Entropy-Driven Molecular Separations in 2D-Nanoporous Materials, with Application to High-Performance Paraffin/Olefin Membrane Separations
K Solvik and JA Weaver and AM Brockway and J Schrier, JOURNAL OF PHYSICAL CHEMISTRY C, 117, 17050-17057 (2013).
Nanometer-scale pores in carbon-based materials such as graphene, carbon nanotubes, and two-dimensional polymers have emerged as a promising approach to high permeance, high selectivity gas separation membranes. In previous studies, quantum-mechanical mass-dependent tunneling, classical size-exclusion and differences in surface adsorption have been used to obtain high selectivity. Here, we illustrate a new classical approach in which an entropic barrier causes the selective separation of gas molecules. Using atomistic molecular dynamics simulations, we study the separation of ethane, ethene, propane, propene, n-butane, isobutane, 1-butene, cis-2-butene, trans-2-butene, isobutene, and 1,3-butadiene through a novel nanoporous two-dimensional hydrocarbon polymer (denoted PG-TP1), as a function of temperature and pressure. Despite the absence of a potential energy barrier for both types of species and the greater surface adsorption of the paraffins, selective passage of olefins results from the greater number of possible conformational and orientational configurations possible within the small pores. This entropic barrier allows for differentiation of similar gases, such as ethane and ethene or propane and propene. Larger branched alkanes and alkenes are completely rejected by size exclusion. The PG-TP1 selectivity exceeds practical requirements for economical separations of propene and 1,3-butadiene; moreover, the high permeances (10 x 10(6) and 17 x 10(6) GPU (gas permeation units), respectively) greatly exceed all existing membrane materials by 5 orders of magnitude.
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