Zeolite window

The diffusivities obtained by Barrer and Brook (i) ranged from 5 X 10" cm /sec for sorption of argon in Li-mordenite to 5 X 10 for nitrogen in Ca-mordenite. A quantitative comparison of results for such different systems is not possible, but considering the relative effective sizes of the gas molecules and zeolite windows, the comparison with the present results is not unreasonable. 

Spectra of other dynamical variables like CoM velocities and zeolite window diameters can also be obtained by Fourier transformation of the appropriate time correlation functions. Calculation of spectra for different spatial components of a time dependent quantity provides useful information about the anisotropy of the corresponding motion. 

Zeolite films were also explored for their use in polenliomctric reactions.[93] The authors determined the potentiometric response of zeolite-modified electrodes towards cations in aqueous phase. Three different preparation routes were used for the formation of the zeolite films zeolites Y (FAU), A (LTA) and mordenite (MOR) were pressed into discs, sodalite (SOD) was grown in a free-standing membrane, and for the pressed discs of LTA a secondary growth phase was added in order to heal defects in the pressed discs. The authors could demonstrate size-selective behaviour in these systems, i.e., cations with diameters exceeding those of the zeolite window openings resulted in no detectable potential response. 

Volumetric measurements of the products evolved when [Fe3(CO)i2] was sorbed in the zeolite, combined with infrared and ultraviolet spectra of the solid, also indicated the formation of [HFe3(CO)n] , as shown in Equation (4.2). The observation that the sorption of [Fe3(CO)i2] was mueh slower than that of [Fe2(CO)9] was inferred to be a consequence of their size differences. The critical molecular dimension of [Fe3(CO)i2] (ca. 10.5 x 7.5 A) is close to the diameter of the zeolite window (about 7.4 A). The anion was inferred to have been generated inside the zeolite supercages. 

Table 4.3.2. Kinetic S sieving diameter of a gas/vapor based on the smallest zeolite window where it can fit...

A number of acid catalyzed reactions have been examined in which the PILCS were compared to zeolites. Shabtai et al. [47] compared the rates of reaction for dealkylation of cumene and 1-isopropylnaphthalene and for cracking of polycyclic naphthenes catalyzed by a pillared montmorillonite with rates of these reactions catalyzed by a Y-type zeolite. In each case the PILC, whether in the H or rare earth form, was found to have a higher activity. When the reactant molecule was larger than the zeolite windows, the rates of the PILC-catalyzed reaction were much greater than those of the zeolite-catalyzed reaction. Some of the data are summarized in Table V. 

The problem of accessibility in microporous solids is extreme in zero-dimensional zeolite structures such as clathrasils, that is, zeolite-related materials consisting of window-connected cages. The pore openings in these caged structures are restricted to six-membered rings of [Si04] units at most, which corresponds to pore diameters of approximately 0.2 nm [58]. These pores are too small for the removal of templates and, afterward, are impenetrable to typical sorptive molecules for characterization such as N2 and Ar or reactants such as hydrocarbons. Therefore, the intrinsic... 

Recently, high-quality SOD membranes for water separation have been developed by Khajavi etal. [21, 52]. These zeolite membranes should allow an absolute separation of water from almost any mixture since only very small molecules such as water, hydrogen, helium, and ammonia can theoretically enter through the six-membered window apertures. Water/alcohol separation factors 10 000 have been reported with reasonable water fluxes up to 2.25 kg nr h at 473 K in pervaporation experiments. 

The probe reaction utilized a 1/1 molar mixture of methanol and isobutanol over H-mordenite, a strongly acidic zeolite comprised of linear one-dimensional channels made up of 12-ring 6.5 by 7.0 A windows [8]. There is a side-pocket system in H-... 

Zeolites with polyhedral cavities which are connected by wide windows or channels that permit the diffusion of foreign ions or molecules. 

For this study, p-xylene and triisopropylcyclohexane (TIPcyC6) were the two molecular probes chosen, using toluene as a solvent. Their molecular dimensions were obtained from the shadow of the three-dimensional molecule projected onto a plane according to the method of Rohrbaurgh et al. [5] (Table 2). A molecular probe is considered not to penetrate into a cylindrical pore if two of its dimensions are greater than the pore diameter [6], As the free diameter of the window of the supercage of the Y zeolite is equal to 0.74 nm, it is considered that only TIPCyC6 cannot penetrate into the zeolite microporosity. 

Zeolite ITQ-21 is a recently discovered zeolite [1], containing Si, Ge and optionally A1 as framework cations. Its three-dimensional structure is formed by three linear 12 ring (12-R) channels that intersect to produce large inner cavities with a nearly spherical shape about 1.18 nm in diameter (Figure 1), similar to those present in the Faujasite structure. However, in the case of ITQ-21 these cavities are accessible through six circular 12-R windows of 0.74 nm wide. 

The ir measurements were carried out with a Perkin Elmer 580 spectrometer and fused silica cell with KBr windows allowing to outgass the zeolitic wafer at a desired temperature and to introduce and further outgass a probe molecule without contact with air. 

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