Gemma Turnes Palomino

Oxidation States of Copper Ions in ZSM-5 Zeolites. A Multitechnique Investigation

The redox behavior of Cu/ZSM-5 zeolites prepared by ion exchange from Cu2+ aqueous solutions has been followed by a variety of spectroscopic techniques to provide a thorough picture of the so-called self-reduction of cupric ions, which occurs upon dehydration of the hydrated system at various temperatures, and of the reverse process of reoxidation as well. Conflicting hypotheses on both these processes are, in fact, present in the literature. The experimental techniques employed in this work are electron paramagnetic resonance (EPR), IR, and optical spectroscopies, extended X-ray absorption fine structure, and X-ray absorption near-edge structure. The early stages of dehydration (from room temperature to about 470 K) involve cupric ion migration and formation of EPR silent moieties but no reduction to Cu+. The onset of this latter phenomenon starts at 470 K and, in the range 470-670 K, involves the majority of copper ions present in the system. Rehydration of Cu+-containing samples does not cause direct Cu+ oxidation to Cu2+ but favor this latter process when O2 is used as oxidant. Oxidation of Cu+ to Cu2+ by molecular oxygen, in fact, does not take place at room temperature if O2 is contacted with the dehydrated material but easily occurs when oxygen is adsorbed on rehydrated samples at the same temperature.

Spectroscopic study in the UV-Vis, near and mid IR of cationic species formed by interaction of thiophene, dithiophene and terthiophene with the zeolite H-Y

The vibrational (in the mid-IR and near-IR) and the UV-Vis spectra of products formed upon interaction of thiophene with H-Y, show the initial formation of neutral hydrogen bonded adducts. In the presence of thiophene excess, protonation slowly occurs with formation of C4H5S+ followed by oligomerization leading to C8H7S2+, C12H9S3+ and more complex species. This conclusion is fully confirmed by the study of the direct interaction of H-Y with dithiophene and terthiophene, where protonated C8H7S2+ and C12H9S3+ are produced in the initial interaction stages. The ascertained occurrence of intramolecular and intermolecular hydrogen transfer points towards a network of complex reactions leading to the final formation of some families of similar isomers. Some of the protonated (oligomeric) species interact via strong hydrogen bonds with the unreacted Bronsted groups of H-Y. The positively charged species can easily react with NH3 with the formation of NH4+ and neutral species. At room temperature the process is partially reversible.

Calorimetric and spectroscopic study of the coordinative unsaturation of copper(I) and silver(I) cations in ZSM-5 zeolite: Room temperature adsorption of NH3

Abstract The room temperature (RT) adsorption of NH 3 was used to probe the coordinative unsaturation of Cu(I) and Ag(I) cations highly dispersed in ZSM-5 zeolites. Adsorption microcalorimetry allowed to characterise both quantitatively and energetically the amino complexes formed at the cationic sites, as revealed by IR spectroscopy. Copper(I) sites were found to form tetra-amino [Cu(NH 3 ) 4 ] + adducts, whereas silver(I) ones do form only di-amino [Ag(NH 3 ) 2 ] + species. Both results are in agreement with the homogeneous chemistry of Cu(I) and Ag(I) cations. The heat of formation of the different amino-complexes was found to be comprised in the 130–50xa0kJ/mol interval for both kind of Me(I) sites, according to the number of ligands progressively bound and in spite of the different stoichiometry of the species formed. The adducts were found to be only partially reversible upon outgassing in the adopted conditions (303xa0K and p ≈10 −5 xa0Torr). Two distinct, nearly equally populated families of sites were revealed in both Me(I)-ZSM-5 samples, corresponding to different coordinative unsaturations of the metal cations. EXAFS data, used to check the local environment of noble metal cations, confirmed the proposed model. In the case of Cu(I) sites, the formation of mixed amino-carbonyl complexes was also studied. The thermodynamic stability of amino- and carbonyl-like species was found not to be the same.

