Alessandro Damin

Reactivity of Ti(IV) sites in Ti-zeolites: An embedded cluster approach

We report a complete cluster/embedded cluster study by means of ab initio methods in the ONIOM scheme, as implemented in GAUSSIAN 98 code, of the reactivity towards water and ammonia of Ti(IV) centers in zeolitic frameworks. For water adsorption, we observe a remarkable increment of the binding energies by moving from 2.1 kJ mol−1 for the unconstrained Ti(OSiH3)4 cluster to 16.9 kJ mol−1 for the TiSi17O26H20 cluster, obtained by cutting a portion of the MFI framework. The same holds for ammonia, where the binding energy increases from 17.4 to 35.4 kJ mol−1, all reported values being BSSE corrected. These results underline the fundamental role played by zeolitic framework constraints, in enhancing the reactivity of Ti(IV) centers towards both H2O and NH3 probes. On the geometrical ground the Ti–O distance of bare clusters and its modification in complexes are in good agreement with the first shell EXAFS data analysis.

Theoretical characterization of dihydrogen adducts with halide anions

The interaction between a hydrogen molecule and the halide anions F(-), Cl(-), Br(-), and I(-) has been studied at different levels of theory and with different basis sets. The most stable configurations of the complexes have a linear geometry, while the t-shaped complexes are saddle points on the potential energy surface, opposite to what is observed for alkali cations. An electrostatic analysis conducted on the resulting adducts has highlighted the predominance of the electrostatic term in the complexation energy and, in particular, of the quadrupole- and dipole-polarizability dependent contributions. Another striking difference with respect to the positive ions, is the fact that although the binding energies have similar values (ranging between 25 and 3 kJ /mol for F(-) and I(-), respectively), the vibrational shift of the nu(H-H) and in general the perturbation of the hydrogen molecule in complexes are much greater in the complexes with anions (Delta nu(H-H) ranges between -720 and -65 cm(-1)). Another difference with respect to the interaction with cations is a larger charge transfer from the anion to the hydrogen molecule. The Delta nu is the result of the cooperative role of the electrostatics and of the charge transfer in the interaction. The correlation between binding energies and vibrational shift is far from linear, contrary to what is observed for cation complexes, in accordance with the higher polarizability and dynamic polarizability of the molecule along the molecular axis. The observed correlation may be valuable in the interpretation of spectra and thermodynamic properties of adsorbed H(2) in storage materials.

MoS2 Nanoparticles Decorating Titanate-Nanotube Surfaces: Combined Microscopy, Spectroscopy, and Catalytic Studies

MoS2/TNTs composites have been obtained by impregnation of titanate nanotubes (TNTs) with a centrifuged solution of nanosized MoS2 particles in isopropyl alcohol (IPA). The characterization has been performed by combining UV-vis-NIR, Raman, AFM, and HRTEM analyses, before and after impregnation. HRTEM images show that the contact between single-layer MoS2 nanoparticles and the support is efficient, so justifying the decoration concept. The volatility of IPA solvent allows the preparation of composites at low temperature and free of carbonaceous impurities. MoS2 nanoparticles have strong excitonic transitions, which are only slightly shifted with respect to the bulk because of quantum size effects. Concentrations of MoS2, less than 0.1 wt %, are enough to induce strong absorption in the visible. Photodegradation of methylene blue (MB) has been performed on TNTs and MoS2/TNTs to verify the effect of the presence of MoS2. The first layer of adsorbed MB is consumed first, followed by clustered MB in the second and more external layers. The presence of low concentrated MoS2 nanoparticles does not substantially alter the photocatalytic properties of TNTs. This result is due to poor overlapping between the high frequency of MoS2 C, D excitonic transitions and the TNTs band gap transition.

Ti-chabazite as a model system of Ti(IV) in Ti-zeolites: A periodic approach

For the first time, full ab initio periodic calculations on a Ti-chabazite with a very low content of Ti (Ti/Si=1/11) were done. The simulated crystal is less computationally demanding and it is similar to the real titanosilicates microporous materials such as TS-1 or Ti-β, where the Ti/Si is lower than 1/30, and hence it can be used as a model system. The geometric and energetic features of the interaction of Ti-CHA with NH3, H2O, H2CO, and CH3CN were studied, principally at the Hartree–Fock level. Bare Ti-CHA and its NH3, H2O, H2CO, and CH3CN complexes were fully optimized at Hartree–Fock level with an upgraded version of the CRYSTAL code [Chem. Phys. Lett. 348, 131 (2001)] and such optimized structures were employed to calculate BEc (binding energy corrected for the basis set superposition error) at the Hartree–Fock and B3-LYP levels.

