Control of microporosity of Al₂O₃-pillared clays: Effect of pH, calcination temperature and clay cation exchange capacity

Effects of the degree of hydrolysis of the Al (expressed as the pH or OH^-/Al of the pillaring solution), the calcination temperature and the ion exchange capacity of the clay on the ultimate microporisity of the Al₂O₃-pillared clays are studied. The results showed that clays prepared from an olimeric solution with an OH^-/Al of 2.2 and calcined at 400°C yielded the highest overall surface area and the largest micropore volume. Micropore volumes of clays calcined at 600°C were consistently lower than those same clays calcined at 400°C. The overall surface area followed the same trend. This is due to a collapse/sintering of the Al₂O₃ pillars with calcination at the higher temperature, which reduces the interlayer spacing as confirmed by XRD. The pillared clays generally contained a bimodal pore size distribution with peak dimensions at 0.4 and 0.55 nm. Two montmorillonites with widely different cation exchange capacities (CEC) were used: Wyoming bentonite (CEC=0.74 meq/g) and Cheto (Arizona) bentonite (CEC=1.4 meq/g). The high CEC resulted in a higher pillar density and consequently a larger ultramicroporosity (with 0.4 nm pores) while the pore volumes of the 0.55 nm pores were the same. In all cases, the peak pore sizes were less than the free interlayer spacing as determined by X-ray diffraction (XRD) analysis. Dubinin-Astakhov transformed isotherms were used to qualitatively characterize the micropore distribution of the clays.