THE EFFECT OF THERMAL-TREATMENT ON THE PROPERTIES OF HYDROXY-AL AND HYDROXY-GA PILLARED MONTMORILLONITE AND BEIDELLITE

The pillaring process of montmorillonite and beidellite with Al and Ga polymers has been studied using XRD, IR, Al-27, Ga-71, and Si-29 MAS NMR, TGA, TEM, N2 adsorption and chemical analyses. The Al adsorption maximum for montmorillonite is close to 5.5 mEq Al/g clay, whereas the maximum for Ga is higher. Basal spacings of both Ga- and Al-pillared clays vary between 16.7 and 18.8 angstrom. Freeze-drying of pillared products followed by calcination yielded more regular pillared structures. Pillaring montmorillonite increased the BET surface area from 35 M2/g to 350 M2/g mainly by the creation of micropores <20 angstrom in diameter. The Al-pillared clays are thermally stable to approximately 700-degrees-C. Calcination of pillared montmorillonite liberates protons from the pillar, which diffuse into the clay sheet, lowering the thermal stability. In pillared beidellite, mainly silanol groups are formed by breaking Si-O-Al bonds. NO reaction is observed between pillars and montmorillonite upon calcination, whereas in pillared beidellite a structural transformation links the pillar to inverted tetrahedra of the tetrahedral sheet. The basal spacing of Ga-pillared montmorillonite collapses to 9.5 angstrom at 350-degrees-C due to the Ga polymer decomposing to Ga3+ cations.