Noble Gas Separation using PG-ESX (X=1, 2, 3) Nanoporous Two-Dimensional Polymers

AM Brockway and J Schrier, JOURNAL OF PHYSICAL CHEMISTRY C, 117, 393-402 (2013).

DOI: 10.1021/jp3101865

Using planewave pseudopotential density functional theory and classical molecular dynamics simulations, we investigate the transport of noble gases through a family of two-dimensional hydrocarbon polymer membranes that generalize the "porous graphene" (PG) material synthesized by Bieri et al. by insertion of (E)-stilbene (ES) groups. We find that density functional theory overestimates the barrier height and empirical dispersion corrections underestimate the barrier height, compared to reference MP2/cc-pVTZ calculations on PG. The barrier height for noble gas transport is greatly reduced from PG to PG-ES1, but additional increases in the size of the pore in PG-ES2 and PG-ES3 lead to an attractive potential well instead of a repulsive barrier. Using the computed potential energy surfaces, we compute pressure- and temperature-driven tunneling probabilities of He isotopes, and refit an improved classical force-field. Using classical molecular dynamics simulations, we find that PG-ES1 has an He permeance of 6 X 10(6) GPU, which is 90 times greater than that of PG, and demonstrate high selectivity for He versus CH4, Ar, and CO2. These results indicate that PG-ES1 is a promising membrane material for separating He from natural gas, and separating He isotopes by tunneling differences.

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