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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 s...

Using aberration-corrected scanning transmission electron microscopy (STEM), we imaged iridium atoms in isolated iridium complexes in the one-dimensional nonintersecting 14-ring channels of zeolite SSZ-53. STEM allows tracking of the movement of atoms in the channels, demonstrating the interaction of iridium with the zeolite framework (channel confinement) and providing a direct visualization of the initial steps of metal nanocluster formation. The results demonstrate how STEM can be used to help design improved catalysts by identifying the catalytic sites and observing how they change in reactive atmospheres.

Structures of zeolite-anchored organorhodium complexes undergoing conversions with gas-phase reactants were characterized by infrared spectra bolstered by calculations with density functional theory and analysis of the gas-phase products. Structurally well-defined zeolite-supported rhodium diethylene complexes were synthesized by chemisorption of Rh(C(2)H(4))(2)(acac) (acac = CH(3)COCHCOCH(3)) on dealuminated Y zeolite, being anchored by two Rh-O bonds, as shown by extended X-ray absorption fine structure (EXAFS) spectroscopy. In contrast to the nonuniformity of metal complexes anchored to metal oxides, the near uniformity of the zeolite-supported species allowed precise determination of their chemistry, including the role of the support as a ligand. The anchored rhodium diethylene complex underwent facile, reversible ligand exchange with deuterated ethylene at 298 K, and ethylene ligands were hydrogenated by reverse spillover of hydrogen from support hydroxyl groups. The supported complexes reacted with CO to form rhodium gem-dicarbonyls, which, in the presence of ethylene, gave rhodium monocarbonyls. The facile removal of ethylene ligands from the complex in H(2)-N(2) mixtures created coordinatively unsaturated rhodium complexes; the coordinative unsaturation was stabilized by the site isolation of the complexes, allowing reaction with N(2) to form rhodium complexes with one and with two N(2) ligands. The results also provide evidence of a new rhodium monohydride species incorporating a C(2)H(4) ligand.

Mononuclear rhodium complexes incorporating two ethylene ligands and anchored to dealuminated zeolite Y by two Rh-O bonds were characterized by transient extended X-ray absorption fine structure (EXAFS) spectroscopy and infrared (IR) spectroscopy as they were converted in the presence of H{sub 2}. EXAFS spectra indicate reduction of the rhodium in the complex at 298 K to form rhodium clusters less than 3 {angstrom} in average diameter. Contacting of the resultant clusters with C{sub 2}H{sub 4} led to their oxidative fragmentation, and the process was reversible. When the H{sub 2} treatment was carried out at a higher temperature (373 K), larger clusters formed. The reduction and oxidation of the rhodium were confirmed by X-ray absorption near edge spectra. During the ethylene treatments, ethyl groups formed on the rhodium, as indicated by IR spectra; treatment in H{sub 2} led to hydrogenation of these groups to form ethane, and the ethyl groups are inferred to be intermediates in the catalytic hydrogenation of ethylene. Ethylene in the gas phase helps to stabilize rhodium in the form of mononuclear complexes on the zeolite during catalysis, hindering the cluster formation.

By anchoring metal complexes to supports, researchers have attempted to combine the high activity and selectivity of molecular homogeneous catalysis with the ease of separation and lack of corrosion of heterogeneous catalysis. However, the intrinsic nonuniformity of supports has limited attempts to make supported catalysts truly uniform. We report the synthesis and performance of such a catalyst, made from [Rh(C{sub 2}H{sub 4}){sub 2}(CH{sub 3}COCHCOCH{sub 3})] and a crystalline support, dealuminated Y zeolite, giving {l_brace}Rh(C{sub 2}H{sub 4}){sub 2}{r_brace} groups anchored by bonds to two zeolite oxygen ions, with the structure determined by extended X-ray absorption fine structure (EXAFS) spectroscopy and the uniformity of the supported complex demonstrated by {sup 13}C NMR spectroscopy. When the ethylene ligands are replaced by acetylene, catalytic cyclotrimerization to benzene ensues. Characterizing the working catalyst, we observed evidence of intermediates in the catalytic cycle by NMR spectroscopy. Calculations at the level of density functional theory confirmed the structure of the as-synthesized supported metal complex determined by EXAFS spectroscopy. With this structure as an anchor, we used the computational results to elucidate the catalytic cycle (including transition states), finding results in agreement with the NMR spectra.

The reaction of Rh(C{sub 2}H{sub 4}){sub 2}(acac) with the partially dehydroxylated surface of dealuminated zeolite Y (calcined at 773 K) and treatments of the resultant surface species in various atmospheres (He, CO, H{sub 2}, and D{sub 2}) were investigated with infrared (IR), extended X-ray absorption fine structure (EXAFS), and {sup 13}C NMR spectroscopies. The IR spectra show that Rh(C{sub 2}H{sub 4}){sub 2}(acac) reacted readily with surface OH groups of the zeolite, leading to loss of acac ligands from the Rh(C{sub 2}H{sub 4}){sub 2}(acac) and formation of supported mononuclear rhodium complexes, confirmed by the lack of Rh-Rh contributions in the EXAFS spectra; each Rh atom was bonded on average to two oxygen atoms of the zeolite surface with a Rh-O distance of 2.19 Angstroms. IR, EXAFS, and {sup 13}C NMR spectra show that the ethylene ligands remained bonded to the Rh center in the supported complex. Treatment of the sample in CO led to the formation of site-isolated Rh(CO){sub 2} complexes bonded to the zeolite. The sharpness of the CO bands in the IR spectrum gives evidence of a nearly uniform supported Rh(CO){sub 2} complex and, by inference, the near uniformity of the mononuclear rhodium complex with ethylene ligands from whichmore » it was formed. The supported complex with ethylene ligands reacted with H{sub 2} to give ethane, and it also catalyzed ethylene hydrogenation at 294 K.« less