FTIR study of CO adsorbed at low temperature on zeolite L

FTIR spectroscopy of CO adsorbed at liquid-nitrogen temperature on zeolite K-L revealed two main IR absorption bands at 2161 and at 2150 cm n -1 n, respectively. These bands were assigned to the C–O-stretching mode of carbon monoxide interacting, through the carbon atom, with K n + n ions present in the main channel of the zeolite and which are present in two different environments. This strongly suggests (in agreement with previous NMR studies) an ordered partition of aluminium atoms in the zeolite framework. When zeolite K-L was partially exchanged with Na n + n ions, the corresponding IR absorption bands were observed at 2174 and at 2157 cm n -1 n. Minor bands, centred at 2137 and ca. 2121 cm n -1 n for CO adsorbed on both zeolite K-L and zeolite Na, K-L were also observed and discussed.

EXAFS Study of Ti, Fe and Ga Substituted Silicalites

The local environment of isomprphous substituted Ti, Fe and Ga metal atoms in silicalite (hereafter indicated as TS-1, Fe-S and Ga-S respectively) has been studied by means of EXAFS spectroscopy. Measurements have been carried out on the samples in presence of template and after template removal at increasing temperatures in order to follow the evolution of the local environment of the metal heteroatoms. Ti(IV) atoms in (TS-1) show a great stability, while Ga(III) and particularly Fe(III) atoms have propensity to migrate from framework into extraframework positions, forming small oxide particles trapped inside the MFI channels upon increasing the template burning temperature. The established stability scale is Ti(IV) > Ga(III) > Fe(III). These conclusions are qualitatively supported by UV-Vis, IR and Raman data.

Amphipathic hydrogen bonding of CO in protonic zeolites

CO interacts with Bronsted acid sites of protonic nzeolites to form both ZH···CO and nZH···OC hydrogen bonds (Z = zeolite framework); by nmeans of variable-temperature FTIR spectroscopy, these hydrogen-bonded nspecies were found to be in a temperature-dependent equilibrium which, for nthe faujasite-type H-Y zeolite, involves an enthalpy change of 4.3 kJ nmol−1.

Zeolite-supported metal carbonyls: sensitive probes for infrared spectroscopic characterization of the zeolite surface

The metal carbonyls [M(CO) n 6 n] (M = Mo or W) nwere found to be molecularly adsorbed on the surface of H-ZSM-5 zeolite ncrystals. Three types of adsorbed species were detected by IR nspectroscopy: weakly physisorbed metal carbonyls and [M(CO) n 6 n] nmolecules interacting (via a CO ligand) with OH groups or with nLewis-acid sites (co-ordinatively unsaturated Al n 3+ n ions). For nphysisorbed species a single C–O stretching mode (T n 1u n) nwas observed, around 1990 cm n -1 n. The (OC) n 5 n nMCO···HOSi nspecies gave a band at 1965 cm n -1 n (anchored CO nligand) and the corresponding E + 2A n 1 n modes at nhigher frequency. O-Bonding between the metal hexacarbonyls and nLewis-acid centres gives rise to a characteristic IR absorption band in nthe range 1750–1850 cm n -1 n, and corresponding nE + 2A n 1 n modes at n n CO n > n 2000 cm n -1 n. Close inspection of these nspectroscopic features, and comparison with IR spectra of adsorbed CO, nenables characterization of the zeolite surface. For medium-pore nzeolites an important feature of this method is that [M(CO) n 6 n] nmolecules cannot penetrate inside the zeolite channels, therefore ndiscrimination between internal and external surface sites can be naccomplished. Thus, it was found that both Bronsted-acid sites n[bridged Si(OH)Al groups] and silanols are located mainly inside the nzeolite channels.

Evidence of very strong [2×ν(NO)] overtones when adsorbing NO in CuI,II-exchanged Y zeolites

In this communication the first vibrational results obtained in transmission mode, concerning NO/Cu-Y systems, containing Cu and Cu I ions, are presented. The IR spectra of these systems are characterized by strong and well defined IR peaks, both in the v(NO) and 2 x v(NO) stretching regions. Plausible physical reasons justifying the unusual and astonishing high intensity of the peaks in the 2 x v(NO) overtone region are discussed.