Immobilization of Oxomolybdenum Species in a Layered Double Hydroxide Pillared by 2,2‘-Bipyridine-5,5‘-dicarboxylate Anions
- 21 July 2004
- journal article
- research article
- Published by American Chemical Society (ACS) in Inorganic Chemistry
- Vol. 43 (17), 5422-5431
- https://doi.org/10.1021/ic049755n
Abstract
A Zn,Al layered double hydroxide (LDH) with Zn/Al = 1.45 and containing nitrate anions was prepared by coprecipitation and characterized by powder X-ray diffraction, Zn K-edge extended X-ray absorption fine structure spectroscopy (EXAFS), thermogravimetric analysis, FTIR and FT Raman spectroscopy, and 27Al MAS NMR spectroscopy. Three Zn···O and four Zn···M (M = Zn, Al) shells could be fitted to the low-temperature (40 K) EXAFS spectrum, in accordance with a model for an ordered cationic sheet. The nitrate anions were easily exchanged by 2,2‘-bipyridine-5,5‘-dicarboxylate anions, resulting in an increase in the basal spacing from 9 to 18 Å. The basal spacing of the pillared derivative indicates that the anions are arranged with their longest dimension nearly perpendicular to the host layers. This material exhibits a high encapsulating ability, as evidenced by its interaction with a dichloromethane solution of the dioxomolybdenum(VI) complex MoO2Cl2(THF)2. A material with a metal loading of 11.2 wt % was obtained. Molybdenum K-edge EXAFS analysis could not substantiate the formation of a supported complex of the type MoO2Cl2(N−N) but instead indicated the formation of unidentate-bridged entities of the type [O2Mo−O−MoO2] with a metal−metal separation of 3.29 Å. The molybdenum-containing LDH was active as a catalyst for the liquid-phase epoxidation of cis-cyclooctene, 1-octene, and trans-2-octene using tert-butyl hydroperoxide as the oxygen source, yielding the corresponding epoxides as the only products. For reactions carried out with no additional solvent (other than n-decane) or in the presence of 1,2-dichloroethane, the solid catalyst could be recycled with no major loss of activity. Other tests confirmed that the systems functioned as true heterogeneous catalysts.Keywords
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