The IR transmission windows of hydrogen bonded complexes in zeolites: A new interpretation of IR data of acetonitrile and water adsorption on zeolitic Broensted sites

The 2800, 2400 and 1700 cm-1 infrared OH bands recently found upon adsorption of many basic molecules on Bronsted sites of zeolites, including acetonitrile and water, are suggested to be analogous to the (A,B,C) triplet of medium-strong molecular X-OHY complexes in vapours, liquids and solids. IR studies of CD3CN and CCI3CN adsorptions on H-ZSM5, H-FeSil, H/D-ZSM5 and H/D-FeSil zeolites show these bands to be in a good agreement with the resonance theory of the (A,B,C) triplet, developed for molecular H-complexes. A new interpretation of IR data of water adsorption on zeolites, using this theory and results of ab initio calculations, suggests the water complex with the bridging OH group of zeolites to be "neutral", the one water OH group interacting by hydrogen bonds both with the bridging OH group and with the nearest AlOSi oxygen and the other OH group being free

Fourier-transform infrared and inelastic neutron-scattering study of hy zeolites

A combination of FTi.r. and INS spectroscopy is used in a vibrational study of the bending and stretching vibrations of the acidic hydroxyl groups of Y zeolites. The influence of the number of acidic Bronsted sites and the Si/Al ratio is discussed. Out-of-plane hydroxyl bending modes are assigned to vibrations centered around 419 cm-1 and in-plane hydroxyl bending modes are assigned to vibrations centered around 1089 cm-1. Upon dealumination, these bands are shifted by approximately 30 cm-1 to lower values. The less intense bands at 319,470,565,765, and 1130 cm-1 are assigned to proton- coupled framework vibrations. Upon dealumination, the mode at 319 cm-1 is shifted to lower frequencies and the modes at 565 and 1130 cm-1 are shifted to higher frequencie

Fourier transform infrared study of the protonation of the zeolitic lattice- influence of silicon - aluminium ratio and structure

An in situ Fourier-transform infrared study is presented for the spectral region from 250 to 1400 cm-1. Using supported samples the changes for the infrared-active lattice modes after protonation are studied for zeolites A, X, L, ZK-5, mordenite and for zeolites Y with Si : Al ratios from 2.4 to 40. Protonation of the lattice results in a shift to higher wavenumbers for the asymmetric and symmetric T-O stretching modes (where T = Al or Si). These shifts are attributed to coupling between T-O lattice modes and in-plane bending modes of acidic hydroxyls. Inter-tetrahedral and intra-tetrahedral T- O stretching modes show a different coupling. Coupling between T-O lattice modes and out-of-plane hydroxyl bending modes appears to be much weaker. Protonation further results in changes of the infrared band intensities for the symmetric T-O stretching modes and for several modes in the region below 650 cm-1. The changes observed for the asymmetric T-O stretching modes are a function of the proton concentration and do not depend significantly on the lattice structure. Changes after protonation for the symmetric T-O stretching modes and especially for the modes in the region below 650 cm-1 are most sensitive to variation in zeolite structur

An in situ Fourier transform infrared study of zeolitic vibrations : dehydration, deammoniation, and reammoniation of ion-exchanged Y zeolites

Dehydration, deammoniation, and reammoniation of ion-exchanged Y zeolites are studied in situ with FTi.r. spectroscopy in the range 40-4000 cm-1. This broad spectroscopic range enables the monitoring of the cationic vibrations, the zeolitic lattice modes, the bending and stretching modes of ammonium ions, and adsorbed water molecules and the stretching modes of the acidic hydroxyls under experimental conditions. Dehydration of zeolites with monovalent cations does not lead to significant migration of these cations and only minor changes are observed for the lattice modes. Dehydration of zeolites with divalent cations results in a migration of these cations into the hexagonal prisms, giving rise to large changes for the zeolitic lattice modes. Deammoniation results in the formation of acidic hydroxyls by protonation of the lattice, which strongly affects the zeolitic lattice modes. The extent of the changes for the lattice modes is related to the amount of protons present. The observed changes for the lattice modes can be explained by an increase in electrostatic interactions between the cations and the zeolitic framework, by deformations of the flexible zeolitic lattice, and by coupling of the lattice modes with the in-plane and out-of-plane bending modes of the acidic bridging hydroxyl group

The Mechanism of the Copper Ion Catalyzed Autoxidation of Cysteine in Alkaline Medium

Quantitative e.s.r. measurements carried out during the copper catalysed alkaline autoxidation of cysteine show that the Cu(II)-dicysteine complex represents almost the total amount of copper. Only a small fraction (<2%) of the copper ions might be present in a state which is not detectable by e.s.r.The kinetics of the catalytic reaction obeys the rate expression The results cannot be explained by a simple Cu(II)/Cu(I) redox mechanism.Instead, a reaction model is proposed, which is based upon the involvement of a thiyl- and a superoxo-activated Cu(II) complex, respectively. These two types of complexes are operative in a chainlike propagation cycle