A comment on “Mechano-chemical effects on surface properties and molybdate exchange on hydrotalcite”, Clay Minerals (2009) 44, 311–317

Abstract

Bonifacio-Martinez et al. (2009) recently reported a study of the mechano-chemical effects on the surface properties and molybdate exchange on hydrotalcite. These authors reported that milling of hydrotalcite leads to a sharp decrease in the specific surface area and a simultaneous decrease in the particle size and pore volume, which they attributed to compaction/shrinkage. The samples had been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive (EDX) and Fourier transform infrared (FTIR) spectroscopy. The ability of the samples to intercalate molybdate after calcinations at 500°C by the so-called reconstruction method was also tested. However, the results reported in that paper lack proper interpretation on scientific grounds, and the reconstruction process with simultaneous incorporation of molybdate in the interlayer of the hydrotalcite phase seems to have failed; I will try to demonstrate why in this letter. The first (at low angle) XRD line recorded for these solids around 2θ values close to 10° is usually ascribed to diffraction by (003) planes. Spacing for this line corresponds to the sum of the width of the brucite-like layers, ∼4.8 A (Drezdzon, 1988), and the gallery height, which depends on the size and orientation of the intercalated species. Carbonate is located with its molecular plane parallel to the brucite-like layers, and the corresponding gallery height result of ∼3.0 A provides a space for the (003) planes close to 7.8 A. However, when carbonate is substituted by anions with a different molecular size, changes in the spacing of the …