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 (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.