1 INTRODUCTION
As is known, hydrogen is one of the green and environmentally friendly
clean fuels in the world, which has the potential to replace traditional
fossil fuels for carbon peaking and carbon neutrality goals. In
industry, a large number of hydrogen-containing mixed gas, mainly
H2 and CO2, are produced through the
steam reforming process of natural gas, thus subsequent purification is
required to obtain high-purity hydrogen.1 Pressure
swing adsorption (PSA) and low-temperature distillation are usually
applied in industrial gas separation and purification, but both are
energy-intensive processes. According to relevant research reports,
energy consumption during the separation stage is extremely high,
accounting for up to 85% of total energy costs.2 To
reduce energy consumption and cost, researchers and engineers have
turned their attention to membrane separation technology, which
typically requires less investment and operating costs than mainstream
alternative technologies such as PSA and cryogenic distillation, which
is also more economical and energy-efficient.3-7However, the most challenging issue in membrane separation is
efficiently preparing membranes with high selectivity/permeability and
stability.
Although the polymeric membranes are easy to prepare at low cost, their
separation performance is usually limited by the Robson Bound, where the
gas permeability and selectivity have a trade-off phenomenon, thus the
corresponding H2/CO2 separation
performance is far away from the requirement for
industrialization.8-13 Many other membranes composed
of metal organic membranes (MOFs), covalent organic frameworks (COFs),
zeolite molecular sieves (Zeolites), or other types of materials are
also restricted from complex and time-consuming preparation process,
difficulty in forming a continuous defect-free membrane layer, high
cost, etc.14-30 In this decade, a kind of lamellar
membrane based on 2D materials has attracted increasing attention, which
shows the potential to break through the performance upper bound of
traditional membranes.31 Various 2D nanosheet
membranes constructed by graphene,32 graphene
oxide,4,33-37 molybdenum
sulfide,38,39 MXenes,40-43 2D
zeolites,44,45 2D MOF,46,47 and 2D
COF,48 etc. have been studied for gas separation,
pervaporation and ion sieving, etc. In our previous work, the lamellar
membranes based on MXene, a type of 2D transition metal carbides or
carbonitrides prepared on the porous disk-shaped anodic aluminum oxide
(AAO) substrate, showed good H2/CO2separation performance.42 In addition, Jin et
al.43 prepared an ultra-thin (20 nm thick) MXene
membrane on an AAO substrate, showing the performance of hydrogen
permeability of 1584 GPU with H2/CO2selectivity of 27. Moreover, the research group proposed an external
force-driven assembly approach (EFDA) to prepare a series of GO
membranes on flaky α-Al2O3 substrates,
which exhibited good molecular sieving performance with
H2/CO2 selectivity of 30 and hydrogen
permeability of 1000 GPU.35 On brittle AAO substrates,
a g-C3N4-GO membrane was developed with
outstanding hydrogen permeation capability (hydrogen permeance: 645 GPU,
H2/CO2 selectivity:
39).36 Furthermore, Lai et al.38prepared GO-MoS2 hybrid membranes via a vacuum filtering
approach on flaky AAO substrates, which exhibited hydrogen permeance of
857 GPU with H2/CO2 selectivity of 44.
H2/CO2 selectivity of 30 and hydrogen
permeance of 70 GPU were observed in self-crosslinked MXene membranes
produced by filtering on yttria-stabilized zirconia hollow
fiber.41
Although many 2D material-based membranes have been reported in the
field of gas separation, the disk-shaped AAO,
α-Al2O3, or some other fragile
substrates were often chosen as the substrates for 2D nanosheet assembly
in most research, which are not suitable for practical application due
to the limited membrane area, difficulty of sealing, high-cost, and
brittleness of the substrates. On the contrary,
tubular membranes are more commonly used in industrial applications,
which present the advantages of a relatively small footprint with large
effective membrane area per unit volume, higher packing density, ease to
seal for gastight, convenient to replace or repair any one from the
entire membrane modules if necessary, etc. There are a few works that
utilized tubular ceramic substrates for membrane construction, which are
still too fragile in practical application for gas
separation.40,41 Till now, seldom study has been
reported to prepare 2D nanosheet membranes using cheap commercial porous
substrates, whose pore size is usually large up to 20 μm with big
curvature, because membrane defects are more likely to occur during the
preparation process, resulting to a poor gas separation
performance.41 Actually, one commercial stainless
steel tube with macro-size pores (~2 μm) is a promising
substrate. Since the porous stainless steel tube exhibits good corrosion
resistance, high-temperature resistance, enough mechanical strength, and
good welding performance compared with other materials, which is
convenient to integrate with the equipment of different unit operations
for H2 production and purification. Besides the
substrate, the membrane preparation method also determines whether it
could be utilized for scale-up production in industrialization, where a
fast and efficient membrane assembly route with acceptable repeatability
is urgently required.
In this work, a series of tubular MXene membranes are successfully
constructed on commercially available macroporous stainless steel via
electrophoresis, which is considered to be a simple preparation process
with high efficiency and good membrane uniformity. The entire membrane
preparation, including substrate modification, can be finished in
2~10 minutes. The membranes had exceptional gas
separation performance with H2/CO2selectivity of 55 and hydrogen permeability of 1290 GPU. The separation
temperature, water vapor and relative humidity, preparation process
repeatability and long-term stability of the tubular MXene membranes
have been investigated in detail, which is helpful and valuable for
future industrial applications. Furthermore, this study would help
promote the amplification of tubular MXene membranes and provide some
experience for other 2D materials to be closer to actual
industrialization.