Structural evolution of SiO₂-ZrO₂ nano-sol intercalated clays upon pillaring reaction

A two-dimensional layered nanohybrid with a high specific surface area has been prepared by ion exchange reactions between the sodium ions in montmorillonite and the positively charged Zr-coated SiO₂ sol particles. The basal spacing increases from 12.5 Å to 26 Å upon intercalation due to the insertion of a SiO₂-ZrO₂ nano-sol into the interlayer space of montmorillonite. Upon calcining at 300°C, it transforms into a porous nanohybrid with a basal spacing of 22.6 Å. The N₂ adsorption-desorption isotherms were characterized as being of type IV according to the BDDT classification, indicating the existence of a large number of micro- and mesopores. From its hysteresis curves, one can classify the nanohybrid as being of the H3 type with slit-shaped pores by the IUPAC classification. The estimated BET specific surface area and average micropore size are about 358 m² g^-1 and 12 Å, respectively, with the latter value is similar to the gallery height of the sample indicating that the SiO₂-ZrO₂ sol particles are intercalated to form a monolayer. The local structural evolution of the Zr species in SiO₂-ZrO₂ sol particle has been investigated systematically by X-ray absorption spectroscopy at the Zr K-edge with respect to the calcination temperature. The EXAFS spectroscopic results confirm that the surface of the nano-sized SiO₂ sol particles is coated with the Zr species.