15064
J. Phys. Chem. C 2007, 111, 15064-15073
A Site-Isolated Iridium Diethylene Complex Supported on Highly Dealuminated Y Zeolite: Synthesis and Characterization Alper Uzun, Vinesh A. Bhirud, Philip W. Kletnieks, James F. Haw, and Bruce C. Gates* Department of Chemical Engineering and Materials Science, UniVersity of California, DaVis, California 95616, and Department of Chemistry, UniVersity of Southern California, Los Angeles, California 90086 ReceiVed: May 1, 2007; In Final Form: June 27, 2007
Highly dealuminated Y zeolite-supported mononuclear iridium complexes with reactive ethylene ligands were synthesized by chemisorption of Ir(C2H4)2(C5H7O2). The resultant structure and its treatment in He, CO, ethylene, and H2 were investigated with infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopies. The IR spectra show that Ir(C2H4)2(C5H7O2) reacted readily with surface OH groups of the zeolite, leading to the removal of C5H7O2 ligands and the formation of supported mononuclear iridium complexes, confirmed by the lack of Ir-Ir contributions in the EXAFS spectra. The EXAFS data show that each Ir atom was bonded to four carbon atoms at an average distance of 2.10 Å, consistent with the presence of two ethylene ligands per Ir atom and in agreement with the IR spectra indicating π-bonded ethylene ligands. The EXAFS data also indicate that each Ir atom was bonded to two oxygen atoms of the zeolite at a distance of 2.15 Å. The supported iridium-ethylene complex reacted with H2 to give ethane, and it also catalyzed ethylene hydrogenation at atmospheric pressure and 294 K. Treatment of the sample in CO led to the formation of Ir(CO)2 complexes bonded to the zeolite. The sharpness of the υCO bands indicates a high degree of uniformity of these complexes on the support. The iridium-ethylene complex on the crystalline zeolite support is inferred to be one of the most nearly uniform supported metal complex catalysts. The results indicate that it is isostructural with a previously reported rhodium complex on the same zeolite; thus, the results are a start to a family of analogous, structurally well-defined supported metal complex catalysts.
Introduction The prospect of combining the advantages of homogeneous and heterogeneous catalysis has motivated the synthesis of solids with surface-bound catalytic groups that are molecular analogues, typically metal clusters or site-isolated mononuclear metal complexes that include the support as a ligand. Such materials offer the prospects of new reactivities and catalytic properties and, when they are made to be nearly uniform in structure, the advantages in fundamental understanding that may emerge from incisive structural characterization, which is not possible with the typical supported catalysts, because they are structurally heterogeneous as a consequence of the intrinsic nonuniformity of most solid surfaces.1,2 Supported metal complexes are typically synthesized from precursors containing reactive ligands (such as alkyl, allyl, carbene, acetylacetonate, and carbonyl), and the reaction of the precursor with the support is usually accompanied by the removal of some of the ligands from the precursor. The resultant supported species are most nearly uniform when the supports are most nearly uniform; the goal of uniform surface species has motivated research with crystalline porous supports, such as zeolites and spherosilicates.3 A supported metal complex typically retains some of its initial ligands, and these may be useful tags for the spectroscopic characterization, and they may also be reactive groups that facilitate entry of the metal complex into a catalytic cycle. For example, Goellner et al.4 used the precursor Rh(acac)(CO)2 (acac is acetylacetonate, CH3COCHCOCH3) to synthesize site-isolated * To whom correspondence should be addressed.
Rh+(CO)2 complexes on dealuminated Y (DAY) zeolite, which has a low density of sites where cationic species bond, favoring their site isolation. In the synthesis, the acac ligands were removed from the rhodium centers, leading to Rh+(CO)2 complexes bonded to the support. The structure of the zeoliteanchored species was characterized by extended X-ray absorption fine structure (EXAFS) and infrared (IR) spectroscopies and density functional theory (DFT).4 Although the carbonyl ligands on the rhodium in this sample are useful structural probes, they make the sample quite unreactive. Consequently, a sample was synthesized instead on the same support from a precursor with reactive ethylene ligands, Rh(C2H4)2(acac). The resultant supported rhodium complex was found to have retained its ethylene ligands and to react with H2 to make ethane and to enter a catalytic cycle for alkene hydrogenation.5 With the goal of extending this class of essentially molecular supported catalysts to other metals, we prepared an analogous iridium complex with reactive ethylene ligands bonded to DAY zeolite. Here, we report its synthesis and characterization by IR and EXAFS spectroscopies. These supported metal complexes are referred to as siteisolated, a term that implies that the metal sites are well separated and do not interact significantly with each other. Experimental Section Materials and Sample Preparation. Sample syntheses and handling were performed with the exclusion of moisture and air. H2 was supplied by Airgas (99.999%) or was generated by electrolysis of water in a Balston generator (99.99%) and was
10.1021/jp073338p CCC: $37.00 © 2007 American Chemical Society Published on Web 09/26/2007
