A Molecular Dynamics Study of Single-Gas and Mixed-Gas N(2) and CH(4) Transport in Triptycene-Based Polyimide Membranes

Fluorinated polyimides incorporated with triptycene units have gained growing attention over the last decade since they present potentially interesting selectivities and a higher free volume with respect to their triptycene-free counterparts. This work examines the transport of single-gas and mixed-gas N(2) and CH(4) in the triptycene-based 6FDA-BAPT homopolyimide and in a block 15,000 g mol(−1)/15,000 g mol(−1) 6FDA-mPDA/BAPT copolyimide by using molecular dynamics (MD) simulations. The void-space analyses reveal that, while the free volume consists of small-to-medium holes in the 6FDA-BAPT homopolyimide, there are more medium-to-large holes in the 6FDA-mPDA/BAPT copolyimide. The single-gas sorption isotherms for N(2) and CH(4) over the 0–70 bar range at 338.5 K show that both gases are more soluble in the block copolyimide, with a higher affinity for methane. CH(4) favours sites with the most favourable energetic interactions, while N(2) probes more sites in the matrices. The volume swellings remain limited since neither N(2) nor CH(4) plasticise penetrants. The transport of a binary-gas 2:1 CH(4)/N(2) mixture is also examined in both polyimides under operating conditions similar to those used in current natural gas processing, i.e., at 65.5 bar and 338.5 K. In the mixed-gas simulations, the solubility selectivities in favour of CH(4) are enhanced similarly in both matrices. Although diffusion is higher in 6FDA-BAPT/6FDA-mPDA, the diffusion selectivities are also close. Both triptycene-based polyimides under study favour, to a similar extent, the transport of methane over that of nitrogen under the conditions studied.