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

The loading dependence of self-diffusion coefficients (D_s) of NO₂, SO₂, and their equimolar binary mixture in MIL-47(V) have been investigated by using classical molecular dynamics (MD) simulations. The D_s of NO₂ are found to be two orders of magnitude greater than SO₂ at low loadings and temperatures, and its D_s decreases monotonically with loading. The D_s of SO₂ exhibit two diffusion patterns, indicating the specific interaction between the gas molecules and the MIL-47(V) lattice. The maximum activation energy (E_a) in the pure component and in the mixture for SO₂ are 16.43 and 18.35 kJ mol^-1, and for NO₂ are 2.69 and 1.89 kJ mol^-1, respectively. It is shown that SO₂ requires more amount of energy than NO₂ to increase the diffusion rate. The radial distribution functions (RDFs) of gas-gas and gas-lattice indicate that the Oh of MIL-47(V) are preferential adsorption site for both NO₂ and SO₂ molecules. However, the presence of the hydrogen bonding (HB) interaction between the O of SO₂ and the H of MIL-47(V) and also their binding angle (θ(OHC)) of 120° with the linkers of lattice indicate a stronger binding interaction between the SO₂ and the MIL-47(V), but it does not occur with NO₂. The jump-diffusion of SO₂ between adsorption sites within the lattice has been confirmed by the 2D density distribution plots. Moreover, the extraordinarily high S_diff for NO₂/SO₂ of 623.4 shows that NO₂ can diffuse through the MIL-47(V) significantly faster than SO₂, especially at low loading and temperature.