Molecules in zeolites

The interaction between the adsorption sites of a zeolite and a probe molecule perturbs the normal vibrations of the molecule and leads to frequency shifts compared to the gas phase which can be interpreted in terms of the strength and nature of the interactions with the adsorption sites of the zeolite. INS is advantageous for observing such interactions for hydrogenous molecules [116,117]. In the infrared spectra the out-of-plane bending modes and the ring vibrations coupled with CH in-plane bending are obscured by the zeolite framework modes below 1300 em .  

The INS spectra of solid benzene and benzene adsorbed in NaY at a concentration of one benzene molecule per supercage are shown in Fig. 7.27 [118], The speetra were also calculated by a foree field method. Peak positions and assignments are given for benzene in Table 7.19. 

These spectra illustrate a number of features of INS spectra. In comparison with the infim-ed speetrum of benzene vapour, in solid benzene the out-of-plane C-H bending modes are shifted to higher wavenumbers (e.g. vn, 673 to 694 cm ). The shift is a consequence of intermolecular forces in the solid. The in-plane modes are not shifted. Out-of-plane C-H bending modes are shifted ftirther to higher wavenumbers for benzene in NaY (vn, 700 em ). 

The Debye-Waller factor is smaller for adsorbed benzene than for solid benzene. This is due to a larger mean square amplitude of the external modes of adsorbed benzene (0.017 A ) than solid benzene (0.011 A ) because the benzene molecules are less strongly bound in the zeolite than in the solid. The external modes of adsorbed benzene are at lower frequency (see Table 7.20) and their contribution to the total displacement amplitude is increased. The intensity of the fundamentals is decreased and is transferred to multiphonon features with consequent 

The adsorption geometry of benzene in the supercage is known from diffraction [119]. Its centre is on the cube diagonal with the ring oriented symmetrically over a sodium ion. From a simulation of the INS spectrum, which included Na-C interactions, a Na-C force constant was calculated. The value (0.06 mdyn A ) was much smaller than for benzene bound to a transition metal centre (e.g. [Cr(CO)3 n -(C6H5)J, 4.5 mdyn A ) implying weaker binding of benzene to sodium consistent with the small perturbation of internal modes. 

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Translation and reorientation of CD4 molecules in zeolites as studied by deuteron NMR relaxation... 

Burton, A., and Zones, S. (2007) Organic molecules in zeolite synthesis their preparation and structure-directing effects, in Introduction to Zeolite Science and Practice, 3rd Revised edn (eds ). Cejka, H. Van Bekkum, A. Cornia, and... 

Mortier, W.J., Pluth, J.J., and Smith, J.V. (1975) Positions of cations and molecules in zeolites with the mordenite-type framework. 1. Dehydrated calcium-exchanged ptilolite. Mat. Res. Bull, 10 (10), 1037-1045. 

Chimie, C.R. (2005) Infrared spectroscopic investigation of diffusion, co-diffusion and counter-diffusion of hydrocarbon molecules in zeolites. Elsevier, Comptes Rendus Chimie, 8, 303-319. 

Although it is not possible for the chemist to absolutely control the movement of individual atoms or molecules in zeolite structures, the nature of the structure itself results in channels that direct the molecular motions (Fig. 13). Furthermore, the sizes and shapes or the channels determine which molecules can form most readily, and which can leave readily. A molecule that cannot leave (Fig. 1.4) is apt to react further. This may have important consequences A catalyst (ZSM-5) that is structurally related to boggsite is used in the alkylation or toluene by methanol to form pui u-xylene. The methanol can provide methyl groups to make all three (ortho, meta. and para)... 

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