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Low-temperature Fourier-transform infrared investigation of the interaction of CO with nanosized ZSM5 and silicalite

Nanosized ZSM5 zeolites with microcrystal dimensions in the 20–120 nm range have been characterized by means of IR spectroscopy and HRTEM microscopy. The vibrational spectrum of the OH groups on the external and internal surfaces of H-ZSM5 and Na-ZSM5 samples of different crystallite dimensions has been investigated. For the sake of comparison the spectra of silicalite samples containing different concentrations of sodium and aluminium are also shown. For this purpose high-purity silicalite samples were prepared following a novel synthesis route.Carbon monoxide (a very weak Lewis base) was used to probe the acidity present on the external and internal surfaces of the zeolites through formation of 1 : 1 adducts with silanols (both internal and external), Bronsted-acid groups (both framework and extraframework), Na+ ions, and Lewis Al3+ centres (in extraframework and framework positions). The IR-active CO stretching modes of the complexes are shifted to higher wavenumber with respect to the free molecule; the positive shift can be used to estimate the acid strength. CO that was physically adsorbed in the zeolite channels has also been investigated.

Cu(I)-ZSM-5 zeolites prepared by reaction of H-ZSM-5 with gaseous CuCl: Spectroscopic characterization and reactivity towards carbon monoxide and nitric oxide☆

Abstract The strongly acidic Bronsted sites of H-ZSM-5 can be quantitatively exchanged with monovalent copper ions by reaction with CuCl at 573 K, as evidenced by the disappearance of the characteristic IR bands of bridged OH groups. Characterization of the Cu-ZSM-5 samples prepared following this route by means of UV-Vis-NIR (diffuse reflectance) and photoluminescence spectroscopies confirms that the protons are substituted by Cu+ ions, which are isolated and located in a few, structurally well defined sites easily accessible to ligand molecules. These Cu+ ions are highly coordinatively unsaturated and can form Cu+ (CO)n (n=1, 2 or 3) carbonylic and Cu+ (NO)n (n=1 or 2) nitrosylic complexes upon dosage of carbon monoxide or nitric oxide at 77 K. The Cu+ (NO)2 dinytrosylic complexes are unstable at room temperature and evolve with formation of nitrous oxide, NO2− and oxidized CuIINO species. This behaviour strongly supports the hypothesis that a redox mechanism is operating in the nitric oxide decomposition reaction leading to nitrogen and oxygen.

FOURIER-TRANSFORM INFRARED AND RAMAN SPECTRA OF PURE AND AL-, B-, TI- AND FE-SUBSTITUTED SILICALITES : STRETCHING-MODE REGION

A systematic investigation of the IR and Raman spectra of pure and Al-, B-, Ti- and Fe-substituted silicalites in the SiO stretching region (1500–700 cm–1) is presented. As well as the characteristic stretching modes of the skeleton, silicalites containing hydroxylated nests show also broad bands at ca. 960 cm–1(IR) and at ca. 976 cm–1(Raman), associated with O3Si—OH group modes, with prevailing Si—OH stretching character. The replacement of Si with heavier elements (like Ti or Fe) causes the appearance of new IR- and Raman-active modes (i) at 960 cm–1(IR and Raman) and at 1127 cm–1(Raman) in Ti silicalite; (ii) at 1015 cm–1(IR) and at 1020 cm–1(Raman) in Fe silicalite. Neither the Raman nor the IR spectra of the skeletal modes are substantially modified by the introduction of Al (ZSM5). The presence of boron induces the appearance in the IR spectra of a complex absorption at 1380 cm–1 and at 960–930 cm–1, which corresponds to the absorptions at 1417 and 976 cm–1 in the Raman spectra. The assignment of the absorptions associated with heteroatoms and hydroxylated nests is discussed in detail.

Framework and Extraframework Ti in Titanium-Silicalite: Investigation by Means of Physical Methods

Abstract The application of UV-Vis diffuse reflectance, IR transmission, Raman and ESR spectroscopies to the determination of extraframework and framework Ti in Titanium-silicalite is discussed in detail. The four methods give complementary information. In particular the utility of the UV-Vis and Raman spectroscopies to detect extraframework Ti is definitely established, while IR proves to be more useful for framework Ti. ESR spectroscopy can be also utilized on reduced samples to distinguish between the two types of Ti.

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.

