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 Cu2+ 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.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Layered ilerite containing structural aluminum were successfully synthesized. ► Thermodynamic data of copper-surface interaction were calorimetrically determined. ► Atomic distances of copper species were calculated by XAFS.