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.