Ag-exchanged Zeolite Research Articles

NH 3-TPD and FTIR spectroscopy of pyridine adsorption studies for characterization of Ag- and Cu-exchanged X zeolites

Pure crystalline NaX zeolites with Si/Al = 1.2 is prepared by the hydrothermal method. The modification is achieved by following the conventional ion-exchange technique to obtain Ag- and Cu-exchanged forms of NaX zeolite with different metal loadings ranging from low to high exchange. Compositional and structural investigations of all samples are performed by atomic absorption spectrometry, elemental analysis, powder XRD, and nitrogen adsorption porosimetry. The acidity of all exchanged zeolites is investigated using both the ammonia temperature-programmed desorption (NH 3-TPD) carried out by microbalance, and the pyridine adsorption performed via Fourier transform infrared spectroscopy (FTIR). XRD analysis shows the global conservation of the zeolitic structure after ionic exchange. The crystallinity loss is more important for zeolites exchanged at high degree of copper. Porosity measurements indicate a decrease of the porous volumes and the specific surface areas when the Ag or Cu exchange degree is raised. Mesopores particularly appear at high exchange degree of copper. NH 3-TPD indicates that the increase of Ag and Cu-exchange degrees leads to an increase of the global acidity, which is more pronounced for Cu-exchanged zeolites. The strength of acid sites for Ag-exchanged zeolites is weaker than for Cu-exchanged zeolites. The FTIR spectroscopy analysis by pyridine adsorption confirms the formation of extra framework aluminum (EFAL) species, inducing an increase in the concentration and the strength of the Lewis acidity. The formation of EFAL is more important for Cu-exchanged zeolites.

Catalytic asymmetric synthesis of a functionalized indolizidine derivative. A useful intermediate suitable for the synthesis of various glycosidase inhibitors

Indolizidine derivative 7 has been sythesized in up 86% ee by an asymmetric Heack reaction (Pd(O)-BPPFOH, Ag-exchanged zeolite) starting with prochiral alkenyl iodide 5. Conversion of 7 to δ-coniceine ( 9) is also described.

Combined ESR-DRS spectroscopies of transition metal ions and metal ion clusters in zeolites

The results of ESR and DRS spectroscopies on zeolites are discussed for three themes: (1) the coordination of Cu 2+ to oxygen six-rings; (2) the formation of Ni +; and (3) of charged Ag-clusters. Cu 2+ trigonally coordinated to oxygen six-rings, is subjected to a dynamical Jahn-Teller effect. Two types of six-rings can be distinguished: the type II six-ring, connecting the supercage with the cubo-octahedron and the type I' six-ring of the hexagonal prism. The spectral properties of Cu 2+ on these two six-rings are independent of the structure type and the chemical composition of the zeolites. Five Ni + species have been identified by ESR in zeolites X and Y. The corresponding DRS spectra have not yet been obtained. The green colour of the partially reduced Ni-zeolites is probably due to diamagnetic Mi-clusters. Paramagnetic Ag-clusters with a nuclearity of one, two and three have been found in Ag-exchanged zeolites A, X and Y after irradiation and—to a lesser extent—after mild reduction with H 2. The Ag 6 + cluster is, up until now, only obtained on zeolite A. In irradiated fully Ag-exchanged samples the cluster is associated with six, seven or eight Ag +, dependent on the residual water content. Upon reduction with H 2, the cluster occurs as an isolated species. The colour changes of Ag-zeolites during activation and reduction have been described, but the assignment of the DRS bands to specific clusters needs independent confirmation.

Comparative electron spin resonance and optical absorption studies of silver-exchanged sodium Y zeolites: silver centers formed on dehydration, oxidation, and subsequent .gamma.-irradiation

Paramagnetic silver centers have been generated in Ag-exchanged zeolite Y with various silver loading by 77 K ..gamma..-irradiation of the zeolites (a) in its hydrated form, (b) following vacuum dehydration at 500/sup 0/C, and (c) following subsequent O/sub 2/ treatment at 500/sup 0/C. The paramagnetic silver centers have been identified by ESR. The data have been interpreted in terms of the silver centers formed during the dehydration and oxidation of Ag-exchanged zeolite Y. The data indicate that dehydration results in extensive clustering of silver species. Subsequent oxidation causes most of the Ag/sup +/ ions to revert to isolated locations in the lattice, although the generation of Ag/sub 2//sup +/ and Ag/sub 3//sup 2 +/ on irradiation suggests significant occupancy, by Ag/sup +/, of adjacent sites. Diffuse reflectance spectra, recorded at room temperature on the same ..gamma..-irradiated samples, allowed some correlations to be made between optical bands and ESR identifiable silver electron capture centers. An absorption at 345 nm, generated on irradiation of oxidized Ag-exchanged zeolite, has been assigned to Ag/sub 2//sup +/. Some doubt is expressed about previous assignments of optical bands to Ag/sup 0/ atoms in Ag-exchanged zeolite Y. 27 references, 4 figures.

Luminescence of Ag-exchanged zeolite 13X

It was found that Ag-exchanged synthetic zeolite 13X shows, under uv excitation, six emission bands with peaks at about 4000 (blue, B 1), 4300 (blue, B 2), 4600 (blue, B 3), 5300 (green), 5700 (yellow) and 6700 (red) Å, respectively. Examination of preparation conditions and excitation spectra led to the conclusions that the three emission bands, blue (B 1), green and red, arise from Ag + centers, while that the other three emission bands, blue (B 2), blue (B 3) and yellow, are due to Ag 0 centers at different sites in 13X lattice.