The distribution of Ga in Ga-pillared montmorillonites: A transmission electron microscopy and microanalysis study

Abstract

The distribution of Ga in the interlayer of montmorillonite pillared with a Ga 13 polyoxocation complex has been studied by transmission electron microscopy, energy-dispersive X-ray microanalysis (EDX), X-ray mapping and powder X-ray diffraction in combination with N 2 adsorption–desorption. To view the clay layers by TEM, the pillared clay was embedded in Spurrs resin in a preferred orientation, and sectioned with an ultramicrotome perpendicular to the layers. Montmorillonites pillared with Al 13 and Al 12Ga complexes were also prepared for microanalysis in the TEM. The Ga X-ray peaks could be easily distinguished in the EDX spectra, allowing concentrations relative to other elements to be determined. Elemental X-ray maps for Ga, Si and Al in the Ga 13-pillared clay cross-sections demonstrated that the Ga was homogeneously distributed throughout the crystal thickness. Comparison of the analytical data with that from the Al 13 and Al 12Ga-pillared clays and the starting material suggested that an approximately constant amount of the intercalated species per amount of Si in the clay became incorporated into the structure in each case. Calculation of the formula for the Ga-pillared montmorillonite showed that 0.89 Ga is present per formula unit containing 8 (Si + Al), which is equivalent to 20 silicate rings, each consisting of 6 tetrahedra, for every Ga 13 pillar. The actual dimension of the pillar, based results from the elemental analyses and XRD is 8.7 Å and the mean distance between the pillars is 44.3 Å, which is in good agreement with the average pore size of 39 Å obtained by N 2 adsorption–desorption measurements. This study shows a new approach for obtaining more detailed information on the pillars in pillared clay by combining analytical data from X-ray microanalysis with measurements by XRD and N 2 adsorption–desorption.