Effects of pH and Mg:Ga ratio on the synthesis of gallium-containing layered double hydroxides and their polyoxometalate anion exchanged products

We have synthesized a series of [Mg-(1-x(2+))M-x(3+)(OH)(2)][TA((x/2)(2-))]. zH(2)O (M3+ = Ga3+ or Al3+ and TA = C6H4(COO-)(2)) from mixed-metal chloride solutions with initial M3+ mole fractions of 0.20, 0.25, and 0.33, at constant pH values of 6.8, 7.7, 8.6, and 10.0. Terephthalate-containing products were obtained only under conditions that resulted in a Mg:M3+ ratio of 2:1 in the mixed metal hydroxide layers, indicating that a high layer charge density is necessary to incorporate terephthalate anions between the inorganic layers. Anion-exchange reactions were subsequently performed to produce LDHs intercalated with PW11O397- and H2W12O406- anions, yielding products that displayed sharp powder diffraction patterns. In each case, the surface area of the POM-intercalated product was found to be approximately 40-45 m(2)/g, indicating that the micropore system is totally blocked and that the materials are virtually nonporous; this may result from the high layer charge density of the LDH-terephthalate precursors.