The Role of Synchrotron-Based Studies in the Elucidation and Design of Active Sites in Titanium−Silica Epoxidation Catalysts

X-ray absorption fine structure (XAFS) measurements unambiguously establish the structure of the active centers in high-performance Ti−SiO2 epoxidation catalysts. Subtleties in structural changes in proceeding from the dispersed titanocene dichloride precursor to the anchored TiIV centers are charte...

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Bibliographic Details
Published in:Accounts of chemical research 2001-07, Vol.34 (7), p.571-581
Main Authors: Thomas, John Meurig, Sankar, Gopinathan
Format: Article
Language:English
Subjects:
Catalysis
Chemistry Techniques, Analytical - methods
Environment
Silicon Dioxide
Titanium
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Summary:X-ray absorption fine structure (XAFS) measurements unambiguously establish the structure of the active centers in high-performance Ti−SiO2 epoxidation catalysts. Subtleties in structural changes in proceeding from the dispersed titanocene dichloride precursor to the anchored TiIV centers are charted, in situ, by both near-edge (XANES) and extended-edge (EXAFS) measurements. The active centers are tripodally attached titanol groups:  (Si−O)3−TiOH. These permit facile expansion of the coordination shell (also monitored by in situ XAFS studies) during catalysis, and thereby lead to the formulation of a plausible mechanism of epoxidation of alkenes that is consonant with recent DFT computations. Previously proposed mechanisms, based on a three-coordinated titanyl group (as well as possible five- or six-coordinated active sites), are discounted. Comparable XAFS studies of soluble catalytically active four-coordinated TiIV-containing molecular entities establish that the heterogeneous Ti−SiO2 catalyst has “single-site” active centers. Equipped with such information, minute changes in hydrophobicity and local composition (e.g., replacement of one Si by a Ge) can be wrought so as to boost the performance of the catalyst. The merits of using combined in situ X-ray absorption spectroscopic and X-ray diffractometric measurements are also highlighted.
ISSN:0001-4842
1520-4898
DOI:10.1021/ar010003w