Gas separation and water desalination performance of defect-free interfacially polymerized para-linked polyamide thin-film composite membranes

The introduction of interfacially polymerized (IP) polyamide thin-film composite (TFC) membranes in the 1980s revolutionized the reverse osmosis desalination industry. However, IP-derived TFCs have not achieved industrial success for gas separation applications due to the presence of membrane defects in their dry state. In this work, we report defect-free crosslinked polyamide thin-film composite membranes prepared from para-substituted aromatic and cycloaliphatic diamines, p-phenylenediamine (PPD) and piperazine (PIP), reacted with trimesoyl chloride (TMC). The key parameters in our modified IP process to mitigate defects are long reaction time (~5 min) and high organic solution temperature (100 °C). The gas separation and desalination properties of the para-linked polyamide membranes were compared to previously reported polyamide TFCs made from meta-phenylenediamine (MPD) and TMC. The gas- and water permeances of the TFCs increased in the order: MPD-TMC < PPD-TMC < PIP-TMC, whereas gas-pair selectivities and salt rejections followed the opposite sequential trend: MPD-TMC > PPD-TMC ≥ PIP-TMC. Elimination of defects allowed exploitation of the ultra-selective nature of polyamide TFCs, specifically for hydrogen and helium separations. At 23 °C, PIP-TMC, PPD-TMC and MPD-TMC exhibited H2/CH4 selectivities of 312, 362 and 1290, respectively, with moderate H2 permeances of 37.4, 32.6 and 25.8 GPU (1 GPU = 10−6 cm3(STP) cm−2 s−1 cmHg−1). Furthermore, the TFCs demonstrated excellent performance for H2/CO2 separation with pure-gas selectivities of 10–14 at 23 °C. The strong size-sieving capability of the polyamide TFCs originated from tight interchain packing induced by strong hydrogen bonding. Wide-angle X-ray diffraction confirmed a dominant fraction of submicropores of less than ~4 Å within PPD-TMC and PIP-TMC polyamide networks.