Infrared emission spectroscopic study of the dehydroxylation of synthetic Mg/Al and Mg/Zn/Al-hydrotalcites

Abstract

The thermal behaviour of synthetic hydrotalcites, Mg₆Al₂(OH)₁₆CO₃·nH₂O and Mg₄Zn₂Al₂(OH)₁₆CO₃·nH₂O, was studied by thermogravimetric analysis and infrared emission spectroscopy. The Mg/Al-hydrotalcite shows a loss of 7.0% m/m physically adsorbed water up to 70°C. Between 70 and 230°C 16.5% m/m is lost due to decarbonisation (DTA endotherm 222°C). The dehydroxylation takes place between 230 and 420°C with 23.6% loss (DTA endotherm 400°C). The steps are more overlapping for Mg/Zn/Al-hydrotalcite. Mg/Al-hydrotalcite shows major changes around 350–400°C corresponding to the second mass loss step in the TGA plot. The 772, 923 and 1029 cm^-1 bands, the Al–OH out-of-plane deformation and doublet deformation modes, disappear. New bands are observed at 713, 797 and 1075 cm^-1. The first and last band accompanied by the 545 cm^-1 band indicate the formation of MgAl₂O₄. Of the 1341, 1398, 1532 and 1582 cm^-1 bands, only the last two bands shift towards lower frequencies. The 1341 and 1398 cm^-1 bands (ν₃ CO₃^2-) shift towards higher frequency between 300 and 350°C followed by either disappearance or a decrease in frequency between 600 and 650°C indicating different carbonate groups. Changing the composition from Mg₆Al₂(OH)₁₆CO₃·nH₂O to Mg₄Zn₂Al₂(OH)₁₆CO₃·nH₂O results in a shift of 40–50 cm^-1 to higher frequencies of the 1463 and 1539 cm^-1 bands. These bands represent combination modes of the 558 and 923 cm^-1 bands (1471 cm^-1) and of the 638 and 923 cm^-1 bands (1551 cm^-1). Similar combinations are observed for the Mg/Zn/Al-hydrotalcite. The OH-stretching region in the infrared emission spectra at 200°C is dominated by a broad band at 3400–3500 cm^-1 (H₂O) with a shoulder at 2950 cm^-1 (CO₃–H₂O bridging). The 3400–3500 cm^-1 band quickly disappears upon heating confirming the DTA/TGA. Upon dehydration a complex band becomes visible around 3600 cm^-1, assigned to M–OH stretching modes. These modes decrease in intensity during heating and disappear at ca. 500°C.

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