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Influence of moisture in CO2 gas for carbon capture using 5A zeolite (MS modelling)

https://doi.org/10.1039/C9RA08334K

“To understand the mechanism of the effect of the trace amount of water vapor on single gas adsorption, the CO2, N2, and water vapor adsorption amounts at different trace amounts of water vapor with and without considering the partial charges of water molecule are investigated.

Fig. 3 shows the CO2 adsorption isotherm in 5A zeolite under different concentrations of water vapor (0, 0.05, 0.1, 0.3, 0.5, 1, 3, and 5 ppm) at pressures ranging from 0 to 120 kPa at 288 K. In all cases, the amount of adsorbed CO2 first increases rapidly with increasing pressure (0–10 kPa), and then increases slowly to reach the saturation adsorption amount for the pressures in the 10–120 kPa range. Furthermore, as the water vapor concentration increases (0.3–5 ppm), the amount of adsorbed CO2 decreases in the entire pressure range. This is due to the competitive adsorption effect for the CO2 and H2O molecules. The H2O molecules have a negative effect on the CO2 adsorption amount in 5A zeolite at a high concentrations of water vapor (>0.5 ppm), similar to that reported in X-type zeolites.35,36 This demonstrates that a large fraction of the strong CO2 adsorption sites will disappear due to the enhancement of the short-range Lennard-Jones repulsive interactions from the water molecules at high water vapor concentrations (>0.5 ppm). Surprisingly, for the water vapor concentrations being 0.1 ppm and 0.05 ppm, the amount of adsorbed CO2 can be enhanced by 0.7–53.4% compared to that in the dry 5A zeolite in the entire pressure range; in particular, in the low pressure range (0–10 kPa), the enhancement is in the 3.0–53.4% range. This can be explained as follows. At a low water vapor concentration (<0.1 ppm), when the water molecules adsorb on the metal sites, their positions will be held relatively fixed due to the bonding of one of the water molecule’s hydrogen atoms directed toward the metal center of the 5A zeolite structure, and due to the electronegative oxygen atom exposed and directed toward the center of the cavity in the 5A zeolite. This allows CO2 to enter the center of the cavity with minimal steric repulsion and to interact favorably through electrostatic interactions with the adsorbed water molecules.37 However, at high water vapor concentrations (>0.3 ppm), few CO2 molecules can enter the cavity, and thus the amount of adsorbed CO2 decreases.”

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Fig. 3 Simulated CO2 adsorption equilibrium isotherms in 5A zeolite under different amounts of trace water vapor.”

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