Tailoring micropore dimensions in pillared clays for enhanced gas adsorption

A systematic investigation has been undertaken for tailoring the micropore structure of the pillared clay. Besides the type of metal oxide (e.g. Al2O3 vs. ZrO2) being used as the pillars, the important factors for determining the micropore structure are OH/Al ratio (for Al2O3-pillared clay), calcination temperature and the starting clay. The effect of the cation exchange capacity (CEC) of the clay on the microporous structure (and consequently the adsorption properties) is reported for the first time. Two clays with widely different CECs are used: Arizona montmorillonite (CEC=1.40 mequiv./g) and Wyoming montmorillonite (CEC=0.76 mequiv./g). The interlayer spacings of the pillared clays from these different clays are essentially the same, since the interlayer spacing is controlled by the sizes of the oligomers that intercalate between the clay layers. However, the pillar density in the pillared clay is substantially higher with a high CEC in the starting clay, and is shown to be approximately proportional to the CEC. Consequently, the interpillar spacing is substantially lower resulting from the higher CEC. The CH4 adsorption on the pillared clay is nearly doubled by the smaller interpillar spacing, due to the back-to-back overlapping potential in the micropores. The N-2 adsorption was not significantly influenced because of its low polarizability (hence low inductive potential). Increasing the calcination temperature of the Al2O3-pillared clay from 400 degrees C to 600 degrees C can decrease the interlayer spacing, but only by 1 Angstrom (from 8.7 Angstrom to 7.7 Angstrom). The CH4/N-2 adsorption ratio of 2.35 is reached on the Al2O3-pillared Arizona clay that is calcined at 600 degrees C. Finally, the surface and pore volume are influenced by the OH/Al ratio (or pH) during pillaring, since this ratio determines the size and charge of the oligomers. A peak surface area is reached at OH/Al=2.2.