Synthesis and Characterization of the Microporosity of Ion-Exchanged Al₂O₃-Pillared Clays

We have studied the effects of postsynthesis ion exchange on the microporous properties of Al₂O₃-pillared clays. These changes were effected by incorporating differently-sized charge-compensating cations from the alkali and alkaline earth series into the pillared clay by standard ion-exchange techniques. The resulting materials were characterized with respect to their surface area, interlayer spacing, and micropore size distribution. The micropore size distributions were determined from their N₂ adsorption isotherms at 77 K using the slit-pore model of the Horvath-Kawazoe equation (and applying the Cheng-Yang correction). The results showed a bimodal micropore size distribution for the unaltered pillared clay and for all of the ion-exchanged pillared clays. In every case, the micropore volume for those pores in the lower distribution (<0.45 nm) increased with increasing ionic radius of the charge-compensating cations used in the ion exchange. In each case, the micropore volume for those pores in the upper distribution (>0.45 nm) decreased with ion exchange. X-ray diffraction (XRD) analysis showed the ion-exchange procedure had no effect on the interlayer spacings of the pillared clays. In all cases, the peak pore sizes were less than the free interlayer spacing as determined by XRD analysis. Dubinin-Astakhov-transformed isotherms were used to qualitatively characterize the micropore distribution of the clays. The Dubinin-Astakhov equation was used to determine the effect of postsynthesis ion exchange on the relative structural and energetic heterogeneity of the material.