Infrared emission spectroscopy of Al-pillared beidellite

Abstract

The structural changes of synthetic beidellite and Al 13-pillared analogues during dehydroxylation were studied using infrared emission spectroscopy (IES). The OH-stretching region is characterised by 2 OH-stretching modes around 3605 cm −1 and 3650 cm −1. These bands strongly decrease in intensity upon dehydroxylation of the beidellite up to 700–750°C. The Al 13-pillared beidellite shows stronger OH-stretching bands, while the silanol band, originally visible in the beidellite spectra, is absent. The OH-stretching mode of the Al 13 is visible around 3450 cm −1 and a very broad band is present around 3200 cm −1 assigned to water molecules in the Al 13 pillar. This band quickly disappears upon heating to 300°C while the OH-stretching band at 3450 cm −1 diminishes in intensity. The presence of two different OH-groups in beidellite is also reflected in the OH-bending modes around 880 cm −1 and 916 cm −1 and in the OH-libration modes around 770 cm −1 and 804 cm −1 in the infrared (IR) absorption spectra. These bands show an intensity decrease upon dehydroxylation of the clay. Similar, slightly shifted, bands are observed in the IR emission spectra. Significant differences between the beidellite and its pillared analogue are the splitting of the single band at 814 cm −1 into two bands at 812 cm −1 and 830 cm −1 and the bands associated with Al–OH modes show increased intensities for the pillared beidellite. These results show that upon calcination the pillars dehydroxylate, thereby releasing protons which are able to diffuse through the tetrahedral sheet into the octahedral sheet where they interact with the structural OH-groups and form water. In addition, the formation of Si–OH– groups is observed in the IR emission spectra of the Al 13-pillared beidellite indicating that these protons can also interact with Si–O–Al bonds upon calcination and form new Si–OH–Al bonds. A reaction between the Al 13 pillar and the protonated Si–OH–Al linkage will yield Al tetrahedral–O–Al pillar linkages.