Dynamic breakthrough for CO2/N2 using ZIF-94 under wet and dry gases

https://doi.org/10.3390/molecules27175608

“The previous experiments demonstrated that ZIF-94 has high CO₂ adsorption capacity and satisfactory CO₂/N₂ selectivity; the phenomena prompted us to investigate the feasibility of using ZIF-94 in the separation process under humid conditions. Two types of wet breakthrough experiments for CO₂/N₂ (15/85; v/v) were conducted in this section (Figure S4), dry gas and wet gas (99.2%), to evaluate the gas-separation performance under ambient conditions. The feed gas of CO₂/N₂ was passed through the adsorption column loaded with activated ZIF powder at a total flow rate of 5 mL/min at 298 K and 1 bar. For the dry gas experiments, as shown in Figure 3a, N₂ was initially eluted through the adsorption column at 1.4 min and then reached saturation, whereas CO₂ was selectively adsorbed on the column at both compositions until 33.2 min. At the same time, the CO₂/N₂ mixture could be efficiently separated to obtain high-purity CO₂ within these time intervals. In addition, the gas adsorption capacity of CO₂ on ZIF-94 reached 32.76 cm³/g in dynamic breakthrough experiment. This indicated that during the dynamic adsorption process of the mixed gas, ZIF-94 displayed a stronger adsorption affinity for CO₂ than for N₂. Afterward, desorption experiments on CO₂/N₂ mixtures purging the column with Ar at a flow rate of 10 mL/min were conducted under ambient conditions. As shown in Figure 3c, the concentration of N₂ dropped sharply to achieve N₂ desorption in preference to CO₂ during the purging time of 9.7 min. Similarly, we could obtain purified CO₂ at the outlet of the adsorption column until purging for 34.0 min. ZIF-94 achieved desorption of CO₂ by room-temperature purging in a short time, which provides the possibility of its repeated use in industry.”

“As a major source of CO₂, flue gas is particularly important with regard to CO₂ capture. However, the presence of water in flue gas imposes higher requirements for the CO₂ adsorbent; it is still necessary to maintain its CO₂ capture capacity under wet conditions. To make the ZIF-94 more suitable for practical application, a mixture of CO₂/N₂/H₂O(v) under aqueous conditions (RH = 99.2%) was studied, and the rest of the conditions were consistent with the above dry experiments. For the wet gas (99.2%) experiments, as shown in Figure 3b, N₂ and CO₂ were first detected through the adsorption column at 1.4 min and 30.4 min, respectively. The dynamic saturated adsorption amounts of CO₂ and N₂ are 30.94 and 5.74 cm³/g, respectively, and the dynamic selectivity is 30.54. We were delighted to discover that the CO₂/N₂ mixture was still well-separated on ZIF-94 when the relative humidity was 99.2%, and the separation time (30.4 min) was similar to the separation time of the dry gas experiments (33.2 min). A possible cause of this phenomenon was that the presence of –CH₃, a hydrophobic group, makes ZIF-94 insensitive to water molecules. At the same time, it showed its poor adsorption capacity for water vapor at room temperature and pressure (Figure S5). It is worth mentioning that the breakthrough curve in a humid environment showed a longer tail, revealing that the adsorption dynamics were also impacted by water, which was attributed to the increased dependence on molecular mass transfer caused by saturation of the outer adsorbent layer [56]. Figure 3d shows CO₂ at the outlet of the adsorption column until purging at 29.4 min in the desorption experiments, a slightly shorter time than in the dry gas experiments (Figure 3c). In summary, the two types of wet breakthrough experiments showed similar characteristics regardless of the humidity, and it was inferred that water vapor adsorption had no effect on the separation of CO₂/N₂; ZIF-94 had excellent water resistance while maintaining its gas separation performance capture capability in wet conditions.”

“Figure 3. Experimental breakthrough and desorption curves for CO₂/N₂ mixture (15/85, v/v) under two humidity conditions: (a,c) dry gas and (b,d) wet gas (99.2% RH).” https://doi.org/10.3390/molecules27175608