Guido Spano

Hydroxyls nests in defective silicalites and strained structures derived upon dehydroxylation: vibrational properties and theoretical modelling

Defective silicalite, an efficient and selective catalyst in the gas phase Beckmann rearrangement reaction, has been characterised by infrared spectroscopy and by molecular modelling techniques. We report a detailed IR study on the effect of outgassing treatments at increasing temperature on silanols bands and on framework modes. The effect of a temperature decrease up to 100 K (during the IR measurement) on the H-bonding interactions has also been investigated. The interaction of silanols with mesitylene, a probe molecule which cannot penetrate the channels, has been studied in order to distinguish between internal and external OH groups. Molecular mechanics and ab initio methods have also been used to model the structure and the vibrational features of a properly designed nest in order to support the assignments of the IR spectrum.

IR spectroscopy of neutral and ionic hydrogen-bonded complexes formed upon interaction of CH3OH, C2H5OH, (CH3)(2)O, (C2H5)(2)O and C4H8O with H-Y, H-ZSM-5 and H-mordenite: Comparison with analogous adducts formed on the H-Nafion superacidic membrane

A complete assignment of the IR spectra of the ionic and neutral hydrogen-bonded species formed by interaction of a series of bases of increasing proton affinity [methanol, ethanol, dimethyl ether, diethyl ether and tetrahydrofuran (THF)] with H-Y, H-ZSM-5, H-mordenite (H-Mord) and H-Nafion is given. Special attention is devoted to the discussion of spectral manifestations associated with structures varying with continuity from the true neutral AH⋯B form (low B proton affinity) to the true ionic A–⋯HB+ forms (high B proton affinity) passing through intermediate situations characterized by proton potentials with a single broad minimum or two minima separated by a low barrier (hesitating proton) and substantial ionic character. The assignment is based more on the comparison of the whole series of spectra than on the minute analysis of the specific properties of each spectrum. The assignment is also based on the comparison of the spectra of the adducts in zeolites with the spectra of analogous species formed in homogeneous conditions between acids and bases of comparable base and acid strengths (used as models).The main conclusions are: (i) adducts formed in H-Y have neutral AH⋯B character (only at the highest filling condition are signs of formation of ionic dimeric B⋯HB+ species observed for methanol and ethanol); (ii) the species formed upon adsorption of methanol and ethanol on H-ZSM-5 and H-Mord are neutral at low coverage and prevalently ionic at the highest filling conditions [because of the formation of (B)n⋯HB+ oligomers: 1, 2, …, n]; (iii) owing to the larger basicity of the ethers and THF, monomeric hydrogen-bonded adducts characterized by nearly symmetric proton potentials and substantial ionic character are directly formed on H-ZSM-5 and H-Mord without the intervention of other adsorbed molecules; (iv) THF and diethyl ether do not enter the H-Mord channels because the strongly adsorbed forms formed at the pore entrances block further penetration; (v) on the superacidic H-Nafion membrane (used as a model of a superacidic solid) all substrates directly form very strong and nearly symmetric hydrogen bonds without the assistance of other basic molecules: consequently the formed species always have prevalently ionic character even at the lowest dosages.

The architecture of catalytically active centers in titanosilicate (TS-1) and related selective-oxidation catalysts

Detailed X-ray spectroscopic studies yield the three-dimensional structure of Ti(IV) centered active sites in a series of TS-1 catalysts, in which titanium ions substitutionally replace a small fraction of the silicon sites in the pentasil structure, Silicalite-1. Advantage has been taken of the atomic structure of Ti(OSiPh3)4 , (determined separately by X-ray diffraction). A detailed EXAFS analysis of the X-ray absorption spectra of Ti(OSiPh3)4 which takes advantage of the full cluster multiple scattering methodology leads to the precise nature of the coordination of the Ti ions in a series of TS-1 samples. It proved possible to distinguish between the tetrapodal [i.e., Ti(OSi)4] and tripodal structure [i.e., Ti(OSi)3OH] shown (see T. Maschmeyer, F. Rey, G. Sankar and J. M. Thomas, Nature, 1995, 378, 159; ref. 1) to be present in high performance anchored titanosilicate epoxidation catalysts.

A calorimetric, IR, XANES and EXAFS study of the adsorption of NH3 on Ti-silicalite as a function of the sample pre-treatment

Abstract The interaction of NH 3 with Ti-silicalite-1 (TS-1) was studied by means of adsorption microcalorimetry IR, XANES and EXAFS techniques. A specific interaction with Ti(IV) sites was revealed and quantitatively and energetically described, in comparison with silicalite taken as a reference material. The adsorption was found to be largely reversible but a minor amount of ammonia was irreversibly held at the surface of TS-1. The energetic spectrum obtained by the heat of adsorption versus coverage plot is quite wide in both TS-1 (40 q ads −1 ) and silicalite (40 q ads −1 ), indicating not only that different processes occur at the surface but that a few highly energetic sites are present on TS-1. The standard pre-treatment of the samples in ammonium acetate aqueous solution lowers the whole adsorption capacity of TS-1 surface (as well of silicalite at about the same degree), but not the number of NH 3 molecules adsorbed per Ti atom, which is close to two for both untreated and treated TS-1 samples. The most energetic sites are preferentially suppressed by the ammonium acetate treatment on both TS-1 and silicalite, as shown by the heat of adsorption values much lower for the treated samples than for the untreated ones. Parallel IR and XANES–EXAFS results were found to support these findings.

