Abstract
The niobium catalysts involving single, pair, and monolayer Nb-sites have been prepared by the reaction between surface OH groups of SiO 2 and Nb(η 3-C 3H 5) 4, [Nb(η 5-C 5H 5)H-μ-(η 5, η 1-C 5H 4)] 2, and Nb(OC 2H 5) 5, respectively. The obtained Nb sites were found to be attached to the surface through oxygen atoms, showing Nb--Si bondings besides NbO bonds as proved by extended X-ray absorption fine structure (EXAFS). The Nb monomers on SiO 2 were active for ethanol dehydrogenation which proceeds on NbO bonds with basic character, while the Nb dimers showed dehydration activity to form ethene and diethyl ether, based on acidic character. The Nb monolayers on SiO 2 produced ethyl acetate from ethanol. The monolayer catalyst also showed a high activity for the formation of ethyl acetate from ethanol and acetic acid. The Nb atoms were suggested to be nearly uniformly supported in a monolayer-growth mode, where the Nb sites showed the Lewis acidic properties available for the catalytic esterification.
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