Zeolite lattice modes

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 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 stretching and bending modes of zeolite lattices have weak Raman cross sections, which makes measuring high quality Raman spectra difficult. Laser induced fluorescence is also a common problem with dehydrated zeolites, although this can be overcome with the Fourier transform technique. As with the corresponding infrared spectra, the frequencies of the Raman active lattice modes depend on both the local structure and the composition of the zeolite lattice. 

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. 

De Man and van Santen ° performed a normal mode analysis of both cluster and periodic models of zeolite lattices using the GVFF developed by Etchepare et al. In an attempt to find a relation between specific normal modes and the presence of particular substructures, de Man and van Santen compared spectra of zeolite lattices with those of lattice substructures, projected eigenvectors of a substructure in the framework onto the eigenvectors of the molecular model of the structure, and constructed the difference and sum spectra of frameworks with and without particular structural units. The study concluded that there is no general justification for correlating the presence of large structural elements with particular features in the vibrational spectra. 

The INS spectra of zeolites have been calculated by force-field methods [106,107]. Note that the hydrogen atoms follow the displacements of the framework atoms due to the lattice modes (480-970 cm ). The lattice modes (Al-0 and Si-O stretching and bending modes) are therefore observed in the INS spectra—another example of hydrogen riding modes. 

The simplest analytic model for an isolated proton in a lattice assumes that it is situated in a potential well centred on an interstitial site. This model is particularly appropriate to protons in a transition metal lattice, where the electron from the hydrogen atom can be accommodated in the d-band of the metal, but is also applicable to many other cases as well - e.g. to molecular hydrogen trapped in ion-exchanged zeolites (see Section 6.8.2 below). The model assumes that there are no interactions between neighbouring hydrogen atoms and that there is little coupling with the lattice modes. This implies that M/wjp 1 where m is the mass of the proton and M is the mass of the lattice atom. In transition metals, with face-centred cubic (FCC), body-centred cubic (BCC) or hexagonal close packed (HCP) lattices, the proton normally sits on either octahedral or tetrahedral sites. In more complex intermetallic... 

The ability of Co° ions in X- and Y-zeolites to form dinitrosylic species has been investigated by means of spectroscopic methods. It has been concluded that, in the complex, the unpaired electron is essentially localized on the Co centre, whose formal oxidation state is very close to zero. The formation of [Co(en)202] adducts by interaction of ethylenediamine and O2 with Co ions in X- and Y-zeolites has been investigated by i.r. and e.s.r. techniques. Due to the presence of the strong i.r. active modes of the zeolite lattice, the stretching mode of the O2 species in the mixed oxygen-ethylenediamine adducts could not be observed. ... 

The mechanism for the polymerisation of acetylene is inherently different from that of aromatic monomers such as pyrrole or thiophene. Whereas the polymerisation of pyrrole or thiophene involves a redox reaction,(77,74) the corresponding reaction of acetylene is probably initiated by acidic properties of the catalyst.(22) In the case of polyacetylene evidence has been obtained to suggest that the nature of the cations in the zeolite lattice is also important.(75) Fig. 1 shows a series of Raman spectra which illustrate the influence of various cations upon the extent of polymerisation, demonstrate the effect of elevating the acetylene pressure and indicate a role for Lewis acid sites in the reaction mechanism. Exposure of acetylene (0.1 MPa) to sodium-mordenite (NaM) at 295 K gave the spectrum displayed in Fig. 1(a). Bands at 398 and 468 cm are ascribed to lattice modes of the mordenite structure(2J), whereas the peak at ca. 1958 cm can be attributed to the Vj vibration of adsorbed monomeric acetylene bound in a side-on" manner to cation sites (16,23). Relatively small maxima at 1112 and 1502 cm are characteristic of trans-polyacetylene (5,18,24,25). Exchange of cesium for the sodium ions in mordenite was found to be beneficial for the formation of polyacetylene, as can be seen in Fig. 1 (b). In addition to the noted intensification of bands typical of rra/iy-polyacetylene at 1112 and... 

