Cationic versus Anionic Pillared Layered Substrates: Comparison of the Pillaring Reactions and the Resulting Porosities

Abstract

Cationic clays and Layered Double Hydroxides (LDHs) are both layeredion exchangers, in which a stable (micro)porosity can be induced via apillaring process. For the cationic clays, the [Zr]-pillaring of hectoritecreates a broad micropore distribution with the maximum at 1.42–2.12nm. The [Al]-equivalent exhibits a narrower distribution, with pores between0.71 and 1.06 nm being dominant. In case of the [Zr]-pillared form a surfacearea of 294 m²/g and a micropore volume of 0.118cm³/g have been obtained. The same reaction on the syntheticlaponite clay reveals a much higher surface area (606 m²/g)and porosity (µPV = 0.336 cm³/g). Forlaponite, extra pores are created in the supermicropore-small mesoporeregion due to the preferential edge-to-face and edge-to-edge stacking of itssmaller sized clay layers.

For the pillaring of MgAl- and ZnAl-LDHs with polyoxometalates (POMs),using large organic anions for pre-swelling purposes forms the mostpromising method for the creation of stable pores. It avoids the formationof sidephases, and gives rise to medium(-high) µPVs. Charge density onthe layers forms the key factor, lowering it improves the porositycharacteristics significantly. [Fe(CN)₆]-MgAl-LDHs exhibitmore spectacular properties, with surface areas and µPVs exceedingthose of pillared hectorite. A variation in the charge density via theM ^II/M ^III ratio optimizes theporosity properties. A M ^II/M ^IIIratio of 3.33 results in a SA of 499 m²/g and a µPV of0.177 cc/g. For LDHs, both types of pillars create mainly small micropores,with a diameter smaller than 0.71 nm.