Abstract
Two-dimensional nanopores are very promising for high-permeance molecular sieving, but the molecular backflow from permeate-side to feed-side is not beneficial for improving molecular permeance. We study the quasi-unidirectional molecular transport through a graphene-hexagonal boron nitride bilayer nanopore, aiming to realize a high-permeance molecular sieving. Molecular dynamics simulations of CO2/CH4 separations show that the bilayer pore presents 3.7 times higher selectivity comparing to the single-layer graphene nanopore with the same size. The quasi-unidirectional molecular transport is attributed to the distinctive adsorption abilities of gas molecules on the two sides of bilayer nanopores and the inhibited molecular backflow from permeate-side to feed-side. This work provides a promising way to realize the ultra-permeable porous membranes with molecular permeance even higher than the single-layer atomic-thickness membranes.
Highlights
Graphene (Geim and Novoselov, 2007; Geim, 2009) and its derivatives have been proposed as a perfect candidate for the molecular separation membranes owing to its atomic thickness, because the permeance of membrane is generally inversely proportional to the thickness of membrane materials
The graphene-Hexagonal boron nitride (hBN) bilayer nanopore exhibits a phenomenon of selective molecular sieving for the separation of CO2/CH4 mixtures
In order to quantitatively analyze the molecular sieving performance of nanopores, the molecular permeance is calculated based on the relationship between the number of permeated molecules N and time t, as follows (Sun and Bai, 2017): N (250 − Nal/2) × 1 − e−8.24×1010Pt where Nal is the average number of molecules adsorbed on both sides of graphene, P is molecular permeance, and the constant 250 is related to the molecular number 500 in the simulation box
Summary
Graphene (Geim and Novoselov, 2007; Geim, 2009) and its derivatives have been proposed as a perfect candidate for the molecular separation membranes owing to its atomic thickness, because the permeance of membrane is generally inversely proportional to the thickness of membrane materials. We systematically study the quasi-unidirectional molecular transport characteristics through a graphene-hBN bilayer nanopore, following the asymmetric two-layer porous membranes proposed by Liu et al (Liu et al, 2020). Through molecular dynamics (MD) simulations of the CO2/CH4 separation, it is shown that the permeance of the graphene-hBN bilayer nanopore is higher than that of the monolayer graphene nanopore with the same size. The selectivity of molecular separation is improved by employing the bilayer twodimensional nanopores This enhancement of the molecular permeance is caused by the relatively stronger CO2 adsorption abilities on the hBN surfaces compared with those on the graphene surface. It is expected that this type of graphene nanopores are promising for high-efficiency membranes for molecular separation as well as other processes involving molecular permeation (Sun et al, 2020b). The stacking of multiple atomic layers makes such membranes more feasible for large-area industrial production
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