Synthesis Of4,4’-methylenedianiline Catalyzed Byacidic Ionic Liquids

4,4’-Methylenedianiline (4,4’-MDA) is an important organic chemical intermediate.There are some problems in the reaction for the synthesis of4,4’-MDA through liquid andsolid acid catalysts, such as corrosion, the difficulty in recycle, high reaction temperature,and low selectivity. In order to overcome these drawbacks, a series of sulfonicacid-functionalized ionic liquids (SFILs) and immobilized ionic liquids were prepared, andtheir structures and acid property were characterized, and then the catalytic performance ofthe ILs was investigated in the condensation reaction of aniline with HCHO.First of all, a series of SFILs were synthesized, and their catalytic performance wasevaluated in the condensation reaction for the synthesis of4,4’-MDA from aniline andHCHO. The result showed that the acid strength of SFILs was consistent with their catalyticactivity. Among the SFILs,[HSO3-bmim]CF3SO3with the highest acid strength showed thebest catalytic activity. Then the influence of reaction conditions on the synthesis of4,4’-MDA was investigated using [HSO3-bmim]CF3SO3. The suitable reaction conditionswere as follows: molar ratio of aniline to HCHO=5, mass ratio of (SFILs)/(HCHO)=3.5,reaction temperature of80℃, and reaction time of8h. Under the above conditions, theconversion of aniline was36.3%, the yield and selectivity of4,4’-MDA were79.4%and87.9%, respectively. After reaction, SFILs was recovered by extraction. The results of UVand FT-IR spectroscopy analyses showed that there was a chemical interaction betweenaniline and SFILs. Consequently, the recovered SFILs were acidized before reused.As-treated SFILs could be reused four times with tolerable lose of catalytic activity.HPLC-MS was used in qualitative analysis of the reaction system for synthesizing4,4’-MDAand a plausible catalytic reaction mechanism was proposed.In order to decrease the ionic liquid dosage and simplify post-treatment process,immobilized ionic liquids were prepared by three reaction pathway and their catalyticperformance was evaluated in the condensation reaction for the synthesis of4,4’-MDAfrom aniline and HCHO. The result showed that the ILs loading was consistent with theircatalytic activity for the synthesis of4,4’-MDA. Among the immobilized ILs,SiO2@[HSO3-ppim]CF3SO3-Ⅰ with the highest loading showed the best catalytic activity.Then the influence of reaction conditions on the synthesis of4,4’-MDA was investigatedusing SiO2@[HSO3-ppim]CF3SO3-Ⅰ catalyst. The suitable reaction conditions were as follows: mass ratio of (cat.)/(HCHO)=1.5, molar ratio of aniline to HCHO=4, reaction timeof7h, and reaction temperature of80℃. Under the above conditions, the conversion ofaniline was40.1%, the yield and selectivity of4,4’-MDA were74.9%and94.5%,respectively. The results of acid-base titration and FT-IR spectroscopy analyses showed thatsulfonic acid group was changed. This further illustrates that there was a chemicalinteraction between aniline and acid center of the catalysts. Similarly, the recoveredcatalysts were acidized before reused. As-treated catalysts could be reused four times withtolerable lose of catalytic activity.Comparing the catalytic performance of SFILs with that of immobilized ionic liquidsin the synthesis of4,4’-MDA, the immobilized ionic liquids have low aniline dosage, highTOF value, high selectivity and simple post-treatment process. This fully reflects theadvantages of immobilized ionic liquids.