Use of Competitive Ion Exchange for Intercalation of Montmorillonite with Hydroxy-Aluminum Species

Abstract

Three montmorillonite samples, from Wyoming (particle size 2 t~m. The X-ray powder diffraction (XRD) pattern showed that most of the material was not intercalated after refluxing at 80"C for 2 hr; several redispersions of the pillared clay in hot water were required to obtain a single sharp XRD line at 18.6 A. The diffusion of the A1 cations therefore limited the cation- exchange process. The addition of NH4 § to the pillaring solution decreased the amount of A1 fixed by the clay from 31 (no NH4 § to 26 wt. % (NH4+/A1 = 10), but increased slightly the microporous volume of the resulting pillared material from 0.13 to 0.14 ml/g. The competition between NH4 + and Al-hydroxy cations for ion-exchange sites forced the A1 species into the interlayer space and increased the homogeneity of the resulting pillared material. The influence of particle size on the amount of Ai exchanged was then reduced. After calcination in air at 700"C the pillared Wyoming clay retained a microporous volume of 0.09 ml/g if no NH4 + was added, compared with 0.i I mug for a NH4+/A1 ratio of I0. The Fe content of the clay affected the thermal stability of pillared clays at 800"C: for two samples prepared by competitive ion exchange (NHa§ = 10) the microporous volume increased from 0.06 ml/g for 6.4 wt. % Fe203 to 0.08 ml/g for 0.5 wt. % Fe203. The acidity of the pillared clay was determined by calorimetric adsorption of ammonia at 150"C, on samples calcined at 500"C. Pillared montmorillonite prepared using non- competitive intercalation showed a weak acid strength: most sites adsorbed ammonia with an enthalpy of 65 kJ/mole. The intercalation of the same original clay using competitive ion exchange yielded a solid that possessed both weak (heat = 60 kJ/mole) and strong (120 kJ/mole) acid sties. Competitive ion exchange appears to be a simple means of preparation of large quantities of pillared clays of reasonable thermal stability and higher acidity than those obtained by conventional methods.