Characterization of the structure and surface acidity of hydroxychromium taeniolites

Hydroxychromium taeniolites were prepared using hydroxychromium solutions with a OH/Cr content of about 1 and 2. To study the influence of oxidation on the properties of the synthesized materials, the samples were heated in air and, for comparison, in vacuum. Structural characteristics were obtained using X-ray diffraction and sorption of nitrogen. The surface chemistry was evaluated using potentiometric titration. The pK(a) distributions calculated from potentiometric titration data revealed peaks characteristic of chromium polymers (Cr-III) for the initial samples and peaks with a pK(a) corresponding to chromic acid (Cr-VI) for the oxidized samples. In addition, the pK(a) distributions also provide evidence for inner-sphere complexation of the interlayer Cr with siloxane oxygen. The materials' acidity and structure, obtained from X-ray diffraction and sorption of nitrogen, demonstrate that chromium species are adsorbed mainly on the external surface of taeniolite and that the properties of the final products depend upon the OH/Cr ratio of hydroxychromium solutions used in their preparation.