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

Abstract

The loading dependence of self-diffusion coefficients (Ds) of NO2, SO2, and their equimolar binary mixture in MIL-47(V) have been investigated by using classical molecular dynamics (MD) simulations. The Ds of NO2 are found to be two orders of magnitude greater than SO2 at low loadings and temperatures, and its Ds decreases monotonically with loading. The Ds of SO2 exhibit two diffusion patterns, indicating the specific interaction between the gas molecules and the MIL-47(V) lattice. The maximum activation energy (Ea) 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 Sdiff 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.