Acidic and Hydrophobic Microporous Clays Pillared with Mixed Metal Oxide Nano-Sols

Silica–metal oxide sol pillared clays have been synthesized from montmorillonite by exchanging interlamellar (Na+) ions with silica sol particles modified with polyhydroxy metal cations. Though the silica sol particle itself is negatively charged in the pH range used in present experiment, 1.5–2.7, the ion-exchange-type intercalation of the silica sol into montmorillonite was realized by modifying the surface charge of the sol particles from negative to positive. The positively charged silica sol particles were prepared by titrating metal aqueous solutions (Mz+=Fe3+, Al3+, Cr3+, and Zr4+) with NaOH in the presence of silica sol particles, which were easily intercalated inbetween the silicate layers of clay. On pillaring of oxide sols and subsequent calcining at 400°C, new porous materials were obtained with high BET surface areas of 320–720 m2/g, pore volumes of 0.24–0.50 ml/g, and basal spacings in the range 40–60 ?. Furthermore, their thermal stability could be remarkably improved up to 700°C. According to the adsorption measurements of nitrogen and solvent vapors, the micropores in the samples of size 9–13 ? dominate due to the multilayer stacking of interlayer sol particles inbetween silicate layers. Temperature-programmed desorption (TPD) spectra of ammonia revealed that the microporous samples are weakly acidic, but with different strengths, depending on the doped metal species.