Study On Preparation Of Lewis Solid Acid By Modification Of Heteropoly Acid And Its Catalytic Performance

The synthesis of biodiesel by esterification of oleic acid and methanol catalyzed by heteropoly acid catalyst has the following advantages:it overcomes the complex process problems caused by the traditional liquid acid-base catalyst in the production of biodiesel.For example,a large number of salt containing wastewater is produced,which is difficult to separate in the later stage,has poor reusability,and requires high quality of raw oil.In this study,the heteropolyacid(H₃PW₁₂O₄₀,H₄SiW₁₂O₄₀)was modified by doping Ce³⁺and Ag⁺,doping Ti⁴⁺and loading it onto the functionalized carbon nanotubes,and doping La³⁺and NH₄⁺and loading it onto the side wall hydroxyl-functionalized carbon nanotubes.A high efficient and stable Br?nsted-Lewis acid type solid acid catalyst was synthesized,and it was used in the esterification of oleic acid and methanol to produce biodiesel,showing a good catalytic activity.The main research contents are as follows:1)A solid acid catalyst Ce-Ag-PW with Lewis acidic active site as the main active sites was prepared from the synergistic modification of phosphotungstic heteropoly acid?HPW?by Ce³⁺and Ag⁺through ultrasonic impregnation.Physicochemical properties of the obtained catalyst were characterized by X-ray diffraction?XRD?,fourier transform infrared spectroscopy?FT-IR?,scanning electron microscopy?SEM?,transmission electron microscopy?TEM?,thermogravimetry?TG?,NH₃ temperature programmed desorption?NH₃-TPD?,pyridine adsorption Fourier-transform infrared?Py-IR?and X-ray photoelectron spectroscopy?XPS?.The results showed that Ce-Ag-PW had high catalytic activity and stability.When the ratio of methanol to oleic acid was 14:1,the amount of catalyst was 2%of the total mass of the reactant,and the reaction temperature was 65?,the conversion of oleic acid was91%after six hours of reaction,and the conversion of oleic acid still reached 80.5%after four cycles of solid acid catalyst.The high catalytic activity and stability of Ce-Ag-PW were attributed to the replacement of the protons in HPW by Ag⁺and the strong electron-withdrawing effect exerted by Ce³⁺,which transformed it from Br?nsted acid catalyst to Lewis acid catalyst.Since the Br?nsted acid site was easily deactivated due to the occurring of hydration reaction between it and the water generated from the esterification reaction,so the formation of Lewis acid sites helped reduce the occurrence of catalyst deactivation.Therefore,Ce-Ag-PW solid acid catalyst based on Lewis acidic active sites had high catalytic activity and stability in the catalytic synthesis of biodiesel from the esterification reaction of oleic acid and methanol.2)A solid acid catalyst Ti-PW₁₂O₄₀/MWCNTs?Ti-PW/MWCNTs?was synthesized by ultrasonic impregnation,the effects of doping modification of H₃PW₁₂O₄₀0 onto the surface of MWCNTs functionalized by polydiene dimethyl ammonium chloride solution?PDDA?were studied.The structural integrity,acidity and pore structure of Keggin of the obtained catalyst were characterized by fourier transform infrared spectroscopy?FT-IR?,X-ray diffraction?XRD?,N₂ adsorption desorption?BET?,NH₃ temperature programmed desorption?NH₃-TPD?,transmission electron microscopy?TEM?and X-ray photoelectron spectroscopy?XPS?.After modified loading,the active particles were uniformly dispersed on the surface of MWCNTs.The(Ti⁴⁺)₃(PW₁₂O₄₀^3-)₄ complex with strong electrostatic adsorption was formed on the surface of carbon nanotubes by Ti doping modification,the adsorption of the complex on the surface of the carbon tube increases the specific surface area of the catalyst and forms Lewis acid sites,this makes the Ti-PW/MWCNTs Br?nsted-Lewis solid acid catalyst.The experimental results showed that when ti-pw/MWCNTs was used as catalysts,when the reaction temperature was 65?,the ratio of methanol to oleic acid was 15:1,the amount of catalyst was 2%of the total mass of the reactant and the reaction time was 6 h,the oleic acid conversion rate was as high as 91%,and the conversion rate remained at 82.5%after repeated use for five times.3)A novel Lewis solid acid catalyst La-NH₄-SiW₁₂O₄₀/SWCNTs?La-NH₄-SiW/SWCNTs?was prepared by sol-gel method using side-wall hydroxy-functionalized single-wall carbon nanotubes?SWCNTs-OH?loaded with H₄SiW₁₂O₄₀,which was synergistically modified by La³⁺and NH₄⁺.Structure of the prepared solid acid catalyst were characterized from experiment and theoretical analysis by X-ray diffraction?XRD?,fourier transform infrared spectroscopy?FT-IR?,N₂ adsorption desorption?BET?,transmission electron microscopy?TEM?,thermogravimetry?TG?,NH₃ temperature programmed desorption?NH₃-TPD?,X-ray photoelectron Spectrosco-py?XPS?.The formation of SiOH₂⁺bond and the strong interaction between SiOH₂⁺bridged and modified heteropolyacids on the wall of carbon tube were obtained by characterization analysis,which was helpful for the uniform dispersion of active substances.The experimental results showed that:the conversion of oleic acid reached 88%reaction time 4 h with La-NH₄-SiW/SWCNTs as catalyst,when the molar ratio of methanol to oleic acid was 12:1,the amount of catalyst dosage was 2.5%of the total mass of the reactant and the reaction temperature was 65?.The conversion of oleic acid still reached 71%,after using the La-NH₄-SiW/SWCNTs catalyst for four cycles.4)The Ti-PW/MWCNTs as catalyst,kinetic simulation of esterification of oleic acid with methanol,the effects of reaction temperature,catalyst amount and alcohol oil ratio on esterification were discussed by eliminating the influence of internal and external diffusion,the reaction mechanism of catalyst was determined to be Eley-Ridel mechanism.It was calculated that the kinetic model of esterification of oleic acid and methanol catalyzed by the catalyst was a pseudo homogeneous second-order kinetic model,the activation energy was Ea=39.59 kJ/mol and pre exponential factor A=1.77x10³ L·mol^-1·min^-1.The results show that:Ti-PW/MWCNTs was the most suitable catalyst for biodiesel synthesis,which had the best catalytic activity and stability.The conversion of oleic acid reached 91%and can be reused five times.The solid acid catalyst prepared by heteropoly acid and carbon nanotubes will be a new catalytic material for the synthesis of biodiesel from oleic acid and methanol.