Zeolite changes

Thus, the addition of compounds to reaction mixtures can influence the catalytic activity of zeolites. The effect may be increased conversion or a shorter reaction time. The effects may be caused by surface modification or by variation in adsorption-desorption in the system reagent-product-zeolite. Sometimes the properties of the zeolite change so radically that it is possible to talk about the action of new catalytic systems. 

The sodium-23 MASNMR results are consistent with the selective removal of sodium cations from the Y zeolite supercages by the partial cation exchange. This demonstrates that this technique can be used to monitor how cation distributions in Y zeolites change with various sample treatments. 

Elutant (Regenerant) Cation-exchange capacity (meq/lOOg zeolite) change in CEC from as rec d state(%)... 

P-26 - Deep-bed dealumination of ZSM-5 zeolites. Changes in structure and catalytic activity... 

Adsorption and desorption isotherms of water vapor on HY zeolites change in shape with increasing dealumination. Thus, for only steamed samples the isotherm is close to the type I whereas for zeolites submitted to an additional acid leaching, it is close to the type IV with a progressive lowering of the point B towards the pressure axis, what emphasizes a more and more hydrophobic character of adsorption. The micropore volume accessible to water and the unit cell parameter are simply correlated with the framework Si/Al ratio. The water molecule does not allow to determine quantitatively structural and secondary pore volumes, but appears to be a selective molecular probe of the structural aluminium ions. For HY zeolites and without any restraint, 8H2O are associated with such a framework aluminic site. 

As an adjective applied to metals base represents the opposite of noble, i.e. a base metal would be attacked by mineral acids, base exchange An old term used to describe the capacity of soils, zeolites, clays, etc. to exchange their cations (Na, K, Ca ) for an equivalent of other cations without undergoing structural change. An example of the general process of ion exchange. ... 

The properties of the zeolite play a significant role in the overall performance of the catalyst. Understanding these properties increases our ability to predict catalyst response to changes in unit operation. From its inception in the catalyst plant, the zeolite must retain its catalytic properties under the hostile conditions of the FCC operation. The reaclor/regenerator environment can cause significant changes in chemical and structural composition of the zeolite. In the regenerator, for instance, the zeolite is subjected to thermal and hydrothermal treatments. In the reactor, it is exposed to feedstock contaminants such as vanadium and sodium. 

The most significant changes associated with adsorption which have been observed to date were the displacements (45) of Raman fundamentals of ethyne on adsorption on zeolite 4A (see Table IX). Such changes constitute a useful monitor of adsorbate-adsorbent interaction for various adsorbents. The appearance of the Raman spectrum of ethyne on zeolites A suggests an... 

Changes in relative peak intensity and marginal line shifts have been observed for benzene adsorbed on porous glass (26). More significantly, infrared spectroscopic evidence had been found in the appearance of inactive fundamentals for the lowering of molecular symmetry of benzene on adsorption on zeolites (47). 

For other centrosymmetric adsorbates such as C02 on zeolites X and Y (1) and ethene on porous Vycor glass (3), only marginal changes in line position were observed. 

The reaction used to test these solid catalysts was the aziridination of styrene with AT-tosyliminophenyliodinane (Phi = NTos) (Scheme 10). In most cases, enantioselectivities were low or moderate (up to 60% ee). The loss of enantioselectivity on changing from ligand 11a to ligand 12 was attributed to the fact that ligand 12 is too big for the copper complex to be accommodated into the zeolite supercages. Further studies carried out with hgands 11a and 11b [62] demonstrated that the reaction is more enantioselective with the supported catalyst (82% ee with 11a and 77% ee with 11b) than in solution (54% ee with 11a and 31% ee with 11b). This trend supports the confinement effect of the zeolite structure on the stereoselectivity of the reaction. 

In the case of the Diels-Alder reaction [68] (Scheme 12), several soUds (AlSBA-15, MCM-41, MSU-2 and zeolite HY) were tested as supports for the bis(oxazoline)-copper complexes. The best enantioselectivity results were obtained with the zeolite HY, although the yield was the poorest (16% yield, 41% ee). As happened with the aziridination reaction, the enantioselectivity changed with time. Short reaction times led to the same major enantiomer as observed in homogeneous reactions. However, at higher conversions, i.e., longer reaction times, the opposite major enantiomer was obtained. 

In the absence of steric constraints in Eq. (1.24a) Eact will not vary. Eads and 9 are the parameters that significantly change with hydrocarbon chain length or zeolite. 

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