PROPERTIES AND CHARACTERIZATION OF AL2O3 AND SIO2-TIO2 PILLARED SAPONITE

A saponite pillared with a single (Al2O3) or a mixed (SiO2-TiO2) oxide exhibited basal spacings of 16-19 and 30-40 angstrom, respectively. The pillared structures were found to be stable up to 700-degrees-C. Water, nitrogen, and high resolution argon adsorption were used to study the effect of thermal treatments on surface chemistry, pore structure, and surface area of these pillared clays. The pillared saponites exhibited a hydrophobic behavior at temperatures > 500-degrees-C, whereas such behavior was observed at greater-than-or-equal-to 300-degrees-C for montmorillonite. Most of the micropores in the Al2O3 pillared clays were < 10 angstrom, whereas the SiO2-TiO2 pillared clays showed a broad distribution of pores in both micropore and mesopore regions. The SiO2-TiO2 pillared samples possessed higher surface area compared with Al2O3 pillared clays. The percent decrease in surface area was smaller for pillared saponites compared with pillared montmorillonites when calcined from 300-degrees to 700-degrees-C, indicating a higher thermal stability of the former. The pillared clays were also characterized by solid state Al-27 and Si-29 magic-angle spinning nuclear magnetic resonance (MAS/NMR) spectroscopy. There was no direct evidence of cross-linking (covalent bonding between the clay layer and pillar) in montmorillonite irrespective of the types of pillars. In saponite, however, a significant structural modification took place. Al-27 spectra of Al2O3 pillared saponite heated at greater-than-or-equal-to 300-degrees-C appear to indicate an increase in Al(VI) as a result, at least in part, of initiation of hydrolytic splitting of Si-O-Al bonds. The actual release of Al from the tetrahedral sheet probably occurred at a temperature > 500-degrees-C and completed around 700-degrees-C with the formation of Si-O-Si linkages. The decreased intensity of peak due to Si(1Al) in Si-29 spectra of the sample heated at 700-degrees-C corroborates the Al-27 MAS/NMR results. Additionally, the Si-29 spectra indicated a cross-linking between SiO4 (clay sheet) with Al2O3 pillars, which could be achieved by inverting some silica tetrahedra into the interlayer. Al-27 and Si-29 spectra Of SiO2-TiO2 pillared saponite also showed the trend similar to that exhibited by Al2O3 pillared saponite, indicating that the crystal chemistry of the host may be more important than the nature of pillars in the structural modification and cross-linking behavior of thermally treated pillared clays.