Shape selectivity definition

Although beyond the scope of the present discussion, another key realization that has shaped the definition of click chemistry in recent years was that while olefins, through their selective oxidative functionalization, provide convenient access to reactive modules, the assembly of these energetic blocks into the final structures is best achieved through cydoaddition reactions involving carbon-het-eroatom bond formation, such as [l,3]-dipolar cydoadditions and hetero-Diels-Al-der reactions. The copper(i)-catalyzed cydoaddition of azides and terminal alkynes [5] is arguably the most powerful and reliable way to date to stitch a broad variety... 

Fischer-Tropsch synthesis could be "tailored by the use of iron, cobalt and ruthenium carbonyl complexes deposited on faujasite Y-type zeolite as starting materials for the preparation of catalysts. Short chain hydrocarbons, i.e. in the C-j-Cq range are obtained. It appears that the formation and the stabilization of small metallic aggregates into the zeolite supercage are the prerequisite to induce a chain length limitation in the hydrocondensation of carbon monoxide. However, the control of this selectivity through either a definite particle size of the metal or a shape selectivity of the zeolite is still a matter of speculation. Further work is needed to solve this dilemna. 

The efiect is thus not related to geometrical constraints induced on complexes anchored in mesoporous charmels (sometimes also called as confinement efiect, even if this definition is not properly correct), neither to shape-selectivity effects as possible in zeolites, since the size of mesoporous charmels is much larger than those of micro-porous materials. Instead, an effective modification on the characteristics of the fluids is observed due to the electrostatic field generated by the charmel walls. This is an enthalpic effect versus an entropic effect as observed when the modification is instead related to limitations in the translation modes of molecules. Recently, it was also demonstrated that wall curvature influence the molecular orientation of the... 

For application of zeolites as catalysts in industrial processes, high activity and easy removal of coke deposits are required. To meet these requirements, small crystals of zeolites (0.5-1.0 (im) should be advantageously used. On the other hand, the para-shape selectivity of zeolites in alkylaromatic transformations is connected especially with large crystals of the ZSM-5 zeolite structure, modified by silicon, boron, magnesium and phosphorus [1-5], However, no definite conclusion has been drawn on the contribution of various species to restricted transport of the bulkier isomers through the zeolite crystals, selectivity of the initial... 

The accessibility of the pores for molecules is subject to definite geometric or steric restrictions. The shape selectivity of zeolites is based on the interaction of re-... 

The variety of new technologies developed in the field of foil and membrane construction and materials is definitely expanding and enriching architectural design options to realise advanced technical solutions and new shapes. Selected options to improve the performance of membrane building envelopes are listed in Fig. 12.26. 

If L is a charac teristic dimension of a ciystal of selected material and shape, the rate of growth of a ciystal face that is perpendicular to L is, by definition,... 

Sphere> pellet > trilobe > hollow extrudate > wagon wheel/ minilith The definitive catalyst size selection will be a compromise between high reaction rate (small partiele, exotic shape), low pressure drop (large particle, exotic shape), large crushing strength... 

Synopsis of Experiment and Results. The material is irradiated during straining and relaxation. The example shows that a nanostructure which is hard to interpret from a series of scattering patterns may clearly reveal its complex domain structure after transformation to the CDF. Different structural entities are identified which respond each in a different way on mechanical load. The shape of the basic particles is identified (cylinders). The arrangement of the cylinders is determined. Thus the semi-quantitative analysis of the CDF provides the information necessary for the selection and definition of a suitable complex model which is required for a... 

As shown by Taylor and Roy (1) the behavior of small-pore zeolites does not necessarily conform to the classical definition of a zeolite. Rather, the properties evidenced by the P-zeolites, and perhaps other classes of small-pore zeolites as well, constitute a basis for possible future technical innovation in selective adsorption and heterogeneous catalysis. The zeolite structure, and hence the size and shape of its cell apertures and cavities and disposition of mobile cations may differ substantially at elevated temperatures from what it is under ambient conditions. 

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