Exchangeability degree

Cu ions are incorporated into molecular sieve as divalent (Cu2+(HzO)6)2+ or monovalent (Cu2+X (H20)s)+ complex cations [11]. Thus, after dehydration and subsequent reduction, Cu ions are placed in viscinity of one or two framework aluminium atoms [1], As two cationic sites with two close aluminium atoms are present in (A1)MCM-41 (cf. Chapters 3.1. and 3.2.), four different coordinations of the Cu+ ions in (A1)MCM-41 represent two cationic sites with two close aluminium atoms and two cationic sites balanced by a single aluminium atom. Thus, besides two cationic sites accommodating divalent cations, two other cationic sites being enable to accommodate only monovalent cations are present in the (A1)MCM-41 molecular sieve. Because only low exchange degree can be reached for divalent cations (cf. Table 1), sites with isolated aluminium atoms represent majority of cationic sites in (A1)MCM-41 with Si/Al > 20. 

The adsorption isotherms of N2 and O2 were measured by constant volume method at ambient temperature. The adsorption properties deduced from the isotherms further confirmed the high crystallinity and Li exchange degree of LiLSX prepared in our lab. 

As mentioned above, Li content in LiNaLSX showed a threshold value at about 2/3, above which the nitrogen adsorption capacity increased rapidly [10], as shown in Fig.3 (a). Ca exchanged LSX can also be a good adsorbent for N2 [7]. We noticed that N2 adsorption capacity of CaNaLSX as a function of Ca exchange degree gives a threshold value at 31% (about 1/3) [7], different from that of LiNaLSX, as shown in Fig.3 (b). 

Figure 1. Dependence of isomerization and hydrogenation activity of mordenite on exchange degree...

The hydrothermal stability of aluminosilicates varies with the structure [1], the Si/Al ratio [2] of their frameworks as well as with the type [3] and the exchange degree [4] of the cations. Corresponding to the reactions... 

Zinc was introduced in H-ZSM-5 (Si/Al=15, synthesized without template by Chemie AG Bitterfeld) by threefold ion exchange with Zn(N03)2 solution at 353 K (proton exchange degree 75 %, corresponding to 2.4 wt% Zn, denoted as ZnH-[Al]MFI). [Zn]MFI was synthesized according to the rapid crystallization method [11] at Kyoto University (Si/Zn=20, 40 and 100). Protonic MFI was obtained after removal of template at 813 K by ion exchange with NH4NO3 solution at 353 K and subsequent calcination at 813 K. MFI structure was confirmed by X-ray diffraction. 

Figure 4. Effect of Cu ion exchange degree in Ga(Imp)Cu(Ex) catalyst at 823K and 11.4 ghmof. ...

In a word, ion-exchange with the assistance of microwaves is feasible, convenient, and fast. It can reach a higher exchange degree than can traditional methods and make the inaccessible ions in traditional methods exchangeable. This method is especially appropriate for the laboratory preparation of ion-exchanged zeolite molecular sieves. The microwave technique is very successful in the synthesis of microporous crystals, modification of the properties of zeolites, secondary synthesis of microporous materials, and the preparation of ultra-fine particles and films, and has attracted the wide interest of chemists in the field of molecular sieves. 

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