Molecular adsorption and self-diffusion of NO₂, SO₂, and their binary mixture in MIL-47(V) material

Area of Science:

• Materials Science
• Chemical Engineering
• Computational Chemistry

Background:

• Understanding gas diffusion in metal-organic frameworks (MOFs) is crucial for gas separation and storage applications.
• MIL-47(V) is a MOF with potential applications in gas adsorption and separation.
• The diffusion behavior of different gas molecules within MOFs can vary significantly due to their unique interactions with the framework.

Purpose of the Study:

• To investigate the loading dependence of self-diffusion coefficients (Ds) for NO2 and SO2 in MIL-47(V).
• To elucidate the underlying mechanisms governing the diffusion of NO2 and SO2, including their interactions with the MOF.
• To compare the diffusion rates and activation energies of NO2 and SO2 within the MIL-47(V) framework.

Main Methods:

• Classical molecular dynamics (MD) simulations were employed to study the diffusion of NO2 and SO2.
• Analysis of self-diffusion coefficients (Ds) as a function of gas loading and temperature.
• Calculation of activation energies (Ea) for diffusion.
• Radial distribution functions (RDFs) were used to analyze gas-gas and gas-lattice interactions.
• 2D density distribution plots were utilized to confirm diffusion mechanisms.

Main Results:

• NO2 diffusion coefficients (Ds) were found to be two orders of magnitude greater than SO2 at low loadings and temperatures.
• SO2 exhibited two distinct diffusion patterns, suggesting specific interactions with the MIL-47(V) lattice.
• SO2 displayed significantly higher maximum activation energies (Ea) for diffusion compared to NO2.
• Radial distribution functions indicated preferential adsorption of both gases at the O atoms of MIL-47(V).
• Hydrogen bonding interactions were observed between SO2 and the MIL-47(V) framework, leading to stronger binding for SO2.
• Jump-diffusion of SO2 between adsorption sites was confirmed.
• An exceptionally high selectivity for NO2 over SO2 (Sdiff = 623.4) was observed, indicating much faster NO2 diffusion.

Conclusions:

• NO2 diffuses significantly faster than SO2 in MIL-47(V), particularly under low loading and temperature conditions.
• The stronger binding interaction of SO2 with the MIL-47(V) framework, attributed to hydrogen bonding, hinders its diffusion.
• MIL-47(V) shows high potential for selective separation of NO2 from SO2 due to the distinct diffusion behaviors.
• Molecular dynamics simulations provide valuable insights into the molecular-level mechanisms of gas diffusion in MOFs.