Decomposition of hexamethylcyclotrisiloxane over solid oxides.

Abstract 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 (Al 2 O 3 , SiO 2 ). 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 m 2  g −1 ), until saturation, at 673 K, is 0.31 g  (HMCTS)  g (Al2O3) −1 , which corresponds to one silicon atom per 9 A 2 , 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 CO 2 , but loose reactivity when in contact with carbon dioxide because of surface carbonation.