Stability of Pillared Clays: Effect of Compaction on the Physicochemical Properties of Al-Pillared Clays

The stability of alumina-pillared montmorillonite clays, prepared at various Al\clay ratios, was investigated by compaction at various pressures. The physical properties of the pillared clays, before and after compaction, were studied by powder XRD and nitrogen sorption. We found that the Al\clay ratio used to prepare the pillared clays (and therefore the resulting pillar density) is not a critical factor in determining the overall mechanical stability. However, pillared clays with a higher density were subject to greater decreases in textural properties (surface area and pore volume) after compaction. It was found that the pillared clays (regardless of pillar density) are generally stable up to ca. 5.7 tons cm-2. Mechanical pressure below 5.7 t cm-2 resulted only in small decreases in basal spacing, surface area, and pore volume. This was, however, accompanied by significant changes in the proportion of micropore surface area and pore volume. Compaction at a pressure of ca. 7.5 t cm-2 decreased the surface area and pore volume by up to 20%, whereas after compaction at 8.3 t cm-2 the structural ordering of the pillared clay is significantly disrupted and the surface area and pore volume reduce by 55%. The porosity of the pillared clays also changes significantly with the micropore surface area and pore volume decreasing by 35% and 70% after compaction at 7.5 and 8.3 t cm-2, respectively, i.e., the pillared clays become less microporous. The (Brnsted) acidity of the pillared clays is affected to a lesser extent by compaction because the acid sites are mainly located on the clay layers rather than on the pillars. Consequently the loss of pillars after compaction has a limited effect on acidity.