Atomic scale imaging of pillared rectorite catalysts with the atomic force microscope

Abstract An atomic force microscope (AFM) has been used to investigate the surface features of two samples of natural rectorite (from Arkansas) pillared with alumina clusters. AFM images of the surface of natural rectorite consists of a collection of bright spots in a well ordered hexagonal pattern with distances between nearest and lateral neighbors that, in contrast to montmorillonites, do not change after exchange with Al 13 clusters and drying. When MgCl 2 solutions are used to flocculate and separate the clay from its impurities, AFM images of the Mg-rectorite show on its surface spheroids spaced at about 9.0 and 13.0 A, believed to be Mg 2+ ions. After pillaring, atomic scale resolution images of the clay surface consist instead of hexagonal arrays of bright spots with nearest and lateral neighbors at 5.4 and 9.7 A, in agreement with the geometry of the basal plane of 2:1 clay minerals. These dimensions are retained even after steaming at 760°C per 5 h. The clay rigidity has been attributed to the presence of mica-like layers in the pillared rectorite structure. The presence of alumina debris or clusters on the silicate layers was not observed in any image examined, suggesting that (as previously observed in pillared montmorillonites) the expended clay coking tendency during gas oil cracking can be attributed mainly to the strong Lewis-type acidity of the alumina pillars between its montmorillonite-like layers.