Layered Double Hydroxides Intercalated by Polyoxometalate Anions with Keggin (α-H2W12O406-), Dawson (α-P2W18O626-), and Finke (Co4(H2O)2(PW9O34)210-) Structures

The pillaring of Mg(3)Al layered double hydroxides (LDHs) by the title polyoxometalates (POMs) was accomplished by ion exchange reaction of the LDH-hydroxide and -adipate precursors with the POM anion at ambient or refluxing temperatures. The structural, thermal and textural properties of the LDH-POM intercalates were elucidated based on XRD, FTIR, TEM, EDS, and N(2) adsorption-desorption studies. A gallery height of approximately 10 Å was observed for the LDH intercalated by the symmetrical Keggin POM, whereas two different gallery heights were found for the cylindrical Dawson (14.5 and 12.8 Å) and Finke (13.3 and 12.6 Å) anions, depending on the preparation temperature. The differences in POM orientations were rationalized in terms of different electrostatic and hydrogen-bonding interactions between the POM pillars and the LDH layers. Upon thermal treatment at >/=100 degrees C, the intercalated Dawson and Finke POM ions exhibited only one gallery orientation, regardless of synthesis conditions. The crystalline microporous structures were retained upon heating each LDH-POM intercalate in N(2) to 200 degrees C. Pillaring in all cases was accompanied by the formation of a poorly ordered Mg(2+)/Al(3+) salt impurity that formed on the external surfaces of the LDH crystals.