Synthesis Of 4,4'-methylenedianiline Catalyzed By Heteroplyacids (Salts) And Supported Heteroplyacids

4,4'-Methylenedianiline?4,4'-MDA?is an important organic chemical intermediate.There are some problems in the reaction for the synthesis of 4,4'-MDA using liquid acid catalysts,such as corrosion and the difficulty in recycle.In order to overcome these drawbacks,heteropolyacid was used to catalyze the condensation reaction of aniline with HCHO to 4,4'-MDA.In order to solve the problem that it is difficult for heteropolyacid to be recycled and reused due to its dissolution in a polar solvent,Cs_2.5H_1.5SiW₁₂O₄₀ and H₄SiW₁₂O₄₀@MIL-100?Fe?was synthesized in this work.First of all,the catalytic performance of commonly used heteropolyacid(H₃PW₁₂O₄₀?H₄SiW₁₂O₄₀ and H₃PMo₁₂O₄₀)were evaluated in the condensation reaction for the synthesis of 4,4'-MDA from aniline and HCHO.The result showed that H4SiW12O40 with a moderate acid strength showed the best catalytic activity.Then the influence of reaction conditions on the synthesis of 4,4'-MDA catalyzed by H₄SiW₁₂O₄₀ was investigated and the suitable reaction conditions were obtained as follows:molar ratio of aniline to HCHO=3,mass ratio of?cat.?/?HCHO?=1.2,reaction temperature of 120?,and reaction time of 6h.Under the above conditions,the conversion of aniline was 63.3%,the yield and selectivity of4,4'-MDA were 80.1%and 81.5%,respectively.In order to solve the problem that H₄SiW₁₂O₄₀0 is difficult to recycle and reuse due to its dissolution in a polar solvent,Cs_2.5H_1.5SiW₁₂O₄₀0 was synthesized in this work.To improve its stability in a polar reaction system,as-prepared Cs_2.5H_1.5SiW₁₂O₄₀ sample was treated with ethanol to remove H₄SiW₁₂O₄₀0 existing in the Cs_2.5H_1.5SiW₁₂O₄₀ sample.The pore volume and pore size was increased after ethanol treatment.Then the catalytic performance of the Cs_2.5H_1.5SiW₁₂O₄₀ was evaluated and the reusability was investigated.The result showed that the catalytic activity of Cs_2.5H_1.5SiW₁₂O₄₀0 treated by ethanol declined because of the disappearance of H₄SiW₁₂O₄₀0 component.Under the appropriate reaction conditions,the conversion of aniline was 63.8%,the yield and selectivity of 4,4'-MDA were 62.4%and65.1%,respectively.The results of BET,FT-IR and NH3-TPD analyses showed that the deactivation of Cs_2.5H_1.5SiW₁₂O₄₀0 was due to the adsorbance of alkaline organic compounds onto Cs_2.5H_1.5SiW₁₂O₄₀,causing the pore blockage and the change of acidic property.The deactivated Cs_2.5H_1.5SiW₁₂O₄₀ could be regenerated by treatment with 30%H₂O₂ and then could be used three times with a tolerable loss of its catalytic activity.In order to get better catalytic activity and reusability,H₄SiW₁₂O₄₀@MIL-100?Fe?was synthesized.H₄SiW₁₂O₄₀@MIL-100?Fe?was characterized by FT-IR,N₂ adsorption,XRD,TG,and then the catalytic performance of the H4SiW12O40@MIL-100?Fe?was evaluated.The characterization results indicate that the H4SiW12O40 molecules were successfully incorporated into the cages of MIL-100?Fe?,and H₄SiW₁₂O₄₀0 molecules kept its Keggin structure.H₄SiW₁₂O₄₀@MIL-100?Fe?showed excellent catalytic performance in the condensation reaction of aniline with HCHO.The influence of reaction conditions was investigated and the suitable reaction conditions were obtained as follows:a mass ratio of?cat.?/?HCHO?=1.2,a molar ratio of aniline to HCHO=5,reaction time of 6h,and reaction temperature of 120?.Under the above conditions,the conversion of aniline was 41.1%,the yield and selectivity of 4,4'-MDA were 81.6%and 79.2%,respectively.The deactivation of H₄SiW₁₂O₄₀@MIL-100?Fe?was due to the adsorbance of alkaline organic compounds such as aniline and 4,4'-MDA onto H₄SiW₁₂O₄₀@MIL-100?Fe?,causing the poreblockageandthechangeofacidicproperty.Thedeactivated H4SiW12O40@MIL-100?Fe?could be regenerated by treatment with DMF and then could be used three times with a tolerable loss of its catalytic activity.