Abstract
Elastic layer-structured metal-organic framework (MOF)-11 {ELM-11: [Cu(BF4)2 (4,4′-bipyridine)2]}, which is a crystalline porous material, is a promising adsorbent for high-throughput and high-efficiency separation processes of CO2 because of its peculiar adsorption characteristics originating from the flexibility of the crystal framework. However, the exposure of ELM-11 to water vapor has been reported to reduce its CO2 capacity, which is problematic for processing feed gases that contain a certain concentration of water vapor. In this study, we investigated the stability of ELM-11 against water vapor exposure to reveal the mechanism of CO2 capacity reduction. Our combined measurements of adsorption isotherms and X-ray diffraction patterns indicated that the CO2 capacity reduction was caused by the partial formation of a crystalline subphase upon adsorption of water molecules and the subsequent formation of an amorphous phase to relax the crystalline grain boundaries. Because a higher supply rate of water molecules resulted in a larger amount of subphase formation, we concluded that the structural subphase was a metastable kinetically controlled structure, formed through the rate-dependent adsorption of water molecules. These results suggest that slowing the adsorption rate is an effective approach to suppress the formation of the subphase; therefore, we proposed the covering of ELM-11 surfaces with porous shells. We used ELM-12 {[Cu(CF3SO3)2 (4,4′-bipyridine)2]} as a shell material because of its robust stability against water adsorption and affinity with ELM-11. The ELM-12 shell decreased the adsorption rate of water molecules compared with that of bare ELM-11, resulting in the suppression of subphase formation and preventing CO2 capacity reduction. Although further optimization of the shell thickness and coverage is required to keep the capacity completely unchanged, controlling the adsorption rate of water molecules is successfully demonstrated to be possible with shell formation, which is key for industrial applications of ELM-11.
Highlights
The separation and recovery of CO2, which is a representative greenhouse gas, is one of the most critical technologies for addressing the global warming issue (Leung et al, 2014)
Pre-ELM-11 transforms into ELM-11 by the removal of water molecules through the activation procedure at 373 K under vacuum (Kondo et al, 2006), and Figure 1C-α shows the adsorption isotherm of CO2 gas in ELM-11 particles at 273 K
We investigated the effects of water vapor exposure of ELM-11 on its crystalline structure, CO2 capacity, and thermal stability
Summary
The separation and recovery of CO2, which is a representative greenhouse gas, is one of the most critical technologies for addressing the global warming issue (Leung et al, 2014). ELM-11 has been demonstrated to enable high-throughput separation of CO2 from a CO2/CH4 gas mixture in a model system of the PSA (Hiraide et al, 2020) This was possible by taking advantage of the stepped isotherms in combination with the fast kinetics of the gating and intrinsically endothermic nature of the framework expansion to compensate for the heat of adsorption. Sotomayor and Lastoskie recently reported that repeated exposure of ELM-11 to water vapor degraded its CO2 capacity (Sotomayor and Lastoskie, 2020), suggesting that the effects of water exposure accumulate during the adsorption and desorption cycles of water molecules This is a significant problem because the adsorption and desorption cycles are repeated in the PSA and TSA processes. We selected ELM-12 {[Cu (CF3SO3) (bpy)2]} (Kondo et al, 2007; Li et al, 2017; Li et al, 2018), which is a structural analog of ELM-11, as the shell material to synthesize ELM-11@ELM-12 core-shell particles and evaluated the effects of shell formation on the variations in crystalline structures and adsorption behaviors against water exposure
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