1. INTRODUCTION
Helium finds wide applications in various industries such as aerospace, automobile and transportation equipment, medical equipment, welding machines, and metal processing. However, despite its abundance in space, helium is scarce on Earth1. Currently, the primary method for obtaining helium industrially is extracting it from natural gas, where the helium content can reach up to 7.5%.1-4 Cryogenic distillation is the main technique for helium purification and separation, but it is known to be energy-intensive.5 In contrast, membrane separation is considered as a green and energy-saving technology without chemical reactions or phase changing process, which offers advantages such as small equipment footprint and easy maintenance.2, 6Therefore, membrane separation has emerged as a promising technology to address the shortage of helium resources.
The membrane material itself plays a key role in the membrane separation process. Till now, plentiful membranes based on materials of metal-organic frameworks (MOFs),7-10zeolites,11 polymers12 and others13 have been used for gas separation14 and desalination1. But the trade-off relationship between gas permeance and selectivity always hinders the further development of membrane separation technic. In recent years, two-dimensional (2D) materials with atomic thickness, such as graphene,15-17 molybdenum disulfide (MoS2),18, 19h-BN,20 MXene,21-23 etc., as a kind of novel membrane materials provide a new way for highly efficient membrane separation. Because on one hand, ultrathin membranes can be aseembled using 2D nanosheets as bliuding blocks aiming for higher gas permeability, on the other hand, the precisely tuning the in-plane nanopores or interlayer spacing between adjacent 2D nanosheets also enhance selectivity.
At present, some studies on 2D materials with nanopores for gas separation have been reported. Zhao et al .24designed a single-layer graphene membrane with high-density pores after O2 plasma and O3 etching for H2 sieving, which exhibited exceptional separation performance with H2 permeance ranging from 1340 to 6045 GPU and H2/CH4 separation factor of 15.6 to 25.1. Huang et al .25 prepared single-layer graphene membranes using nanoporous-carbon-assisted transfer technique, whose H2/CH4 selectivity was 25 with He permeance of 1220 GPU. However, experimental studies of gas transport through 2D material with in-plane nanopores is difficult and only a few reported, nevertheless it can be investigated at the atomic level through computational simulation methods. Bai’s group explored the influence of pore shape,26 surface charge,27, 28 and functional groups29, 30 of graphene nanopores on gas transport, revealing that gas separation performance is primarily controlled by the adsorption of gas molecules onto the graphene surface31 or molecular sieving.26Blankschtein et al. implemented in-depth study of gas molecule diffusion on graphene surface and found that changes in pore size caused different mechanisms through graphene pores.32,33 Till now, most studies on gas separation with 2D materials using in-plane nanopores are mainly focused on graphene or graphene oxides, other kinds of 2D materials are barely studied. Furthermore, whether the revealed gas transport mechinisums through the nanopores based on graphene/graphene oxide are suitable for other kind of 2D materials is still unknown, which is worth studying.
As a rising star in 2D materials family, MXenes are a group of transition metal carbides/nitrides, or carbonitrides, which were first discovered in 201134 and they have become significant members for rapid expansion class of 2D materials. A large number of membranes based on MXene have been prepared, benefited from their high mechanical strength, good thermal and chemical stability, which were widely utilized in gas separation, organic solvent separation, ions sieving and seawater desalination,22, 35-37 etc. Even more, Meidani et al .35 created pores on MXene nanosheet to shorten the mass transfer path and applied it in seawater desalination for excellent performance. Yadav et al .38 investigated the translocation features of different DNA bases across MXene nanopores for DNA detection.
To the best of our knowledge, MXene nanopores for gas separation is extremely rare. As a kind of typical 2D material, it is necessary to explore its gas separation proces. However, it will be very difficult and time-consuming to experimentally investigate the gas transport behavior in various MXene nanopores and corresponding separation mechanism. Molecular dynamics is a tool that can rapidly evaluate membrane separation performance and provide a microscopic explanation of the separation mechanism.27, 39, 40 Therefore, in this work, we simulated the He/CH4 separation performance based on four different kinds of MXene models with nanopores using MD and analyzed the gas transport mechanism. And the gas selectivity was further quantitatively explained by the potential of mean force combined with transition state theory of gas passing through MXene nanopores. Moreover, the effects of the surface functional groups, pore diameter (d ) and pore density on the gas separation performance were also explored.