Study Of Esterification Catalyzed By Br(?)nsted Acid Ionic Liquids

Br(o|¨)nsted acidic ionic liquid is a new kind of environmental-benign material that deserves great considerations as it can dissolve most organic and inorganic substances, having adjustable acidity and can be used as an alternative for traditional acid catalysts. In this dissertation, several ionic liquids were designed and synthesized basing on the relationship of the molecular structure and acid strength of ionic liquids, and have been used to catalyze the esterification of ethanol with fatty acids (acetic acid, caproic acid, lauratic acid and stearatic acid), in which the catalytic activity, recycle life and the reaction kinetics of the ionic liquids were considered. Furthermore, in order to reduce the amount of ionic liquid needed in the reaction and obtain an easier separation process for the products, the synthesized ionic liquids were also immobilized on rigid carriers by several immobilization methods to form heterogenous catalysts for the esterification.In the field of synthesis and immobilization of ionic liquids, three imidazole-based ionic liquids [MIm(CH₂)₄SO₃H][HSO₄],[MIm(CH₂)₄SO₃H][pTSO],[AlIm(CH₂)₄SO₃H] [CF₃SO₃] and another simple ionic liquid [(en)₃NH][HSO₄] were synthesized and characterized by ¹H-NMR and FT-IR. Afterward, [MIm(CH₂)₄SO₃H][HSO₄] was immobilized on silica gel by impregnation and sol-gel methods while [AlIm(CH₂)₄SO₃H][CF₃SO₃] was immobilized by covalently grafting method. Such immobilized ionic liquids were characterized by ICP for element analysis and ASAP for specific surface area.In the field of esterification catalyzed by ionic liquids, the catalytic activity order of the used ionic liquids was: [MIm(CH₂)₄SO₃H][HSO₄]≈[AlIm(CH₂)₄SO₃H][CF₃SO₃]≈[MIm(CH₂)₄SO₃H][pTSO] > [MImB][CF₃SO₃] > [MImB][BF₄]. It was found the catalytic activity of the ionic liquid is mainly affected by its solubility to reactants and products as well as by the acid strength. When [MIm(CH₂)₄SO₃H][HSO₄] used as a catalyst, the optimized reaction conditions were: T 85℃, t 6 hr, ratio of acid/alcohol/ionic liquid=2:4:1. The yields of ethyl acetate, ethyl caproate, ethyl laurate, ethyl stearate were 91.4%,100.0%,90.9% and 97.0%, respectively. The ionic liquid could be recycled by simply a vacumn drying process and after reusing of it for 4 times, the yield of ethyl caproate is still as high as 86.7%. When [AlIm(CH₂)₄SO₃H][CF₃SO₃] used as catalyst, the optimized reaction conditions were: T 85℃, t 6 hr, the ratio of acid/ethanol/ionic liquid =5 : 10 : 1. At which the yields of ethyl acetate, ethyl caproate, ethyl laurate, ethyl stearate were 95.4%, 92.6%, 94.5% and 90.0%, respectively. After reuse of the ionic liquid for 7 times, the yield of ethyl caproate is still as high as 88.9%.In the field of reaction kinetics of esterification reactions catalyzed by [MIm(CH₂)₄SO₃H][HSO₄], reactions of enthanol with acetic acid, caproic acid, lauratic acid and stearatic acid were conducted at different temperatures. The activation energy of these four esterification reactions are found to be 25.5,33.6,47.4 and 57.7 kJ/mol, respectively. It can be concluded that the larger number of carbon, the more activating energy is.In the field of immobilization of ionic liquids, our results showed sol-gel method is the best one for immobilizing ionic liquid on silica gel, which has the highest loading amount of ionic liquid, highest catalytic activity for esterification, and high stability. The optimized ratio of ionic liquid: acid: alcohol is 1 : 3.5 : 14 when the reaction was carried out in a stirred reactor. And after reused for 10 times, the yield of ethyl caproate was still as high as 92.7%. [AlIm(CH₂)₄SO₃H][CF₃SO₃] immobilized by covalently grafting method also showed higher catalytic activity for esterification and better stability. The optimized ratio of ionic liquid: acid: alcohol is 1 : 20 : 40. And after reuse of 8 times, the yield of ethyl caproate was still 92.0%, although it costs too much.