Al and Zr pillared clays: a study of the structure based on thermal change using XRD and XANES

JUAN MANUEL MARTÍNEZ; M. SUSANA CONCONI; LEANDRO ANDRINI; NICOLÁS M. RENDTORFF

Otro; Annual Users Meeting LNLS/CNPEM; 2019

Brazilian Synchrotron Light Laboratory

Pillaredclays (PILCs) are obtained by intercalating polycations of different speciesand thermal treatment. They have multiple technological applications due totheir high specific surface and tailorable pore size; characteristicsattributable to the natural nanostructure of the parental clay. In the naturalclay, these characteristics are lost with thermal treatment, while conserved inthe pillared clay due to its higher thermal stability. Furthermore, calcinationis necessary because the intercalated clay is metastable, as the interleavedpolycations themselves. Thermal treatment transforms the polycations intostable oxi-hydroxide phases named pillars. X-ray absorption near-edge structure(XANES) has demostrated to be a suitable for clay minerals and clay-basedmaterials characterizations. Different Species have been reported as been usedas precursors of said pillars, being the most studied the Al13-Kegginpolycation. Another interesting option for the pillaring are Zr precursors, asZrO2 is a promising pillar-material due to its high thermalstability and high acidity. This work looks to describe the structure of twopillared clays: an Al13 and a Zr-pillared bentonite at different firingconditions, comparing those results with the original clay. Particular focuswas made in the analysis of the local structure of Al and Si. Both pillaredclays were prepared using an Argentinian unpurified bentonite. Pillaringreagents used for the pillared clays were a) a polymeric hydroxylAl solutionand b) a partially hidrolized ZrOCl2 solution. In order to characterize the material, X-raydiffraction (XRD), scanning electron microscopy (SEM) and simultaneousthermogravimetric and differential thermal analysis (DTA-TG) of the materialswere complemented with XANES. The d001 of the montmorillonite structurecollapsed at 400°C, while the Zr-pillared clay maintained this (shifted) valueup to 600 °C and the Al-pillared clay up to 800°C. Further thermal treatmentproduced the loss of the characteristic clay structure, and only a glassy phasewith a low amount of quartz and feldspar was observed by XRD in all samples. Thermaltreatment caused greater mass loss in the pillared clay due to the presence ofthe Zr-OH and AlOH oligomers allocated in the interlayer space of themontmorillonite, being the mass loss higher in the Al pillared clay due to thehigher amount of OH- and H2O in the structural formula pillaring precursor. TheAl K XANES spectra confirmed the octahedral aluminum coordination in nativemontmorillonite, being this coordination progressively lost with thermaltreatments at increasing temperatures, giving place to the presence of a fourfolded coordination. In the intermediate treatments, it presented some threeand five coordination slight contributions. Also, the Si K XANES spectra do notchange with temperature, indicating the stability of Si environments. Theresults confirm the potential of synchrotron-based techniques for characterizationof natural materials and its derived composites, in particular ones with lowcrystallinity. In particular, Zr-pillared montmorillonites aluminums Si and AlK-XANES spectra have not been reported before.