Combined CO2 capture and catalytic methanation over highly performing Li-Ru/Al2O3 dual function materials

Highly optimized Carbon Capture and Utilization (CCU) technologies are required to reach the ambitious goals of net-zero greenhouse gas emissions in the European Union by 2050. However, current multistage CCU technologies using renewable electricity to yield fuels suffer from low energy efficiency and require large capital expenditures. An innovative solution is represented by the Combined Capture and hydrogenation of CO2 to Methane (CCCM). This is a catalyst-assisted chemical looping process wherein CO2 from a point-source (e.g. combustion flue gases, anaerobic digestion, or even air) is captured using a Dual Function Material (DFM) composed of a supported adsorbent which also includes a catalytically active phase to produce CH4 upon reaction with H2 from renewable sources, by this way completing its regeneration. The two process steps (CO2 adsorption and methanation) require a temporal or a spatial separation and are mediated by the solid DFM, which is a CO2-carrier and a methanation catalyst. As for all emerging chemical looping processes, the key to success is the DFM mediator that needs to fulfil a number of stringent characteristics, including high catalytic activity and selectivity at low temperature, high mechanical stability and long durability under cyclic operation, easy reducibility, large and selective CO2 adsorption capability and easy regeneration. In this work, we prepared a set of novel DFMs with low (ca. 1% wt.) Ru loading at fixed dispersion and variable Li contents (up to 5% wt.) dispersed on ?-Al2O3 spheres. DFMs were characterized by BET, PSD, XRD, H2 chemisorption, CO2-TPD, TG-MS, H2-TPSRx, in-situ DRIFT and CO2 catalytic methanation under continuous flow. Transient CO2 storage/methanation cycles were studied in a fixed bed reactor operated with alternate feed conditions. Fig. 1 shows results obtained at 263 and 293°C: after the CO2 capture step on the DFM, CH4 production occurred quickly as soon as H2 was admitted to the reactor and it was highly repeatable during following cycles whereas the formation of CO was negligible. The CO2 conversion during thehydrogenation phases was as high as 98% at 263°C and ca 97% at 293 °C. Eventually, a parametric study demonstrated that CH4 productivity as high as 0.5 mmol/g can be achieved with the Li-Ru/A catalysts that are optimally operated at milder temperatures (around 280 °C) than the previous state of the art DFMs, and guarantee a very high methane selectivity and an outstanding methane turn-over (5.7 molCH4/molRu) due to their limited Ru content. It was also shown that by splitting the Sabatier reaction into two half phases mediated by the solid DFM, the heat release during the methanation step can be reduced by ca. 50%, thus substantially simplifying the thermal management of the methanator reactor.