de Ajm Andries Man

The relation between zeolite framework structure and vibrational spectra

Abstract Using a general valence force field method, infrared and Raman spectra of pure silica zeolite crystals and molecular substructures of zeolites are calculated. Computed spectra of the substructures are directly compared with computed spectra of the crystal. Also, a comparison of the atomic displacement vectors of the vibrational modes is made. No general theoretical basis for a correlation between the presence of large structural elements and spectral features, sometimes reported, is found to exist. An extensive comparison between experimental and theoretical infrared and Raman spectra of the SiO 2 forms of sodalite and faujasite is made. Other applications of vibrational spectroscopy to zeolite structure research, such as the Si O Si angle distribution and the location of acid sites, are evaluated as well.

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.

The stability and vibrational spectra of three-ring containing zeolitic silica polymorphs

Abstract Zeolitic silica polymorphs containing three-membered rings are modeled using three different atomistic potential sets for lattice energy minimization calculations of their structure and using a general valence force field to calculate vibrational spectra. In some cases, large deviations from the experimental symmetry occur upon energy minimization. The lattice energy calculations show that structures with tetrahedra that participate in one or two three-rings can exist in the pure silica form. A hypothetical structure in which all tetrahedrally coordinated atoms are part of a three ring, and with a corresponding very low framework density, is shown to be significantly less stable. The calculated vibrational spectra are very sensitive to small-scale structural features like bond lengths and bond angles. Because three-ring structures generally have small SiOSi angles compared with other zeolites, relatively low-frequency asymmetric stretch models are predicted. No specific spectral peak can be assigned to a three-ring mode. The differences in structural detail computed using the three potential sets for silica produce variations in predicted infrared spectra that are larger than found between different structural polymorphs. This indicates the need for further improvement of potential parameters applicable to structure relaxation.

On lattice dynamics, stability and acidity of zeolites

Theoretical approaches to chemical bonding and the stability of zeolites can be distinguished in quantum mechanical methods based on electronic structure calculations1,2 and methods that start with known potentials3.

Chemical Bonding in Zeolites

The current status of chemical bonding of zeolites and AlPO’s is reviewed. Short range covalent bonding dominates, the tetrahedra have to be considered relatively rigid and the Si-O-Si bond angle flexible. Differences in energy of the SiO2 or AlPO4 polymorphs are small. The relative stability sensitively depends on a proper accounting of the small changes in electrostatic energy. The deprotonation energy is also mainly determined by short range covalent interactions. These are only properly accounted for when full lattice relaxation is included in the calculations. Isomorphous substitution effects are also dominated by changes in covalent interaction energy.

AN INFRARED STUDY OF STRUCTURE RELAXATION IN MOLECULAR SIEVES

ABSTRACT The lattice vibrations of the isostructural analogues AIPO 4 -5 and SSZ-24 have been studied by infrared spectroscopy. The vibrational modes have been assigned based on theoretical relaxed structures using a valence force field model to compute the lattice modes. The influence of the substitution of two silicon atoms by one phosporus and aluminium atom is discussed in relation with the experimental spectra. The influence of protonation and exchangeable cations on lattice vibrations is demonstrated by in-situ infrared transmission studies of the reammoniation of HY and the dehydration of NiY. The effect of substitution of the framework atoms or the extra-framework species on the lattice modes results In relaxation of the zeolitic structure, which can be monitored with infrared spectroscopy. Further in the case of protonation of the lattice coupling between the zeolitic lattice modes and the bending modes of the acidic hydroxyls is observed.