Nest effect

Other Types of Shape Selectivity. Various other types of shape selectivity have been proposed, some of them requiring additional demonstration. This is not the case for the shape selectivity of the external surface of zeohte crystallites nest effect, pore mouth, and key lock catalysis, which is discussed in the examples in the next section. 

Gwinner, H., Oltrogge, M., Trost, L., and Nienaber, U. (2002). Green plants in starling nests effects on nesxlmgs. AnimalBehaviour 59,301-309. 

The catalytic isomerization of 1-methylnaphthalene and all lation of 2-methylnaphtha-lene with methanol were studied at ambient pressure in a flow-type fixed bed reactor. Acid zeolites with a Spaciousness Index between ca. 2 and 16 were found to be excellent isomerization catalysts which completely suppress the undesired disproportionation into nwhthalene and dimethylnaphthalenes due to transition state shape selectivity. Examples are HZSM-12, H-EU-1 and H-Beta. Optimum catalysts for the shape selective methylation of 2-methylnaphthalene are HZSM-5 and HZSM-li. All experimental finding concerning this reaction can be readily accounted for by conventional product shape selectivity combined with coke selectivation, so there is no need for invoking shape selectivity effects at the external surface or "nest effects", at variance with recent pubhcations from other groups. 

The alkylation of naphthalene and 2-methylnaphthalene with methanol and their ammoxidation were investigated by F r a e n k e 1 et al. [22-25] on zeolites ZSM-5, mordenite and Y. In the alkylation over HZSM-5 - unlike on H-mordenite or HY - the slim isomers, namely 2-methylnaphthalene as well as 2,6- and 2,7-dimethylnaphthalene, again clearly predominated. These authors suggest that such shape selective reactions of naphthalene derivatives occur at the external surface of zeolite ZSM-5, in so-called "half-cavities" [22, 24, 25]. D e r o u a n e et al. [26,27] went even further and generalized the concept of shape selectivity at the external surface. Based, in part, on Fraenkel s experimental results, Derouane [26] coined the term "nest effect". This whole concept, however, is by no means fully accepted and has recently been severely questioned in the light of results obtained in catalytic studies with a much broader assortment of ten-membered ring zeolites [28]. 

Fraenkel et al. (54) were the first to propose that the external surface of zeolites could be responsible for shape selective catalysis. Acid sites located in the half cavities on the external surface of HMFI would be responsible for the selective formation of 2,6- and 2,7-dimethylnaphthalene during naphthalene methylation (nest effect). This explanation was afterwards rejected on the basis of adsorption experiments. However, a nest effect was recently proposed to be responsible for the shape selective properties of the MCM-22 (MWW) zeolite and its delaminated analog (ITQ-4) in aromatics alkylation (55). 

Even emerging concepts such as those related to the shape selectivity of the external surface of the crystallites (nest effect, pore mouth catalysis,. ..) have already been applied to develop new catalytic processes isodewaxing, selective... 

Interface Module and Model Nesting Effects on Air Quality Simulation... 

Fraenkel et al. postulated that H-ZSM-5 crystals contain two types of Brdnsted acid sites [43]. The internal sites are accessible for molecules which can cross the 10-MR barriers and have kinetic diameters lower than 0.58 run. A second type of sites is accessible for molecules with kinetic diameters up to 0.62 nm, and was assigned to half chatmel intersections located on (001) crystal planes. These special pore mouths were thought to be responsible e.g. for the discrimination between ortho- and meta-ethyltoluene isomers and cymene. The key observation at the basis of this hypothesis was that in the alkylation of naphthalene with methanol the "slim" isomers 2-methyl- and 2,6- and 2,7-dimethylnaphthalene were dominating [44,45] over HZSM-5 and HZSM-11 as catalysts, in contrast to what was observed on zeolites with larger pores as H-Mordenite. lire authors suggest that this shape selectivity occurs at the special sites at the external surface of the ZSM-5 or 2 M-11 crystals and advanced the concept of shape selectivity at the external surface. Derouane et al. [46,47] generalized this concept and coined to it the term "nest effect" [46]. 

TTiis concept recently has been severely questioned by Neuber and Weitkamp [48,49], mainly based on results obtained in the same reaction over a whole series of 10-MR zeolites. On all catalysts, the observations of Fraenkel et al. were confirmed. Moreover, it was demonstrated unambiguously that 2-methylnaphthalene does enter the ZSM-5 pores at temperatures as low as 373 K and that the selectivity for the slim dimethylnaphthalenes increased upon coking of the catalyst [49]. As "coke selectivation" is a well-known effect in in zeolite catalysis [50], the authors convincingly state that the methylnaphthalene selectivity is better explained by the classical product selectivity enhanced by coke selectivation, rather than by the so-called nest effect in half-cavities at the external surface. 

The permeability of the preform specimens that are prepared from the same fabric roll by the same person may have significant variations. This is mainly due to the inconsistency in the fiber structure and racetracking channels which may vary from one experiment to the next. Slight variations in the preform ply layup and nesting effects may cause significant variations in the permeability. Hence it is necessary to perform experiments on a large number of specimens, and document the mean and standard deviation of the results. 

From sorption experiments the efficacy of a sorbate has been measured as heat of adsorption and described as nest effect , relating size and shape of the sorbate with the surface curvature of the pore [48]. Recently, host-guest complexes have been formulated quantitatively in terms of van der Waals interactions. Lewis et al. [47] calculated the nonbonded interactions energy of the SDA within the cavities of different silica zeoHtes, which was in good agreement with the experimental synthesis experience. The computational strategy developed in this study should stimulate the systematic search for new effective SDAs for the synthesis of new porosil structures with tailored pore geometry [49]. 

E.G. Derouane, Shape selectivity in catalysis by zeolites the nest effect. J. Catal. 100,541-544 (1986)... 

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