Entropy-Driven Molecular Separations in 2D-Nanoporous Materials, with Application to High-Performance Paraffin/Olefin Membrane Separations

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 × 106 and 17 × 106 GPU (gas permeation units), respectively) greatly exceed
all existing membrane materials by 5 orders of magnitude.