Molecular Characterization of Membrane Gas Separation under Very High Temperatures and Pressure: Single- and Mixed-Gas CO2/CH4 and CO2/N2 Permselectivities in Hybrid Networks

This work illustrates the potential of using atomistic molecular dynamics (MD) and grandcanonical Monte Carlo (GCMC) simulations prior to experiments in order to pre-screen candidate membrane structures for gas separation, under harsh conditions of temperature and pressure. It compares at 300 degrees C and 400 degrees C the CO2/CH4 and CO2/N-2 sieving properties of a series of hybrid networks based on inorganic silsesquioxanes hyper-cross-linked with small organic PMDA or 6FDA imides. The inorganic precursors are the octa(aminopropyl)silsesquioxane (POSS), which degrades above 300 degrees C, and the octa(aminophenyl)silsesquioxane (OAPS), which has three possible meta, Para or ortho isomers and is expected to resist well above 400 degrees C. As such, the polyPOSS-imide networks were tested at 300 degrees C only, while the polyOAPS-imide networks were tested at both 300 degrees C and 400 degrees C. The feed gas pressure was set to 60 bar in all the simulations. The morphologies and densities of the pure model networks at 300 degrees C and 400 degrees C are strongly dependent on their precursors, with the amount of significant free volume ranging from similar to 2% to similar to 20%. Since measurements at high temperatures and pressures are difficult to carry out in a laboratory, six isomer-specific polyOAPS-imides and two polyPOSS-imides were simulated in order to assess their N-2, CH4 and CO2 permselectivities under such harsh conditions. The models were first analyzed under single-gas conditions, but to be closer to the real processes, the networks that maintained CO2/CH4 and CO2/N-2 ideal permselectivities above 2 were also tested with binary-gas 90%/10% CH4/CO2 and N-2/CO2 feeds. At very high temperatures, the single-gas solubility coefficients vary in the same order as their critical temperatures, but the differences between the penetrants are attenuated and the plasticizing effect of CO2 is strongly reduced. The single-gas diffusion coefficients correlate well with the amount of available free volume in the matrices. Some OAPS-based networks exhibit a nanoporous behavior, while the others are less permeable and show higher ideal permselectivities. Four of the networks were further tested under mixed-gas conditions. The solubility coefficient improved for CO2, while the diffusion selectivity remained similar for the CO2/CH4 pair and disappeared for the CO2/N-2 pair. The real separation factor is, thus, mostly governed by the solubility. Two polyOAPS-imide networks, i.e., the polyorthoOAPS-PMDA and the polymetaOAPS-6FDA, seem to be able to maintain their CO2/CH4 and CO2/N-2 sieving abilities above 2 at 400 degrees C. These are outstanding performances for polymer-based membranes, and consequently, it is important to be able to produce isomer-specific polyOAPS-imides for use as gas separation membranes under harsh conditions.

Automated summary

This work demonstrates the potential of using atomistic molecular dynamics (MD) and grandcanonical Monte Carlo (GCMC) simulations to pre-screen candidate membrane structures for gas separation under harsh conditions. The study compares the CO2/CH4 and CO2/N-2 sieving properties of hybrid networks based on different inorganic precursors at different temperatures. The simulations provide insights into the performance of the membranes under high temperature and pressure conditions, which are difficult to measure experimentally. The results contribute to the development of efficient gas separation membranes for industrial applications.