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.