Molecular sieve membranes made from 3D microparticles and 2D nanosheets allow for the separation of gas mixtures -- for example, by sieving hydrogen (H2) from carbon dioxide (CO2). However, weak connections between the 3D and 2D building blocks of these membranes tend to result in intercrystalline gaps, causing the overall selectivity of these separations to suffer. A research group from the Dalian Institute of Chemical Physics (DICP) at the Chinese Academy of Sciences (CAS) sought to improve the selectivity of molecular sieve membranes and avoid these non-selective gaps by assembling new zero-dimensional membranes from 2-methylimidazole (mim) molecules.
The team used solvent-free vapor processing on a metal-organic framework to arrange the mim molecules into unprecedented supramolecule array membranes (SAMs). The zero-dimensional building blocks, together with supramolecule interactions, resulted in the absence of intercrystalline gaps and ensured movement would occur through intermolecular spacings. Transport through the approximately 0.30 nm intermolecular spacing of mim enabled extremely precise sieving of H2 from CO2, with H2/CO2 selectivity of the SAMs calculated to be one order of magnitude higher than the selectivities of classical 3D/2D membranes. The research was recently published in Angewandte Chemie International Edition.
“Our study opens the doors to create a variety of SAMs to distinguish the subtle size/shape differences of a pair of gas molecules,” said lead author Weishen Yang. “In the future, we will tailor the intermolecular spacing, control the assembly process, and enable a wide range of application of SAMs to energy-efficient chemical separation processes.”
The development of highly-selective molecular sieve membranes such as SAMs are important for separation processes such as precombustion carbon capture, and SAMs with variable intermolecular channels could be developed for diversified separations.
Photo: A partial schematic illustration of SAM assembly. Credit: DICP