Removal of siloxane (L2) from biogas using methyl-functionalised silica gel as adsorbent

Abstract

Volatile methylsiloxanes (VOSiC) are impurities present in trace amounts in biogas produced by the anaerobic digestion of organic wastes in landfill and sewage plants. Efficient removal of these impurities is required due to the restrictions posed by them on the application of biogas as an alternative energy resource. In this study, methyl-functionalised silica gels (MFS) have been prepared via a hydrolysis and condensation approach using double precursor (TEOS and MTS) in methanol–water medium for achieving effective removal of a model VOSiC - hexamethyldisiloxane (L2). The developed gels have been characterized by N2-BET, FTIR, SEM-EDS and XRD. The results reveal that the as-synthesized MFS materials have well-developed micropore structure and superhydrophobicity. Among the developed materials, MFS2 sample obtained with a MTS/TEOS ratio of 1/1 (v/v) exhibits the highest specific surface area (1261.3 m2 g−1), micropore volume (0.52 cm3 g−1) and total pore volume (1.03 cm3 g−1), along with superior water contact angle (123.5°). The non-crystalline nature and sponge-like structure of MFS are confirmed by XRD and SEM analyses. The adsorptive performance of MFS materials for L2 siloxane is measured to be 14.9–18.3 times higher than that of the unmodified silica in terms of the breakthrough adsorption capacity (QB). Also, the adsorption capacities of MFS materials are noted to be linearly related to the specific surface area and micropore volume, along with positively affected by the total pore volume and water contact angle. From a series of the experimental results of adsorption of L2 by MFS2, an empirical equation is developed for reflecting the correlation of the breakthrough time and inlet concentration. Use of lower temperature is concluded to result in better overall siloxane adsorption levels. The QB reaches an optimal value of 346.3 mgL2 g−1 at 0 °C using 83.82 mg L−1 of L2 with N2 as the carrier gas. The developed MFS adsorbent can be readily regenerated for reuse by heating at 80 °C, and an insignificant fluctuation in the dynamic adsorption behavior is observed even after consecutive 5 adsorption/desorption cycles.