AL AND ZR PILLARED CLAYS: A STUDY OF THE STRUCTURE BASED ON THERMAL CHANGE USING XRD AND XANES

L. ANDRINI; J.M. MARTÍNEZ; N. RENDTORFF; M.S. CONCONI

Campinhas

Congreso; 29th RAU- Reunion Anual de Usuarios LNLS; 2019

LNLS

Pillared clays (PILCs) are obtained by intercalating polycations of different species and thermal treatment [1].They have multiple technological applications due to their high specific surface and tailorable pore size;characteristics attributable to the natural nanostructure of the parental clay. In the natural clay, thesecharacteristics are lost with thermal treatment, while conserved in the pillared clay due to its higher thermalstability. Furthermore, calcination is necessary because the intercalated clay is metastable, as theinterleaved polycations themselves. Thermal treatment transforms the polycations into stable oxi-hydroxidephases named pillars. X-ray absorption near-edge structure (XANES) has demostrated to be a suitable forclay minerals and clay-based materials characterizations [2]. Different Species have been reported as beenused as precursors of said pillars, being the most studied the Al13-Keggin polycation. Another interestingoption for the pillaring are Zr precursors, as ZrO2 is a promising pillar-material due to its high thermal stabilityand high acidity. This work looks to describe the structure of two pillared clays: an Al13 and a Zr-pillaredbentonite at different firing conditions, comparing those results with the original clay. Particular focus wasmade in the analysis of the local structure of Al and Si. Both pillared clays were prepared using anArgentinian unpurified bentonite. Pillaring reagents used for the pillared clays were a) a polymeric hydroxyl-Al solution and b) a partially hidrolized ZrOCl2 solution. In order to characterize the material, X-ray diffraction(XRD), scanning electron microscopy (SEM) and simultaneous thermogravimetric and differential thermalanalysis (DTA-TG) of the materials were complemented with XANES. The d001 of the montmorillonitestructure collapsed at 400°C, while the Zr-pillared clay maintained this (shifted) value up to 600 °C and theAl-pillared clay up to 800°C. Further thermal treatment produced the loss of the characteristic clay structure,and only a glassy phase with a low amount of quartz and feldspar was observed by XRD in all samples.Thermal treatment caused greater mass loss in the pillared clay due to the presence of the Zr-OH and Al-OH oligomers allocated in the interlayer space of the montmorillonite, being the mass loss higher in the Alpillared clay due to the higher amount of OH- and H2O in the structural formula pillaring precursor. The Al KXANES spectra confirmed the octahedral aluminum coordination in native montmorillonite, being thiscoordination progressively lost with thermal treatments at increasing temperatures, giving place to thepresence of a four folded coordination. In the intermediate treatments, it presented some three and fivecoordination slight contributions. Also, the Si K XANES spectra do not change with temperature, indicatingthe stability of Si environments. The results confirm the potential of synchrotron-based techniques forcharacterization of natural materials and its derived composites, in particular ones with low crystallinity. Inparticular, Zr-pillared montmorillonites aluminums Si and Al K-XANES spectra have not been reportedbefore.