Ammonia-Promoted Rhenium-Cluster Formation in CH3ReO3-Encapsulated H-ZSM-5 Relevant to the Performance of the Catalytically Selective Oxidation/Ammoxidation of Propene

HZcvd catalysts, which were prepared by chemical vapor deposition (CVD) of methyl trioxorhenium CH 3 ReO 3 on H-ZSM-5 (HZ) and subsequent treatment at 673 K, exhibited significant activities in the selective oxidation/ammoxidation of propene. The catalytic selective oxidation of propene to acrolein never proceeded in the absence of ammonia. In the HZcvd catalysts, the hydrophilic nature of HZ increased, and the crystallinity of the HZ framework decreased as indicated by XRD. The detailed characterizations by XRD, 29 Si and 27 Al MAS NMR, XANES, and EXAFS at the sequential stages of the catalyst preparation demonstrated the occurrence of structural changes of Re species during the preparation and under the catalytic reaction conditions. CH 3 ReO 3 interacted with protons of HZ at 333 K, the subsequent treatment at 673 K formed tetrahedral [ReO 4 ] monomer, and ammonia promoted the formation of a new [Re 6 O 17 ] cluster at 673 K. A proposed cluster structure constituted an octahedral Re 6 framework with an Re−Re bond at 0.276 nm and terminal and bridge oxygen atoms (Re O at 0.172 nm and Re−O at 0.203 nm, respectively). Thirty-two percent of these oxygen atoms were active for the selective oxidation of propene to acrolein. The active [Re 6 O 17 ] clusters were converted to the inactive [ReO 4 ] monomers in the absence of ammonia.