Preparation and characterization of H4SiW12O40@MIL-100(Fe) and its catalytic performance for synthesis of 4,4′-MDA

The catalytic performance of commonly used heteropolyacids (H3PW12O40, H3PW12O40 and H3PMo12O40) for the synthesis of 4,4'-methylenedianiline (4,4'-MDA) from aniline and formaldehyde was evaluated and the result showed that H4SiW12O40 with moderate acid strength exhibited the best catalytic performance. Then H3PW12O40@MIL-100(Fe) was prepared by encapsulating H3PW12O40 within the pores of MIL-100(Fe) to facilitate its recovery and reuse. The prepared H4SiW12O40@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 H3PW12O40 was highly dispersed in the pores of MIL-100(Fe), and both the Keggin structure of H3PW12O40 and the crystal skeleton structure of MIL-100(Fe) could be effectively preserved. Furthermore, H3PW12O40@ 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 degrees C 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 H3PW12O40@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. (C) 2017 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.