CO2 capture and conversion to methane with Ni-substituted hydrotalcite dual function extrudates

Abstract

Nickel-substituted hydrotalcites promoted with cesium were synthesized, and their dual-function ability for acting as a high-temperature CO2 adsorbent and methanation catalyst was assessed. The Ni-substituted hydrotalcites were synthesized by the co-precipitation method with (Ni + Mg)/Al and Ni/(Ni + Mg) molar ratios of 3 and 0.4, respectively. Extrudates of these materials were prepared and tested for CO2 capture from a synthetic flue gas mixture (with 15 vol% CO2) and subsequent methanation. Two wetness impregnation routes with cesium were considered: i) powder impregnation before the extrudates shaping (PI) or ii) the direct impregnation of the extrudates (EI). EI materials were able to sorb more CO2 due to their higher number of basic sites, especially those with strong strength. However, while the amount of CH4 produced by both types of samples is similar, EI materials are less selective to methane (i.e., they present a slightly higher formation of CO). For this reason, Cs-doped PI extrudates were selected and used in the parametric study. Under cyclic steady-state and operation at 350 °C and 1 bar, the reactor packed with this material allowed capturing ca. 0.24 molCO2 kgDFM-1 and the complete conversion of the captured CO2 into CH4. However, a substantial amount of H2 is released from the reactor without being converted, hindering the CH4 purity level. This indicator can be increased at the expense of some product loss by performing a short-duration purge with CH4 between the capture and methanation stages. Remarkedly, this material showed outstanding stability during the continuous cyclic operation, without CO formation.