Synthesis of Titanium Chabazite: A New Shape Selective Oxidation Catalyst with Small Pore Openings and Application in the Production of Methyl Formate from Methanol

Isomorphous substitution of a small fraction of titanium(IV) in the tetrahedral sites of zeolitic frameworks provides materials with excellent catalytic properties for partial oxidation reactions with H2O2. [1] The original TS-1 material performs with an enzyme like activity and selectivity and is of great industrial importance in partial oxidation reactions of small organic substrates. Titanium incorporation in silicates has been extensively studied both on amorphous phases (dispersed in microporous silica and grafted to mesoporous molecular sieves), in layered delaminated zeolite precursors and in zeolitic frameworks; such as Ti-Beta, TS-2, Ti-ZSM-48, Ti-MWW, and Ti-MTF. Here we report the synthesis of Ti-CHA zeolites with and without aluminum and demonstrate their activity in partial oxidation reactions with H2O2. The pores in most titanium silicates are medium to large; however in this material they are 8-membered rings. This opens up new possibilities within shape-selective oxidation catalysis. CHA is a zeolite topology with a 3dimensional pore structure comprising an elongated cage with 8-ring windows. Different CHA materials (SAPO-34 and SSZ-13) have been studied as catalysts in the methanol-to-olefin process due to their shape selective properties. The CHA topology is rare in that it contains only one crystallographic independent T site and is, therefore, an interesting model material to study Ti insertion in the framework, as testified by previous theoretical periodic studies. It was recently reported that CHA zeolites can be prepared over a wide range of silicon/aluminum ratios, including the purely siliceous. This creates new doping options, for example in Ti-CHA and Ti-Al-CHA, in which both redox and acidic properties can work together. For example, a possible application of the Ti-Al-CHA material is in the production of methyl formate from methanol. Methyl formate is of great importance as an intermediate in the production of chemicals such as formamide, dimethyl formamide, and formic acid. Formic acid alone is produced in several hundred thousands of tons per year. In contrast to the conventional synthetic route, this method operates at very mild conditions, making it economically and environmentally advantageous. Detailed results are reported for two Ti-containing samples (Ti-CHA with Si/Ti = 246 and Ti-Al-CHA with Si/Ti = 95 and Si/ Al = 17) with comparison to their respective blank matrices (pure siliceousand Al-CHA with Si/Al = 20). The elemental analysis reported in Table 1 shows that the Ti loading is low, as already observed in other Ti-containing zeolites, and that Ti in-

Enhancing the Initial Rate of Polymerisation of the Reduced Phillips Catalyst by One Order of Magnitude

