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Supported Rhenium Complexes: Almost Uniform Rhenium Tricarbonyls Synthesized from CH3Re(CO)5 and HY Zeolite† Rodrigo J. Lobo-Lapidus‡,§ and Bruce C. Gates*,‡ ‡
Department of Chemical Engineering and Materials Science, University of California, Davis, California 95616, and §Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439 Received April 28, 2010. Revised Manuscript Received May 27, 2010
Supported rhenium complexes were prepared from CH3Re(CO)5 and dealuminated HY zeolite or NaY zeolite, each with a Si/Al atomic ratio of 30. The samples were characterized with infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopies. EXAFS data characterizing the sample formed by the reaction of CH3Re(CO)5 with dealuminated HY zeolite show that the rhenium complexes were bonded to the zeolite frame, incorporating, on average, three carbonyl ligands per Re atom (as shown by Re-C and multiple-scattering Re-O EXAFS contributions). The IR spectra, consistent with this result, show that the supported rhenium carbonyls were bonded near aluminum sites of the zeolite, as shown by the decrease in intensity of the IR bands characterizing the acidic silanol groups resulting from the reaction of the rhenium carbonyl with the zeolite. This supported metal complex was characterized by narrow peaks in the νCO region of the IR spectrum, indicating highly uniform species. In contrast, the species formed from CH3Re(CO)5 on NaY zeolite lost fewer carbonyl ligands than those formed on HY zeolite and were significantly less uniform, as indicated by the greater breadth of the νCO bands in the IR spectra. The results show the importance of zeolite Hþ sites for the formation of uniform supported rhenium carbonyls from CH3Re(CO)5; the formation of such uniform complexes did not occur on the NaY zeolite.
1. Introduction Notwithstanding the importance of solid catalysts in technology, their lack of uniformity often makes them unselective, and it also makes challenging the determination of structures of the catalytic species and thus the dependence of catalyst performance on structure.1 The nonuniformity of a supported catalyst can often be traced to the nonuniformity of the support surface on which the catalytically active species reside, because the surface sites influence the catalytic properties of such species. Because they have highly ordered structures, zeolites are supports that present surface sites with a high degree of uniformity, providing almost ideal scaffolds on which to construct uniform catalytic sites. The goal of preparing uniform, well-defined species on support surfaces is facilitated by the use of organometallic precursors, as they have well-defined structures and can be prepared with a variety of ligands that may allow tuning of the reactivity of the precursor with specific support surface sites.2 For example, Rh(CO)2(acac) (acac is acetylacetonate),3 Rh(η2-C2H4)2(acac),4 Ir(η2C2H4)2(acac),5 and cis-Ru(η2-C2H4)2(acac)26 have been used to prepare highly uniform metal complexes on dealuminated HY zeolite by reactions in which the acac ligands were replaced by oxygen ligands of the zeolite. The uniformity of the supported species was demonstrated by extremely narrow peaks in the νCO † Part of the Molecular Surface Chemistry and Its Applications special issue. *To whom correspondence should be addressed. E-mail: bcgates@ucdavis. edu.
(1) Gates, B. C. Nat. Nanotechnol. 2008, 3, 583. (2) Shriver, D. F.; Atkins, P. W. Inorganic Chemistry, 3rd ed.; Oxford University Press: New York, 1999. (3) Goellner, J. F.; Gates, B. C.; Vayssilov, G. N.; R€osch, N. J. Am. Chem. Soc. 2000, 122, 8056. (4) Liang, A. J.; Bhirud, V. A.; Ehresmann, J. O.; Kletnieks, P. W.; Haw, J. F.; Gates, B. C. J. Phys. Chem. B 2005, 109, 24236. (5) Uzun, A.; Bhirud, V. A.; Kletnieks, P. W.; Haw, J. F.; Gates, B. C. J. Phys. Chem. C 2007, 111, 15064. (6) Ogino, I.; Gates, B. C. Chem.;Eur. J. 2009, 15, 6827.
