Performances of rhenium oxide-encapsulated ZSM-5 catalysts in propene selective oxidation/ammoxidation Original Research Article

Rhenium oxide-encapsulated zeolites were prepared by chemical vapor deposition (CVD), impregnation (imp) and physical mixing (phy) methods using HZSM-5, NaZSM-5 and HY zeolites. The samples were pretreated under He at 673 for 4 h in situ before use as catalysts. Among the catalysts employed for propene selective oxidation/ammoxidation, the catalyst prepared by CVD of methyl trioxo rhenium CH3ReO3 (MTO) on HZSM-5 (HZcvd) exhibited the highest selectivity (88.2%) to acrolein (main)+acrylonitrile at 648 K. The HYcvd catalyst showed less selectivity of 46%. However, the other ReOx/HZSM-5 samples prepared by impregnation and physical mixing, NaZcvd prepared by CVD on NaZSM-5, and HZSM-5 and HY zeolites without Re exhibited very low or no activity for the selective oxidation/ammoxidation. The order of the selectivity to acrolein+acrylonitrile for the catalysts is as follows: HZcvd>HYcvd>HZimp=HZphy=HYimp>NaZcvd>HZ=HY. The results indicate that CVD of MTO complex is an effective method for Re incorporation into zeolites, that proton sites are required for interaction of Re species with the zeolite framework, and that the pore size geometry of HZSM-5 is more suitable for the formation of active sites involving Re oxide species than that of HY. The HZcvd catalyst also exhibited the stable performance during the period of 36 h. It was found that the coexistence of ammonia was prerequisite for the performance of the ReOx/zeolite catalysts, where no selective oxidation reaction proceeded in the absence of ammonia.