Titanium K-edge XANES analysis to unravel the local structure of alkene epoxidation titanium-polysiloxane homogeneous catalysts

We describe the synthesis of highly effective homogeneous catalysts for the epoxidation of alkenes with organic hydroperoxide and their detailed characterization with the X-ray absorption near-edge spectroscopy (XANES) technique. Prepeaks at Ti K-edge XANES spectra of the catalysts showed a shift towards higher photon energy and a simultaneous drop in relative intensity with the increasing number of nearest oxygen neighbours around titanium. PCA analysis of Ti K-edge XANES spectra revealed that the catalysts can be grouped in four distinct regions according to their structural characteristics. These structural differences revealed by Ti K-edge XANES spectra can be ascribed to the procedure employed in catalyst synthesis. In a preliminary attempt to explain catalytic performance, FTIR, UV-visible and XPS spectroscopic techniques were used to characterize the chemical structures developed on the catalysts; however, no correlation between reactivity and surface and/or bulk structures was found. Only the Ti K-edge XANES spectra of these Ti-polysiloxanes afforded a successful description of the local structure of Ti-IV centers due to their sensitivity in distinguishing different electronic structures for Ti atoms even under slight variations in their surroundings.