Synthesis Of Biodiesel By Phosphotungstic Acid-based Solid Acid Catalyzed Esterification And Transesterification

In the process of preparing biodiesel with waste oil as raw material,although the alkali catalyst has high catalytic efficiency,the free fatty acid in the waste oil will undergo saponification reaction with alkali catalysis,which makes it difficult to purify the product in the later stage and the catalyst is difficult to recycle.Cost of production.The use of acid catalysts can effectively avoid the problem of saponification.However,in the process of homogeneous acid catalysis,there are often problems of equipment corrosion,a large amount of waste water and the failure of catalyst recovery.Therefore,solid acid catalysts have become the mainstream of catalytic synthesis of biodiesel.Compared with other inorganic strong acids such as hydrochloric acid and sulfuric acid,heteropoly acid has stronger catalytic activity and stability,and is pollution-free and less corrosive to production equipment.In this paper,a series of Br?nsted-Lewis double-acid solid acid catalysts were prepared and synthesized using Keggin-type phosphotungstic acid(H₃PW₁₂O₄₀)as the matrix for the study of catalytic synthesis of biodiesel（fatty acid methyl ester）.（1）The rare earth ion Ce³⁺and strong electronegativity F^-were introduced into Keggin-type phosphotungstic acid to make insoluble heteropolysalt Cex-PW-F（x=1/3,2/3,1,4/3;x is Ce/HPW molar ratio）.Ce³⁺replaces the H⁺of the strong Br?nsted acid phosphotungstic acid,the Lewis acidity increases,and the Br?nsted acidity decreases.The addition of strongly electronegative F^-exacerbates the charge imbalance trend of the catalyst and further strengthens the Lewis acidity of the catalyst.The structure,morphology,valence state,elemental composition,acidity of Cex-PW-F catalyst were analyzed by a series of characterization methods:FT-IR,XRD,SEM,TEM,XPS,TG,BET,NH₃-TPD and Py-IR.strength and type.The acidified castor oil was used as the raw material for esterification and transesterification to synthesize biodiesel,and the catalytic performance and stability of the catalyst were investigated.Through single-factor experiments,it was found that the Ce（1/3）-PW-F catalyst has the best catalytic stability.The molar ratio of alcohol to oil is 18:1,the catalyst dosage is4wt%,the reaction temperature is 110℃,and the reaction time is 90 min.,the yield of fatty acid methyl esters can reach 93%.The activity was 75.9%after 5 repeated cycles.（2）Considering that the low specific surface area of heteropolyacid salts leads to insufficient utilization of active sites,loading heteropolyacid salts on carriers with high specific surface area and suitable pore volume will help the dispersion of active sites of heteropolyacid salts,improve the catalytic efficiency.La-PW/SBA-16,a solid acid catalyst with diacid sites,was prepared by hydrothermally loading La³⁺-modified phosphotungstic acid on ordered mesoporous silica molecular sieve SBA-16.The comparative experiments and single-factor variable experiments show that La-PW/SBA-16（20%）has the best catalytic stability,and when the alcohol-oil ratio is 24:1,the catalyst dosage is 5%,and the reaction temperature is 140°C,the reaction At 5h,the yield of fatty acid methyl ester was 90.2%,and the activity could still reach 80.9%after 5 repeated cycles..（3）By preparing the metal framework organic compound UIO-66 with unsaturated coordination sites,the unsaturated coordination sites Zr4+accepting electron pairs generate Lewis acid sites.Incorporation of phosphotungstic acid and hydroxylated multi-walled carbon nanotubes into ligand-deficient UIO-66,the strong electron-withdrawing effect of Zr4+on the highly delocalizedπbonds of MWCNTs-OH promotes the formation of a large number of strong Lewis acid sites,a solid acid catalyst PW-CNTs/UIO containing Br?nsted and Lewis acids was prepared.The central composite design of surface response method was used to study the synthesis of biodiesel by PW-CNTs/UIO catalyzed esterification of oleic acid and methanol,and the optimal reaction parameters were optimized:the molar ratio of alcohol to oil was 14.2:1,the catalyst dosage was 6.6%,and the reaction When the temperature is 69.2°C,the yield of methyl oleate can reach 92.2%,and the yield of methyl oleate can still reach 82.3%after the catalyst is reused five times.The above studies have shown that the phosphotungstic acid catalyst with Keggin structure as the matrix,modified by metal ions or loaded on the carrier,has a high catalytic activity in catalyzing esterification and transesterification reactions.