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

Dissolution of large platelet-like (similar to 100 mum) crystals of the fau jasitic zincophosphate-X (ZnPO-X) were examined using Raman microprobe spec troscopy in the presence of a series of monovalent ions. Sodium ions did no t alter the ZnPO-X framework, whereas in the presence of H+, Li and Cs+, th e ZnPO-X transformed to hopeite (Zn-2(PO4)(3)), the zincophosphate analog o f LiA(BW) and CsZnFO4, respectively. The broadening of the symmetric C-N st retching 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 distor tions. Ion-exchange led to distortions of the framework and promoted the hy drolytic instability of the ZnPO-X framework. It is being proposed that in the case of H+ exchange, protons penetrate into the structure and dissoluti on occurs by solubilization of the topmost layers of the crystal which lead s to a local supersaturated state that results in nucleation of hopeite. Gr owth of hopeite occurs via nutrients available from dissolution of succeedi ng layers of the ZnPO-X. For the Li+ and Cs+, considering the neutral pH du ring exchange, proton penetration into the framework should be negligible. Dissolution then must begin at the outermost crystal surface and the solubi lized 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.