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

Abstract

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, quantummechanical 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 × 10 6 and 17 × 10 6 GPU (gas permeation units), respectively) greatly exceed all existing membrane materials by 5 orders of magnitude. ■ INTRODUCTIONEthene, propene, and butadiene are essential industrial feedstocks used in plastics and chemical production. These olefins are produced from paraffins (ethane, propane, and butane) by steam cracking; this process results in a paraffin/ olefin mixture, which must then be separated through cryodistillation of the liquified gas mixture. The multiple phase changes make this separation highly energy-intensive; paraffin/olefin distillations consume more than 10 11 MJ annually in the United States. 1 Membrane-based gas separation methods have a much lower thermodynamic cost than distillation and adsorption processes 2 and thus can substantially reduce the energy needed. 3 In principle, membrane-based or hybrid distillation-membrane processes have the potential to reduce the energy usage and cost of paraffin/olefin separations by 20−30% compared to existing distillation methods. 4 Several recent reviews discuss the use of membranes in the petrochemical industry (including paraffin/olefin separation), 5,6 and a history of the membranes used for paraffin/olefin separations is presented in a technical report by Merkel et al. 7 A growing number of theoretical studies have investigated the use of two-dimensional nanoporous membranes to perform chemical, 8−21 ionic, 22,23 and isotopic 10,21,24−28 separations. Recent experiments have demonstrated the use of oxidative etching to produce small size-selective pores for gas separation, 29 and intrinsic defe...

Hydrophobic dipeptide crystals: a promising Ag-free class of ultramicroporous materials showing argon/oxygen adsorption selectivity

“…This factor is the basis for some separations using microporous membranes. 59,60 The much bigger VV pores allow such rotational freedom that the increasing trend in K N H observed for Ar is finally established.…”

“…In the case of O2 and N2, it is possible the highly hindered non-axial rotation in VI potentiates faster translation inside the pores, while the slightly larger IA and IV allow greater rotational freedom, thus limiting translation inside the pore. This factor is the basis for some separations using microporous membranes 59,60 . The much bigger VV pores allow such rotational freedom that the increasing trend in ∞ observed for Ar is finally established.…”

Selective Permeation through One-Atom-Thick Nanoporous Carbon Membranes: Theory Reveals Excellent Design Strategies!

“…Copyright Springer Nature. Part c is adapted with permission from ref , and parts e and f are adapted with permission from ref . Copyright American Chemical Society.…”

Crystallization of gas-selective nanoporous graphene by competitive etching and growth: a modeling study

“…The lattice etching needs to be carried out in a controlled manner to open only those pores that are selective to the gas molecule of interest, and to avoid large non-selective pores. Here, selectivity could be size-selectivity (molecular sieving) 15 , shape-selectivity (entropic selectivity) 16 , and adsorption-selectivity 17 . Among these, size-selectivity has the potential to yield the highest separation selectivity, where a complete blockage of larger gas molecules can be achieved.…”