Decomposition of hexamethylcyclotrisiloxane over solid oxides

The decomposition of hexamethylcyclotrisiloxane (HMCTS) has been studied at room temperature and in
the range 473–673 K over the surface of basic (CaO, MgO) and acidic oxides (Al2O3, SiO2). Alumina allows
the complete removal of HMCTS from synthetic biogases at 673 K. A reactive adsorption occurs with surface
silication and release of methane. The adsorption capacity of our alumina adsorbent (180 m2 g1),
until saturation, at 673 K, is 0.31 g (HMCTS) g(Al2O3)
1, which corresponds to one silicon atom per 9 Å2, i.e.
the silication monolayer capacity.
On the contrary, silica, which is an excellent adsorbent for siloxanes at room temperature, looses its
adsorption ability at high temperature as it is typical of a molecular adsorption behavior. Basic oxides
such as MgO and CaO have strong reactivity in decomposing siloxanes in the absence of CO2, but loose
reactivity when in contact with carbon dioxide because of surface carbonation.