Heating stage Raman and infrared emission spectroscopic study of the dehydroxylation of synthetic Mg-hydrotalcite

The thermal behaviour of synthetic hydrotalcite, Mg5.6Al2.4(OH)16(CO3,NO3)·nH2O, has been studied by Infrared Emission Spectroscopy (IES) and heating stage Raman microscopy. Heating stage Raman microscopy reveals that upon heating and subsequent dehydration the bands at 553, 1052, 3503, 3603 and 3689 cm−1 associated with the (Mg,Al)3–OH translation, deformation and stretching vibrations decrease in intensity due to changes in the stacking order of the hydroxide layers. During this rearrangement around 150–175°C the free interlayer nitrate forms a type of bridging nitrato complex with the metals in the hydroxide layers as evidenced by the disappearance of the normal free nitrate vibrations at 716, 1067 and 1386 cm−1 and the formation of a new band at 1039 cm−1. Further heating to 300°C results in the dehydroxylation and decarbonisation of the hydrotalcite, which is only partially reversed upon cooling in air over a period of more than 12 h. The Mg-hydrotalcite IES spectra show major changes around 350–400°C indicating the end of the dehydroxylation. In this temperature range, “Al”–OH bands at 772, 923 and 1029 cm−1 disappear. New bands are observed around 713, 797 and 1075 cm−1. The first and the last bands plus the 545 cm−1 band indicate the formation of spinel (MgAl2O4). The 713 cm−1 band is also close to the νLO position of MgO at 717 cm−1, which is another product formed after dehydroxylation. However, decarbonisation is not complete at this stage as evidenced by both a continuing weight loss in the TGA up to at least 625°C and a decreasing carbonate signal in the IES up to 800°C.