Adsorption of linear and cyclic siloxanes on activated carbons for biogas purification: Sorbents regenerability

Abstract

Two commercial activated carbons, namely STIX and AP4, were selected to investigate their ability for siloxane adsorption and further thermal regeneration in relation with their different physico-chemical properties. In the frame of biogas purification, the studied siloxanes were L2, L3, L4 (linear molecules) and D4, D5 (cyclic molecules). The maximum capacity of adsorption was estimated by gas chromatography and gravimetric methods. AP4 presents better adsorptive properties comparing with STIX irrespective of the type of siloxane. Thermal regeneration after siloxanes adsorption was followed up to 400 °C by both gas chromatography and in situ DRIFTS. Except L2, the siloxane polymerization proceeds on the AC containing alkali metals (K, Na) as revealed by the observed release of their decomposition products during the thermodesorption treatment. The same sites are responsible for the cleavage of SiO bonds in linear molecules and formation of L2 as primary product above 100 °C. The presence of such sites, possibly strong basic sites, is detrimental to the regenerability of the spent adsorbent and to the adsorption which could be limited by partial blockage of the AC porosity. Interestingly the regenerability of the alkali-free AC depends only on the volatility of the siloxane.