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 2+ (1 - x ) M 3+ x (OH) 2 ][TA 2 - (x/2) ]·zH 2 O (M 3+ = Ga 3+ or Al 3+ and TA = C 6 H 4 (COO - ) 2 ) from mixed-metal chloride solutions with initial M 3+ 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:M 3+ 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 PW 11 O 39 7 - and H 2 W 12 O 40 6 - 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.