Effect Of Caffeic Acid-based Deep Eutectic Solvent On The Synthesis And Reaction Rules Of Caffeoyl Esters

Caffeic acid(CA)is a natural phenolic acid,which can be found in some fruits,vegetables and chinese herbs,such as blueberry,carrot,mint and Salvia miltiorrhiza.CA has attracted more attentions due to its bioactivity and functions,for examples,antioxidant,UV-absorption,antimicrobial,etc.However,the poor solubility of CA in oil/water system limited its application in food,pharmaceutical industries.Therefore,modifications of CA,using glycerol,glyceryl monooleate(GMO)and fatty alcohol,can enhance the application of CA.At present,although the solid CA as caffeoyl donor is used for caffeoyl esters preparation,the reaction activity is low.In order to improve low reaction efficiency,the effects of caffeic acid-based deep eutectic solvent(CA-based DES)as caffeoyl donor on the reactions with glycerol,GMO and fatty alcohol to prepare caffeoyl esters were investigated,which were compared with those of solid CA as caffeoyl donor.The effects of CA-based DES on the reaction rules for caffeoyl esters preparation were studied.The reaction thermodynamics and mechanism were also evaluated.Fristly,the esterification of CA-based DES as caffeoyl donor with glycerol for GMC preparation was studied,which was compared with that of solid CA as caffeoyl donor.A-35 was found as the best catalyst for the esterification.Effects of different reaction parameters on the esterification were also evaluated.Results showed that,when CA-based DES was used as caffeoyl donor,hydrophilic glyceryl monocaffeate(GMC)and lipophilic glyceryl dicaffeate(GDC)were both formed.However,for solid CA as caffeoyl donor,only GMC was formed in the reaction system.When CA-based DES was caffeoyl donor,under the reaction conditions(90?,A-35 load 7%(w/w,relative to the weight of CA and glycerol),CA:glycerol=1:5(mol/mol),24 h),CA conversion and yield of GMC were 82.23±2.02% and 69.75±1.03%,respectively,which were higher than those(57.14±2.10% and 56.75±1.95%)of solid CA as caffeoyl donor.The activation energy(Ea)of CA conversion and GMC formation were 43.07 k J/mol and 43.71 k J/mol,respectively,which were lower than those(75.05 kJ/mol and 81.20 k J/mol)of solid CA as caffeoyl donor.Compared with solid CA,CA-based DES has faster reaction rate and better compatibility with glycerol.Secondly,the reactions of CA-based DES or solid CA as caffeoyl donors with GMO for caffeoyl structured lipids preparation were studied and compared.CA-based DES as caffeoyl donor was favor for hydrophilic GMC and lipophilic GDC formations.However,when solid CA was used as caffeoyl donor,in the products,lipophilic caffeoylated acylglycerols(CAGs)and hydrophilic GMC were formed.Among all tested solid acid catalyst,NKC-9 showed the best catalytic efficiency.The results showed that,when CA-based DES was used as caffeoyl donor,under the reaction conditions(NKC-9 load 10%(w/w,relative to the weight of CA and GMO),100?,CA:GMO=1:5(mol/mol),48 h),CA conversion,GMC and GDC yields were 93.37±1.79%,55.31±1.24% and 16.27±1.21%,respectively.When solid CA was caffeoyl donor,CA conversion,CAGs yield and GMC yield were 55.05±2.06%,43.38±1.91% and 5.54±1.68%,respectively.According to Arrhenius law,when CA-based DES was used as caffeoyl donor,the Ea of CA conversion,GMC formation were 70.66 and 65.38 kJ/mol,which were lower than those(83.31 and 80.41 kJ/mol)of solid CA as caffeoyl donor.And when solid CA was used as caffeoyl donor,the Ea of CAGs formation was 92.78 kJ/mol.Finally,the esterifications of CA-based DES with fatty alcohols for caffeoyl fatty alcohol esters preparation were investigated,which were compared with those of solid CA as caffeoyl donor.A-35 and stearyl alcohol were found as the best catalyst and caffeoyl acceptor.Under the reaction conditions(85?,A-35 load 5%(w/w,relative to the weight of CA and stearyl alcohol),CA:stearyl alcohol=1:8(mol/mol),24 h),CA conversion and octodecyl caffeate(OC)yield were 95.17±2.76% and 90.69±2.71%,respectively,which were higher than those(44.26±1.69% and 40.97±2.37%)of solid CA as caffeoyl donor.Ea of CA conversion(67.57 k J/mol)was lower that(90.19 k J/mol)of solid CA as caffeoyl donor.CA-based DES was more appropriate caffeoyl donor for caffeoyl fatty alcohol esters preparation.