Influence of iron removal on the synthesis of pillared clays: A surface study by nitrogen adsorption, XRD and EPR

The present paper reports a surface study on the synthesis of pillared clays before and after removal of iron oxides (with sodium dithionite) from the clay used as starting material. Four pillared clays were synthesised with Al- or Al-Fe-polyhydroxocationic solutions: two solids were prepared from the iron-removed clay and the others from the non-pre-treated clay. All the solids were characterised by X-ray diffraction (XRD), electron paramagnetic resonance (EPR), differential adsorption potential distributions (DAPDs) and fractal dimension from nitrogen adsorption. The natural clay was also analysed by transmission electron microscopy (TEM) to verify iron oxide clusters. XRD and EPR analyses revealed that iron-reduction with sodium dithionite, under the studied conditions, did not affect the clay mineral structure and the clay became successfully pillared in all cases; however, nitrogen adsorption showed a decrease of external surface areas and an increase of micropore areas and volumes as a consequence of chemical pre-treatment. DAPDs indicated that iron-oxide removal reduced the surface heterogeneity of the pillared clays but enhanced the micropore fraction in the solids.