Valeria Dellarocca

Structure–functionality relationships of grafted Ti-MCM41 silicas. Spectroscopic and catalytic studies

Detailed spectroscopic measurements have been used to elucidate the nature of Ti(IV)-centred active sites that were anchored to a mesoporous silica surface (MCM41) by two distinct routes from titanocene precursors. The catalyst prepared in dry argon (Ti-MCM41 [Ar]) is more active than that prepared in air (in the presence of water vapour) in its activity in the epoxidation of cyclohexene with tert-butylhydroperoxide (TBHP). The degree of loading of the titanium onto the silica support also influences the precise nature of the active sites. In Ti-MCM41 [Ar] samples containing less that 2 wt.% of Ti, the most abundant species are tetrahedrally bonded Ti(IV) active sites which absorb at 210–230 nm in diffuse reflectance (DR) UV–Vis spectra and exhibit an emission at 430 and 490 nm when excited with a 250 nm light. Samples of catalyst with greater than 4 wt.% loading are shown to contain TiO2-like microclusters. These species absorb at λ250 nm in the DR UV–Vis and emit very weakly in the 500–600 nm region. In the case of Ti-MCM41 [air] catalysts, which were prepared in the presence of atmospheric water at the MCM41 surface, even at the lowest Ti loading (e.g. ⩽0.5%) an incipient formation of oligomers occurs. It is proposed that dimers or very small oligomers which absorb at around 250 nm in the DR spectra are responsible for a very strong emission at around 500 nm in the photoluminescence spectra. The abundant presence of these species might well explain the lower catalytic performance of Ti-MCM41 [air] in comparison with that of Ti-MCM41 [Ar]. The anchoring of the Ti species at the surface silanol groups of MCM41 was in all cases followed by FTIR spectroscopy.

Elucidating the local environment of Ti(IV) active sites in Ti-MCM-48 : a comparison between silylated and calcined catalysts

Abstract A mesoporous Ti-MCM-48 material was silylated using hexamethyldisilazane (HDMS) and a remarkable increase in the catalytic performances in the cyclohexene epoxidation with tert-butylhydroperoxide obtained. We report on a combined FTIR, diffuse reflectance UV–Vis–NIR, photoluminescence and 29Si MAS NMR study, augmented by TGA and XRD analysis, of the local environment of Ti sites in both calcined and silylated Ti-MCM-48 catalysts. Spectroscopic studies were also performed on the as-synthesised material and showed that both silanols and titanols are formed during the decomposition of the cetyltrimethylammonium hydroxide surfactant used to template the mesoporous structure. Unequivocal evidences that a further silica condensation of the MCM-48 framework structure occurs both during thermal treatments in vacuum and after calcination at 540°C are also reported. Upon silylation, a more hydrophobic surface is obtained where trimethylsilyl groups (Si(CH3)3) are anchored on to the silica surface by reaction of the silylating agent, HDMS, with silanols, and in addition, (SiO)3TiOH sites transformed into (SiO)3TiOSi(CH3)3. It is proposed that the more hydrophobic character of the silylated material prevents the deactivation of the catalyst by decreasing the amount of glycols which may be formed by reaction of the epoxide with water adsorbed on the surface of the Ti-MCM-48, and that poison the titanium catalytic centres.

NH3 adsorption on MCM-41 and Ti-grafted MCM-41. FTIR, DR UV–Vis–NIR and photoluminescence studies

NH3 was used as molecular probe to monitor the nature of both Bronsted and Lewis acid sites as well as co-ordination sphere of Ti(IV) centres in Ti-grafted MCM-41 by means of FTIR, DR UV-Vis-NIR and photoluminescence spectroscopies. A bare MCM-41 sample was also studied for comparison. Diffuse reflectance UV–Vis–NIR spectroscopy was used to investigate both electronic and vibrational properties of Ti-MCM-41. The UV–Vis region (55 000–14 300 cm−1) gave information on the co-ordination of Ti(IV) ions and the NIR region (14 300–4000 cm−1) on overtones and combinations of fundamental modes of surface OH and adsorbed NH3 falling in the medium IR (MIR) region. Significant effect related to the anarmonicity of the oscillators were observed. MCM-41, after calcination at 550 °C in O2, exhibits only isolated Si–OH groups (fundamental νOH stretching band at 3745 cm−1, overtone at 7321 cm−1 and combination of stretching and bending modes at 4530 cm−1). Ti-containing MCM-41 shows, besides isolated Si–OH groups, OH groups likely close to Ti Lewis acid sites (fundamental at 3725 cm−1, overtone at 7300 cm−1 and combination of stretching and bending modes at 4530 cm−1). NH3 was used as molecular probe to study the acidity of such materials by analysing the vibrational features of adsorbed ammonia in the NIR and MIR region. FTIR spectroscopy also revealed that the Bronsted groups are sufficiently acidic to protonate some ammonia molecules, to produce ammonium ions. DR UV–Vis and photoluminescence studies of Ti-MCM-41 revealed the presence of Ti(IV) Lewis sites in tetrahedral co-ordination, which, by the insertion of NH3 molecules, enlarge the co-ordination sphere from tetrahedral to octahedral.

