The_Catalyst_Review_February_2024 - 18

Shearing Liquid-crystalline MXene into Lamellar Membranes with Super-aligned Nanochannels
for Ion Sieving
The use of ion-selective membranes has emerged as a
critical technique for separating monovalent/ multivalent
ions. However, efficient monovalent/monovalent ion
sieving has proved challenging due to their same
valence and similar radii. Herein, the authors discuss
the preparation and use of a two-dimensional MXene
membrane with super-aligned slit-shaped nanochannels
with ultrahigh monovalent ion selectivity. This membrane
exhibited ultrahigh selectivities toward Li+
K+, and Li+
/Na+
/Rb+
outperforming state-of-the-art membranes.
MXenes are formed from layered MAX-phase materials
(such as Ti3
AlC2) possessing the generic formula Mn+1
Xn
Tx
where M represents an early transition metal, X denotes
carbon and/or nitrogen, T refers to the terminating
surface functional groups (- OH, =O, and -F), and n
varies from 1 to 4. These researchers discovered that
applying shear pressures to a liquid-crystalline (LC)
MXene nanosheet dispersion (Figure 1a) resulted in
the formation of defect-free membranes with superaligned
nanochannels, good in-plane stacking order,
and accurate interlayer spacing of 6 Å. These LC MXene
membranes exhibited rapid and selective transport of Li+
over other monovalent ions M+
, resulting in superior Li+
M+ separation performance.
A series of MXene membranes were prepared from
MXene nanosheet dispersion of different concentrations.
In addition, MXene membranes were generated from
the non-liquid-crystal MXene dispersion with a lower
MXene concentration (NLCMM). Evaluation of the liquidcrystalline
Mxene membrane (LCMM) indicated that the
number of permeated ions increases linearly with time
(Figure 2a), with the permeation rate of Li+
, (0.0079, 0.0070, and 0.0057 mol m-2
h-1
) However, for the NLCMM, the permeation rate of Li+
is as low as ~ 0.025 mol m-2
associated with the MXene nanosheet stacking order
of the membranes. The super-aligned structure endows
the LCMM with a precise channel size of ~ 6 Å, while
disordered nanosheet stacking might lead to a broader
distribution of channel sizes for NLCMM.
To better understand the ion sieving mechanism, these
investigators performed density functional theory (DFT)
and classical molecular dynamics (MD) simulations of the
ion hydration states and ion transport through simulated
LCMM and NLCMM channels. The results indicate that in
MXene nanochannels, the hydrated Li+
with a tetrahedral
shape has the smallest diameter among the monovalent
ions, contributing to the highest mobility. In addition, the
weakest interaction was found between hydrated Li+
and
MXene channels, which also contributes to the ultrafast
permeation of Li+
through the super-aligned MXene
channels. Lingzhi H, Haoyu W, Li D, et al. 2024. Angew.
Chem. Int. Ed. doi.org/10.1002/anie.202314638
18
The Catalyst Review
February 2024
(Figure 2 b) a result likely
/
, Li+
separation and a permeation rate
/
Figure 1. Preparation of the LCMM. (a) Schematic illustration of the fabrication
of LCMM by shearing LC MXene nanosheets and the corresponding separation
process. (b) Scanning electron microscopy (SEM) images of MXene nanosheets
and (c) size distribution based on 200 nanosheets. (d) Polarized optical microscopy
(POM) images of MXene dispersions for the concentrations of 5, 10,
15, and 20 mg ml−1
with the illustrations of MXene nanosheets changing from
isotropic to nematic state. (e) Viscosity versus shear rate for MXene nanosheet
dispersions at different concentrations. Digital photos for (f) the viscous LC
MXene dispersions at 15 mg ml−1
and (g) the as-prepared LCMM.
being as high
as ~ 0.35 mol m-2 h-1, compared to much lower values for
Na+, K+, and Rb+
h-1
Figure 2. Ion separation performance. (a) Permeated ions (Li+
/Na+
, Li+/K+
, and Li+/Rb+
/K+
, Na+,/K+
, and
Rb+) through the LCMM as a function of testing time. (b) Comparisons of the
permeation rates through LCMM and NLCMM. (c) Permeation selectivity of
Li+
ion selectivity of LCMM and NLCMM. (d) Long-term stability of the LCMM. (e)
Comparisons of Li+
through LCMM and NLCMM. Inset is the binary
selectivity of various membranes
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