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The ionic exchange of the NaX zeolite by Cu 2+ and/or Zn 2+ cations is progressively driven while following the development of some of its characteristic: crystallinity by XR diffraction, profile of isotherms, RI criterion, isosteric adsorption heat and microporous volume using both the Dubinin–Radushkevich (DR) equation and the t -plot through the Lippens–de Boer method which also makes it possible to determine the external surface area. Results show that the cationic exchange process, in the case of Cu 2+ introduced at higher degree, is accompanied by crystalline degradation for Cu( x )X, in contrast to Zn 2+ -exchanged zeolite X. T his degradation occurs without significant presence of mesopores, because the RI criterion values were found to be much lower than 2.2. A comparison between the binary and ternary exchanges shows that the curves of CuZn( x )X are clearly below those of Zn( x )X and Cu( x )X, whatever the examined parameter. On the other hand, the curves relating to CuZn( x )X tend towards those of Cu( x )X. This would again confirm the sensitivity of the crystalline structure of CuZn( x )X with respect to the introduction of Cu 2+ cations. An original result is the distortion of the zeolitic framework of X zeolites at middle exchange degree, when Cu 2+ competes with another divalent cation, such as Zn 2+ , for the occupancy of sites distributed within zeolitic cavities. In other words, the ternary exchange accentuates the crystalline degradation of X zeolites. An unexpected result also is the noncorrelation between crystal damage and the external surface area. |
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