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) PV₂W₁₀O₄₀]^5–, Mo₇O₂₄]^6–, V₁₀O₂₈]^6– and H₂W₁₂O₄₀]^6–, using large organic anions like terephthalate for pre-swelling the LDH structure forms a promising method for the controlled creation of small micropores. The use of the terephthalate precursor ((T)-ZnAl-LDH) avoided almost completely the formation of undesired side phases during pillaring, although anion exchange with the large POM complexes proceeded with more difficulty than in the case where (NO₃)-LDHs were used as a starting material. Direct pillaring via the (NO₃)-LDHs resulted in multiphased materials, and no correlation was found between the M(II)/M(III) ratios in the starting LDHs and the created porosity. For the POM]-ZnAl-LDHs pillared via the terephthalate precursor, the layer charge density arising from the amount of isomorphically substituted Al³⁺ in the LDH layers forms the crucial parameter with regard to the created microporosity. Improving the surface area (SA) and micropore volume (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 m²/g and a PV of 0.047 cm³/g was 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⁺ and V₁₀O₂₈]⁶⁺, an increase in PV and SA was not accompanied by a detectable shift in average pore size, which was the case for the second group of complexes, PV₂W₁₀O₄₀]^5– and H₂W₁₂O₄₀]^6–. Due to their larger dimensions, 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.