However, the influence of the exchangeable cation on the framework vibrations has not been systematically investigated. From x-ray diffraction studies (2) on zeolites it is known that most of the exchangeable cations are firmly bound onto the negatively charged framework. Therefore these cations might have some influence on the lattice vibrational modes. 

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. 

For pure Si-MCM-41. this band has been assigned to the Si-O stretching vibrations and the presence of this band in the pure siliceous is due to the great amount of silanol groups present. A characteristic absorption band at about 970 cm 1 has been observed in all the framework IR spectra of titanium-silicalites. It was also reported that the intensity of 970 cm 1 band increased as a function of titanium in the lattice[17] and this absorption band is attributed to an asymmetric stretching mode of tetrahetral Si-O-Ti linkages [18] in the zeolitic framework. The increase in intensity of this peak with the Ti content has been taken as a proof of incorporation of titanium into the framework. 

Comparatively, the walls of a reaction cavity of an inclusion complex are less rigid but more variegated than those of a zeolite. Depending upon the constituent molecules of the host lattice, the guest molecules may experience an environment which is tolerant or intolerant of the motions that lead from an initial ketone conformation to its Norrish II photoproducts and which either can direct those motions via selective attractive (NB, hydrogen bonding) and/or repulsive (steric) interactions. The specificity of the reaction cavity is dependent upon the structure of the host molecule, the mode of guest inclusion, and the mode of crystallization of the host. 

Despite many advances in analytical methods in recent years, the structural characterization of materials that only occur as microcrystals less than about 30 l in diameter remains difficult and laborious. High resolution electron microscopy in the lattice imaging mode is by far the most powerful tool in giving the direct evidence of structural details essential for modelling clues, as has been demonstrated in the cases of recent zeolite structure solutions of theta-l/ZSM-23 (26) and beta (27), in addition to ECR-1. X-ray diffraction methods provide the essential confirmatory data, and sorption molecular probing and various well established spectroscopic methods are useful ancillary tools. 

When the proton is attached to a mobile ion, two modes of transport have been proposed. (1) Polyatomic ions like H3O+ or NH4+ may migrate by a simple jump from site to site in the lattice, as has been claimed in the jS-aluminas and in the zeolites.(2) A more involved mechanism based on the simultaneous difiiision of two types of polyatomic units, the so-called vehicular mechanism, was suggested... 

Not only the CN region gives information about the acidity but also the shift of the OH stretching modes of the zeolites upon adsorption of a base (Table 2). The high-frequency shift of the v(OH) from 3610-3640 cm after isomorphous substitution of the lattice corresponds with the decreasing heat of ammonia adsorption (Al>Fe>In). But more important the decreasing heats combine with a lower shift of the v(OH), see column 2 and... 

Lattice-dynamical calculations for unit cells of natrolite and edingtonite were performed. It was shown that strongest Raman bands of natrolite at 534 cm" and edingtonite at 530 cm are related to breathing modes of 4-membered rings. Assignment of vibrational spectra of used zeolites, presented here for symmetric modes of natrolite, may provide a base for interpretation of vibrations in other zeolites. Calculated natrolite crystal structure exhibits instability at about 5.5 GPa, which corresponds to amorphization observed at pressure range of 4-7 GPa. 

Zeolites Y dealuminated by Si/Al substitution using SiCU (DAY-S) and dealuminated thermochemically in steam (DAY-T) were investigated by X-ray powder diffraction, infrared spectroscopy and wet chemical methods. The dependence of lattice constants (a) on the molar ratio X = (1+Si/Al) show non-ideal solid solution behaviour. In a first approximation the change in a (in nm) can be described as a = 0.187x+2.412, for 0.1 < x < 0.5. For x < 0.1 the change in lattice constant saturates towards a = 2.425 nm. A similar shift in the double ring mode (wdr) is observed, tailing off. 

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