Isomorphic silicon/aluminum substitution on layered ilerite - Structural study and calorimetry of copper interaction

Layered ilerite (Na-il) and ilerite with structural aluminum incorporated (Na-[Al]il) through isomorphic substitution of silicon atom in the silicic network were synthesized by hydrothermal procedures. X-ray diffraction (XRD) and scanning electron microscopy confirmed the structure and morphology. The incorporated metal was detected and quantified by nuclear magnetic resonance in the solid state and X-ray fluorescence. Both classes of materials sorbed copper and these interactions were calorimetrically followed through isothermal titrations. The thermodynamic data for Na-il and Na-[Al]il gave ΔG -4.2 ± 0.4 and -4.7 ± 0.5 kJ mol -1, ΔH 1.54 ± 0.02 and 2.9 ± 0.01 kJ mol -1 and ΔS 19 ± 1 and 25 ± 1 J K -1 mol -1, respectively. These values are relatively close and the differences are explained by the higher acidity for the Na-[Al]il surface, which leads to stronger interaction between the copper species and the solid material. The copper-ilerite structures were characterized by XRD, the environment surrounding copper has been studied by X-ray absorption spectroscopy and EXAFS was used to determine copper coordination, metal-metal and copper-oxygen distances. For both materials octahedral coordination was determined, whose axial and equatorial distances for Cu-O gave 196 and 234 pm for Na-il, respectively. For the Na-[Al]il sample two equatorial distances, 189 and 196 ppm and an axial distance of 247 pm were found. These results indicate that hydrated Cu 2+ species interact with silicate layers by electrostatic forces. The XAS results are in agreement with the calorimetric results and with acid-base principles, as well described for these systems. © 2012 Elsevier Inc. All rights reserved.