Isotopic separation of helium through graphyne membranes: a ring polymer molecular dynamics study.

Somnath Bhowmick,Marta I Hernández,José Campos-Martínez,Yury V Suleimanov

doi:10.1039/d1cp02121d

Abstract

Microscopic-level understanding of the separation mechanism for two-dimensional (2D) membranes is an active area of research due to potential implications of this class of membranes for various technological processes. Helium (He) purification from the natural resources is of particular interest due to the shortfall in its production. In this work, we applied the ring polymer molecular dynamics (RPMD) method to graphdiyne (Gr2) and graphtriyne (Gr3) 2D membranes having variable pore sizes for the separation of He isotopes, and compare for the first time with rigorous quantum calculations. We found that the transmission rate through Gr3 is many orders of magnitude greater than Gr2. The selectivity of either isotope at low temperatures is a consequence of a delicate balance between the zero-point energy effect and tunneling of 4He and 3He. In particular, a remarkable tunneling effect is reported on the Gr2 membrane at 10 K, leading to a much larger permeation of the lighter species as compared to the heavier isotope. RPMD provides an efficient approach for studying the separation of He isotopes, taking into account quantum effects of light nuclei motions at low temperatures, which classical methods fail to capture.

Highlights

Besides using 2D membranes with fabricated nanopores, some compounds contain nanopores in their structures at different positions and sizes; they are more naturally suited for filtering at the molecular level
The variation of the ring polymer molecular dynamics (RPMD) potential of mean force W(s) for 3He and 4He along the reaction coordinate s at T = 10–250 K are plotted in Fig. 4 (Gr2) and Fig. 5 (Gr3), and the corresponding kQTST values are reported in the Table S1 of Electronic supplementary information (ESI).† These PMF profiles include both potential energy and temperaturedependent entropic contributions
We recall that the He–Gr3 interaction potential does not have any barrier;[51] rather, a well is built around the transmission zone. This behavior is reflected in the free energy profiles, within the temperature range 10–30 K, in which the thermodynamic barrier in the TS region has a negative value compared to the reactant site

Summary

Introduction

Besides using 2D membranes with fabricated nanopores, some compounds contain nanopores in their structures at different positions and sizes; they are more naturally suited for filtering at the molecular level. The chemical and mechanical properties of these graphyne membranes have many useful features, such as they are chemically inert and stable at ambient temperatures,[27,28,29] and flexible enough to withstand deformations induced by high pressures.[30,31] The above-mentioned

Objectives

Results

Conclusion