20036
J. Phys. Chem. C 2009, 113, 20036–20043
Transient Spectroscopic Characterization of the Genesis of a Ruthenium Complex Catalyst Supported on Zeolite Y Isao Ogino and Bruce C. Gates* Department of Chemical Engineering and Materials Science, UniVersity of California, DaVis, California 95616 ReceiVed: August 14, 2009; ReVised Manuscript ReceiVed: September 21, 2009
A mononuclear ruthenium complex anchored to dealuminated zeolite HY, Ru(acac)(C2H4)2+ (acac ) acetylacetonate, C5H7O2-), was characterized in flow reactors by transient infrared (IR) spectroscopy and Ru K edge X-ray absorption spectroscopy. The combined results show how the supported complex was converted into a form that catalyzes ethene conversion to butene. The formation of these species resulted from the removal of acac ligands from the ruthenium (as shown by IR and extended X-ray absorption fine structure (EXAFS) spectra) and the simultaneous decrease in the symmetry of the ruthenium complex, with the ruthenium remaining mononuclear and its oxidation state remaining essentially unchanged (as shown by EXAFS and X-ray absorption near-edge structure spectra). The removal of anionic acac ligands from the ruthenium was evidently compensated by the bonding of other anionic ligands, such as hydride from H2 in the feed stream, to form species suggested to be Ru(H)(C2H4)2+, which is coordinatively unsaturated and inferred to react with ethene, leading to the observed formation of butene in a catalytic process. Introduction
Experimental Methods
The structures of supported metal complexes, an important class of industrial catalyst,1,2 are typically challenging to determine because the complexes are dispersed on nonuniform supports. However, when the supports are crystalline porous materials, exemplified by zeolites,3-8 supported metal complexes with a high degree of structural uniformity can be formed, exemplified by the species formed by the reaction of Rh(acac)(η2-C2H4)23,4,7(acac is acetylacetonate, C5H7O2-) or Ir(acac)(η2C2H4)25 or cis-Ru(acac)2(C2H4)2 (I)6,8 with dealuminated zeolite HY. The near uniformity of the supported metal complexes in these samples provides opportunities for precise determination of their chemistry by spectroscopic methods such as extended X-ray absorption fine structure (EXAFS) and infrared (IR), guided by electronic structure calculations at the level of density functional theory.4,7 Our goal was to gain an understanding of the genesis of the catalytically active species formed from I on the zeolite by characterizing the changing structures that are bonded to the zeolite during their transformation into catalytically active species. Our earlier investigation with this sample showed that the supported ruthenium complexes, Ru(acac)(C2H4)2+, react with ethene, forming butenes catalytically in the presence of H2.6,8 We now present data showing how the ruthenium complexes in the initially prepared sample are transformed into catalytically active species for the ethene dimerization by timeresolved IR, EXAFS, and X-ray absorption near-edge structure (XANES) spectroscopies. The supported samples were mounted in flow systems because they offer the advantage that, during treatments in various gases, gas-phase products formed from the supported species can be swept away and analyzed, without equilibrium limitations in the conversion of the surface species;7 identification of the evolved species by mass spectrometry and the correlations of the evolution of these species with the spectra of the solid sample have provided insight into the chemistry.
Materials and Procedures. The air-sensitive samples were handled with standard Schlenk techniques and stored in a glovebox under dry argon. The O2 and moisture contents in the glovebox were
