Preparation and characterization of H_4SiW_(12)O_(40)@MIL-100(Fe) and its catalytic performance for synthesis of 4,4'-MDA

The catalytic performance of commonly used heteropoly acids(H_3 PW_(12)O_(40). H_4 SiW_(12)O_(40) and H_3 PMo_(12)O_(40)) for the synthesis of 4,4'-methylenedianiline(4.4'-MDA) from aniline and formaldehyde was evaluated and the result showed that H_4 SiW_(12)O_(40) with moderate acid strength exhibited the best catalytic performance. Then H_4 SiW_(12)O_(40)@MIL-100(Fe) was prepared by encapsulating H_4 SiW_(12)O_(40) within the pores of MIL-100(Fe) to facilitate its recovery and reuse. The prepared H_4 SiW_(12)O_(40)@MIL-100(Fe) was characterized by means of FT-IR, N_2 adsorption-desorption, XRD, TG and then the catalytic performance was evaluated. The result showed that H_4 SiW_(12)O_(40) was highly dispersed in the pores of MIL-100(Fe), and both the Keggin structure of H_4 SiW_(12)O_(40)and the crystal skeleton structure of MIL-100(Fe) could be effectively preserved. Furthermore. H_4 SiW_(12)O_(40)@MIL-100(Fe) showed excellent catalytic performance under the following reaction conditions: a molar ratio of aniline to formaldehyde = 5, a mass ratio of catalyst to formaldehyde = 1.2, a reaction temperature of 120 ℃and a reaction time of 6 h. Under the above reaction conditions, the conversion of aniline was 41.1%, and the yield and selectivity of 4.4'-MDA were 81.6% and 79.2%, respectively. Unfortunately, an appreciable loss in the catalytic activity of the recovered H_4 SiW_(12)O_(40)@MIL-100(Fe) was observed because of the blocking of the pores and the change of the acidity resulted from the adsorption of alkaline organics such as aniline and 4,4'-MDA.The adsorbed alkaline organics could be cleaned up when the recovered catalyst was washed by methanol and DMF. Then the catalyst was effectively reused up to three cycles without much loss in its activity.