MgO based DFMs for ICCU-methanation

Using integrated CO₂ capture and utilisation concept to produce methane is one solution to recycle CO₂. The advantage of this process is that the produced methane has a mature market and infrastructure. However, the product value is low, in particular, compared to H₂. Even, if renewable H₂ is used, the economics might not be feasible.

From the perspective of research, 2D-layered Ni–MgO–Al₂O₃ nanosheets were prepared for ICCU and used at temperatures around 250 °C (https://doi.org/10.1002/cssc.201902828). The authors claimed almost 100% CO2 capture efficiency and 100% conversion of captured CO2. However, the presented results are not quantified and the reaction time is very short (~20 s). The high conversion of CO₂ might be related to the low CO₂ capture capacity in this work (~1.42wt.%). This work provides an important message. By shortening the time of CO2 capture, 100% conversion of the captured CO₂ could be achieved. There is a balance between the capacity of CO₂ capture and the conversion of the captured CO₂.

A relatively excellent ICCU-methanation performance was reported with 3.24 mmol/g CO₂ capture capacity and 2.21 mmol/g CH₄ formation, using NaNO₃/MgO sorbents mixed with Ru/Al₂O₃ catalysts (https://doi.org/10.1016/j.cej.2021.130369). MgO normally has a low practical CO₂ capture capacity. In this work, mesoporous MgO was prepared and also doped with NaNO₃. However, excessive sintering of MgO was observed and responsible for the reduced capacity of CO₂ capture when the number of sorption cycles increased. The potential evaporation or redistribution of the NaNO3 promoter might also contribute to the decreased CO₂ capture capacity. Around 60% of the captured CO2 was converted to methane in this work. However, the use of Ru-based catalysts for ICCU might not be feasible, as ICCU materials need to be used directly in flue gas, where the pollutants could easily deactivate the Ru-based catalysts. It is noted that interesting kinetic studies were reported in this work. This is rare in the current ICCU literature.

MgO was also used as the adsorbent for ICCU-methanation, and Ru/CeO₂ with different morphologies acted as the catalyst. Only 0.3 mmol/g CH₄ production was reported. The conversion of captured CO₂ is around 60%, which is consistent with the above paper. It is noted that cube-CeO₂ demonstrated very poor conversion of the capture CO₂ (around 2.7%). Interestingly, over 95% CO₂ conversion was obtained after 5 cycles of ICCU.

The authors provide useful suggestions for reaction mechanisms and enhancing ICCU-methanation performance as below:

“In situ DRIFTS study indicates that ICCM over Ru/rod-CeO₂-MgO proceeds via the formates and CO₂ dissociation (Ru-CO species) pathways. The formates pathway do not require gas phase CO₂ to release from carbonates. Therefore, it could contribute to achieving ideal ICCM performance (100% CO₂ conversion and 100% CH₄ selectivity). Improving the formates pathway and inhibiting gas-phase CO₂ release in the methanation step could be a promising strategy to achieve perfect ICCM.”