Study On Br (?) Nsted Acidic Ionic Liquid For Biodiesel Preparation

In today’s society, fossil fuel resources are increasingly scarce and environmental problems have become more serious. It is very important to develop clean and renewable fuel substitute-biodiesel. However, strong acid and alkali catalysts used in traditional biodiesel production easily corrode equipment and cause environmental pollutions. Ionic liquids as green solvents and catalysts, due to the excellent chemical and thermal stability, the performance of no vapor pressure at room temperature, have been extensively studied and used in various chemical fields.The transesterification reaction in the process of preparation of biodiesel catalyzed by Br(?)nsted acidic ionic liquids was taken as the research object in this paper. Three Br(?)nsted acidic ionic liquids containing cation caprolactam were first successfully prepared according to the literatures:caprolactam hydrosulfate ([CP]HSO4), caprolactam benzenesulfonate ([CP]BSA) and caprolactam p-toluenesulfonate ([CP]pTSA). The chemical structures were characterized by the FT-IR and NMR. The three ionic liquids could be miscible with the polar solvents like water, methanol, ethanol and so on, but almost not dissolve 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, the acidic strength of the three ionic liquids was in the order of [CP]HSO4> [CP]pTSA> [CP]BSA. The three ionic liquids were used to catalyze the transesterification of cottonseed oil with methanol respectively, the catalytic activity of ionic liquids was in the order:[CP]HSO4> [CP]pTSA> [CP]BSA, which was in accordance with the acidic strength. The reaction conditions of transesterification were futher optimized with the caprolactam hydrosulfate as the catalyst. When the reaction temperature was 65℃, the reaction time was 5h, the molar ratio of methanol to oil was 12:1. the amount of ionic liquid was 2%(the mass ratio of ionic liquid to cottonseed oil), the yield of biodiesel reached 90.2%. Compared to the traditional process, the transesterification reaction catalyzed by caprolactam hydrosulfate reduced the energy consumption, and produced little waste acid, so it can meet the requirements of green chemical process.Task-specific ionic liquid was a kind of novel efficient catalytic material, it refers to the ionic liquid that was introduced one or more functional groups to the anion or cation or itself had a specific structure to make the ionic liquid have a particular function or characteristic. The ionic liquids were functionalized aiming to improve the acidic strength in this paper. Six kinds of Br(?)nsted acidic task-specific ionic liquids were synthesized according to the two-step synthesis method:N-carboxymethyl imidazole hydrosulfate([MimCH2COOH]HSO4), N-carboxymethyl pyridine hydrosulfate([PyCH2COOH]HSO4), carboxymethyl caprolactam hydrosulfate ([CPCH2COOH]HSO4), carboxymethyl caprolactam nitrate([CPCH2COOH]NO3), 1-(3-sulfonic acid) propyl caprolactam hydrogen sulfate([(CH2);,SO3HCP]HSO4) and 1-(4-sulfonic acid) butyl caprolactam hydrogen sulfate([(CH2)4SO3HCP]HSO4). The chemical structures were characterized by the FT-IR and NMR. The six task-specific ionic liquids also could be miscible with the polar solvents like water, methanol, ethanol and so on. but almost not dissolve in the weakly polar solvents such as carboxylic acid esters, ethers and others. The acidic strength of task-specific ionic liquids was compared by the Hammett-UV method, the acidic strength of the six ionic liquids was in the order of [(CH2)4SO3HCP]HSO4> [(CH2)3SO3HCP]HSO4 [CPCH2COOH]HSO4> [CPCH2COOH]NO3> [PyCH;COOH]HSO4> [MimCH:COOH]HSO4. The six task-specific ionic liquids were used to catalyze the transesterification of cottonseed oil with methanol respectively, the catalytic activity of ionic liquids was in the order: [(CH2)4SO3HCP]HSO4> [(CH2)3SO3HCP]HSO4> [CPCH2COOH] HSO4> [CPCH2COOH] NO3> [PyCH2COOH]HSO4> [MimCH2COOH]HSO4, which was in accordance with the acidic strength. The reaction conditions of transesterification were futher optimized with the 1-(4-sulfonic acid) butyl caprolactam hydrogen sulfate as the catalyst. When the reaction temperature was 60℃, the reaction time was 4h, the molar ratio of methanol to oil was 10:1, the amount of catalyst was 1.5%(the mass ratio of ionic liquid to cottonseed oil), the yield of biodiesel reached 98.8%. Besides, the ionic liquid could be recycled for 5 times after regeneration without noticeably decreasing the catalytic activity. Therefore, the task-specific ionic liquid used as catalyst on transesterification reaction showed higher catalytic activity and good performance in recycling, so it was a novel and efficient green catalyst for the preparation technology of biodiesel.