Evidence of the Presence of Two Different Framework Ti(IV) Species in Ti−Silicalite-1 in Vacuo Conditions:  an EXAFS and a Photoluminescence Study

Abstract

In a previous contribution (J. Phys. Chem. 1994, 98, 4125) we reported EXAFS data collected at ADONE on a high quality Ti-silicalite (TS-1) sample (Ti ) 1.47 wt %) dehydrated in a carefully controlled atmosphere at 400 K. The importance of that paper was to definitively clarify that Titanium atoms occupy silicon substitutional framework positions. However, due to the large incertitude associated with the estimated first shell Ti-coordination number (N ) 4.4 ( 0.6), it was not possible to clarify if only perfect "closed" Ti(OSi) 4 sites are present or if a considerable fraction of defective "open" Ti(OSi) 3 (OH) sites must also be considered (in an ideal EXAFS measurement N is expected to be five if only open sites are present, due to the insertion of a fifth oxygen of the silanol formed in the adjacent Si center when the Ti-O-Si bridge is broken). We report more recent EXAFS data collected at LURE DCI on a high quality TS-1 (Ti ) 2.03 wt %) sample: the higher photon flux of that facility and the higher Ti content of the sample has allowed us to remarkably increase the signal/noise ratio with a consequent reduction of the errors associated with the fitted parameters. The quality of the fit, performed on a wider ∆k range, has also been strongly improved. Our new measurements indicate that a first shell coordination number higher than four (4.44 ( 0.25). This suggests that, even for samples dehydrated in a carefully controlled atmosphere at 400 K, a considerable fraction of sites exhibits the substitution of a bridged oxygen with two OH groups. Being the value of the error bars associated to the EXAFS analysis of fundamental importance for the solidity of our conclusions, both statistical and systematic errors associated with our measurement and data analysis have been deeply discussed. EXAFS results are also qualitatively confirmed by a parallel photoluminescence study, where the presence, in both emission and excitation spectra, of two distinct bands clearly indicates that we are dealing with two family of Ti sites characterized by slightly different environments. For both techniques, the comparison with measurements performed on the Ti(OSi(CH 3 ) 3 ) 4 model compound, where Ti has both the first and the second coordination shells identical to those of Ti (perfect sites) in TS-1, strongly validates our conclusions.

Vibrational Spectroscopy of Zeolites

“…a frequency where the characteristic oxygen to titanium charge transfer absorption of tetrahedral Ti in Ti-Silicalite occurs. 19,20 In fact the peak at 1125 cm 1 is enhanced by a factor between 10 1 and 10 2 , as expected for a resonance enhanced mode where the UV excitation causes the simultaneous and symmetric elongation of all the four TiO distances.…”

The effect of electron irradiation on the optical properties of the natural armenian zeolite-clinoptilolite

“…The results obtained, agree with those of previous similar studies [10,18] whereby the influence of γ-irradiation on the IR absorption of zeolites showed dehydroxylation of the Zeolite surface and that the formation of water molecules increased with the γ-radiation dose. This process is followed by infrared tracking of isolated OH-groups (3550, 3650 cm −1 ).…”

Titanosilicate Beads with Hierarchical Porosity: Synthesis and Application as Epoxidation Catalysts

“…This is ascribed to the presence of various types of tetrahedral Ti sites, such as TiA C H T U N G T R E N N U N G (OSi) 4 and Ti(OH)A C H T U N G T R E N N U N G (OSi) 3, which generate the broad band centered at 230 nm. [29,30] Moreover, a small amount of higher-coordinated Ti species coexist with the tetrahedral Ti sites in both materials, as indicated by the shoulder between 270 and 290 nm, which is assigned to reversible penta-coordinated Ti species that form from the interaction of Ti species with moisture and/or to polymerized hexa-coordinated Ti species. [28] The lack of the absorption band characteristic of octahedral extra-framework titanium at about 300-330 nm in both materials shows that no separate titania (anatase) phase is formed during the synthesis.…”