Enrica Gianotti

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.

One-step synthesis of a highly active, mesoporous, titanium-containing silica by using bifunctional templating.

A highly active, three-dimensional, mesoporous titanosilica, TiTUD-1, with comparable properties to Ti-grafted mesoporous silica MCM-41, has been prepared in a one-step synthesis. A non-surfactant chemical, triethanolamine, was used as a template molecule. Triethanolamine easily forms complexes with titanium alkoxides, yielding titanatrane complexes, which together with free triethanolamine form meso-sized aggregates that template mesopores upon increasing the temperature of the synthesis mixture. Triethanolamine served as both mesopore template and ligand for the titanium complexes, which represent the majority of the catalytic-site precursors. The formation of the silica network and the titanium insertion were followed by a combination of diffuse reflectance UV/ Vis/NIR and FTIR spectroscopy. A titanium-rich phase was obtained on the mesopore surfaces during calcination, allowing for easy accessibility of the reactants to the catalytic sites. TiTUD-1 is about six times more active than framework-substituted Ti-MCM-41 and has similar activity to Ti-grafted MCM-41.

Heterogeneous catalytic epoxidation of fatty acid methyl esters on titanium-grafted silicas

Three different titanium-grafted silicates, with different morphological features were compared in the epoxidation, using tert-butylhydroperoxide as oxidant, of methyl oleate, methyl elaidate and of a mixture of methyl esters, obtained from high-oleic sunflower oil. Ti-MCM-41, a well-ordered mesoporous catalyst, showed very high conversions and excellent selectivity on all the substrates. The fatty acid methyl esters derived from renewable raw material were epoxidised selectively with very high yields in a reaction medium completely free from organic acidic compounds. The influence of the pore order of the catalysts on the catalytic performance was also examined. The interactions between the organic substrate and the ordered array of mesopores in Ti-MCM-41 were evidenced.

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.

New catalytic liquid-phase ammoxidation approach to the preparation of niacin (vitamin B3).

New highly dispersed bimetallic nanoscale catalysts based on rhenium combined with antimony or bismuth have been shown to be highly effective for the ammoxidation of 3-picoline to nicotinonitrile (precursor for vitamin B3) under mild conditions in the liquid phase.

The identity of titanium centres in microporous aluminophosphates compared with Ti-MCM-41 mesoporous catalyst and titanosilsesquioxane dimer molecular complex: a spectroscopy study

Abstract Titanium-aluminophosphate and silico-aluminophosphate with chabasite-like structures (TAPO-34 and TAPSO-34) have been hydrothermally synthesised using morpholine as structure-directing agent. The local environment of Ti(IV) sites in these materials was studied using diffuse-reflectance UV-Vis, photoluminescence and Raman spectroscopies. This study, combined with the study of Ti-grafted MCM-41 mesoporous catalyst and a titanosilsesquioxane dimer molecular complex with a well-defined co-ordination of the Ti(IV) centers, revealed that a fraction of Ti(IV) ions are present in tetrahedral co-ordination, consistent with some Ti(IV) ions being incorporated into the aluminophosphate frameworks. However, Raman spectroscopy showed that around 20% of the total amount of the titanium used in the gel synthesis is present in a TiO 2 -like extra-phase.

Spectroscopic characterisation of microporous aluminophosphate materials with potential application in environmental catalysis

Abstract This review article deals with a spectroscopic characterisation, including FTIR, UV-Vis-NIR and NMR, of acid and redox microporous aluminophosphate catalysts with chabasite-related structure. These materials show high thermal and hydrothermal stability, and for this reason are attractive catalysts for environmental application. An extended investigation of acid SAPO-18 and SAPO-34, which are selective catalysts in methanol-to-olefins (MTO) and oxidative dehydrogenation (ODH) of light alkanes processes, will be presented. These catalysts can also be used as supports for the preparation of metal-containing molecular sieves for De-NOx reactions. Catalytic studies of NO oxidation to NO2 and N2O decomposition performed on cobalt- and copper-containing materials will be illustrated and correlated to spectroscopic results.

Toward understanding the catalytic synergy in the design of bimetallic molecular sieves for selective aerobic oxidations

Structure-property correlations and mechanistic implications are important in the design of single-site catalysts for the activation of molecular oxygen. In this study we rationalize trends in catalytic synergy to elucidate the nature of the active site through structural and spectroscopic correlations. In particular, the redox behavior and coordination geometry in isomorphously substituted, bimetallic VTiAlPO-5 catalysts are investigated with a view to specifically engineering and enhancing their reactivity and selectivity in aerobic oxidations. By using a combination of HYSCORE EPR and in situ FTIR studies, we show that the well-defined and isolated oxophilic tetrahedral titanium centers coupled with redox-active VO(2+) ions at proximal framework positions provide the loci for the activation of oxidant that leads to a concomitant increase in catalytic activity compared to analogous monometallic systems.

Acidic and Basic Sites in Nax aNd Nay fAujasites Investigated By Nh3, Co2 and Co Molecular Probes

The acid-base properties of samples of NaY and NaX faujasites have been investigated by adsorbing different probe molecules and measuring IR spectra. In the case of the NaY sample, only Na+ ions were found to be involved in the adsorption of CO2 and CO, confirming the overwhelming Lewis acid character of this material. In contrast, carbonate-like species were formed by adsorbing carbon dioxide on the NaX sample, due to the reaction of basic framework oxygen atoms with CO2 molecules polarised on neighbour Na+ ions. The spectroscopic analysis of NaX also showed evidence of Brønsted acid hydroxyls and OH groups bonded to extra-framework Al atoms. Ammonia adsorption revealed that the amount of Brønsted acid hydroxyls is significantly lower than the Lewis acid Na+ countercations. Moreover, small oxide particles, carrying carbonate-like species on their surface, are present in the zeolitic cavities. These particles could be responsible for the basic reactivity towards CO observed after outgassing the NaX sample at high temperature.

NIR Persistent Luminescence of Lanthanide Ion-Doped Rare-Earth Oxycarbonates: The Effect of Dopants

A series of luminescent rare-earth ion-doped hexagonal II-type Gd oxycarbonate phosphors Gd2-xRExO2CO3 (RE = Eu(3+), Yb(3+), Dy(3+)) have been successfully synthesized by thermal decomposition of the corresponding mixed oxalates. The Yb(3+) doped Gd-oxycarbonate has evidenced a high persistent luminescence in the NIR region, that is independent from the temperature and makes this materials particular attractive as optical probes for bioimaging.