CAGE AND WINDOW EFFECT

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. 

A review on cage and window effects in mainly hydroconversion of alkanes with zeolites [73] shows that bi- and even trinodal distributions of product carbon numbers can be formed. In the erionite (ERI) cage Cs hydrocarbon fragments are selectively trapped, thus undergoing intense secondary cracking. This effect was confirmed in the ketonization of carboxylic acids [74]. Alternatively, in cases of slow diffusion (and counterdiffusion), viz. in the liquid-phase propylation of benzene in mordenite, the possibihty of having pore-mouth catalysis was advanced [75]. Multinodal product distributions from... 

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. 

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... 

In the case of zeolite A, three different cation sites have been identified most of the cations occupy comer sites in the central cavity (Type I sites), but some of the total of twelve univalent ions (e.g. Na+ or K+) ions per cage must occupy sites within the eight-ring windows and therefore partially obstruct the channels. The effective window size of NaA (i.e. the 4A sieve) is thereby reduced from 0.42 nm to c. 0.38 nm. Since the K+ ion is somewhat larger, the window size becomes even smaller (i.e the 3A sieve). When the Na+ cations are exchanged for Ca2+ or Mg2+, the number of requisite cations is reduced and the effective aperture size and pore volume are both increased (the 5A sieve). 

In the majority of publications the effective diffusion coefficient takes into account the pore diffusion in the fluid phase (free or molecular diffusion and Knudsen diffusion). In the case of narrow windows of zeolite cages and large adsorptive molecules, micropore diffusion can be rate controlling. Some authors have extended the LDF model in the following way ... 

Calculation results at 177 K led to the prediction that methane diffuses from one a-cage to another at a rate of 1.9 x 1010 per sorbate per second in NaY zeolite and 24.3 x 1010 per sorbate per second in NaCaA zeolite. Thus, this is another example of a guest diffusing faster through a host with smaller pore windows (the so-called ring effect ), a phenomenon that had been observed previously only for noble gas atoms. 

FIGURE 9.6 Photosensitized hydrogen abstraction from triethylamine DH by 9,10-anthra-quinone A (for the formulas, see Chart 9.3). The rate constant of in-cage deprotonation as obtained from the polarity pattern, is shown as a function of the relativity permittivity e of the reaction medium (mixtures of acetonitrile and chloroform). The timescale of the CIDNP effect provides a kinetic window, within which such a quantitative treatment is apphcable. Further explanation, see text. 

On the other hand, our knowledge of configurational diffusion in zeolites is far from being adequate. While Fick s law on diffusion has been commonly used to obtain diffusion coefficients in zeolites, there is experimental data such as the window or cage effect observed in erionite which cannot be interpreted by such equations. Data on diffusion of high molecular weight molecules in zeolites are almost nonexistent. Discrepancies also remain unresolved between diffusion coefficients determined by NMR and uptake data (22-23). Needless to say, much remains to be investigated, Uoth in theory (24-25) and in experimental measurement (26). 

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