CHROMIUM-BASED MIL-101 METAL ORGANIC FRAMEWORK AS A FULLY REGENERABLE ADSORBENT FOR SILOXANES

Biogas, produced by anaerobic decomposition of different kinds of biomasses, is particularly interesting because of the widespread accessibility to raw materials, byproducts and wastes suitable for its production, and of the relatively low cost of anaerobic digestion technologies. Because of its high methane content, biogas can be used as fuel; however, it contains a lot of impurities, including volatile organic silicon compounds. Among these, siloxanes, even if present in traces, are one of the major poisoning contaminants for internal biogas-reforming fuel cells.1 For these reasons, removal of siloxanes from biogas, even if quite challenging, is highly recommended. In this work, the potentialities of the chromium-based MIL-101 metal organic framework (MOF) as a high capacity, fully regenerable siloxane adsorbent were investigated. MIL-101 has been synthesized in our laboratories and its adsorption capacities were tested towards octamethylcyclotetrasiloxane (D4), the major siloxane species in biogas. In particular, three replicates of D4 adsorption kinetics on MIL-101 at T = 298 K and p = 0.3 Torr, obtained by means of a McBain-type balance, have been carried out. A single MIL-101 sample was used to perform all reported runs: indeed, it was activated under vacuum at 423K for two hours and, after every run, the spent sample was re-activated under the same conditions. The reported curves are clearly superimposable; moreover, they all reached the same adsorption equilibrium capacity qmax, which turned out to be about 2.8 mmol/g. These observations suggest that, differently from more traditional adsorbents1, MIL-101 is a completely regenerable D4 adsorbent.