| As a green catalyst of high activity and selectivity ionic liquid has been wildly used to catalyze various reactions. But large-scale application of ionic liquid in industrialization is limited by its difficult separation and purification, high material cost, wastage and energy consumption during recycling process. When supported on polymer, the Ionic liquid will be changed from homogeneous catalyst to heterogeneous catalyst and obtain fine prospect of industrial application. Because this supported ionic liquid can not only be as much of catalytic activity and selectivity as is the ionic liquid, but also solve the limitation mentioned above.In this study, sulfopropyimidazolium based sulfate ionic liquids were supported in Merrifield resin and crosslinked polystyrene microsphere. Besides, other four imidazolium based sulfate ionic liquids of different side chains were supported in crosslinked polystyrene microsphere respectively. Those above six polymer-supported imidazolium based ionic liquids are almost insoluble and swelling in liquids phase. Their compositions and structures were characterized via infrared spectrometer, scanning electron microscope and elemental analyzer. And their acid strength were determined by Solid Nuclear Magnetic Resonance. The result analysis showed that side chain structures of the imidazolium-based ionic liquids had significant impact on acid strength and catalytic activity of polymer-supported imidazolium based ionic liquids with the same supporter and ionic liquid anion. The factors of the additional H+ carried by side chains and the structure of side chains both effected the acid strength of catalysts. And the structure of side chains decided electronegativity of imidazolyl; the electronegativity of imidazolyl decided hydrogen bonding strength between cation and anion of the catalysts; and the hydrogen bonding strength decided the amount of H+ which can be released additionally.In this paper, using the esterification reaction of acetylsalicylic acid synthesis as probe reaction, we investigated the catalytic effect and the recycling of the imidazolium based ionic liquids supported in Merrifield resin and crosslinked polystyrene microsphere. The experiment results showed that when using Merrifield resin-supported sulfopropyimidazolium based sulfate ionic liquid as catalyst to synthesize acetylsalicylic acid, the optimum reaction conditions were as follows:the mass ratio of salicylic acid and catalyst was 10:1; the molar ratio of salicylic acid and acetic anhydride was 1:2; the reaction time was 50min; the reaction temperature was 75℃ and the yield of acetylsalicylic acid was up to 84.8%. After five times recycling the yields had little decrease. When using crosslinked polystyrene microsphere-supported imidazolium based sulfate ionic liquids as catalyst, crosslinked polystyrene microsphere-supported sulfopropyimidazolium based sulfate ionic liquid showed the best catalytic activity. The optimum reaction conditions were as follows:the mass ratio of salicylic acid and catalyst was 12:1; the molar ratio of salicylic acid and acetic anhydride was 1:2; the reaction time was 45min; the reaction temperature was 75℃ and the yield of acetylsalicylic acid was 89.2%. And the catalytic activity still remained almost the same condition after five times recycling. |
