Francesco Geobaldo

DRS UV-VIS AND EPR SPECTROSCOPY OF HYDROPEROXO AND SUPEROXO COMPLEXES IN TITANIUM SILICALITE

The most important spectroscopic features in the UV-Vis of framework and extraframework Ti(IV) in titanium silicalite are briefly summarized. The spectroscopic manifestations of the complexes formed by framework Ti(IV) in presence of H2O2 are reported. The formation of EPR active species is also considered.

The vibrational spectroscopy of H2, N2, CO and NO adsorbed on the titanosilicate molecular sieve ETS-10

Dihydrogen, dinitrogen, carbon monoxide and nitric oxide have been adsorbed, at nominally liquid nitrogen temperature, on Na+- and K+-exchanged ETS-10. IR spectroscopy shows formation of M+···(H2), M+···(N2)n, M+···(CO)n and M+···(NO) (n=1, 2, . . . ; M+=Na, K) adducts prevalently involving alkali-metal cations located in the 12-membered channels. These adducts give main IR absorption bands in the range 4050–4150 cm-1 for H2, 2331–2333 cm-1 for N2, 2148–2176 cm-1 for CO, and 1820–1900 cm-1 for NO, which are assigned to the fundamental stretching mode of the diatomic molecules polarized by the electric field created by the metal ions. On Na-exchanged samples, the Na+···(N2) and Na+···(CO) species, formed at lowest dosage, evolve into Na+···(N2)n and Na+···(CO)n (n=2, 3) species upon increasing the gas phase pressure. This reversible ‘solvation’ process is not observed for K-exchanged samples. For adsorbed CO, the high intensity of the IR spectra allowed us to observe and assign overtone bands and combination modes. This result does not find a comparable precedent for CO adsorbed on other zeolites. From bands corresponding to the combination of metal–carbon with carbon–oxygen fundamental stretching modes, the wavenumber values corresponding to metal–carbon stretching vibrations in Na+···CO and K+···CO were deduced: 122 and 107 cm-1, respectively. Due to the strong tendency of NO towards dimerization, IR spectroscopy of adsorbed NO is complicated by the manifestations of cis and trans dimers interacting with alkali cations, and Na+···(NO) and K+···(NO) species could be distinctly observed only at the very initial stages of the adsorption process. Finally, as far as the H2/ETS-10 system is concerned, the most relevant result is that the ortho and para forms are clearly detected in the Na+-ETS-10 sample.

STRETCHING FREQUENCIES OF CATION-CO ADDUCTS IN ALKALI-METAL EXCHANGED ZEOLITES : AN ELEMENTARY ELECTROSTATIC APPROACH

The C–O stretching frequencies, obtained by a 77 K Fourier transform infrared study of the interaction of CO with the extraframework cations in alkali–metal exchanged M‐ZSM‐5 and M‐mordenites (M=Na,K,Rb,Cs) have been used to evaluate the local electric field strength tested by the probe molecule. For all the investigated zeolites, the dipolar CO molecule was found to interact specifically with extra‐framework metal ions, and its stretching frequency was found to increase continuously from Cs+ to Na+ samples. The observed frequency shifts, with respect to that of the free CO molecule (νCO=2143 cm−1), were used to evaluate the electrostatic field created by the cation sites. Electric fields in the range of 2 to 6 Vu2009nm−1 were obtained; experimental values are inferior to those expected if generated in vacuo, at the distance of the probe molecule, from a point charge of +‖e‖. The observed fields can be reproduced by the sum of a positive contribution of the cation itself, and by two negative contributions, t...

XANES, EXAFS and FTIR characterization of copper-exchanged mordenite

Over-exchanged CuI-mordenite has been prepared by gas phase reaction between the protonic form of the zeolite and CuICl. No evidence was found for the presence of CuII ions or copper species in microaggregates. EXAFS analysis revealed that CuI ions in the outgassed zeolite are surrounded by 2.5 ± 0.35 oxygen atoms of the zeolite framework, the average CuI–O distance being 1.96 (±0.02) A. Both X-ray absorption and FTIR spectroscopy showed that the zeolite adsorbs CO at room temperature with formation of CuI(CO)n adducts (n = 1, 2). At liquid nitrogen temperature, tricarbonyl species were also formed. The IR spectra of adsorbed CO showed broad and complex bands, which is consistent with the fact that the zeolite contains different types of cation sites accessible to carbon monoxide. This was further demonstrated by comparison with corresponding IR and EXAFS spectra of CO adsorbed on CuI-ZSM-5.

