Infrared Emission Spectroscopic Study of the Dehydroxylation via Surface Silanol Groups of Synthetic and Natural Beidellite

Abstract

The structural changes of synthetic and natural beidellites during dehydroxylation have been studied using infrared emission spectroscopy of the OH-stretching and bending regions. The OH-stretching region is characterized by two OH-stretching modes around 3600–3615 cm−1and around 3650 cm−1. These bands strongly decrease in intensity upon dehydroxylation up to 600°C for the natural beidellite and 700–750°C for the synthetic ones. The differences in bandwidth, intensity, and dehydroxylation behavior are interpreted as due to differences in crystallinity with crystallinity increasing in the order natural beidellite < synthetic beidellite BSK3 < synthetic beidellite E498. Above 400°C a new band attributed to silanol groups becomes visible in all samples due to transfer of the hydroxyls from the octahedral layer to the siloxane layer before they are lost. The broad band around 3300–3400 cm−1is assigned to both H-bonding in H2O and H-bonding to Si-O-Al linkages. The presence of two different OH groups is also reflected in the OH-bending modes around 875–895 cm−1and 915–925 cm−1and in the OH-libration modes around 780 and 800–820 cm−1. These bands show a decrease in intensity upon heating and dehydroxylation of the clay structure. Here again the same order can be observed for the disappearance of the bands as for the OH-stretching region.