Examination of Coked Surfaces of Pillared Rectorite Catalysts with the Atomic Force Microscope

Abstract

The reaction of a sample of natural rectorite with an aluminum chlorhydroxide solution containing the dodecameric [Al13O4(OH)24 (H2O)12]7+ ion generates a heat-stable microporous structure with surface area available to sorption and catalysis. An atomic force microscope (AFM) operating in contact mode has produced images showing that the pillared rectorite surface is free from Al species, implying that all the [Al13O4(OH)24(H2O)12]7+ ions are located inside the clay montmorillonite-like layers. As expected, nanometer-scale images of the clay tetrahedral sheet show a surface that consists of rings of basal oxygens arranged in a hexagonal symmetry. White spots, representative of the three basal oxygens of SiO4 tetrahedra, have repeat distances of 0.54 and 0.97 nm, respectively, in close agreement with the rectorite unit cell dimensions. Thermal and hydrothermal treatments used to prepare the pillared rectorite for microactivity testing (MAT) have little effect on the molecular parameters of the clay surface. After cracking gas oil under MAT conditions, AFM images of the spent clay catalyst reveal that surface carbon preferentially deposited on or near the three basal oxygens of the SiO4 units that constitute the clay silicate layer. As a result, nanometer-scale images of the coked pillared clay surface contain well-defined hexagonal arrangements of white spots having next-neighbor and lateral distances of 0.70 and 1.23 nm that exclude the possibility of graphite formation. After regeneration in flowing air at 760°C/2 h to remove carbon deposits, the pillared clay catalyst resumes its original coloration and the nanometer-scale parameters of the rectorite surface are restored.