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

Rhenium oxide-encapsulated zeolites were prepared by chemical vapor deposit ion (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 prop ene selective oxidation/ammoxidation, the catalyst prepared by CVD of methy l trioxo rhenium CH3ReO3 (MTO) on HZSM-5 (HZ(cvd)) exhibited the highest se lectivity (88.2%) to acrolein (main) + acrylonitrile at 648 K. The HYcvd ca talyst showed less selectivity of 46%. However, the other ReOx/HZSM-5 sampl es prepared by impregnation and physical mixing, NaZ(cvd) prepared by CVD o n NaZSM-5, and HZSM-5 and HY zeolites without Re exhibited very low or no a ctivity for the selective oxidation/ammoxidation. The order of the selectiv ity to acrolein + acrylonitrile for the catalysts is as follows: HZ(cvd) > HYcvd > HZ(imp) = HZ(phy) = HYimp > NaZ(cvd) > HZ = HY. The results indicat e 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 s uitable for the formation of active sites involving Re oxide species than t hat of HY The HZ(cvd) catalyst also exhibited the stable performance during the period of 36 h. It was found that the coexistence of ammonia was prere quisite for the performance of the ReOx/zeolite catalysts, where no selecti ve oxidation reaction proceeded in the absence of ammonia. (C) 2001 Elsevie r Science B.V. All rights reserved.