N2/CH4 separation via mixed matrix composite membranes: Crystallinity tuning and diffusion pathways enhancement strategy

Natural gas, known for its cleanliness and cost-effectiveness, is primed to become a key energy source for the future. However, the high N 2 content in many natural gas reservoirs presents a significant challenge, necessitating the development of advanced N 2 rejection technologies. N 2 -selective membranes provide a cost-effective solution for reducing CH 4 recompression in natural gas purification, although further improvements in N 2 /CH 4 separation efficiency are still required. This work investigates the separation capability of advanced mixed PI-matrix composite (MMC) membranes for selective nitrogen (N 2 ) removal from methane (CH 4 ). A polyimide/polyacrylonitrile (PI/PAN)-based thin-film composite (TFC) membrane was fabricated, exhibiting enhanced interfacial compatibility and modified crystallinity, leading to a more ordered polymer network with a reduced intersegmental spacing of 4.1 Å, in contrast to 5.8 Å observed in the pristine PI. PALS analysis confirmed that incorporating TYUT-96Cr nanoparticles optimally improved fractional free volume (FFV) while decreasing the PI-matrix accessible pore size to below 0.39 nm. This structural adjustment results in a sevenfold increase in annihilation intensity for smaller microcavities (I 3  > I 4 ), suggesting improved N 2 over CH 4 separation via enhanced molecular sieving and faster N 2 diffusion. At a pressure of 1.5 bar and a TYUT-96Cr loading of 30 wt%, the membrane displayed an N 2 permeance of 3.68 GPU and a N 2 /CH 4 selectivity of 63.78. Upon decreasing the selective layer thickness from 19 to 13 μm, the N 2 permeance increased to 12.47 GPU, however, the N 2 /CH 4 selectivity declined to 51.96. These findings significantly advance membrane gas separation, with strong implications for efficient natural gas purification.