EXAFS studies on MFI‐type gallosilicate molecular sieves

EXAFS spectroscopy shows that gallium atoms incorporated into the framework of MFI‐type ZSM‐5 gallosilicates have a typical Ga–O bond length of 1.82 ± 0.01 Å corresponding to a tetrahedral coordination. Accordingly, the coordination number, N, was found to take the value N = 4.2 ± 0.3. Template burning (at 773 K) causes little disturbance to the zeolite framework, and most of the gallium remains tetrahedrally coordinated. However, after a heat treatment at 923 K the apparent coordination number was found to decrease, as well as the corresponding Ga–O bond length. These results were interpreted in terms of formation of extra‐framework gallium oxide species which appear to be EXAFS silent. The thermal stability of the gallosilicate framework was found to increase with increasing Si/Ga ratio.

Acetonitrile as probe molecule for an integrated 1H NMR and FTIR study of zeolitic Brønsted acidity: Interaction with zeolites H-ferrierite and H-beta

In order to investigate the potentialities of an integrated IR and 1H NMR study of the acidity of Bronsted sites, a spectroscopic study of the interaction of CD3CN (at different coverages) with zeolites H-ferrierite and H-beta is reported. The formation of H-bonded CD3CN adducts of different strength with silanols and with strong Bronsted sites is described. It is shown that an integrated spectroscopic study gives complementary information on the observed H-bond adducts. While the presence of isosbestic points in IR spectra allows a clear demonstration of the existence of equilibria between species, 1H NMR spectra give information about dynamic adsorbate exchange phenomena.

Characterisation of defective silicalites

The structure and the adsorptive properties of internal defects in two chemically very pure silicalite samples, characterised by a high and low concentration of defects (internal SiOH nests generated by Si vacancies), are investigated by means of several physical methods (neutron diffraction, IR spectroscopy of adsorbed NH3, microcalorimetry of NH3 adsorption and quantum chemical computation). The role of cooperative effects in hydrogen bonded chains and rings of silanols covering the internal defective nanocavities in influencing the absorptive properties of the material is elucidated. This study provides an explanation of the less hydrophobic character of defective silicalite relative to perfect silicalite and sheds light on the reason why defective silicalite acts as an efficient and selective catalyst for the gas phase Beckman rearrangement reaction where cyclohexanone oxime is converted into caprolactam.

Voltammetric characterization of structural titanium species in zeotypes

Abstract Ti 4+ ions structurally contained in titanium silicalite (TS-1) and ETS-10 samples, two nanoporous materials, have been studied by means of voltammetric measurements. Both structurally tetrahedral and octahedral Ti 4+ ions show electrochemical response. In particular, by using acid solutions, it was possible to distinguish, in TS-1, between signals coming from different titanium species. Finally, it has been proven that the use of acid solutions allows us to discriminate between Ti 4+ ions in TS-1 and in ETS-10 since only the former is able to coordinate water molecules, the reduction of which can be monitored by the presence of a peak at −600 mV/SSE.

14-P-34-On the interaction of H2O with TS-1: a spectroscopic and ab-initio study

Publisher Summary This chapter presents an infrared (IR), extended X-ray absorption fine structure (EXAFS), and ab initio study on the interaction of TS-1 with water. IR spectroscopy shows that water is reversibly adsorbed at room temperature (RT), indicating a weak interaction between H 2 O molecules and TS-1. EXAFS confirms an elongation of the four Ti–O bonds of only 0.03 A upon water adsorption. On a theoretical ground, the adsorption of a water molecule has been investigated on three clusters of increasing size—namely, [Ti(OH) 4 )], [Ti(OSiH 3 ) 4 ], and [TiO 8 Si 6 H 12 ]. On the two bigger clusters, a binding energy in the range of 10–15 kJ/mol has been obtained.

15-O-05 - The beckmann rearrangement catalyzed by silicalite: a spectroscopic and computational study

Publisher Summary This chapter discusses the rearrangement of cyclohexanone oxime to caprolactam over zeolites with MFI and FAU structure by means of spectroscopical (IR) and computational methods. Hydrogen bond interactions, proton transfer, and rearrangement are detected. The thermal reaction mechanism and that catalyzed by H + are studied with the quantum-mechanics (QM) methods of B3LYP/6-31+G (d,p) quality. The same reaction path in presence of HCl and silanol is described to model the acids of different strength. The H + -catalyzed mechanism is found to be a multistep mechanism in which proton transfer from N to O in the oxime is the rate-determining step.