Template extraction from porous clay heterostructures: Influence on the porosity and the hydrothermal stability of the materials

Abstract

The template extraction of two porous clay heterostructures (PCHs), derived from natural montmorillonite (PMH) and synthetic saponite (PSH), has been studied. Extraction in methanol solvent in the presence of cations (H⁺) removes up to 90% of the templates in a reproducible way without a subsequent calcination step for both PMH and PSH. The extracted materials have a high surface area (896 m² g⁻¹ for PMH and 1122 m² g⁻¹ for PSH) and pore volume (0.84 cm³ g⁻¹ for PMH and 1.13 cm³ g⁻¹ for PSH) and are structurally very stable when exposed to hydrothermal conditions. The number of silanol groups on the surface of PCH materials have been titrated by a surface reaction with hexamethyldisilazane, followed by quantification of the liberated NH₃. Based on this method both extracted PMH and PSH have slightly higher silanol numbers than their calcined forms, a very important feature with respect to the amount of catalytic sites that can eventually be grafted onto the surface. The hydrothermal stability under a N₂ flow containing 25% steam for 5 days at different temperatures between 400 °C and 700 °C for both calcined and extracted PMH and PSH was investigated using X-ray diffraction and N₂-adsorption data. A remarkable hydrothermal stability was observed for all samples until 600 °C, a promising characteristic for applications of porous clay heterostructures.