Activated carbons as adsorbents of siloxanes: Adsorption and desorption study by in-situ FTIR.
Résumé
Biogas produced from sludge in waste water treatment plant, as well as from landfills, can be collected and utilized as a clean energy source. However, a special issue regarding biogas concerns the presence of traces siloxanes, semi-volatile methylated organosilicon (VMS) compounds containing Si–O bonds originating from hygiene, health care and industrial products [1]. During the biogas combustion, siloxanes are converted into crystalline silica inducing the engine damage.
The present work aims to investigate the adsorption and thermodesorption of decamethyltetrasiloxane (L4) over two commercial activated carbon (AC) samples (STIX and AP4-50 from ChemEnvirocarb) using the Diffuse Reflection Infrared Spectroscopy (DRIFTS). The physicochemical properties of ACs were characterized using various techniques like N2 adsorption at 77K, XRD, SEM and TGA. The adsorption experiments were performed by contacting a stream of L4 (40 ppm) in nitrogen with a mixture of AC/KBr at room temperature. The spectra recorded during this step are dominated by the response of the gas phase. Therefore, the adsorbed phase can be hardly observed. Upon saturation, the cell was purged under N2 flow during 16h to eliminate L4 in the gas phase. The exhausted ACs underwent thermal programmed desorption from 25°C to 300°C (for STIX) or 400°C (for AP4-50).
At 25°C, the spectra of ACs exhibit very weak bands of L4 in adsorbed phase. Upon heating, STIX sample shows mainly the decomposition of L4 into L2 between 80 and 200°C accompanied by a small desorption of L4. Further heating leads to the polymerization of L4 into PolyDimethylSiloxane (PDMS) [2]. This compound is weakly volatile and cannot be completely removed at 300°C. For AP4-50, the release of L4 into the gas phase can be observed throughout the heating process (mainly between 175 and 350°C). Small formation of L2 is noticed when heating up to 145°C. No evidence of the polymerization can be found for this sample.
The transformation of L4 during the thermodesorption can be related to its reaction with the surface functional groups of ACs. The determination of these groups which are responsible for the formation of PDMS seems to be important for improving the regenerability properties and the reuse of ACs.
References
[1]. R. Dewil, L. Appels, J. Baeyens, Energy. Convers. Manag. 47 (2006) 1711–1722.
[2]. V.T.L. Tran, P. Gélin, C. Ferronato, L. Fine, J.M. Chovelon, G. Postole, Catal. Today. (2017) http://dx.doi.org/10.1016/j.cattod.2017.01.006