Quantitative Analysis of Photoluminescence Quenching of Silica-Supported Molybdena Catalysts. Relation to Photocatalytic Reduction of Nitric Oxide by Carbon Monoxide

The quenching effect of NO, O2, CO, and N2O on the photoluminescence of Mo6+/SiO2 has been studied at room temperature as a function of gas pressure. Nonlinear plots of the relative photoluminescence intensity I0/I (I0 is the initial intensity under vacuum) vs. pressure of the quenching gases were rationalized assuming that only adsorbed molecules efficiently quench the (Mo5+–O-)* excited state and that the fraction of adsorbed quenching molecules can be determined from the classical Langmuir-type adsorption isotherm. The ratio of the quenching rate constants for NO and CO calculated from the computer best fits of the experimental I0/I–pressure dependence is in agreement with earlier data on the kinetics of the photocatalytic reduction of NO by carbon monoxide on Mo6+/SiO2.

Acidity of mesoporous aluminophosphates and silicas MCM-41. A combined FTIR and UV-Vis-NIR study

The surface acidity of new mesoporous aluminophosphates, silicoaluminophosphates and [Si]-MCM-41 was studied by means of FTIR and DR UV-Vis-NR spectroscopies using ammonia as probe molecule. Moreover, a novel Ti-grafted mesoporous ALPO was synthesised and studied by the use of DR-UV-Vis spectroscopy.

Unequivocal evidence of the presence of titanols in Ti-MCM-48 mesoporous materials. A combined diffuse reflectance UV-Vis-Nir and 29Si-MAS-NMR study

Mesoporous Ti-MCM-48 materials were synthesised with titanium loading varying from 1 to 3 wt% TiO2. Post-synthesis silylation treatment of the silica surface led to the transformation of silanols, (SiO)3SiOH, into (SiO)3SiOSi(CH3)3 groups, which were identified by means of 29Si-MAS-NMR and diffuse reflectance (DR) Nir spectroscopy. DR UV-Vis spectroscopy was performed to clarify the nature of Ti(IV) sites present in these samples. The UV-Vis spectra show a band at 210 nm with a shoulder at 230 nm, typically assigned to oxygen to tetrahedral Ti(IV) (LMCT) electronic transitions. Interestingly, the component at 230 nm, which was assigned to an electronic transition that involves titanium sites linked to OH groups (named as titanols), strongly decreased upon the silylation treatment. This is a clear evidence that, beside silanols, also titanols were silylated, leading to (SiO)3TiOSi(CH3)3 sites. This fact can be taken as an unequivocal assignment of the shoulder at 230 nm due to oxygen to tetrahedral Ti(IV) electronic transition related to titanol species.

Photoluminescence study of mesoporous MCM-41 and Ti-grafted MCM-41

Photoluminescence combined with diffuse reflectance UV—Vis spectroscopy was used to characterise both the pure siliceous MCM-41 and Ti-MCM-41 containing tetrahedral Ti(IV) species anchored onto the inner walls of the siliceous MCM-41. Both Ti-MCM-41 and MCM-41 showed complex photoemission signals and this study allows, for the first time, the distinction of the emission signals of tetrahedral Ti9IV) ions from those due to silica surface centres.

29-P-30-Ti-MCM-48 with different titanium loading: synthesis, spectroscopic characterization and catalytic activity

Publisher Summary This chapter describes the synthesis, spectroscopic characterization, and catalytic activity of titanium (Ti)-MCM-48 with different titanium loading. A series of Ti-MCM-48 materials with different Ti contents has been synthesized and characterized in the as-prepared and calcined states.The catalytic activity for the epoxidation of cyclohexene with tertbutylhydroperoxide on the different Ti-MCM-48 samples is tested. It has been possible to establish the dispersion of Ti (IV) sites on the silica surface by means of diffuse reflectance ultraviolet-visible (DR-UV-Vis) spectroscopy. These characterization results have been correlated with the final catalytic activity found for these mesoporous catalysts.