DIFFUSE-REFLECTANCE SPECTROSCOPY - STUDY OF THE SOLIDS OBTAINED BY THE PILLARING OF SAPONITE WITH MIXED AL-CR POLYCATIONS

Chromium and chromium-aluminium pillared saponites were obtained by re action between the corresponding Cr-Al mixed polyoxocations and saponi te from Yunclillos (Toledo, Spain). Series in which the pillaring solu tions had mole stoichiometries of Cr-1.0, Cr0.9Al0.1, Cr0.8Al0.2, Cr0. 5Al0.5, Cr0.2Al0.8 and Cr0.1Al0.9 were synthesized. Both the intercala ted precursors (obtained after cation exchange reaction) and the pilla red solids obtained by calcination of the intercalated solids at 200 a nd 500 degrees C were characterized. The environment of Cr3+ cations w as studied by Diffuse Reflectance Spectroscopy (DRS), important differ ences being found depending on the Chromium content in the solids and on the calcination temperature. The crystal field stabilization energy was higher in intercalated than in pillared solids, and the same tend ency was observed in the Racah parameter. The intercalated precursors had very ionic Cr-ligands (water molecules and hydroxyl groups) bonds, while the pillaring process led to the formation of Cr2O3 oxide pilla rs with highly covalent Cr-O bonds.