Shape selectivity factors influencing

In this Section, possible factors influencing the selectivity other than shape similarity and shape-specific weak interactions (Sect. 2.4) are discussed. These mainly include intermolecular association, exchange reactions, and hydrophobic interaction. In connection with intermolecular association and crystalline 1 1 complex formation (Sect. 2.3), tetrameric intermediates are also discussed. 

Farcasiu, D., Hutchison, J. and Luowei, L. Analysis of factors that influence shape selectivity in the craking of long-chain alkanes on zeolites catalysts, J. Catal., 1990,122, 34-43. 

Thus, the rate and selectivity of catalyzed reactions over molecular sieve catalysts are influenced by factors that are affiliated with the specific interface chemistry (chemical induced selectivity) and the constraints induced by the steric limitations (shape selectivity). This advantage, however, also induces drawbacks that can only be partly overcome by adjustment of the mesoscopic properties of the molecular sieve. 

It is not easy to give an explanation for this behaviour, but it is possible to discuss the influence or not of several factors. First of all it is important to note that the exchanged or supported copper is responsible for the catalytic activity, in fact the solids without copper did not catalyse the reaction. The influence of shape-selectivity must be discarded given that the normal trans preference is obtained with the microporous Y zeolite and the more sterically... 

Zeolites are especially suitable as support materials for active components such as metals and rare earths. With rare earths, the activity of the catalyst and its stability towards steam and heat can be increased. Suitable metals are effective catalysts for hydrogenations and oxidations, whereby the shape selectivity of the carrier is retained. Important factors influencing the reactions of such bifiinctional catalysts are the location of the metal, the particle size, and the metal-support interaction. 

The selection of the method of application depends on the adhesive form, whether liquid, paste, powder, film, or hot melt. Other factors influencing the choice of application method are the size and shape of parts to be bonded, the areas where the adhesive is to be applied, and production volume and rate. ... 

A number of factors influence the separation which can be achieved. The shape of the molecule and its polarity determine whether it will be able to enter the hydrophobic cavity of the cyclodextrin. Vt varying the concentration of cyclodextrin, one can influence the partition of the complex between the selective and bulk phase. One can also modify the cyclodextrin in a manner that changes its solubility or affects its electrophoretic mobility. Bile salts and crown ethers have also been used as enantioselective agents. 

Shape selectivity t/o Retention factor ratio between triphenylene and o-terphenyl, = ky/kQ. This descriptor is a measure of the shape selectivity, which is influenced by the spacing of the ligands and the functionality of the silylating reagent. 

Size, shape, and density The shielding effects of dendritic shells can likewise be caused by steric factors. Thus, the access of foreign molecules to the central functional unit can be hindered or prevented according to size and density of the dendritic shell. Sometimes, even a certain size selectivity is observed. These effects are especially interesting for electrochemically, catalytically active, redox-and photo-active functional units, since interactions with foreign molecules, such as oxygen quenching of the luminescence (photo-active units) or the access of substrates (catalytically active units) can be influenced.14 11 17,221... 

The problems associated with the measurement and control of the flowrate of solids are much more complicated than those in the corresponding situation with liquids. The flow characteristics will depend, not only on particle size, size range and shape, but also on how densely the particles are packed. In addition, surface and electrical properties and moisture content all exert a strong influence on flow behaviour, and the combined effect of these factors is almost impossible to predict in advance. It is therefore desirable to carry out a preliminary qualitative assessment before making a selection of the, most appropriate technique for controlling and measuring flowrate for any particular application. 

Separation depends on the selection of a process in which the behaviour of the material is influenced to a very marked degree by some physical property. Thus, if a material is to be separated into various size fractions, a sieving method may be used because this process depends primarily on the size of the particles, though other physical properties such as the shape of the particles and their tendency to agglomerate may also be involved. Other methods of separation depend on the differences in the behaviour of the particles in a moving fluid, and in this case the size and the density of the particles are the most important factors and shape is of secondary importance. Other processes make use of differences in electrical or magnetic properties of the materials or in their surface properties. 

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