Lattice vibrational frequencies zeolite

The recent studies on the relationship between activation temperature and carbonium ion type catalytic activity of both decationized and cation exchanged zeolites show that at arid above the temperature required for the removal of all observable hydroxyls with vibrational frequencies between 3700-3500 cm" the activity sharply declines. The lowest concentration of acidic lattice hydroxyl required for carbonium ion activity seems to depend on the reaction involved. For example, dehydroxylation of La-exchanged Y to a level at which hydroxyl content was unobservable by currently-used infrared techniques led to total loss of activity to crack n-butane, but only partial loss of activity to crack cumene (vide infra) and to alkylate toluene with propylene (74). The activity and hydroxyl content lost on dehydroxylation can be restored upon subsequent treatment with water (11). Furthermore, alkali metal zeolites, which have little or no carbonium ion type activity can be made to show strong activity by the addition of a proton source, such as alkyl chlorides (51, 58). The similarity of the products obtained with the... 

The vibrational frequencies of the so-called lattice modes of aluminosilicate zeolites (stretching and bending modes of the T-0 linkages, plus specific vibrations of discrete structural units) were first studied in detail by Flanigen more than 20 years ago [21], The lattice modes are sensitive to both the composition of the lattice and the structure. For example, Jacobs et al. showed that the T-0 stretching... 

The sensitivity of lattice modes to structural changes is illustrated by the recent study of Mueller and Connor [25] on the effects of cyclohexane adsorption on the structure of MFI zeolites. The adsorption of molecules such as paraxylene and benzene into MFI zeolites causes a structural transition from monoclinic to orthorhombic symmetry, which has been characterized by X-ray powder diffraction and 29 si NMR spectroscopy [26]. Cyclohexane has a slightly larger kinetic diameter than benzene or paraxylene (0.60 nm compared with 0.585nm), but does not cause the same structural transition. Cyclohexane adsorption does however affect the zeolite lattice mode vibrational frequencies. Figure 7 shows spectra taken from reference 25 before and after (upper spectrum) adsorption of cyclohexane in a low aluminium MFI zeolite. 

The influence of exchangeable monovalent cations on the framework vibrations for the hydrated zeolites Linde A and X has been investigated. An approximately linear relationship is found between the frequency of some absorption bands and the inverse of the sum of the cation and framework oxygen ionic radii. It is proposed that the shift in framework vibrations is largely caused by those cations which are strongly interacting with the zeolite framework. Thus the linear relationship indicates that these monovalent cations are all similarly sited in the zeolite lattice. This is consistent with the presently available x-ray analyses on some of these zeolites. Since Rb + and Cs + are only partially exchangeable in both Linde A and Linde X, these cations deviate from this linear relationship. 

The exchangeable monovalent cations have a marked influence on the framework vibrations of hydrated Linde A and X. For some vibrational modes the frequency shifts appear to give a quantitative measure of the interaction between cations and lattice. A regularity is found for Li+, Na+, Ag+, K+, and T1+ exchanged forms which implies a similar distribution of cation sites for both zeolites. It is further deduced that in the Cs+ and Rb+ exchanged forms there is only a relatively weak interaction between the cations and the zeolite framework. This technique can be readily extended to study cation siting in other zeolites in both hydrated and dehydrated forms. 

Introduction of aluminium into a zeolite lattice broadens the lattice modes, but also introduces additional bands in the Raman spectra at low frequencies due to cation vibrations, completely analogous to the far infrared bands described in section 3.3. Figure 18 shows, for example, Raman spectra taken from the work of Bremard and Le Maire [53] of zeolite Y exchanged with different alkali metal cations. The arrows indicate bands assigned to translational modes of the cations these move to lower frequency as the mass of the cations increases, just as in the far infared spectra. 

Assuming an ionic-like interaction potential between the framework atoms and cations, Smirnov et al. performed an MD study to assess the cation dynamics in zeolite A. The calculation of the power spectra for Na" and cations at each site revealed that no specific spectral pattern can be attributed to a particular cation position. Vibrations of cations in all sites occur over a frequency region of 30-300 cm In addition, the spectra calculated with a flexible framework showed a substantial coupling between the cationic and lattice degrees of freedom. The results of this work °° have brought into question the assignments proposed for the bands in the far-infrared spectra of cationic forms of zeolites. 

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