Acidic Ionic Liquid Synthesis And Catalytic Esterification Of Applied Research

Ionic liquids have many excellent features, so it is considered to be the ideal green reaction medium, and has been researched or used in the field of chemical synthesis, separation engineering and nano-materials preparation. In the field of catalysts, ionic liquids also have many unmatched advantages compared with traditional catalysts, so it has gained more and more attention as the green catalyst.The esterification reaction catalyzed by ionic liquids was taken as the research object in this paper. Three Brcpnsted acidic ionic liquids containing anion HSO4-were first successfully prepared by one-step synthesis method according to the literatures:N-methyl pyrrolidonium hydrosulfate ([Hnmp]HS04), N-methyl imidazolium hydrosulfate ([Hmim]HS04) and pyridinium hydrosulfate ([Hpy]HSO4). The chemical structures were characterized by the FT-IR and NMR. The preparation processes of directly protonated acidic ionic liquids were relatively simple, and the synthesis conditions were mild, moreover, the yield and purity were both high. Three ionic liquids all had a good thermal stability, and could be miscible with the polar solvents like water, methanol, ethanol and so on, but almost not be dissolved in the weakly polar solvents such as carboxylic acid esters, ethers and others. The acidic strength of ionic liquids was compared by the Hammett-UV method, which was found that when the anion of the ionic liquids was the same, its cationic structure could significantly affect the Brφnsted acidic strength.With gallic acid and n-propanol as the raw materials, the synthesis of propyl gallate was selected as the probe reaction to investigate the relationship between the catalytic activity and structure of the three directly protonated acidic ionic liquids from acidic, polarity, solubility and so on. The results showed that when the anion was HSO4-, by changing the cation, the activity order of the catalysts was [Hnmp]HSO4>[HPy]HSO4>[Hmim]HSO4. The ionic liquid [Hnmp]HSO4showed higher acidic strength and catalytic activity, so it was chosen as the better catalyst to further optimize the reaction process conditions for the synthesis of PG. The optimum reaction conditions were as follows:the molar ratio of n-propanol and gallic acid and catalyst was5:1:0.3, reaction time was5h and the yield of propyl gallate was up to89.8%without water-carrying agent at95-105℃. Compared with conventional processes, this synthesis method showed shorter reaction time without organic solvents as the reaction medium. It was environment-friendly, safe and efficient to synthesise the propyl gallate with lower energy consumption, which was in line with the development direction of green chemistry.Task-specific ionic liquid was that the anion or cation was given one or more functional groups or itself had a specific structure to make the ionic liquid have a particular function or characteristic. Task-specific ionic liquid, as a reaction medium or catalyst in organic reaction, could change the reaction mechanism, make the catalyst activity and stability better, and let the selectivity and conversion rate higher. It was one of the most promising ionic liquids. Ionic liquids with sulfo group, the strongest Brφnsted acidic ionic liquids, had more active sites. In order to further improve the catalytic performances of the acidic ionic liquids, common cations like N-methylimidazole and pyridine were functionalized with the alkyl side chains. Two task-specific ionic liquids functionalized with sulfo group were prepared by two-step synthesis method:1-methyl-3-(3-sulfopropyl) imidazolium hydrosulfate ([HSO3-pmim]HSO4),1-(3-sulfopropyl) pyrrolidonium hydrosulfate ([HSO3-pPy]HSO4). The chemical structures were characterized by the FT-IR and NMR. The acidic strength of ionic liquids was compared by the Hammett-UV method, which was found that the acidity of task-specific ionic liquids was very close to concentrated sulfuric acid. So the task-specific ionic liquids had the application prospect for catalyzing esterification reaction instead of concentrated sulfuric acid.The synthesis reaction of trioctyl trimellitate was taken as a probe to study the effects of the catalytic activity of task-specific acidic ionic liquids containing the sulfonic acid group. Task-specific acidic ionic liquids had a certain polarity, so the product ester could be automatically separated from the system, which simplified the separation process of catalyst and product. The ionic liquid separated from the system could be reused many times, and the catalytic activity reduced less, which showed good stability. The task-specific ionic liquid [HSO3-pmim]HSO4with higher catalytic activity was selected as the catalyst to further optimize the esterification reaction conditions of trioctyl trimellitate. The optimum reaction conditions were as follows:9.61g trimellitic anhydride, nacid:n alcohol:ncat=1:4:0.15, reaction time was2h at140～165℃, and the yield of trioctyl trimellitate was up to96.5%. The task-specific ionic liquids, with significant advantages like higher catalytic activity and stability, easier separation process and reusability compared with general ionic liquids, would become the new generation of excellent catalysts for the esterification reaction.