Optimisation of the preparation of an Al-pillared clay: thermal stability and surface acidity

The intercalation of montmorillonite by aluminium requires the use of aluminium chloride as a precursor for the intercalation solution. The best samples were obtained with an Al/OH ratio = 2, a pH of 5.6 and an Al/clay ratio = 5 mmol/g. High surface area materials were obtained with good thermal stability after calcination at 773 K. A study of the acidity of surface groups showed that the Al,, polycation is transformed into a specific form of Al oxyhydroxide with amphoteric properties. The data, when treated in the '1 site, 2 pK' model, may be interpreted by the existence of surface species of type {AlOH(OH2)}, having two constants of deprotonation K-a1 = 10(-5) and K-a2 = 0.32.10(-6). An alternate treatment in terms of the 'multisite, 1 pK' model recently developed by Jagiello et al. is also discussed. (C) 1997 Elsevier Science B.V.