Preparation Of Several Solid Acids And Their Application In Catalyzing Oleic Acid Esterification

In the petrochemical and fine chemical fields,acid catalysis occupies a very important position,such as acid catalyzed esterification reaction systems.The synthesis of low-carbon esters in industry generally uses traditional liquid acids as catalysts,such as H₂SO₄ and HF.They usually have high catalytic activity,but they also have a series of problems when used,such as generating a large amount of waste liquid and slag,serious corrosion of equipment,difficult separation of catalysts,difficulty in continuous production and dangerous operation in technology.Compared with liquid acid,solid acid has the advantages of easy recovery,high recycling rate,less corrosion to equipment and less environmental pollution.Therefore,solid acid is favored by researchers in the field of catalytic esterification reaction.In this paper,three solid acid catalysts of niobium-tin bimetallic phosphate,phosphotungstic acid-supported iron oxide and sulfonated mesoporous silica nanospheres were prepared to catalyze the esterification reaction of oleic acid and methanol,and X-rays were used.Diffraction(XRD),N₂ adsorption-desorption test,scanning electron microscope(SEM),transmission electron microscope(TEM),infrared spectroscopy(FT-IR),X-ray atomic energy spectroscopy(XPS),thermal analysis(TG),ammonia adsorption and desorption test(NH₃-TPD)and other techniques are used to characterize the catalyst.The main contents are as follows:(1)Amorphous layered NbSnP with high specific surface area is the optimal catalyst,and its catalytic activity is significantly higher than that of monometallic SnP and NbP and bimetallic NbSn₂P and Nb₂SnP.This may be attributed to the increase in acid content and acid strength,as well as the synergistic catalysis of Sn⁴⁺and Nb⁵⁺in an appropriate amount of octahedral coordination.When doped with a lower concentration of Nb element,although the acid content of niobium-tin bimetallic phosphate is higher than that of single metal SnP,at the same time Nb⁵⁺will block the mesoporous channel of SnP,so NbSn₂P promotes the catalytic effect limited ability.With the addition of Nb content,the protons of Nb⁵⁺and P-OH are exchanged normally,and part of the phosphate ions are polymerized into pyrophosphate,and NbSnP has super acidic sites,which may be a key factor in increasing the activity.In addition,the DR-UV-vis spectra and XPS spectra showed that the appropriate amount of octahedral coordination Sn⁴⁺and Nb⁵⁺may have a synergistic catalytic effect.Although Nb₂SnP also exhibits super acidic sites when Nb is excessive,Nb mostly exists in the form of crystals and has no activity or can't form an active phase with other components,resulting in the disappearance of the promotion effect.(2)Load strong acidic H₃PW₁₂O₄₀ on Fe₂O₃ with high temperature resistance,strong hydrophobicity and non-toxicity by impregnation method,making it a heterogeneous catalyst that is easy to recover,and used to catalyze the esterification reaction of oleic acid and methanol.The results showed that compared with pure Fe₂O₃,Fe₂O₃ with H₃PW₁₂O₄₀catalyzed the esterification of oleic acid with methanol significantly improved.Among them,30%H₃PW₁₂O₄₀/Fe₂O₃-200 is the optimal catalyst.Under optimized conditions(0.08 g 30%H₃PW₁₂O₄₀/Fe₂O₃-200,3 mL methanol,1.047 g oleic acid,60?,10 h),the yield of methyl oleate is as high as 95.3%.But when the calcination temperature is too high(>450?),the supported H₃PW₁₂O₄₀ structure begins to decompose and the catalyst activity decreases.(3)The sulfonated mesoporous silica nanosphere MS-PrSO₃H was prepared by template method.The results showed that the sulfonated mesoporous silica nanospheres MS-PrSO₃H catalyzed the esterification of methanol with oleic acid,and the activity was significantly higher than that of MS and MS-PrSH before modification.In addition,under optimized conditions(0.10 g MS-PrSO₃H catalyst,3 mL methanol,1.047 g oleic acid,60°C,10 h),the yield of methyl oleate can reach 92.7%,and the activity of the catalyst is not significantly reduced after being reused for 3 times.It is beneficial to that MS-PrSO₃H retains the original mesoporous morphology,high specific surface area and thermal stability of MS,which is conducive to the diffusion of substrate molecules and the exposure of active sites,and plays a certain role in promoting catalytic activity.In addition,the successful introduction of-SO₃H into the mesoporous silica framework through silylation and strong oxidation reactions significantly increases the acid content and acid strength of the material,which is conducive to the protonation of the carbonyl group in the carboxyl group of oleic acid;finally,due to the use of chemical grafting method,the active sites are not easy to lose,and the catalyst has a certain degree of reusability.