Zeolite lattice vibrations

Absorbance Spectra In Figures 3 and 4, spectra of the freshly activated, unloaded zeolite sample A, as well as of the otherwise unchanged sample after admitting a partial pressure of 3.1 mbar of n-hexane are given. The experimental resolution was 2 cm. Although 25 spectra were accumulated, the signal-to-noise ratio is, due to the small sample area of about 20 x 20 pm rather low. The band at 2350 cm is attributed to COj present in the beam path within the IR microscope. The spectrum of the freshly activated sample exhibits IR bands at 2007, 1882 and 1644 cm which may be attributed to overtones of zeolite lattice vibrations [11]. The broad structure at 3500 cm is due to SiOH groups of lattice defect sites [12]. After equilibration of the sample with 3.1 mbar of n-hexane, the positions and relative intensities of the IR... 

Moreover, the FTIR bands at 3610,2157 and 966 cm vary in accordance (in linearity), as it has been demonstrated earlier [7]. Any infrared band at around 930-910 cm referring to Cu species can not be obtained in the spectra. The very weak band at ca. 885 cm in Figure 1 rather seems to be related to protonated residual water than to any ionic copper species, according to some relevant experiments [10]. The isobestic point at 1003-1000 cm" in Figure 1 su ests that (i) the i.r. band at 966 cm" originates from a shift of the band at 1025-1010 cm, i.e. a perturbation of zeolite lattice vibrations by Cu and (ii) the band at 1025-1010 cm" may be related indirectly to Cu° in ZSM-5. The decrease in intensity of this... 

For describing measured zeolite lattice vibrations, the empirical estimation of SiO and AlO stretching force constants from Badger s rule [32] has proven to be one of the most successful tools. It gives a relationship between bond lengths (r) and force constants of the form... 

The KBr spectrum is employed in the internal-reference technique. In this case, a weak absorption band of the adsorbent (e.g., the overtones of the silica or zeolite lattice vibrations, 2100-1600 cm [119]) acts as the internal reference, and the ratio of adsorbate absorption to the reference absorption is proportional to the quantity of molecules adsorbed. The advantage of this is that it does not require to control precisely the sample weights [120]. 

The variation in the lattice vibration of the solid products was examined by utilizing the FT-IR technique at successive DGC process times and the results are presented in Fig. 5. The absorption bands at 550 cm and 450 cm" are assigned to the vibration of the MFI-type zeolite and the internal vibration of tetrahedral inorganic atoms. The band 960 cm" has been assigned to the 0-Si stretching vibration associated with the incorporation of titanium species into silica lattice [4], This indicates that the amorphous wall of Ti-MCM-41 was transformed into the TS-1 structure. 

Crystal lattice packing, 12 249-250 Crystal lattice vibrations, 14 236 Crystalline adsorbents, 1 586, 589. See also Molecular sieves Zeolites for gas separation, 1 631 properties and applications, l 588t Crystalline alkali silicates, atomic structure of, 22 454-455 Crystalline cellulose, 5 373-379 Crystalline epoxy resins, 10 373-374 Crystalline flake graphite, 12 793 manufacture and processing of, 12 781-784... 

Work with the objective of comparing oxo-ions with oxide particles in order to test the validity of this reasoning has been reported by Chen et al. who used a catalyst that initially contains Fe oxo-ions, [HO-Fe-0-Fe-OH] +. These sites were first converted to Fc203 particies by a simpie chemical treatment. This was followed by another treatment, which redispersed these Fc203 particies back to oxo-ions. The change in particle size was monitored by a spectroscopic method based on the observation that in zeolites metal ions and oxo-ions, that are attached to the wall of a cage, give rise to a typical IR band caused by the perturbation of the vibrations of the zeolite lattice. 

The utilily of measuring lattice vibrations for obtaining information about zeolites has been widely demonstrated. Applications include determining the structure of zeobtes by the identification of the structural units present, measuring changes in the framework Si/Al within materials with the same zeolite structure and tracking the formation of zeolite during synthesis. 

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. 

