Physical and chemical stabilities of pillared clays containing transition metal ions

Chromium(III) and iron (III) ions exchanged both into the pillars and into the micropore structure of pillared clays are compared during thermal treatments from 120°–480°C for various times and in different atmospheres (air and nitrogen). Thermal gravimetric analysis (TGA) is used to monitor the %H₂ O lost from 20°–720°C and X-ray diffraction (XRD) is used to elucidate structural information. Both untreated and heat-treated clays are then examined for resistance to acid-attack using solutions of HCl having a concentration range of 1.25 × 10⁻⁴ N to 1N. Transition-metal leaching during acid treatment is monitored via atomic absorption (AA), and found to increase as the concentration of HCl increases. The % loadings of transition metal (TM) is seen to be an important factor with respect to stability of the clays. Since iron-containing materials in most cases contain higher concentrations of TM than chromium-containing clays, these materials have undergone more decomposition of the clay lattice and are less stable to acid attack. The exception is Fe + ACH-BL where iron(III) ions have been incorporated into the pillars of an alumina pillared clay. When this material is heat-treated it becomes more resistant to 0.05N HCl than ACH-BL without TM ions.