel in the B-La PW12O40/Si O2 catalyst and acted as carrier. It will be favorable for the improvent of the surface area of B-La PW12O40/Si O2 since Si O2 has high surface area,so the surface area of B-La PW12O40/SiO2 has increased from the 1. 4 m2/g of H3PW12O40 to the 31. 3 m2/g. And more,La PW12O40/SiO2 has been determined from the Py-FTIR spectra of pyridine adsorption analysis,which is a Brnsted-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 Br9 nsted acid site and H2 O,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＋）（ H2PW12O-40） to form La PW12O40/SiO2.
出处
《燃料化学学报》
EI
CAS
CSCD
北大核心
2017年第8期939-949,共11页
Journal of Fuel Chemistry and Technology
基金
国家自然科学基金(21206062
21466013)
江西省自然科学(青年)基金计划重大项目(20143ACB21018)
江西理工大学清江青年英才支持计划资助~~
