NEW VARIETIES OF ZINC-CHROMIUM-SULFATE LAMELLAR DOUBLE HYDROXIDES

The synthesis of zinc-chromium lamellar double hydroxides (LDHs) inter calated by sulfate anions is carried out with a onestep method that co mbines coprecipitation and anionic exchange, The materials obtained in a reactor containing a sodium sulfate solution are compared with thos e prepared from lithium, potassium, and ammonium sulfate solutions. Th e compounds are characterized by chemical analysis, powder X-ray diffr action, and infra-red spectroscopy. Depending on pH conditions, the wa shing process, and the nature of the monovalent cation, the sulfate in tercalated compounds exhibit two 2H hexagonal varieties with different layer spacings d approximate to 8.9 Angstrom and d approximate to 11 Angstrom at room temperature; this last phase is related to the insert ion of alkaline interlamellar cations. Under dry air, the loss of inte rcalated water leads to a 3R rhombohedral variety with d approximate t o 8.2 Angstrom. A second rhombohedral variety with d approximate to 10 .9 Angstrom is reversibly obtained from the ''8.9 Angstrom'' phase und er high relative humidity. These four [Zn-CrSO4] LDHs differ by their stacking sequences and interlamellar distances and can be selectively obtained. (C) 1997 Academic Press.