Dehydration and structural development of mullite precursors: an FTIR spectroscopic study

Mullite type, I and III precursors were prepared by a sol-gel process using tetraethoxysilane and aluminum sec.-butoxide as starting materials. The precursors were treated by 15 h heating steps in intervals of 100°C from 200 to 1000°C. Type III precursors are characterised by a more discontinuous decrease of the analytical water content compared to that of type I precursors. The FTIR powder spectra of the preheated precursors show weak absorption bands at 5160 and 4540 cm−1 which are due to H2O and OH combination modes, thus proving the presence of both, H2O molecules and OH groups as structural components of the precursors. (H2O, OH) stretching modes are centered at around 3430 cm−1 and H2O bending modes at 1635 cm−1. The deconvolution of the stretching mode bands reveals non-bridging and bridging H2O molecules and OH groups. Close similarities in the pattern of the 1400–400 cm−1 vibrational region between type I and III precursors exist up to preheating temperatures of 800°C. Significant differences are evident at temperatures of 900°C, where the spectrum of the type III precursor still corresponds to that at 800°C, while the type I precursor reveals a spectrum with features present in the spectrum of mullite. Bands centered around 1110 and 1010 cm−1 are assigned to Si–O stretching vibrations of the SiO4 tetrahedral units and are strongly shifted in 900°C treated type I precursors to higher wavenumbers. This band shift is a strong indication for an increasing degree of network condensation and for changes in the Si–O distances to tetrahedra dimensions similar to those of mullite. A significant absorption around 860 cm−1 is assigned to Al–O stretching modes of AlO4 tetrahedral units, a band around 570 cm−1 is assigned to Al–O stretching vibrations of AlO6 octahedral units. A slightly decreasing intensity of this band with increasing preheating temperatures, along with a strongly increasing intensity of the 860 cm−1 band demonstrates a clear preference of Al for a four-fold coordinated structural position in the precursors preheated at high temperatures. This process is correlated to the dehydration process occurring in the medium- to high-temperature field of network condensation starting at around 400°C.