ALUMINA-PILLARED MONTMORILLONITE - EFFECT OF THERMAL AND HYDROTHERMAL TREATMENT ON THE ACCESSIBLE MICROPORE VOLUME

In the search for materials having larger pores than zeolites, the concept of pillared clays gradually emerged. The substitution of zeolite-based commercial catalysts by pillared clays requires the improvement of the thermal and hydrothermal stability of the latter. Characterization of pillared clays has essentially consisted of nitrogen BET specific surface area and X-ray diffraction powder measurements. The study of the texture of pillared smectites really requires the assessment of their microporosity. However, little detailed information is available on the evolution of microporosity with thermal and hydrothermal treatments of these materials. The aim of this work was to supply such information on an alumina-pillared montmorillonite. The effect of La(III) addition was also studied. Up to now the thermal stability of the pillars has been over-estimated from measurement of the basal spacing. However, it has been found that the accessible volume decreases with increasing calcination temperature at a much greater rate than does the basal spacing. Therefore, the generally accepted conclusion that alumina pillars are stable at temperatures even higher than 500-degrees-C may give a misleading picture of pore structure evolution with calcination temperature. Lanthanum appears to exert a small positive effect on the thermal and hydrothermal stability of the pillared montmorillonite.