Potential application for direct air capture and conversion

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“Apart from the widely investigated ICCC for CO2 at high concentration levels in industrially relevant processes as listed in Table 5, DMFs were also reported to be promising for direct air capture (CO2 at ppm levels) and conversion by a few studies. It was demonstrated that the Ni/Na-γ-Al2O3 could convert >90% of the captured CO2 from a gas stream containing 100–400 ppm CO2 to more concentrated CH4 even in the presence of 20% O2 as shown in Fig. 21a (Kosaka et al., 2021). Ru-based DFMs were also investigated for the CO2 direct air capture and methanation. Jeong-Potter et al. used 0.5%Ru-6.1%Na2O/γ-Al2O3 for the integrated capture of diluted CO2 in a simulated air stream (400 ppm) and methanation at 320°C (Jeong-Potter and Farrauto, 2021). More than 82% of the absorbed CO2 can be effectively converted to CH4 with a yield of 0.172 mol/kg and selectivity of 100% under dry conditions. The developed DFMs show great potential for combining direct air capture and methanation in one process. Nonetheless, the sorption of CO2 was conducted at 320°C meaning the energy consumption for heating up the air stream. The energy required for the temperature swing mode of the direct air capture and methanation was estimated to be 9 MJ/m3-CH4 in another study, although with the CO2 sorbent and catalyst deployed in two separate reactors (Veselovskaya et al., 2020). Lower operation temperatures need to be conducted to reduce the energy requirement (Jeong-Potter and Farrauto, 2021). In a further study, the 1%Ru-10%Na2O/γ-Al2O3 was used for the direct air capture of CO2 at ambient temperature and methanation at 300°C as shown in Fig. 21b (Jeong-Potter et al., 2022). The CH4 yield of 1.067 mol/kg with 100% selectivity was achieved. It was found that with the exposure of 0.5%Ru-6.1%Na2O/γ-Al2O3 to a high content of O2 (∼21%) for a long time, the Ru in DFM can be completely reactivated in the methanation step (Jeong-Potter and Farrauto, 2021). In the presence of 2.0% H2O in air, it can significantly improve both the sorption and methanation performances of 1%Ru-10%Na2O/γ-Al2O3, which were 2.4 and 3.5 times greater respectively than that under dry condition. This was supposed to be caused by the formation of amorphous NaHCO3 in the presence of H2O. On Ru-CaO/γ-Al2O3 DFM, on the contrary, humid air was found to adversely affect the sorption of CO2 (Jeong-Potter et al., 2022). Considering the lower operation temperature of Na2O-DFM compared to CaO-DFM due to the higher stability of formed carbonates on CaO (Arellano-Treviño et al., 2019aBermejo-López et al., 2019a), it appears that Na2O containing Ru-based DFMs is more promising for direct air capture/methanation under realistic conditions (Jeong-Potter et al., 2022).”


Fig. 21. Schematic of diluted CO2 (atmospheric level) capture and conversion using DFMs in (a) isothermal and (b) temperature swing operation mode (Jeong-Potter et al., 2022Kosaka et al., 2021).”

“Nonetheless, intensive investigations are needed for integrated direct air capture and conversion. For example, more DFMs need to be developed and tested for DAC applicability, thus guiding the frame of promising materials combination. Detailed and extended effects of O2, H2O, and other prevalent components on the sorption and catalytic activity of DFMs remain to be tested. Prolonged experimental tests should be conducted in ambient air conditions to evaluate the robustness of the developed DFMs. Methane is currently the only conversion product for the integrated DAC process. Other possible products can be investigated to achieve the diversity of the process. Appropriate operation temperatures for the sorption and subsequent conversion should be studied for process optimization. Since the CO2 concentration is much lower, thus the sorption duration of CO2 capture was much longer than that for the industrial flue gases to reach reasonable CO2 uptake. However, the time needed for the conversion was usually much lower than the sorption process (Kosaka et al., 2021). Therefore, the matching between these two consecutive processes should be properly designed and optimized for practical application.”

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