16368 DOI: 10.1021/la101344t
region of the infrared (IR) spectra characterizing the samples after they were treated in CO to form supported metal carbonyls. To be good catalysts, supported metal complexes must be stable. Most examples of supported metal complex catalysts have been made from complexes of group-8 metals, typified by zeolitesupported complexes;3-6 however, the noble group-8 metals in such catalysts easily undergo reduction and aggregation,7 easily leading to nonuniform structures, typically including clusters or particles metal of various shapes and sizes. The reduction and aggregation occur even at mild temperatures and especially in the presence of reducing agents such as H2. Thus, metals that are more oxophilic than group-8 metals present opportunities to prepare supported metal species that are more stable than those formed from group-8 metal complexes because the oxophilic metals bond more strongly to the support surface and are intrinsically more difficult to reduce.8 Among the group-7 metals, rhenium is especially important as a catalyst. Supported rhenium is applied in industrial catalysts for reactions including alkene metathesis9 and naphtha reforming.10 Rhenium is known to form surface sites on metal oxides that are highly resistant to reduction,11 and it has been inferred that rhenium in supported bimetallic cluster catalysts such as the RePt catalysts used for naphtha reforming is present in a cationic form when the more noble metal is present in a zerovalent form; the cationic rhenium may be preferentially present at the metal-support interface and help to disperse and stabilize the dispersion of a group-8 metal.12 (7) Liang, A. J.; Gates, B. C. J. Phys. Chem. C 2008, 112, 18039. (8) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals, 2nd ed.; Wiley: New York, 1994. (9) Mol, J. Catal. Today 1999, 51, 289. (10) Sinfelt, J. H. In Handbook of Heterogeneous Catalysis; Ertl, G., Kn€ozinger, H., Weitkamp, J., Eds.; VCH: Weinheim, 1997; Vol. 4, p 1939. (11) Yao, H. C.; Shelef, M. J. Catal. 1976, 44, 392. (12) Fung, A. S.; McDevitt, M. R.; Tooley, P. A.; Kelley, M. J.; Koningsberger, D. C.; Gates, B. C. J. Catal. 1993, 140, 190.
Published on Web 06/18/2010
Langmuir 2010, 26(21), 16368–16374
Lobo-Lapidus and Gates
Our goal was to use organorhenium precursors and zeolite supports to form highly uniform supported rhenium complexes and to determine the influence of the zeolite surface chemistry on the properties and degree of the uniformity of the supported species. To understand such materials, it is useful to start with organometallic precursors that incorporate ligands that can be readily identified and that, via their spectra, provide insight into the structure and degree of uniformity of the resultant supported structures. CO is such a ligand, as IR spectra in the νCO region are sensitive to the structure and nature of the metal atom to which the CO is bonded,13 and the widths of the νCO peaks indicate the degree of uniformity of the surface species.14 The family of rhenium carbonyls includes compounds with a variety of ligands in addition to CO15,16 and offers numerous opportunities to probe support surface sites. We chose CH3Re(CO)5 as a precursor because of the expected reactivity of the methyl ligands with zeolite OH groups, and we chose dealuminated HY zeolite as a support because it has been used before to prepare highly uniform supported metal complexes, as summarized above. We chose to vary the zeolite composition by selecting two different exchange ions, Hþ and Naþ, chosen because the former can potentially react with the CH3- ligand on the precursor to form CH4, whereas the latter cannot. We used IR spectroscopy to characterize the rheniumcontaining species formed on the zeolites and extended X-ray absorption fine structure (EXAFS) spectroscopy to characterize the Re atoms and their surroundings. A specific goal was to understand how the two different zeolite surface sites affect the reactivity of the surface with CH3Re(CO)5 and to determine the structure and degree of uniformity of the resulting supported species.
2. Experimental Methods Synthesis. Zeolite Ion Exchange. A zeolite-Y sample with
Article moisture, to an argon-filled glovebox, where the sample was stored until it was used (the O2 and H2O contents were
