Theory of Broensted acidity in zeolites

The nature of the chem. bond of protons in a zeolite is analyzed from theor. and spectroscopic results. Of interest is the dependence on zeolite structure as well as compn. The zeolitic OH bond is mainly covalent. Proton attachment to the zeolite lattice causes a weakening of neighboring Si-O and Al-O bonds. The effective increase in vol. of the bridging O atom causes a local deformation, that changes the strength of the lattice-chem. bonds over a few bond distances. Proton concn. effects as well as lattice-compn. effects can be understood from the lattice-relaxation model. The energetics of proton transfer is controlled by the need to stabilize the resulting Zwitter-ion. The pos. charge on the cation becomes stabilized by contact with basic lattice-O atom

Spectroscopy, energetics and siting of NH+4 in zeolites; theory and experiment

The adsorption of an NH3 mol. on an acidic zeolite and the proton transfer from the zeolite to the NH3 are studied by quantum chem. methods. The NH+4 is adsorbed with 2 or 3 H bonds to the zeolite. The calcd. vibrational frequencies explain the exptl. IR spectra. [on SciFinder (R)

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

Modeling of structure and vibrational spectra of aluminophosphate AlPO4 5 and its silica analog SSZ 24

In this study, the structural and vibrational properties of the AlPO4 structure AlPO4-5 and the silica structure SSZ-24 are compared. Lattice energy calcns. are done using existing potential parameter sets suitable for silicas and AlPO4's. For the computation of vibrational spectra of silica systems, force consts. derived by Etchepare et al. are used. For AlPO4 spectra simulations, a new force field is presented that is based on a fit on vibrational frequencies of a-berlinite, the AlPO4 analog of a-quartz. Lattice energy calcns. result in a symmetry of AlPO4-5 and SSZ-24 that is lower than derived exptl. A shift of layers is obsd. for both structures when a potential with partial charges is used. These results are indicative for an underestn. 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 AlPO4-5 and SSZ-24 are due to the interat. force const

A Quasi-Elastic Neutron Scattering Study of the Ammonium Ions in CsNH4-Y Zeolite

Quasi-elastic neutron scattering was used to study the reorientation mechanism for ammonium ions in Cs-exchanged zeolite Y. This sample contains ammonium ions preferentially located in the sodalite cages at site 1'. These ions reorient with 120 or 180 Deg jumps around fixed axes through the N atom. This corresponds to a bonding geometry in which the ammonium ion interacts via 3 or 2 H atoms with the lattice O atoms. Addnl., a reorientation around a fixed axis through 2 H atoms of the ammonium ion may occur. The ammonium ion then jumps between 2 equil. positions: one position involves a 2-fold bonding geometry and the other a 3-fold bonding geometry. At 300 K the effective time interval between 2 jumps is t = 1.2 +- 0.5 ps. Compared to zeolite Rho, the ammonium ions in the Cs-exchanged zeolite Y show a more restricted reorientational behavior, resulting in a larger elastic incoherent structure factor. [on SciFinder (R)

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-lattic

Inelastic neutron scattering study of NH4Y zeolites

An inelastic neutron scattering study of NH4Y zeolites is presented 2-350 meV (16-2800 cm-1). The spectra are interpreted in terms of translational and librational motions of ammonium ions and NH3 and H2O mols. For the hydrated samples, the translational modes of H2O species are obsd. at 12 meV and librational modes are found at .gtorsim. 50 meV. The translational and librational modes of the ammonium ions also depend on the cation locations. Ammonium species give rise to (ion-lattice) translational modes at 10-15 meV and at 15-25 meV for species localized in sodalite cages and supercages, resp. The corresponding librational modes are obsd. at .apprx. 8 meV and in the region 3-6 meV, resp. Strongly hindered librations are obsd. at 50-70 meV and 30-50 meV, resp., for the ions in the 2 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 obsd. 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 NH3, librational and translational motions of NH3 species can be obsd. A heterogeneity in the reorientational barriers is present, indicating the presence of different NH3 species. [on SciFinder (R)