MECHANISM OF THE TOPOTACTIC FORMATION OF GAMMA-ZIRCONIUM PHOSPHATE COVALENTLY PILLARED WITH DIPHOSPHONATE GROUPS

The topotactic reaction of gamma-ZrPO4[O2P(OH)(2)]. 2H(2)O (gamma-ZrP) with benzenediphosphonic acid was examined in water and in acetone-wa ter mixtures. This reaction was found to take place in water only on t he external surface of the microcrystals, and pillared compounds were never obtained, even after very long reaction times. On the contrary, covalently pillared compounds were quickly obtained in acetone-water m ixtures. The mechanism of the latter topotactic reaction was investiga ted by determining the rate of the phosphate groups released and the r ate of the benzenediphosphonates taken up by gamma-ZrP over a long tim e (50 days). These data showed that pillared derivatives of gamma-ZrP can be obtained because colloidal dispersions of exfoliated lamellae a re formed in acetone-water mixtures. The diphosphonate group acts init ially as a monovalent species, replacing only one dihydrogen phosphate group on the surface of the exfoliated gamma-lamellae. The colloidal and partially derivatized lamellae thus formed can interact with each other by forming polylamellar pillared systems. When the number of pil lared lamellae exceeds a given value (usually 5-6), flocculation of th e colloidal gamma-ZrP takes place. Topotactic reactions between packet s of pillared lamellae may also continue in the flocculated system. Th erefore, the average number of the pillared lamellae slowly increases over time.