Integrated CO2 capture and methanation over Li/Na-Ru dual function materials

Reversing climate change and global temperature rise requires mitigating greenhouse gas emissions from industry and energy sectors (decarbonization) and reducing atmospheric CO2 levels with CO2 removal technologies (including Direct Air Capture, DAC). Carbon Capture and Utilization (CCU) is especially promising because it enables a net CO2 consumption and at the same time the recycling of the carbon content of CO2. The challenge is to sustainably convert CO2 from industrial processes or from air into synthetic fuels using renewable energy-driven processes approaching a circular carbon economy. This is also a valuable and efficient solution to store excess energy that is generally seasonal and fluctuating. A possible innovative solution to reduce the costs and increase the efficiency of the current multi-step CO2 Capture and Utilization (CCU) processes is represented by the integration of the CO2 capture and hydrogenation into a single chemical looping process performed with Dual Function Materials . DFMs contain both a CO2 adsorbent and a hydrogenation catalyst, which are intimately coupled and generally nanodispersed on a high surface area oxide support . Therefore, the DFM can work as the chemical looping mediator, being exposed to alternate steps of CO2 capture and reduction with H2 (regeneration) in cyclic operations. As for any emerging chemical looping processes, the key to success relies on the development of advanced materials (sorbents/catalysts/dual function) strictly coupled to optimized process design and reaction engineering. The ideal sorbent can selectively adsorb CO2 at moderate strength to retain the CO2 over a range of temperatures and allow for CO2 methanation at the target temperature by a highly selective catalyst. In this work we set out to investigate and compare the performance of two alternative Ru-based DFMs containing either Li or Na as the sorbent phase. Ruthenium (ca.1 % wt.) and then alkali metal (3% wt) were sequentially and uniformly dispersed inside ?-Al2O3 spherical particles by impregnation with nitrate precursors followed by calcination in air at 350 °C and reduction in H2 at 450 °C. The loadings of Ru and the alkali in the DFM were deliberately kept low given the economic concerns for the use of critical raw materials, and to highlight the possible deactivation effects during a prolonged parametric study involving several capture and methanation cycles in a fixed bed reactor operated in the temperature range 260 - 320 °C with alternate feed conditions under realistic conditions including the presence of H2O, O2, and possibly SO2 in the process gas stream. Temperature-programmed methanation tests performed co-feeding CO2 and H2 over DFMs and their parent Ru/Al2O3 catalyst showed a peculiar promoting effect characteristic of Li, which boosted the intrinsic catalytic activity by factor ranging from 2.5 to 6, at variance to Na, which slightly lowered the process selectivity to methane. Moreover, the CO2 adsorption capacity of the Li-Ru DFM measured at 300 °C was ca 20-25% higher than for the corresponding Na-based sample with similar alkali content. All DFMs showed highly repeatable performances across several cycles of alternated CO2 capture and methanation in the temperature range 260 - 320 °C (Figure 1), but Li-Ru formulation was significantly better in terms of maximum methane production (+35%) and selectivity (very close to 100%). Notably, the favourable and unique synergy existing between Li and Ru phases in the DFM boosts the catalytic activity and guarantees a 20-60 °C reduction for the optimal operating temperature of the integrated cycles with respect to Na-Ru DFM, which is particularly advantageous in view of the higher CO2 capacity and methane yield, and enhanced long-term durability. Li- and Na- DFMs proved to be highly tolerant to operating under realistic conditions in the presence of SO2 impurities (up to 100 ppm) in the flue gas. Both DFMs completely removed SO2 storing it as alkali sulfates, which were generally stable during the subsequent hydrogenation stage, thus avoiding any severe poisoning of the catalytic Ru sites. However, the accumulation of sulfates progressively lowered the CO2 capture capacity, and, in turn, the CH4 production, due to the saturation of the adsorption sites, which were not regenerated. Eventually, we investigated the opportunity to further enhance the CO2 capture capacity and, in turn, the corresponding methane production by the addition of 3% wt Na to the Li-Ru formulation.