Site-Isolated Molecular Iridium Complex Catalyst Supported in the 1-Dimensional Channels of Zeolite HSSZ-53: Characterization by Spectroscopy and Aberration-Corrected Scanning Transmission Electron Microscopy

Zeolite HSSZ-53, which has 1-dimensional channels with 14-ring extra-large pores, was used as a support for a molecular iridium complex synthesized from Ir(C2H4)2(C5H7O2) and characterized with infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopies and atomic-resolution aberration-corrected scanning transmission electron microscopy (STEM). The spectra show that Ir(C2H4)2(C5H7O2) reacted readily with the bridging OH groups of the zeolite, leading to the removal of C5H7O2 ligands and the formation of mononuclear Ir(C2H4)2 complexes bonded to the zeolite by Ir–O bonds at the framework aluminum sites. STEM images confirm the spectra, showing site-isolated iridium centers within the zeolite channels, with no evidence of iridium clusters. The samples constitute a highly uniform, well-defined array of essentially molecular catalytic species in a highly uniform, confined environment, allowing precise investigations of the chemistry of the iridium complex in the absence of solvents. IR spectra show that the supported Ir(C2H4)2 complexes were converted to Ir(C2H5)2, Ir(CO)2, Ir(CO)(C2H4), and Ir(CO)(C2H4)2 as various mixtures of H2, CO, and C2H4 reacted with the sample. The sample was tested as a catalyst for ethylene hydrogenation and for H–D exchange in the reaction of H2 + D2. The data, combined with results reported for isostructural iridium complexes bonded to zeolite HY and to MgO, demonstrate how the catalytic activity can be tuned by choice of the support, with the support being characterized as a ligand with electron-donating or electron-withdrawing properties. The results demonstrate that the rate of ethylene hydrogenation catalyzed by the supported iridium complexes is limited by H2 activation when the iridium is electron rich (on the MgO support), whereas the rate-limiting step is C2H4 adsorption when the iridium is electron deficient (on either zeolite support).