Studies On The Structure-property Relationship Of Sulfonic Acid-Functionalized Ionic Liquids And Their Application In Esterification Reactions

Esterification is the key technology in the high-value utilization of natural bioactive compounds.It can not only enhance the activity of natural bioactive compounds such as polyunsaturated fatty acid and a-tocopherol,but also improve the stability and storability.But traditional liquid catalysts used in the esterification exist a lot of serious drawbacks,including equipment corrosion caused by acidic media,environmental pollution,and difficulties with separation and purification of the product.Sulfonic acid-functionalized ionic liquids(ILs)have been considered to be novel environment friendly liquid acid catalysts with high catalytic activity and excellent reuse performance,which are expected to replace the traditional inorganic acid catalysts.In addition,they are easy to be separated from products.In this paper,sulfonic acid-functionalized ILs were used as catalysts in the systhesis of polyunsaturated fatty acid ethyl ester and a-tocopherol acetate.The structure-property relationship and catalytic activity of sulfonic acid-functionalized ILs were studied.In this paper,sulfonic acid-functionalized ILs with different structures of cations were synthesized,and their properties were characterized,including acidity and solubility.The structure and property relationship of sulfonic acid-functionalized ILs has been studied by means of quantum chemical calculation and the conductor-like screening model for realistic salvation(COSMO-RS)method.According to the results,in the same organic solvents,the solubility of quaternary ammonium-type protic acidic ILs is much higher than that of sulfonic acid-functionalized ILs,and the solubility of N-sulfopropyl pyridinium hydrogen sulfate([PSPy][HSO4])is lower than that of 1-sulfopropyl-3-methylimidazolium hydrogen sulfate([mpsim][HSO4]).The solubility in the different organic solvents have the order as follows:water>ethanol>ethyl acetate>n-hexane>n-heptane,and the solubility of sulfonic acid-functionalized ILs increased with increasing the polarity of organic solvents.The relationship of solubilities with the interaction of ILs and organic solvents were discussed with COSMO-RS method.The results show that,the misfit interaction energy(Emisfit)of ILs and organic solvents is the key fator in ILs' solubility in organic solvents,and the solubility of studied ILs in organic solvents decreases as Emisfit increases.Sulfonic acid-functionalized ILs were utilized as novel acidic catalysts for the esterification reaction of polyunsaturated fatty acid with ethanol,which show high catalytic activity in the esterification reaction.Effects of cationic structures and reaction conditions were investigated.The optimal conditions of the synthesis of polyunsaturated fatty acid ethyl ester were:mole ratio of polyunsaturated fatty acid and ethanol 1/4,reaction temperature 80 ?,and the conversion of all-cis-5,8,11,14,17-eicosapentaenoic acid(EPA)and all-cis-4,7,10,13,16,19-docosahexaenoic acid(DHA)reached up to 96%,and the yield of EPA and DHA acetate were as high as 84.3%and 90.8%,respectively.In addition,the feasibility with separation and purification of EPA and DHA ester from fish oil with ILs as extractants was studied.ILs demonstrate excellent separation selectivity of EPA and DHA ester.Sulfonic acid-functionalized ILs were utilized as novel acidic catalysts for the esterification reaction of ?-tocopherol with acetic anhydride,and the effect of cationic structures on the catalytic activity was studied.In addition,influences of reaction conditions were investigated through orthogonal test.Sulfonic acid-functionalized ILs manifest high catalytic activity and cationic structures have little effect on the catalytic activity.When[PSPy][HSO4]was utilized as the catalyst,the conversion of a-tocopherol and the yield of a-tocopherol acetate were as high as 100%and 93%,respectively.The optimal conditions of the synthesis of a-tocopherol acetate were:mole ratio of?-tocopherol/acetic anhydride/IL 1/1.3/0.05,reaction temperature 30 ?,reaction time 1.5 h.The relationship between reuse performance and solubility was investigated.Sulfonic acid-functionalized ILs show fair reusability,which can be reused five times and the catalytic activity was unchanged.[PSPy][HSO4]has best reuse performance owing to its poor solubility in the n-heptane and the ester.