Window effect

Stmcture Cation Typical formula of unit cell or pseudoceU Window Effective channel diameter, nm Apphcations... 

Fn = correction factor to allow for the effect of the number of vertical tube rows, Fw = window effect correction factor,... 

Framework Cationic Form Formula of Typical Unit Cell Window Effective Channel Diameter (A) Application... 

This dimensionless rate constant contains typical parameters of the process (i.e., the heterogeneous rate constant k°, the diffusion coefficient, and the experiment time), thus reflecting that the behavior of the process is the result of a combination of intrinsic (kinetics and diffusion) and extrinsic (time window) effects. The effect of Kplane in the voltammograms obtained when both species (a) or only oxidized species O (b) are initially present can be seen in Fig. 3.3. 

Figure 6.5 a Product distribution for hydrocracking catalyzed by an ERI-type zeolite, showing the window effect observed by Chen et ai. b simulation of n-C- in an ERI cage (window size = 0.36 nm xO.51 nm). Thanks to Dr. David Dubbeldam for the zeolite simulation snapshot. 

Ruthven, D.M. (2006) The window effect in zeolitic diffusion. Micropor. Mesopor. Mater., 96, 262. 

The cage or window effect was proposed by Gorring (48) to explain the nonlinear effect of chain length observed in hydrocracking of various n alkanes over T zeolite, chabazite (CHA) and erionite (ERI). Thus, when a nC22 alkane is cracked over erionite, there are two maxima in the size distribution of the product molecules at carbon numbers of 4 and 11 and a minimum at carbon number of 8. The diffiisivities of n-alkanes also change in a similar periodic manner by over two orders of magnitude between the minimum at C8 and the maxima. This shows that for diffusion, and hence for shape selective effects, not only the size but also the structure of the reactant and product molecules need to be considered. 

In a capillary tube, the applied electric field E is expressed by the ratio VILj, where V is the potential difference in volts across the capillary tube of length Lj (in meters). The velocity of the electro-osmotic flow, Veo (in meters per second), can be evaluated from the migration time t of (in seconds) of an electrically neutral marker substance and the distance L, (in meters) from the end of the capillary where the samples are introduced to the detection windows (effective length of the capillary). This indicates that, experimentally, the electro-osmotic mobility can be easily calculated using the Helmholtz-von Smoluchowski equation in the following form ... 

The ZSM-5 family of zeolites show further interesting shape-selective effects. Both normal and methyl-substituted paraffins have access to interior sites, so both hexane and 3-methylpentane are cracked by ZSM-5, but steric constraints cause hexane to be cracked faster than 3-methylpentane. Further shape selectivity was found between 3-methylpentane and 2,3-dimethylbutane. No window effect with paraffin chain length was found with ZSM-5. In the conversion of methanol to hydrocarbons over ZSM-5 catalysts, the distribution 94,152,195 of aromatic products ends at Cio- The distribution of tetramethylbenzenes is not far from equilibrium, but has excess 1,2,4,5-tetramethylbenzene. Measurements of diffusion coefficients of alkyl benzenes show rapid decrease, by orders of magnitude, as ring substitution increases. 

Sachet dry use oil perfume medicinal culinary philtre. Protection if placed in doorways and windows effective against negativity provokes others to come to terms. 

There is no direct way to correct for the effect of the window. Commercial detectors are made with very thin windows, but the investigator should examine the importance of the window effect for the particular measurement performed. If there is a need for an energy-loss correction, it is applied separately to the energy spectrum. If, however, there is a need to correct for the number of particles stopped by the window, that correction is incorporated into the detector efficiency. 

Abstract Neutron scattering was first used to derive the self-diffusivities of hydrocarbons in zeolites, but transport diffusivities of deuterated molecules and of molecules which do not contain hydrogen atoms can now be measured. The technique allows one to probe diffusion over space scales ranging from a few A to hundreds of A. The mechanism of diffusion can, thus, be followed from the elementary jumps between adsorption sites to Lickian diffusion. The neutron spin-echo technique pushes down the lower limit of diffusion coefficients, traditionally accessible by neutron methods, by two orders of magnitude. The neutron scattering results indicate that the corrected diffusivity is rarely constant and that it follows neither the Darken approximation nor the lattice gas model. The clear minimum and maximum in diffusivity observed by neutron spin-echo for n-alkanes in 5A zeolite is reminiscent of the controversial window effect . 

One of the most controversial issues in the field of diffusion in zeohtes is the so-called window effect . This term was coined by Gorring to interpret the anomalous transport results obtained for hnear alkanes in zeoHte T [46], but his experimental conditions have been criticized. More recent macroscopic studies could not reproduce the periodic variation in diffusivity, and a monotonic decline with carbon number was reported [47,48]. However, a microscopic technique, such as neutron scattering, is better suited to probe anomalous diffusion mechanisms on a molecular scale, since it is much less sensitive to the influence of defects or internal transport barriers within the zeolite crystals. 

Anomalous diffusion in zeohtes is expected to happen only in structures which possess cages separated by windows, and the concept of the window effect depends both on the cage and window sizes. An unusual behavior may occur in systems where the sizes of the molecule and of the aperture between cavities are similar, and when the characteristic length scales of the molecifle and of the cavity are comparable. If a molecule is too long to fit comfortably... 

As shown in Fig. 12, a monotonous decrease in the self-diffusion coefficient was measured by PPG NMR for a series of n-alkanes in Na-X [50]. A similar trend was observed in ZSM-5 by QENS. From the NSE experiments performed in 5A, one finds that Dt drops to a minimum at Cs and has a clear maximiun at Ci2. A similar variation is obtained for Do after correcting from the thermodynamic correction factor (the number of carbon atoms per cavity is the same). Recent PEG NMR results indicate also a small minimum for Ds at Cg and a small maximum at Cm [51]. The NSE data obtained for longer n-alkanes in 5A are in contradiction with simulations which predict increasing diffusivities from C12 to Ci7 [52] whereas a decreasing trend is observed (Fig. 12). Finally, the activation energies derived from the NSE measurements show a minimum for C12, in agreement with the explanation in terms of the window effect. These results are related to similar concepts such as resonant diffusion [53] or the levitation effect, which corresponds to a maximum in self-diffusivity when the size of the diffusant is comparable to the diameter of the void [54]. 

ABSTRACT. The amount of published work on molecular shape-selective catalysis with zeolites is vast. In this paper, a brief overview of the general principles involved in molecular shape-selectivity is provided. The recently proposed distinction between primary and secondary shape-selectivity is discussed. Whereas primary shape-selectivity is the result of the interaction of a reactant with a micropore system, secondary shape-selectivity is caused by mutual interactions of reactant molecules in micropores. The potential of diffusion/reaction kinetic analysis and molecular graphics for rationalizing molecular shape-selectivity is illustrated, and an alternative explanation for the cage and window effect in cracking and hydrocracking is proposed. Pore mouth catalysis is a speculative mechanism advanced for some systems (a combination of a specific zeolite and a reactant), which exhibit peculiar selectivities and for which the intracrystalline diffusion rates of reactants are very low. 

For the classic types of molecular shape-selectivity in zeolites, the reader is referred to the excellent review papers in literature [18-25]. In this paper we elaborate on the recently proposed distinction between primary and secondary shape-selectivity [26], and on the more or less abused concept of cage and window effects in cracl g and hydrocracking. In addition, some evidence available in literature for the speculative mechanism of pore mouth catalysis is presented. 

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