Zeolites capacity

The recovery and purification of cesium-137 from Purex acid waste using a synthetic zeolite has been studied. Zeolite capacity and selectivity for cesium were determined. Stability of the synthetic zeolite to high radiation fields and chemical attack was adequately demonstrated. Kilocurie quantities of cesium-137 of 98+% chemical purity were prepared using zeolite ion exchange. 

Several sodium zeolites were prepared in essentially the same manner as the ammonium zeolites. Their ignition loss was assumed to consist exclusively of water. The values for zeolitic capacity and water content of these preparations fall on the same curves as those obtained for the ammonium zeolites containing up to 20 per cent alumina. 

The slopes of the zeolite-capacity lines and the water-content lines for gels containing up to 20% alumina indicate that each aluminum ion is associated in these gels with one equivalent of ammonium or sodium... 

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. ... 

Figure C2.12.1. Origin of ion exchange capacity in zeolites. Since every oxygen atom contributes one negative charge to the tetrahedron incoriDorated in the framework, the silicon tetrahedron carries no net charge while the aluminium tetrahedron carries a net charge of-1 which is compensated by cations M.

Alongside tliese teclmiques, microbalance measurements of adsorjDtion capacities and kinetics, microcalorimetric measurements of adsorjDtion processes and temperature-programmed desorjDtion of base molecules have provided useful infonnation about tire tliennochemistry of adsorjDtion processes and tire acidity characteristics of zeolites [46]. 

Presently, the most successful adsorbents arc microporous carbons, but there is considerable interest in other possible adsorbents, mainly porous polymers, silica based xerogels or zeolite type materials. Regardless of the type of material, the above principles still apply to achieving a satisfactory storage capacity. The limiting storage uptake will be directly proportional to the accessible micropore volume per volume of storage capacity. 

Compared to amorphous silica-alumina catalysts, the zeolite catalysts are more active and more selective. The higher activity and selectivity translate to more profitable liquid product yields and additional cracking capacity. To take full advantage of the zeolite catalyst, refiners have revamped older units to crack more of the heavier, lower-value feedstocks. 

It may be noted that the ion exchange capacity is directly connected with the Al-content (each Al provides one negative charge). The zeolite used in detergent formulations (over 10 t/a) is NaA with Si/Al = 1, so with maximum exchange abi ity. 

In contrast to NaZSM-5 zeolite, introduction of CoZSM-5 or HZSM-5 zeolite in the reaction system shifts the "light-off" temperature and modifies the chemistry now not only NO but Nj is formed. Hence, some intermediate species required for Nj formation must be stabilized on the catalyst surface. The "light-off"temperature shifts observed with CoZSM-5 and HZSM-5 catalysts may result from the enhanced redox capacity provided by these catalysts or from the NOj/NO equilibrium achieved more readily than with NaZSM-5. Moreover, equilibrium is approached at a somewhat lower temperature over CoZSM-5 than HZSM-5, and much lower than with the empty reactor (see Fig. 1 of Ref. lOl.The decomposition reaction of NOj into NO -t- occurs readily on these catalysts and the "light-off" temperature of both combustion and SCR is lower in comparison with that of the homogeneous reaction. 

The use of zeolites can also be very helpful in removing a reaction product that unfavourably influences the yield of the desired product. Thus, in the manufacture of antibiotic cefoxitin, the amide acylation results in the generation of HCI, which can be removed by the addition of molecular sieve 3 A or 4 A, which has a large capacity for HCI (Weinstock, 1986). Other examples are reactions in which products like methanol or water retard the rate and prevent the reaction to reach the desired degree of completion. Molecular sieves capture methanol or water very well. 

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