Cu- and Mg- activated carbon honeycombs monoliths for H2S removal from biogas at low temperature

Hydrogen sulfide is one of the main impurities present in reformate, natural gas and raw biogas. The purification of these gas streams is a mandatory step before their use in downstream processes as H2S is toxic, corrosive for engines and fuel processing devices and poisonous to catalysts even in small concentrations. One of the most effective techniques for H2S removal is the adsorption due to its easy management, high efficiency and low cost. The surface chemistry, the high internal surface area and porosity are the main features that make Activated Carbons (AC) good H2S sorbents. Application of ACs, generally produced as powders, is limited when high pressures are required. Therefore, the use of ACs in structured form is preferred but this generally requires the use of binders that while they improve the mechanical properties of sorbents they reduce the adsorption capacity due to the partial blockage of porosity. Otherwise, the introduction of suitable metals can lead to the improvement of original ACs properties [1, 2]. In this work, copper and magnesium oxides were dispersed both separately and mixed on commercial honeycomb ACs to improve the H2S capture capacity. Dynamic catalytic tests (100ppmv H2S) have been carried out at room temperature in the co-presence of humidity and O2 to evaluate the sulfur capture capacity and to investigate the oxidation reactions involved by varying the composition of the two metals. The adsorption performance and the activated mechanism were significantly different depending on the nature of the metal. The basic MgO active sites promote the formation of elemental sulfur through a path faster than the deeper oxidation mechanism promoted by CuO, resulting in the formation of sulfates/sulfuric acid along with elemental sulfur. The easy dissociation of H2S into HS- and H+, occurring for Mg-modified sorbents, is the reason for the higher capture capacity of these materials in the presence of wet streams compared to Cu-modified ACs for the same load level. Nevertheless, the Cu-Mg catalytic sorbents coupled the fast kinetic features of MgO, promoted by the presence of water vapor, to the deeper oxidation activity of CuO, exerted also under dry conditions, thus representing materials with tunable properties. Both fresh and spent honeycomb sorbents were characterized by N2-adsorption at -196°C, PSD, XPS, TG-MS in order to investigate the specific nature of S-species formed during the catalytic adsorption. A thermal treatment up to 620°C under inert gas flow of the bimetallic spent sorbents restores the original porous structure of the modified AC monoliths due to the decomposition of sulfate species at lower temperature and the evaporation of elemental sulfur at higher temperature. The honeycomb monoliths can operate several capture cycles without detectable loss of capture capacity releasing only SO2 and S by the thermal regeneration. This makes these structured metal-modified ACs very good candidates for H2S capture process at room temperature for natural and raw biogas purification.