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Theory of Brønsted Acidity in Zeolites

The nature of the chemical bond of protons in a zeolite is analysed on the basis of theoretical and spectroscopic results. 0f interest is the dependence on zeolite structure as well as composition. The zeolitic OH bond is mainly covalent. Proton attachment to the zeolite lattice causes a weakening of neighbouring Si-O and Al-O bonds. The effective increase in volume of the bridging oxygen atom causes a local deformation, that changes the strength of the lattice-chemical bonds over a few bond distcinces. Proton concentration effects as well as lattice-composition effects can be understood on the basis of the lattice-relaxation model. The energetics of proton transfer is controlled by the need to stabilize the resulting Zwitter-ion. The positive charge on the cation becomes stabilized by contact with basic lattice-oxygen atoms.

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

Abstract Dehydration, deammoniation, and reammoniation of ion-exchanged Y zeolites are studied in situ withFTi.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 groups.

Fourier transform infrared and inelastic neutron scattering study of HY zeolites

Abstract A combination of FT i.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 Bro¨nsted sites and theSi/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 frequencies.

A vibrational study of the OH and OD bending modes of the Brönsted acid sites in zeolites

Abstract Deuteration of Bronsted acid sites in zeolites and SAPOs results in in-plane OD bending modes that can be studied with FT i.r. spectroscopy. A band maximum near 870 cm −1 and a shoulder near 890 cm −1 are related to the high-frequency (HF) and low-frequency (LF) OH or OD stretching modes observed for zeolites Y and ZK-5 and for SAPO-17 and SAPO-34. For mordenite. zeolite β, ZSM-5, and SAPO-11, only one type of bending mode is observed near 890 cm −1 . Structure type and composition influence the frequencies for these bands. Increasing the sodium content of the samples results in a shift of the fundamentals to lower frequencies. Increasing the Si AI ratio shifts the bands to slightly higher frequencies. The in-plane bending modes are more sensitive to structural differences than are the corresponding stretching modes. The first overtones of the OD bending modes are observed near 1580 cm −1 for the zeolites and near 1540 cm −1 for the SAPOs. Simulations of the inelastic neutron scattering spectra indicate that the in-plane bending modes are localized modes, whereas the out-of-plane bending modes are coupled to the lattice modes. The in-plane OD bending modes are shifted to higher frequencies due to Fermi resonance with the zeolite TO lattice modes. The observed shift is 80 cm −1 for the zeolites and 110 cm −1 for the SAPOs.

Lattice relaxation of zeolites

Quantum-chemical cluster calculations as well as solid-state chemical lattice-calculations indicate that zeolitic SiO2- and AlPO4-structures are flexible structures. The structures reflect the subtle balance of electrostatic and covalent interactions. The different electrostatic interactions lower the symmetry of layered AlPO4-structures compared to that of the corresponding SiO2-compounds. The result is a smaller zeolite-channel dimension for the AlPO4-structure compared to that of the corresponding SiO2-network. Deprotonation of the zeolite-lattice leads to large local changes in geometry that changes acidity compared to that predicted for a non-flexible lattice. Changes in lattice vibrational frequencies are consistent with the theoretically predicted relaxation of the zeolite-lattice.

Modeling of structure and vibrational spectra of AIPO4-5 and its silica analog SSZ-24

Abstract In this study, the structural and vibrational properties of the AlPO 4 structure AlPO 4 -5 and the silica structure SSS-24 are compared. Lattice energy calculations are done using existing potential parameter sets suitable for silicas and AlPO 4 s. For the computation of vibrational spectra of silica systems, force constants derived by Etchepare et al. are used. For AlPO 4 spectra simulations, a new force field is presented that is based on a fit on vibrational frequencies of α-berlinite, the AlPO 4 analog of α-quartz. Lattice energy calculations result in a symmetry of AlPO 4 -5 and SSZ-24 that is lower than derived experimentally. A shift of layers is observed for both structures when a potential with partial charges is used. This shift is not found for a formally charged shell model calculation of SSZ-24. These results are indicative for an underestimation of the charges used in the partial charge model. The influence of structure on spectra is shown to be rather weak. The main differences between the spectra of AlPO 4 -5 and SSZ-24 are due to the interatomic force constants.

Infrared spectroscopic study of the bending modes of the Brønsted acidic sites in deuterated zeolites

Abstract Deuteration of Bronsted acidic sites in zeolites gives observable infrared in-plane OD bending modes. For zeolite Y and ZK-5, a maximum near 870 cm −1 with a shoulder near 890 cm −1 is observed. These signals are related to, respectively, the high-frequency and the low-frequency stretching modes. Mordenite, zeolite β and ZSM-5 possess a single mode near 890 cm −1 . The zeolite structure type and composition influences the position of the modes strongly. A large anharmonicity parameter, x OD δδ (−80 cm −1 ), is found due to Fermi resonance with the zeolite lattice modes.

Inelastic neutron scattering study of NH4Y zeolites

An inelastic neutron scattering study of NH4Y and CsNH4Y zeolites is presented from 2 to 350 meV (16–2800 cm–1). The spectra are interpreted in terms of translational and librational motions of ammonium ions, ammonia and water molecules. For the hydrated samples the translational modes of water species are observed at 12 meV and librational modes are found above ca. 50 meV. The translational and librational modes of the ammonium ions also depend on the location of the cations. Ammonium species give rise to (ion–lattice) translational modes at 10–15 meV and at 15–25 meV for species localised in sodalite cages and supercages, respectively. The corresponding librational modes are observed at ca. 8 meV and in the region 3–6 meV, respectively. Strongly hindered librations are observed at 50–70 meV and 30–50 meV, respectively, for the ions in the two different cages. Low-frequency as well as high-frequency librational modes of the ammonium species may occur caused by the presence of different ammonium species.Differences in reorientational motions are observed for hydrated zeolites and for reammoniated zeolites. For the latter, a stronger interaction of the ammonium ions with the zeolitic lattice is present and the low-frequency librational modes are shifted to higher energy-transfer values.When increasing the loading with ammonia, librational and translational motions of ammonia species could be observed. Again, a heterogeneity in the reorientational barriers is present, indicating the presence of different ammonia species.

Vibrational relaxation of hydroxyls in Zeolite Y: H-bonding, dipole-dipole coupling and accepting modes

Excited vibrational population lifetimes of hydroxyls in zeolite Y are shown to provide insight in the (dynamic) coupling of the hydroxyls to the zeolitic lattice as well as to other O—H oscillators.

SPECTROSCOPY, ENERGETICS AND SITING OF NH+4 IN ZEOLITES; THEORY AND EXPERIMENT

ABSTRACT: The adsorption of an ammonia molecule onto an acidic zeolite and the proton transfer from the zeolite to the ammonia are studied by quantum chemical methods. The NH+4 is adsorbed with two or three hydrogen bonds to the zeolite. The calculated vibrational frequencies explain the experimental infra–red spectra.