Raman microprobe studies of dissolution of microporous faujasitic-like zincophosphate crystals

Dissolution of large platelet-like (similar to 100 mum) crystals of the faujasitic zincophosphate-X (ZnPO-X) were examined using Raman microprobe spectroscopy in the presence of a series of monovalent ions. Sodium ions did not alter the ZnPO-X framework, whereas in the presence of H+, Li and Cs+, the ZnPO-X transformed to hopeite (Zn-2(PO4)(3)), the zincophosphate analog of LiA(BW) and CsZnFO4, respectively. The broadening of the symmetric C-N stretching band (similar to 766 cm(-1)) of tetramethylammonium trapped in the sodalite cages of ZnPO-X was found to be a marker for the framework distortions. Ion-exchange led to distortions of the framework and promoted the hydrolytic instability of the ZnPO-X framework. It is being proposed that in the case of H+ exchange, protons penetrate into the structure and dissolution occurs by solubilization of the topmost layers of the crystal which leads to a local supersaturated state that results in nucleation of hopeite. Growth of hopeite occurs via nutrients available from dissolution of succeeding layers of the ZnPO-X. For the Li+ and Cs+, considering the neutral pH during exchange, proton penetration into the framework should be negligible. Dissolution then must begin at the outermost crystal surface and the solubilized zinc and phosphate ions combine with the cations in solution to form LiA(BM? and CsZnPO4. (C) 2001 Elsevier Science B.V. All rights reserved.