More than 50 % of the high-density polyethylene produced worldwide is obtained by using the Phillips catalyst, which is prepared by impregnation of an amorphous silica gel with an aqueous solution of an appropriate Cr compound to give a metal loading of about 1 % by weight. After thermal activation, a grafted chromate species is obtained (species 1 in Scheme 1), which can be used directly as the catalyst precursor. In this case, ethylene polymerisation is performed in the temperature range 353–373 K (Scheme 1) and a variable induction period is usually observed that can last from a few minutes to more than 1 hour and during which Cr is slowly reduced to Cr and organic oxygen-containing redox products are desorbed. The mechanism of formation of the reduced state from the chromate precursor in the presence of ethylene is still the subject of ongoing debate, despite well over 30 years of research competently carried out by several research groups. Although there is broad agreement that Cr is an active precursor constituting the catalytic site, a Cr oxidation state has frequently been suggested to be an intermediate species and even cycling between two oxidation states during the catalytic process has frequently been debated in the specialised literature. 13, 14] The above-mentioned induction period becomes a problem when ethylene polymerisation must be performed at a lower temperature (e.g., for production of ultra-high-molecular-weight polyethylene). In these cases, the catalyst is previously reduced by CO (giving species 2 in Scheme 1). Although in this system nearly all of the surface-grafted chromium centres are in the divalent state, only a fraction of them 11a,15] remain highly coordinatively unsaturated. This unsaturation is a key feature for ethylene polymerisation because it confers on the transition-metal ion the ability to bind (simultaneously) both the growing polyethylene chain and the monomer molecule. Ethylene polymerisation readily starts on these sites at room temperature without an induction period. The remaining Cr sites, which become partially buried in the silica framework and surrounded by weak siloxane ligands, are dormant spectators. Recently we showed that amorphous Cr/SiO2 can be an efficient catalyst for reducing nitrogen oxides with CO and (in the case of N2O) the attendant formation of a Cr – oxo complex (species 3 in Scheme 1) was proposed. Based on this experience we realised that controlled oxidation of the reduced Phillips catalyst by N2O could afford a means to increase catalyst activity as the formation of species 3 would help to pull chromium ions out of their dormant sites. Note also that, in contrast to species 1, species 3 retains an open coordination site on the metal cation, which would be ready for inserting an incoming ethylene molecule (Scheme 1). These considerations prompted the study reported herein on ethylene polymerisation on a catalyst (hereafter modified catalyst) obtained by controlled oxidation of the reduced Phillips catalyst (hereafter standard catalyst) with N2O, as described in the Experimental Section. The main finding of our work was a largely increased initial [a] Dr. E. Groppo, Dr. A. Damin, Prof. C. Otero Arean, Prof. A. Zecchina Department of Inorganic, Physical and Material Chemistry NIS Centre of Excellence and INSTM Unit di Torino University of Torino via Quarello 11, 10135 Torino (Italy) Fax: (+39) 011-6707855 E-mail : elena.groppo@unito.it [b] Prof. C. Otero Arean On leave from the Department of Chemistry University of the Balearic Islands, 07122 Palma de Mallorca (Spain) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201101714. Scheme 1. Schematic structure of silica-grafted chromate in the fully oxidised Phillips catalyst 1, the corresponding CO-reduced Phillips catalyst precursor 2 and the hypothesised Cr–oxo surface species 3 on the modified catalyst. Chromium, oxygen and silicon atoms are shown in blue, red and yellow, respectively.

Ti-STT: a new zeotype shape selective oxidation catalyst

A new zeotype titanium silicate oxidation catalyst with the STT topology has been synthesized from direct synthesis. Ti-STT has a microporous structure with small pore openings, allowing shape selective oxidation catalysis. The isomorphous substitution of Si by Ti in the framework has been confirmed by Raman, FT-IR, UV-VIS and XANES spectroscopies.

Modeling CO and N2 adsorption at Cr surface species of Phillips catalyst by hybrid density functionals: effect of Hartree-Fock exchange percentage.

In this article, we present a computational study of the structure and vibrational properties of the species formed by the interaction between Cr sites of Phillips catalyst and probe molecules (CO, N(2)). The vibrational properties of these surface species, intensively investigated in the past, form a very rich and ideal set of experimental data to test computational approaches. By adopting the X(4)Si(2)O(3)Cr (X = H, OH, F) cluster as a simplified model of the ([triple bond]SiO)(2)Cr(II) species present at the surface of the real catalyst, we found that the B3LYP hybrid functional (containing 20% of Hartree-Fock exchange), when applied to this model, is unable to reproduce with reasonable accuracy the currently available experimental data (principally coming from IR spectroscopy). Better agreement is obtained when the percentage of Hartree-Fock exchange is increased (up to 35-40%).

AM-6: a microporous one-dimensional ferromagnet

The vanadosilicate zeolite AM-6 is shown by a combination of spectroscopic techniques (UV-vis, Raman, XPS, XAS and EPR) to contain linear chains of alternating V=O and V-O bonds. The V(IV) ions in these chains are ferromagnetically coupled, and an excellent fit a to the susceptibility data with a one-dimensional Heisenberg model is obtained with J = 0.66(1) cm(-1). AM-6 is thus the first reported example of a microporous material incorporating one-dimensional ferromagnetic chains.

Healing of defects in ETS-10 by selective UV irradiation: a Raman study

The UV–Raman spectra of a crystalline and a defective ETS-10 sample have shown that the two materials possess different stability under the laser beam. These results are interpreted in terms of a possible light-induced healing of defects in the case of the defective sample, through annealing of two vicinal TiOH groups along the Ti–O–Ti chains. Possible implications for the preparation of highly ordered materials are envisaged.