Comparative study of the synthesis and properties of polyoxometalate pillared layered double hydroxides (POM-LDHs)

The pillaring of (NO3)-ZnAl-LDHs with the polyoxometalates (POMs) [PV2W10O4 0](5-), [Mo7O24](6-), [V10O28](6-) and [H2W12O40](6-), using large organic anions like terephthalate for pre-swelling the LDH structure forms a promis ing method for the controlled creation of small micropores. The use of the terephthalate precursor ((T)-ZnAl-LDH) avoided almost completely the format ion of undesired side phases during pillaring, although anion exchange with the large POM complexes proceeded with more difficulty than in the case wh ere (NO3)-LDHs were used as a starting material. Direct pillaring via the ( NO3)-LDHs resulted in multiphased materials, and no correlation was found b etween the M(II)/M(III) ratios in the starting LDHs and the created porosit y. For the [POM]-ZnAl-LDHs pillared via the terephthalate precursor, the la yer charge density arising from the amount of isomorphically substituted Al 3+ in the LDH layers forms the crucial parameter with regard to the created microporosity. Improving the surface area (SA) and micropore volume (mu PV ) values was accomplished by lowering the charge density on the LDH layers (increasing the Zn2+/Al3+ ratio). In this way, a [PV2W10O40]-ZnAl-LDH (Zn2/Al3+ = 4.26) with a SA (BET) of 166 m(2)/g and a mu PV of 0.047 cm(3)/g wa s formed. For the different types of pillars, small micropores were formed due to the pillaring process. In the case of the smaller POM complexes [Mo7O24](6+) a nd [V10O28](6+), an increase in mu PV and SA was not accompanied by a detec table shift in average pore size, which was the case for the second group o f complexes, [PV2W10O40](5-) and [H2W12O40](6-). Due to their larger dimens ions, mainly micropores between 0.71 and 1.06 nm were created at high Zn2+/ Al3+ ratios, together with a substantial amount of pores smaller than 0.71 nm.