Site-Isolated Iridium Diethylene Complex purified by passage through traps containing reduced Cu/Al2O3 and activated zeolite 4A to remove traces of O2 and moisture, respectively. He (Airgas, 99.999%) and C2H4 (Airgas, 99.99%) were purified by passage through similar traps. CO (Matheson, 99.999%), in a 10% mixture in He, was purified by passage through a trap containing activated R-Al2O3 particles and zeolite 4A to remove any traces of metal carbonyls from high-pressure gas cylinders and moisture, respectively. Ethylene-2-13C (99% 13C) was purchased from Cambridge Isotopes. The highly dealuminated HY zeolite (DAY zeolite) (Zeolyst International, CBV760), with a Si/Al atomic ratio of approximately 30, was calcined in O2 at 773 K for 4 h and was evacuated for 16 h at 773 K, was isolated, and was stored in a Vacuum Atmospheres HE-63-P N2-filled glove box until it was used. n-Pentane solvent (Fisher, 99%) was dried and purified by refluxing over sodium benzophenone ketyl and was deoxygenated by sparging with nitrogen. The precursor Ir(C2H4)2(acac) [acetylacetonatobis(ethylene)iridium(I)] was synthesized as described elsewhere,6 with a detailed characterization by X-ray diffraction crystallography, 1H and 13C NMR, Raman, and IR spectroscopies. The reference compound acetylacetone (Hacac) (Sigma Aldrich, 99%) was used as supplied. To prepare the supported iridium complex, Ir(C2H4)2(acac) and the calcined zeolite powder in a Schlenk flask were slurried in dried n-pentane that was initially at dry ice temperature. The stirred slurry was warmed to room temperature, and after 1 day, the solvent was removed by evacuation for 1 day, so that all the iridium remained in the zeolite. The resultant solid, containing 1 wt % Ir, was gray in color. It was stored in the N2-filled glove box. In another preparation to make a material for comparison, the reaction of Hacac with DAY zeolite powder was carried out in n-pentane with exclusion of air and moisture; the slurry was stirred for 1 day and the solvent was removed by evacuation for 1 day. IR Spectroscopy. A Bruker IFS 66v/S spectrometer with a spectral resolution of 2 cm-1 was used to collect transmission IR spectra of powder samples. The sample was present in a cell with reactive gases flowing through it. Each sample (typically, 10 mg) was pressed into a thin wafer and was loaded into the cell (In-situ Research Institute, Inc., South Bend, IN) in the N2-filled glove box. The cell was connected to a vacuum system with a base pressure of 1.3 × 10-4 mbar, which allowed recording of spectra while the reactant and inert gas (CO, H2, He, and C2H4) flowed through the cell at the reaction temperature. Each spectrum is the average of 64 scans. NMR Spectroscopy. Samples were prepared with a shallowbed CAVERN apparatus.7 Typically, 0.3 g of sample in a glove box was loaded into the CAVERN, which was removed and connected to a vacuum line. The sample was evacuated to a final pressure of less than 7 × 10-5 mbar. Adsorption of ethylene-13C2 was carried out at room temperature by two different loading procedures. The first involved adsorption of an excess (20 mbar) of ethylene-13C2, and the second was done by multiple steps whereby ca. 2 mbar of ethylene was introduced into the sample for 2 min followed by evacuation for 2 min. Ten of these exposures to ethylene were carried out, after which the sample was loaded into a 7.5-mm zirconia rotor and was capped within the CAVERN. Pulse sequences applied to the samples were typically conventional 13C CP/MAS (cross-polarization magic-angle spinning) and Bloch decay with proton decoupling carried out by using a Chemagnetics CMX 300 (7.05 T) spectrometer equipped with a modified Chemagnetics 7.5 mm MAS probe.
J. Phys. Chem. C, Vol. 111, No. 41, 2007 15065 Samples were referenced relative to hexamethylbenzene as a secondary standard (methyl signal 17.35 ppm relative TMS); the hexamethylbenzene sample was also used to calibrate the 90° flip for both carbon and the proton. Cross-polarization spectra were acquired with 10 000 scans and a 10 s pulse delay. The MAS spinning speed was 5.0 kHz. 27Al MAS NMR of the DAY zeolite support material was carried out prior to its use in preparation of the final catalyst. The zeolite was in a hydrated state for the NMR measurements. The spectrum was acquired by using a single pulse with a 15° tip angle and a spinning speed of 5 kHz. The spectrum was referenced to aqueous Al(NO3)3. X-ray Absorption Spectroscopy. EXAFS spectra were collected at beamline X-18B at the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory, Upton, NY, and at beamline 2-3 at the Stanford Synchrotron Radiation Laboratory (SSRL), Stanford Linear Accelerator Center, Stanford, CA. The storage ring electron energy was 2.8 GeV at NSLS and 3 GeV at SSRL. The ring currents were 110-250 mA and 50-100 mA at NSLS and SSRL, respectively. In a glove box at each synchrotron (filled with argon at NSLS and N2 at SSRL), powder samples were loaded into an EXAFS cell.8 The cell was evacuated to a pressure less than 1.3 × 10-5 mbar and was aligned in the X-ray beam. Spectra were collected in transmission mode at the Ir LIII edge (11 215 eV) with the sample cooled to approximately liquid-nitrogen temperature. In some experiments, spectra were collected at 298 K during treatment of the sample in flowing He, CO, C2H4, H2, or H2 + C2H4 in the EXAFS cell described elsewhere.9 For measurements of a sample working as an ethylene hydrogenation catalyst, the sample was initially scanned in flowing helium, and then the flow of reactant gases was begun (40 mbar each of ethylene and H2, with the remainder being helium and the pressure being atmospheric). After the measurement of an initial EXAFS spectrum, the steady flow of reactants continued for 10 h, whereupon four EXAFS spectra were collected. Ethylene Hydrogenation Catalysis in a Tubular Plug-Flow Reactor. Ethylene hydrogenation catalysis was also carried out in a conventional laboratory once-through tubular plug-flow reactor. The catalyst (30 mg), diluted with particles of inert, nonporous R-Al2O3 in a mass ratio of Al2O3 to catalyst of 30: 1, was loaded into the reactor in a glove box. The feed ethylene and H2 partial pressures were each 40 mbar, and the temperature was 294 ( 1 K. Details of the reaction experiments and product analysis by gas chromatography are as described elsewhere.10 Conversions of ethylene to ethane were
