THERMAL CHANGES OF EXCHANGEABLE AND ADSORBED METHYLENE BLUE (MB) ON MONTMORILLONITE SURFACES

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IR Spectral changes occurred during heating of MB-and MB-Na-montmorillonites (25-500°C) in N₂ flow have been considered in this work. Within a temperature range of 25-150°C, the H-O-H deformation band at 1635 cm^-1 diminishes progressively and MB bands shift towards low frequencies. Therefore, the thermal changes at this stage are limited to increases in the attraction between the exchangeable MB⁺ and the clay surfaces.
Within a temperature range of 200-500°C the exchangeable MB⁺ is completely decomposed. The decomposition starts near 200°C and is almost completed at 500°C. The decomposition of exchangeable MB⁺ leaves a residue consisting of a mixture of aromatic species, methyl groups, NH₃ and NH₄⁺ ions on the clay surface. Pyrolysis of these species takes place at the end of this stage.
Structural water of the clay participates in decomposition of MB⁺ on the clay surfaces. This was observed in IR spectra and confirmed by differential thermal microanalysis (DTMA).
Thermal decomposition of molecular MB adsorbed on dehydroxylated montmorillonite seems to be very similar to that of exchangeable MB⁺on normal clay surfaces.