Effects of H2O, NO and SO2 on MIL-101(Cr) based CO2 capture

https://doi.org/10.1038/srep02916

“The importance of testing the impact of humidity lies in ascertaining the stability of the framework structure and retention of adsorption capacity after exposure to water in gas flows. Figure 4(a) depicts the relationship between adsorption capacity and relative humidity (RH). Results show that adsorption capacity increases slightly to 0.509 mmol/g at 10% RH, but decreases from 10% RH to 100% RH. The increased CO₂ capacity may be attributed to electrostatic interactions between water bound to Cr³⁺ sites and the quadrupole moment of CO₂³⁰. The slightly decreased CO₂ capacity above 10% RH is due to the competitive adsorption of water and CO₂. Unlike other MOFs such as HKUST-1 that lost about 25% of its adsorption capacity after exposure to a 30% RH gas flow or Mg₂(dobdc) that experienced a 84% decrease in capacity, MIL-101(Cr) performed much better in humidified environments^16,30.”

“Besides the gases H₂O, CO₂ and N₂ that constitute 95% or more of typical flue gas, the more minor components like SO₂ and NO can play a major role in affecting adsorption processes¹. The effect of NO on adsorption capacity is shown in Figure 4(b). Capacity follows a gradually declining trend when the concentration of NO rises from 0 to 2000 ppm. The adsorption stability of MIL-101(Cr) in the presence of SO₂ is illustrated in Figure 4(c). Results show that adsorption capacity changes very little when the concentration of SO₂ rises from 0 to 2000 ppm. At SO₂ concentrations of 0, 200, 500, 1000 and 2000 ppm, MIL-101(Cr) adsorption capacities are 0.495, 0.481, 0.486, 0.478 and 0.49 mmol/g.

The experimental results for the impact of SO₂ and NO on MIL-101 are supported by the results of a simulation study. However, the influence of H₂O determined here differs from the conclusion of that study, where the models predicted H₂O would completely occupy the coordinatively unsaturated “open metal” sites¹⁷. A possible explanation for the relatively small effect of trace contaminants on CO₂ adsorption may be as follows. Gas adsorption onto MIL-101(Cr) is a physical adsorption process; thus MIL-101(Cr) lacks the reactive functional groups to chemically adsorb the flue gas contaminants. Secondly, because concentrations of contaminants are always significantly less than that of CO₂ in flue gas, trace gases that do adsorb onto the framework will be largely substituted by the much higher-concentrated CO₂.”

“Figure 4 Effects of (a) moisture, (b) NO and (c) SO₂ on 10 vol% CO₂ adsorption on MIL-101(Cr) at 298 K.”