Evidence from NMR and EXAFS Studies of a Dynamically Uniform Mononuclear Single-Site Zeolite-Supported Rhodium Catalyst

Characterization of a Molecule Partially Confined at the Pore Mouth of a Zeotype

We investigate the interaction between a molecule and a pore mouth-a critical step in adsorption processes-by characterizing the conformation of a macrocyclic calix[4]arene-Ti^IV complex, which is grafted on the external surface of a zeotype (*-SVY). X-ray absorption and ¹³ C{¹ H} CPMAS NMR spectroscopies independently detect a unique conformation of this complex when it is grafted at crystallographically equivalent locations that lie at the interface of 7 Å hemispherical microporous cavities and the external surface. Electronic structure calculations support the presence of this unique conformation, and suggest that it is brought about by a specific orientation of the macrocycle that maximizes non-covalent interactions between calix[4]arene upper-rim tert-butyl substituents and the microporous-cavity walls. Our comparative study provides a rare "snapshot" of a molecule partially confined at a pore mouth, an essential intermediate for adsorption into micropores, and demonstrates how surrounding environment controls this confinement in a sensitive fashion.

Role of the support in catalysis: activation of a mononuclear ruthenium complex for ethene dimerization by chemisorption on dealuminated zeolite Y

A set of supported ruthenium complexes with systematically varied ratios of chemisorbed to physisorbed species was formed by contacting cis-[Ru(acac)(2)(C(2)H(4))(2)] (I; acac = C(5)H(7)O(2) (-)) with dealuminated zeolite Y. Extended X-ray absorption fine structure (EXAFS) spectra used to characterize the samples confirmed the systematic variation in the loadings of the two supported species and demonstrated that removal of bidentate acac ligands from I accompanied chemisorption to form [Ru(acac)(C(2)H(4))(2)](+) attached through two Ru-O bonds to the Al sites of the zeolite. A high degree of uniformity in the chemisorbed species was demonstrated by sharp bands in the infrared (IR) spectrum characteristic of ruthenium dicarbonyls that formed when CO reacted with the anchored complex. When the ruthenium loading exceeded 1.0 wt % (Ru/Al approximately 1:6), the additional adsorbed species were simply physisorbed. Ethene ligands on the chemisorbed species reacted to form butenes when the temperature was raised to approximately 393 K; acac ligands remained bonded to Ru. In contrast, ethene ligands on the physisorbed complex simply desorbed under the same conditions. The chemisorption activated the ruthenium complex and facilitated dimerization of the ethene, which occurred catalytically. IR and EXAFS spectra of the supported samples indicate that 1) Ru centers in the chemisorbed species are more electron deficient than those in the physisorbed species and 2) Ru-ethene bonds in the chemisorbed species are less symmetric than those in the physisorbed species, which implies the presence of a preferred configuration for the catalytic dimerization.

Molecular Heterogeneous Catalysis: A Single-Site Zeolite-Supported Rhodium Complex for Acetylene Cyclotrimerization

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(2)H(4))(2)(CH(3)COCHCOCH(3))] and a crystalline support, dealuminated Y zeolite, giving {Rh(C(2)H(4))(2)} 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 (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.