The cation exchange capacity of a natural phillipsite-rich sample from the Neapolitan Yellow Tuff, Southern Italy (treated in order to obtain a 95 wt% zeolite-rich sample composed mainly of phillipsite and minor chabazite) for Cu was evaluated using the batch exchange method. The sample had previously been exchanged into its monocationic form (Na), and then used for the equilibrium studies of the exchange reaction 2Na+ ⇆ Cu2+. Reversibility ion exchange tests were performed. The isotherm displays an evident hysteresis loop. Interestingly, the final Cu-exchanged polycrystalline material was green-bluish. Natural, Na- and Cu-exchanged forms were analyzed by X-ray powder diffraction, and the Cu-phillipsite was also investigated by transmission electron microscopy (TEM). Structure refinement of Cu-phillipsite was performed by the Rietveld method using synchrotron data, and it indicates a small, but significant, fraction of Cu sharing with Na two–three independent extra-framework sites. The TEM experiment shows sub-spherical nano-clusters of crystalline species (with average size of 5 nm) lying on the surfaces of zeolite crystals or dispersed in the amorphous fraction, with electron diffraction patterns corresponding to those of CuO (tenorite-like structure) and Cu(OH)2 (spertiniite-like structure). X-ray and TEM investigations show that Cu is mainly concentrated in different species (crystalline or amorphous) within the sample, not only in phillipsite. The experimental findings based on X-ray and TEM investigations, along with the hysteresis loop of the ion exchange tests, are discussed and some general considerations about the mechanisms of exchange reactions involving divalent cations with high hydration energy are provided.
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