(355e) Fabrication of Highly Permeable Matrimid Substrates for a CO2-Selective Composite Membrane

A more permeable substrate with a higher surface porosity is more desirable to reduce the mass transfer resistance in the substrate and the lateral diffusion resistance in the selective layer of a thin-film composite (TFC) membrane. In this study, highly permeable Matrimid substrates with a bicontinuous surface layer and macrovoids in the bulk were prepared by vapor-induced phase separation to enhance the CO₂ permeance of an amine-containing facilitated transport membrane. With the addition of LiCl in the Matrimid (thermoplastic polyimide)/N-methyl-2-pyrrolidone (NMP) casting solution, the casting solution viscosity was increased significantly so that a lower Matrimid concentration could be used for the casting on the nonwoven fabric without causing penetration. In addition, LiCl could decrease the thermodynamic stability of the casting solution significantly. As a result, the phase separation was induced via the spinodal decomposition mechanism, and a bicontinuous surface layer could be formed. The casting solution composition, including Matrimid, LiCl and polyvinylpyrrolidone (hydrophilic additive) concentrations, was optimized with respect to the substrate morphology and CO₂ permeance. Compared with a benchmark polyethersulfone substrate that has been previously used for TFC membrane synthesis, the best Matrimid substrate showed a high surface porosity of 20.3% (vs. 13.4% of PES substrate). Also, the new substrate exhibited a high CO₂ permeance of 260,057 GPU, which was 11.8 times more permeable than the PES substrate. This high permeance was contributed mainly by the absence of a top dense layer and the presence of macrovoids in the bulk. By using this substrate, the prepared TFC facilitated transport membrane showed a CO₂ permeance of 932 GPU, which was 72 GPU higher than the counterpart coated on the PES substrate. Meanwhile, the CO₂/N₂ selectivity was retained at 158 at 57°C. This permeance improvement can be explained well using the resistance-in-series model, attributing to significant reductions in both substrate and lateral diffusion mass transfer resistances.