Study On Synthesis And Synthetic Process Of Citric Esters Catalyzed By Acidic Functionalized Ionic Liquids

Several acid functionalized ionic liquids were used as the catalysts of the esterification to obtain citric ester . The catalytic and reusable performance of various ionic liquids was studied and some of them showed good capabilities. Therefore an environmental friendly approach for the synthesis of citric ester is provided.Three kinds of neutral ionic liquids: [C₄mim]Cl, [C₄mim]Br, [C₄mim][BF₄]; four kinds of acidic ionic liquids: [C4mim][HSO₄], [C₄mim][H₂PO₄], [Hmim][HSO₄], [Hpyro][HSO₄]; and six kinds of acidic functionalized ionic liquids: [HSO₃-pmim][HSO₄],[HSO₃-pmim][H₂PO₄],[HSO₃-pmim][CH₃-ph-SO₃],[HSO₃-pmim][ph-SO₃],[HSO₃-pPydin][HSO₄], [HSO₃-N(C₂H₅)₃][HSO₄] were prepared based on standard literature method. The ionic liquids which have good catalytic and reusable performance were characterized via IR, ¹H-NMR, ¹³C-NMR and TG-DSC.Among the ionic liquids used in synthesis of tributyl citrate (TBC), by researching the catalytic activity and the reusability of various ionic liquids, two acidic functionalized ionic liquids, i.e.[HSO₃-pmim][HSO₄] and [HSO₃-N(C₂H₅)₃][HSO₄] were chosen as the best catalysts in the synthesis of TBC. Then the reaction conditions were optimized. The experimental results indicated that (1) when [HSO3-pmim] [HSO4] was used as catalyst, the optimal reaction condition were: Wt (catalyst /reactants)% =8.0%, n ( alcohol): n(citric acid)=5.5:1, reaction temperature was 110～150℃, reaction time=3h, Under the optimal reaction conditions, the conversion could reach 99.0%. After separating from the product,[HSO₃-pmim][HSO₄]without any disposal could be reused 8 times and the conversion was still 95.2%.(2) with [HSO₃-N(C₂H₅)₃][HSO₄] as catalyst, Wt (catalyst /reactants)%=9.6%, n ( alcohol): n (citric acid)=5.5:1, reaction temperature was 110～150℃, reaction time= 3h, Under the optimal reaction conditions, the conversion could reach 98.7%. After separating from the product, [HSO₃-N(C₂H₅)₃][HSO₄] without any disposal could be reused 12 times upward and the conversion was still up to 97.8%.Among the ionic liquids used in synthesis of acetyl tributyl citrate (ATBC), The optimal conditions of ATBC catalyzed by [HSO₃-N(C₂H₅)₃] [HSO₄] were as follows: Wt (catalyst /reactants)%=2.0%, M.R =1.5:1, reaction temperature 75℃, reaction time= 65min. Under the optimal reaction conditions, the conversion could be 98.19%. After separating from the product, [HSO₃-N(C₂H₅)₃] [HSO₄] without any disposal could be reused 6 times and the conversion could reach 97.14%.Among the ionic liquids used in synthesis of triethyl citrate (TEC), [HSO₃-pmim] [CH₃-ph-SO₃] was chosen to synthesize TEC as the catalyst. The optimal conditions were as follows: citric acid 0.15mol, Wt (catalyst /reactants)%=10.0%, n (ethyl alcohol): n (citric acid)=6.0:1, toluene 75ml, reaction temperature was 80～120℃, reaction time =8h. Under the optimal reaction conditions, the yield could be 94.5%. After separating from the product, [HSO₃-pmim] [CH₃-ph-SO₃] without any disposal could be reused 10 times and the yield could reach 94.2%.Among the ionic liquids used in synthesis of trioctyl citrate (TOC), [HSO₃-pmim][HSO₄] was chosen to synthesize TOC as the catalyst. The optimal conditions were as follows: citric acid 0.15mol, Wt (catalyst /reactants)%=5.0%, n (ethyl alcohol): n (citric acid)=4.5.:1, reaction temperature under 165℃, reaction time =1.5h. Under the optimal reaction conditions, the conversion could be 96.7 %. After separating from the product, [HSO₃-pmim][HSO₄] without any disposal could be reused 7 times and the yield could reach 96.6 %.