The role of synchrotron-based studies in the elucidation and design of active sites in titanium-silica epoxidation catalysts.

Abstract

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 Ti(IV) 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 Ti(IV)-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.