Unusual water vapor adsorption behavior of montmorillonite pillared with ceramic oxides

A montmorillonite was pillared with oxide pillars such as Al₂O₃, ZrO₂ and TiO₂ to generate permanent microporosity in the interlayers. The X-ray powder diffraction analysis and nitrogen and water adsorption measurements indicated that the pillars were stable at least up to 600°C. The slit widths (pore size) are on the order of 7-9 Å for Al₂O₃, 8-10 Å for ZrO₂ and 14-17 Å for TiO₂ pillared clay. The TiO₂ pillared clay exhibited Brunauer type IV isotherm that fits BET linear equation for both nitrogen and water. Although Al₂O₃ and ZrO₂ pillared clays followed Brunauer type I isotherm with nitrogen, their water adsorption isotherms exhibited unusual shapes and did not fit either Langmuir or BET equation. The unique pore sizes of these pillared clays and low affinity of their surfaces for water, which determine the amount and rate of water adsorption at particular relative pressure (P/P_O), were demonstrated to be responsible for the unusual shapes of the isotherms. The water capacities of these pillared clays were found to be 200-300 mg/g of degassed sample, which are comparable to commercially available desiccants such as zeolites and silica gels. The application of dessicant materials in gas-fired cooling and dehumidification equipments (air conditioning systems) demands an ideal isotherm shape (moderate type I), a quality lacking both in zeolites and silica gels. Pillaring led to improved water adsorption isotherm shape as compared to the original montmorillonite. Further improvement in the shape of isotherm may be achieved by introducing higher degree of hydrophilic or polarity components in these pillared clays.