Zeolite window diameters

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/zeotype Structure type code No. of tetrahedra in ring Window diameter (pm) Cavity diameter (pm)... 

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. 

The microkinetic interpretation of the origin of these different patterns may be confirmed by molecular dynamics (MD) simulations. As an example. Fig. 11 presents the result of an MD study with methane in a cation-free zeolite of type LTA [117,118]. By increasing the Lennard-Jones distance a between the methane molecules and the oxygen of the zeolite lattice one is able to simulate the influence of a reduction of the window diameter on the dif-... 

As a consequence of the much smaller window diameters, window blocking in zeolite A is easier to be achieved than in zeolite X. However, there are examples, where the co-adsorption of a second molecular species (e.g., water... 

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...

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... 

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. 

Not all frameworks built from tetrahedra as described above are considered to be zeolites. Dense phases are not considered to be zeolites, only those phases with some porosity. Generally, materials with pores accessible by windows defined by six T-atoms or less (six-rings) are not considered to be zeolites. In fact, the boundary between zeolites and dense phases is somewhat nebulous. lUPAC defines [1] zeolites as a subset of microporous or mesoporous materials containing voids arranged in an ordered manner and with a free volume larger than a 0.25 nm diameter sphere. The Structure Commission of the International Zeolite Association uses the criterion of framework density (T-atoms per lOOOA ) with the maximum framework density for zeolites ranging from 19 to 21. 

Zeolites are aluminosilicate crystallines consisting of pores of molecular dimensions, interconnected by small windows(5-8A diameter). Strict regularity of the pore structure enables higher slectivities to be achieved in both catalysis and sorption processes. The intrazeolite circumstances alike a "solid-solvent" accomodate the selected reactant molecules and promote some inorganic and organic synthetic reactions, similarly in solution. 

The channels in zeolite A run parallel to the three cubic axial directions, and are entered by a port of diameter 410 pm, determined by an 8-ring window this is still considerably smaller than the diameter of the internal cavity, which measures 1140 pm across. The computer model of zeolite A in Figure 7.10 clearly illustrates the 8-ring windows, the channels running through the structure and the cavities created by their intersection. 

ZSM-5 is a catalyst now widely used in the industrial world. Its structure is generated from the pentasil unit depicted in Figure 7.11 (as are the others of this group). These units link into chains, which join to make layers. Appropriate stacking of these layers gives the various pentasil structures. Both ZSM-5 and ZSM-11 are characterized by channels controlled by 10-ring windows with diameters of about 550 pm. The pore systems in these zeolites do not link big cavities, but they do contain intersections where larger amounts of free space are available for molecular... 

At -196 "C, oxygen is freely adsorbed by Ca-zeolite A, whereas nitrogen is essentially excluded. The two molecules are not very different in size O2 has a diameter of 346 pm whereas that of N2 is 364 pm. As the temperature rises, the adsorption of N2 increases to a maximum at around -i00 "C. The main reason is probably due to the thermal vibrations of the oxygen atoms in the window. Over a range of 80 to 300 K, a variation of vibrational amplitude of 10 to 20 pm could well be expected. Thus, a variation of 30 pm... 

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