High-Throughput Screening of COF Membranes and COF/Polymer MMMs for Helium Separation and Hydrogen Purification

Hundreds of covalent
organic frameworks (COFs) have been synthesized,
and thousands of them have been computationally designed. However,
it is impractical to experimentally test each material as a membrane
for gas separations. In this work, we focused on the membrane-based
gas separation performances of experimentally synthesized COFs and
hypothetical COFs (hypoCOFs). Gas permeabilities of COFs were computed
by combining the results of grand canonical Monte Carlo (GCMC) and
molecular dynamics (MD) simulations, and many COF membranes were found
to overcome the upper bound of polymeric membranes for He/H2, N2/CH4, H2/N2, He/CH4, H2/CH4, and He/N2 separations.
We then examined the structure–permeability relations of the
COF membranes that are above the upper bound for each of the six gas
separations, and based on these relations, we proposed an efficient
approach for the selection of the best hypoCOFs from a very large
database. Molecular simulations showed that 120 hypoCOFs that we identified
to be promising based on these structure–performance relations
exceed the upper bound for He/CH4, He/N2, H2/CH4, and H2/N2 separations.
Both real and hypothetical COFs were then studied as fillers in 25
different polymers, leading to a total of 29 020 COF/polymer
and hypoCOF/polymer mixed matrix membranes (MMMs), representing the
largest number of COF-based MMMs investigated to date. Permeabilities
and selectivities of COF/polymer MMMs were computed for six different
gas separations, and results revealed that 18 of the 25 polymers can
be carried above the upper bound when COFs were used as fillers. The
comprehensive analysis of COFs provided in this work will fully unlock
the potential of COF membranes and COF/polymer MMMs for helium separation
and hydrogen purification.