Adsorptive removal of siloxanes from biogas: recent advances in catalyst reusability and water content effect

Abstract

The valorization of biogas as a renewable energy source faces a major obstacle regarding its purification. Siloxane is one of the impurities that cause problems such as damages to equipment of combustion engines, turbines, and boilers used for biogas conversion to heat and electricity. In this review, adsorption for siloxane removal is widely discussed, with two specific approaches: adsorbents sensitivity to water and regeneration, two essential points for industrial application. Thus, determining factors in adsorbents capacity, reusability, and water tolerance including textural properties, surface functional groups, and hydrophobicity are deeply analyzed. Studies oriented to the optimization of traditional adsorbents such as activated carbon, silica gel, and aluminosilicates as well as newly emerging adsorbents such as metal organic frameworks, graphene oxides, and waste-derived materials are studied in detail in terms of reusability and water tolerance. Although activated carbon is commercially used, its low selectivity, pore blockage due to siloxane polymerization, and unsuccessful regeneration make it disadvantageous. Silica gel, however, shows better reusability as a result of less adsorbent-adsorbate dissociation energy. In addition, aluminosilicates, despite its low adsorption capacity, proved to be more practical for real biogas due to their high hydrophobicity. Graphene oxide cost and energy efficiency in their synthesis make them more industrially appealing candidates despite their low adsorption capacity. Finally, metal organic frameworks demonstrated high selectivity, high adsorption capacity, and more efficient regeneration and therefore have more advantages and less drawbacks, although the number of published studies is still limited.

Recent Advances in the Catalytic Conversion of Methane to Methanol: From the Challenges of Traditional Catalysts to the Use of Nanomaterials and Metal-Organic Frameworks

“…An undoubtedly significant issue is the question of the stability and reusability of the catalyst. As observed, in most of the studies in this review, the stability and reusability of the catalyst have not been investigated except for a few works [120,121,[123][124][125]127]. Generally, these studies showed good results for long operation times in the range of a few hours.…”

“…MOFs offer considerable opportunities for the incorporation of active sites for catalysis that mimic methane monooxygenases, with high tailorability of the pore structures and environmental conditions in the proximity of the active sites. In this regard, there is already a considerable amount of literature in which various aspects of MOF are well reviewed: synthesis and post-synthetic modifications [103][104][105], active sites and their characterization [106], structure [107][108][109][110], the inclusion of defects [111][112][113][114], water stability [115,116], scale-up of synthesis [117][118][119], multiple functionalities [120,121], application for CO 2 and biomass conversion [122][123][124][125]. In this review, a deep comparison of the MOF-based catalysts for the conversion of methane to methanol is performed in terms of methane conversion, methanol selectivity, and space-time yield (STY) (Table 2).…”

Removal of Hexamethyldisiloxane via a Novel Hydrophobic (3–Aminopropyl)Trimethoxysilane-Modified Activated Porous Carbon

“…Raman spectroscopy (Figure 2g) is an experimental technique which is sensitive to the carbon state; therefore, it can be used to analyze the structure of APCs. The characteristic peaks in the Raman spectrum of carbon materials are mainly D and G peaks, where the D peak is attributed to the disordered or defected sp 3 -bonded carbon, and the G peak originates from the in-plane vibrations of the sp 2 -bonded carbon [40]. The intensity ratio of D and G peaks (I D /I G ) presents the graphitization degree of carbon materials.…”

“…In addition to containing methane, biogas comprises some other components, such as carbon dioxide (CO 2 , 30–50 percent vol. ), hydrogen sulfide (H 2 S, 5000–10,000 ppm), ammonia (NH 3 , 100 ppm), and volatile methyl siloxanes (VMS, traces, 3–150 mg m −3 ) [ 3 , 4 , 5 ]. With the improvement of urbanization, silicone materials are widely used, and more than 90% of all the VMS entering the environment end up in the atmosphere [ 6 ].…”