Molecular motion in zeolites

To understand the principles of shape-selective adsorption and catalysis, a detailed knowledge of the microdynamics of the molecules inside the zeolitic pore system is required. Garcia and Weisz 4) pointed out the relevance of NMR methods to yield a unifying picture for the phenomena, mechanisms, and magnitudes of diffusivities. In the past few years, a deeper insight into molecular motions in zeolites has been achieved, especially by combining investigations of molecular translation with studies of reorientation processes on different time scales 5-14). 

Computational methods have proved to be helpful tools for the investigation of molecular motions in zeolite structures [1]. It was shown that possible adsorption sites, ener es and difiusion properties could be correlated with experimental results.The aim of our investigations was to enlighten the dynamical behavior of chlorobenzenes in zeolite HY using molecular dynamics techniques. Complementary Monte Carlo computations were performed on the same system. 

The first attempt to measure molecular motions in zeolites was done as far back as 1966 by Egelstaff, Downes and White [39] on water, ammonia and alcohols on K-A zeolite. From these preliminary experiments, the authors could only deduce that the molecules are immobilized for about lO s, vibrating with respect to the internal surfaces of the cavities. Almost at the same time, Pokotilovskii studied the inelastic scattering of water in several zeolites [40] a well marked peak at 63 meV (1 meV = 8 cm-1) was attributed to the torsional vibrations of the hydrogen-bonded molecules and a broad band between 6 and 30 meV was assumed to be characteristic of translational motions and bending vibrations of the H-bond. After these studies, it took almost ten years to see new publications on this kind of experiment. 

Figure 6.7 Proposed restriction on molecular motion in zeolites.

NMR has been extensively applied to carbonyl compounds in acidic zeolites and other solid acids. The unshared pairs of electrons on the oxygen can interact with either Brpnsted or Lewis sites, and aldol condensation reactions are commonly observed. Acetone was first studied on a zeolite by Bosacek and co-workers (146) followed by Haw and co-workers (147) and later by Gorte and co-workers (148). The conclusion of an earlier acetone paper of Gorte and co-workers (149) was that acetone forms a static complex on the Brdnsted site of HZSM-5 at room temperature, but this claim was later revised (150) upon the realization that molecular motion in the complex is not halted except at appreciably lower temperatures. 

Paramagnetic ions are now being used quite extensively to study adsorption phenomena. Mn ions have been used as probes for studying molecular motion in synthetic zeolites, (350) Co and Ni ions have been used for studying the complexation of molecular hydrogen on the surface of zeolites, (351) and these same ions have been used in a variety of studies of adsorption on Aerosil surfaces. (352-358) Adsorbed molecules studied include olefins, saturated hydrocarbons, alcohols, and benzene. From the measured line-shifts the number of active surface sites can be deduced in favourable cases. (357, 358)... 

It has been demonstrated that the combined application of various NMR techniques for observing molecular rotations and migrations on different time scales can contribute to a deeper understanding of the elementary steps of molecular diffusion in zeolite catalysts. The NMR results (self-diffusion coefficients, anisotropic diffiisivities, jump lengths, and residence times) can be correlated with corresponding neutron scattering data and sorption kinetics as well as molecular dynamics calculations, thus giving a comprehensive picture of molecular motions in porous solids. 

Molecular dynamics simulation is perhaps the most powerful computational technique available for obtaining information on time dependent properties of molecular or atomic motion in zeolite crystals. It is used to obtain thermodynamic quantities and detailed dynamical information on sorption and diffusion processes in zeolite systems. For instance, the extent to which intramolecular vibration and framework motion assist sorption and diffusion of molecules can be simulated. The major limitation is its inability to model diffusion of larger sorbed molecules and electronic polarisability due to the huge amount of computer time and memory requirements. However, with the improvement in supercomputers and improved computing facilities, the full application of M.D. simulation to zeolite studies is becoming feasible. 

Deuterium NMR has recently been used to study molecular motion of organic adsorbates on alumina (1.) and in framework aluminosilicates (2). The advantage of NMR is that the quadrupole interaction dominates the spectrum. This intramolecular interaction depends on the average ordering and dynamics of the individual molecules. In the present work we describe NMR measurements of deuterated benzene in (Na)X and (Cs,Na)X zeolite. 

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)... 

Quadrupolar interactions can offer direct information on the dynamics of organics within zeolite crystals. Eckman and Vega (304) studied the 2H quadrupolar echo decay in perdeuterated p-xylene adsorbed on zeolite ZSM-5. The deuterium quadrupolar interaction usually dominates the spin Hamiltonian, so that the powder pattern can be used as a test for models of molecular motion. At -75°C and 25°C typical rigid-lattice spectra were obtained. At 100°C however, the resonance arising from the aromatic deuterons was motionally narrowed, while the methyl resonance was not, The authors conclude that p-xylene molecules reorient about an axis which passes through the C3 axes of the methyl groups. 

The results reported here and in earlier publications in this series suggest that cavity size and limitations to molecular motion play a dominant role in the photochemistry and photophysics of alkyl aryl ketones included in zeolites. In the case of Silicalite the size and polarity of various substituted 8-phenylpropiophenones seem to determine the efficiency of inclusion and ultimately of luminescence. The same factors, relating to size and mobility can be expected to play an important role in the use of zeolites as catalysts for other reactions, whether these are photochemical or thermal processes. In this sense studies with 8-phenylpropiophenones may lead to considerable information on adsorption sites and on the freedom (or lack of it) of molecular motion as well as on the accessibility of these sites to other reactants. Recent work from Turro s laboratory has shown that pyrene aldehyde can be used to probe the nature of inclusion sites in various zeolites (27) dibenzyl-ketones were also used as probes on porous silica (28). 

The comparison between the 7, values of the two systems (Figure 10.5) shows that their behavior compared to HTT is different. At 280°C, the 7 , value of the system with the zeolite is twice as large as that for the APP/PER system. It was concluded that the materials formed at this temperature are structurally different. Indeed a slower 7) means that the molecular motions may be hindered and therefore that the structure may be more rigid. Between 280°C and 350°C, the Tx values decrease and become identical. At 350°C, both systems are organized in stacks of polyaromatic species and it may be proposed that the two materials are structurally, in the NMR sense, similar. At higher temperatures, the 7 , values of the systems with zeolite are always larger. The carbonization process of the two intumescent materials develops in different ways. The zeolite allows molecular motion and the carbonaceous shields keep mobile structures. 

Diffusional motion. Many rotational and translational diffusion processes for hydrocarbons within zeolites fall within the time scale that is measurable by quasielastic neutron scattering (QENS). Measurements of methane in zeolite 5A (24) yielded a diffusion coefficient, D= 6 x lO" cm at 300K, in agreement with measurements by pulsed-field gradient nmr. Measurements of the EISF are reported to be consistent with fast reorientations about the unique axis for benzene in ZSM-5 (54) and mordenite (26). and with 180 rotations of ethylene about the normal to the molecular plane in sodium zeolite X (55). Similar measurements on methanol in ZSM-5 were interpreted as consistent with two types of methanol species (56). 

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