Titanium K-Edge XANES Analysis to Unravel the Local Structure of Alkene Epoxidation Titanium-Polysiloxane Homogeneous Catalysts

GEMA BLANCO-BRIEVA; MARI CARMEN CAPEL-SANCHEZ; MARI CARMEN CAPEL-SANCHEZ; JOSE L. G. FIERRO; EDUARDO J. LEDE; L. ANDRINI; FELIX G. REQUEJO

ADVANCED SYNTHESIS & CATALYSIS (PRINT)

Wiley-VCH Verlag GmbH & Co. KGaA

Año: 2003 vol. 345 p. 1314 - 1320

1615-4150

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 TiIV centers due to their sensitivity in distinguishing different electronic structures for Ti atoms even under slight variations in their surroundings.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 TiIV centers due to their sensitivity in distinguishing different electronic structures for Ti atoms even under slight variations in their surroundings.