Sorption and Diffusion of CO2/N2 in gas mixture in thermally-rearranged polymeric membranes: A molecular investigation

In this work, we study the adsorption and diffusion of nitrogen and carbon dioxide through an atomistically detailed model of a thermally rearranged polybenzoxazole (TR-PBO) polymer membranes, via equilibrium molecular dynamics (MD) simulations. This work represents a first explicit molecular modelling of the behavior of CO2/N-2 binary mixture in TR-PBO and demonstrates how diffusivity and solubility in mixtures can be coherently obtained. In particular, the number of molecules present in the polymer matrix is estimated using the Gran Canonical Monte Carlo approach. As for the sorption in mixture conditions, MD simulations are used in a synergistic pairing with GCMC and Ideal Adsorption Solution Theory (LAST). For this purpose, the single gas isotherms calculated from GCMC simulations are fitted with Langmuir and Dual-Langmuir adsorption models to obtain the parameters needed for the LAST simulations. As for diffusion, single-gas and mixture (Maxwell-Stefan) diffusion coefficients are performed by MD simulations. As main results, it is observed that the evaluated diffusion coefficients of CO2 and N-2 are in a satisfactory agreement with the values estimated using the available experimental permeability data. More specifically, the CO2 diffusivity in mixture conditions is found to be the same as that in the single-gas one, whereas the N-2 diffusivity is slightly higher. These differences are explained in terms of the effect of both the mutual gas diffusion and the competing occupancy of the available free space preferentially occupied by the CO2 molecules in mixture.