Infrared studies of the interaction of carbon monoxide and dinitrogen with ferrisilicate MFI-type zeolites

Fourier transform IR spectroscopy of CO and N2, adsorbed at liquid nitrogen temperature, was used to characterize an MFI-type H-FeZSM-5 ferrisilicate which was synthesized with a Si/Fe ratio of 50. Thermal treatment of this material at 773 and 973 K was performed in order to follow formation of extraframework species. On samples fired at 773 K Brønsted acid sites were present which gave an O-H stretching band at 3630 cm−1. These hydroxyl groups formed adducts with both probe molecules, which were monitored by the corresponding bathochromic shift, Δν. Corresponding values for adsorbed CO and N2 were Δν = −270 and Δν = − 100, respectively. Increasing the firing temperature up to 973 K led to complete removal of iron from the zeolite framework, and consequent disappearance of Brønsted acidity. In this process, extraframework iron oxide species were formed which were also characterized by IR spectroscopy of the adsorbed probe molecules. Both CO and N2 gave Lewis-type adducts with coordinatively unsaturated Fe3+ ions present in the extraframework material. A comparison is made with results of a previous study on the H-GaZSM-5 isomorph.

Ammoximation of Cyclohexanone on Titanium Silicalite: Investigation of the Reaction Mechanism

Abstract In a series of combined adsorption, reactivity and spectroscopic experiments in solution with ketones of different size and shape, the fundamental role of the hydroxylamine pathway in the cyclohexanone ammoximation is established. In presence of H 2 O 2 and NH 3 , framework Ti(IV) forms octahedrally distorted aquohydroperoxo and ammonia-hydroperoxo mixed complexes: the latter one being the precursor of the hydroxylamine formation. This intermediate immediately reacts with cyclohexanone in the channels or diffuses to the external solution where it gives oximation reaction with larger ketones which could not penetrate the channels.

AN EPR STUDY ON THE FORMATION OF THE SUPEROXIDE RADICAL ION ON MONOCLINIC ZIRCONIA

Abstract Zirconium oxide is capable of forming and stabilizing the Superoxide radical ion O 2 − at its surface according to various routes. These include both an electron transfer from the solid to molecular oxygen and the capture of superoxide ions formed along a different surface process like the catalytic disproportionation of hydrogen peroxide. The EPR spectrum is characterized by an anisotropic g tensor with g zz = 2.0336, g yy = 2.0096 and g xx = 2.0034. A less abundant species is sometimes observed with g zz = 2.0255. The superoxide formation from molecular oxygen is favored by partial rehydration of the surface.

The interaction between hydrogen peroxide and metal oxides: EPR investigations

The interaction of hydrogen peroxide with various metal oxides has been followed by EPR in the aim of investigating the paramagnetic species formed along the complex processes of H2O2 decomposition and interaction with the oxide. Data are reported about three systems (MgO, ZrO2 and Me2+/Y zeolite) which are representative of three classes of oxides (insulators, semiconductors and aluminosilicate molecular sieves). Several radical species have been put into evidence in the various cases. The superoxide O2− ion has been observed in all cases while in the case of MgO also the OH radical and the O− radical ion have been identified. In the case of Zn2+/Y zeolite, beside the superoxide radical anion, a new signal is observed which shows up upon treating the sample at 640 K and which is preliminarily assigned to a species in the triplet state in the zeolitic cavity.

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.

Host- guest interactions in zeolite cavities

Abstract When molecules of low-medium proton affinity (B = N2, CO, C2H4, C2H2, propene, methylacetylene, acetonitrile and H2O) are adsorbed on acidic zeolites, the resulting host-guest interactions in the cavities and in the channels are dominated by hydrogen bonding in neutral Z-H…B adducts between the Bronsted groups Z-bH (Z = zeolite framework) and the base. If the interaction is sufficiently strong and the protonated products stable (ethylene, acetylene, propene and methylacetylene) the hydrogen bonded precursors are slowly consumed with formation of olygomeric products entrapped into the zeolite cavities. Acetonitrile gives a stronger hydrogen bonding interaction not followed by protonation. Water initially gives neutral complexes; however when more than one water molecule per protonic site is present, H5O2+·nH2O species are formed. When molecules of high proton affinities are adsorbed (NH3 and Py), B-H+···Z− ionic pairs readily appear. The modifications induced on the spectrum of B-H+ by interaction with Z− can be used to probe the basicity of the negatively charged framework. At higher filling conditions B-H+···B dimers are also observed. The basic IR spectroscopy of all these hydrogen bonded systems is discussed in detail.