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 object of the present investigation was to study systematically the effect of monovalent cations on the lattice vibrations in the synthetic zeolites Linde A and Linde X. It was reasoned that mid-infrared spectroscopy might yield information on cation siting in these zeolites. 

Cation Siting in Linde A. At the time this work was completed, x-ray studies on hydrated NaA (3, 4) and hydrated KA (5) had shown that 8 of the 12 exchangeable cations per unit cell are firmly bound to the zeolite framework and would therefore be expected to have the major influence on the lattice vibrations. These cations are sited in front of the sodalite... 

Whether the rhodium dicarbonyl was attached to the zeolite lattice or to an extra-framework anion such as OH, 0 or a labile ion, could be also decided upon using IR spectroscopy. Indeed lattice vibration between 1300 and 300 cm- characteristic of an NaY zeolite (16) are sensitive to the interaction of lattice oxide ions with cations. In particular, it was observed that an IR absorption band at 877 cm- grew simultaneously with the growth of CO absorptions at 2115-2048 characteristic of the dicarbonyl (13).This... 

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. 

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. 

Parameters of GVFF for silicates and aluminosilicates obtained from first-principles calculations were reported by Ermoshin et al. ° ° Having assumed that the dynamics of zeolite lattices can be described in terms of vibrations of the TO4 tetrahedra (T = Si, Al) and shared 03T-0(H)-T03 tetrahedra, the authors calculated the matrix of second derivatives of the total energy in Cartesian coordinates, the Hessian matrix H, for molecular models of such units. The matrix H was then transformed into a matrix of force constants in internal coordinates F... 

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. 

IR spectroscopy was mainly used to characterize the sorbed species. The zeolite powder was pressed into self supporting wafers and analyzed in situ during all treatments (i.e., activation, sorption, reaction) by means of transmission absorption IR spectroscopy using a BRUKER IPS 88 FTIR spectrometer (resolution 4 cm" ). For the sorption experiments, an IR cell equipped with IR transparent windows which could be evacuated to pressures below 10" mbar was used [11]. The activated zeolite wafer was contacted with a constant partial pressure (0.001 mbar) of the adsorbate at 308 K until adsorption-desorption equilibrium was reached (which was monitored by time resolved IR spectroscopy). For the coadsorption experiments, the catalysts were equilibrated with 0.001 mbar of both adsorbates admitted in sequentional order. The spectra were normalized for the sample thickness by comparing the intensities of the absorption bands of the adsorbate with the integral intensity of the lattice vibration bands of the zeolite between 2090 and 1740 cm". The surface coverage was quantified by calibration with gravimetric measurements (under conditions identical to the IR spectroscopic experiments). 

However, at low H2 pressure the reverse process, the reoxidation of Cu° atoms to copper ions by zeolite protons takes place at higher temperatures. It has been showed [6-8,10] that in the spectral range of lattice vibration of ZSM-5 a new i.r. band at 967-958 cm is... 

Infrared spectroscopy can be used to monitor the crystallinity of ZSM-5 preparations through observation of bands due to vibrations of the zeolite lattice. 

It is known (16) that there is a linear relationship between the IR wavenumbers of the T-0 vibrations of the zeolite lattice and the aluminum content. The aging treatments shift the FTIR bands towards higher wavenumbers which indicates a partial dealumination of the lattice (Table 3). The variations remain however small because of the low initial Si/Al ratio (for the 1227-28 cm band and after aging at 1173 K Av = 7 cm for H-ZSM-5 and 4 cm for Cu-ZSM-5). Let us recall that, when the Sl/Al ratio decreases from 319 to 19, the 1235 cm band shifts to 1220 cm (A= 15 cm ). 

This work discusses the vibrational features associated with the insertion of tetrahedral Ti in the MFI zeolite lattice. The understanding of these features forms the base for the technical characterization of Ti containing silicate catalysts using spectroscopic methods, which is of capital importance in industrial catalysis. A combination of spectroscopic and computational techniques is used in order to assign the main vibrational features of Ti-silicalite, also taking into account the presence of hydroxylated defects. 

Natrolite, applications Natrolite, ion exchange Natrolite, lattice vibrations Natural zeolites, Antarctica Natural zeolites applications PL-2... 

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