Surface Structures of Oxides and Halides and Their Relationships to Catalytic Properties

In this review, the relationships between structure, morphology, and surface reac-tivity of microcrystals of oxides and halides are assessed. The investigated systems we discuss include alkali halides, alkaline earth oxides, NiO, CoO, NiOMgO, CoOMgO solid solutions, ZnO, spinels, cuprous oxide, chromia, ferric oxide, alumina, lanthana, perovskites, anatase, rutile, and chromia/silica. A combination of high-resolution transmission electron microscopy with vibrational spectroscopy of adsorbed probes and of reaction intermediates and calorimetric methods was used to characterize the surface properties. A few examples of reactions catalyzed by oxides are also reported.

An in situ temperature dependent IR, EPR and high resolution XANES study on the NO/Cu+–ZSM-5 interaction

We report an in situ IR, EPR, and high resolution XANES study on the Cu+(NO) and Cu+(NO)2 complexes formed at 80 K in Cu+–ZSM-5. Bare Cu2+ ions (<1%) exhibit an axial EPR spectrum, while bare Cu+ ions exhibit well resolved 1s→4pxy and 1s→4pz transitions (pxy/pz splitting of 3.1 eV), reflecting the same local symmetry. Cu+(NO) complex exhibits an anisotropic EPR signal, while XANES spectrum of dinitrosyl adducts presents three resolved components ascribed to the 1s→4px, 1s→4py, and 1s→4pz transitions (px/py and py/pz splitting of 3.8 and 2.6 eV, respectively) testifying the rupture of the axial symmetry of Cu+ bare cations upon NO adsorption. Upon increasing the temperature to 300 K, NO oxidizes a fraction of 75–85% of the cuprous ions.

The role of Al in the structure and reactivity of iron centers in Fe-ZSM-5-based catalysts: a statistically based infrared study

Abstract A statistical analysis of the infrared data obtained upon absorption of NO on a large number of Al-free Fe-silicalite and Fe-ZSM-5 samples highlighted the influence of Al on the formation and stabilization of extraframework iron species and shed more light on the superior activity of Fe-ZSM-5-based catalysts. It was concluded that Al favors the dispersion of extraframework iron species and that isolated Fe II species with one or two Al atoms in the immediate vicinity are the active sites in partial oxidation reactions. The structure of the α -oxygen species adsorbed on iron centers is also briefly discussed.

Acetylene, methylacetylene and ethylacetylene polymerization on H-ZSM5: a spectroscopic study

Acetylene, methylacetylene and ethylacetylene interact with the Bronsted acid sites of H-ZSM5 with the formation of hydrogen-bonded (precursor) species characterized by well defined IR properties. These precursors are then protonated to give intensely coloured carbocationic species. The speed of protonation is in the order: C2H5—CCH > CH3—CCH > HCCH. Insertion of the monomer into the first protonation product, leads to an oligomeric species with carbocationic character. The IR and UV–VIS spectra of the carbocationic species are discussed in detail and found to correspond to those of analogous species generated in the homogeneous phase. The π-delocalization of the positive charge on the backbone is responsible for the peculiar and intense spectroscopic manifestations both in the IR and UV–VIS. The positive charge in the carbocationic species can be captured by bases (NH3, pyridine) and subtracted to π-delocalization. This is accompanied by the disappearance of the spectroscopic manifestations associated with the positive charge and by the appearance of new absorptions associated with neutral oligomeric chains. The process can be fully reversed.The dimensions and shape of the oligomers are discussed in terms of the steric constraints imposed by the zeolitic framework, by means of computer graphics and molecular mechanics modelling.

Interaction of N2, CO and NO with Cu-exchanged ETS-10: a compared FTIR study with other Cu-zeolites and with dispersed Cu2O ☆

Abstract After a brief overview of the reasons why, in spite of the high fraction of framework Ti(IV) atoms, Engelhard titanosilicate (ETS-10) cannot be used as competitive catalyst in partial oxidation reactions, we draw the attention on the fact that the high cation density of ETS-10 can be the key property for potential new catalytic applications of this recent material. Among all, cation exchange with Cu2+ can yield to Cu-ETS-10, a promising material for environmental catalysis. We so present a detailed characterization of this material using N2, CO and NO as probe molecules. In spite of the rather high complexity of the obtained spectra, a comparison with similar experiments (described in the literature or ad hoc performed for this work) on other Cu-exchanged zeolites and on Cu2O dispersed on silica and on MCM-41, allows a full interpretation of the spectroscopic properties. It is shown that copper is present both as counterion and in the form of Cu2O nanoclusters dispersed in the ETS-10 channels and in the external surface. Finally, IR spectroscopy has been used to demonstrate that Cu-ETS-10 is active in the decomposition of NO.