Abstract
Monovalent
cation permselective membranes (MCPMs) are highly desirable for the
extraction of Li+ and Na+ ions from
earth-abundant sources, such as salt lakes and seawater. Metal–organic
frameworks (MOFs) are promising functional nanomaterials with excellent
potential for ion separation technologies owing to their regular
structure and tunable pore sizes. However, the successful use of MOFs in
ion separation membranes is still challenging owing to the numerous
difficulties in preparing ultrathin and defect-free MOF membranes. Here,
we proposed a facile
post-synthetic method for the preparation of
UiO-66(Zr/Ti)-NH2 and subsequently immobilized
UiO-66(Zr/Ti)-NH2 in an ultrathin polyamide layer
(~100 nm). The resulting thin-film nanocomposite
membranes presented high monovalent cation permeation and excellent
selectivity for mono-/di-valent cations.
The\(\text{\ P}_{\text{Na}^{+}{/\text{Mg}}^{2+}}\) and\(P_{\text{Li}^{+}{/\text{Mg}}^{2+}}\) permselectivities of the
best-performing thin-film nanocomposite membrane were 13.44 and 11.38,
respectively, which were 3.8 and 5.1 times higher, respectively, than
those of the commercial state-of-art CSO membrane.
Keywords: metal-organic frameworks; post-synthetic; ion
separation; interfacial polymerization;
thin-film
nanocomposite membranes
The efficient extraction and permselective separation of valuable metal
cations, such as Li+ and Na+, from
salt lakes and seawater are critical aspects in membrane research and
should be promptly addressed.1, 2 Recently, monovalent
cation permselective membranes (MCPMs) have been widely investigated for
cation separation owing to their facile scalability and low energy
consumption. Changing the electrostatic repulsive forces via the surface
modification of the cation exchange membranes is the most common
approach used to fabricate efficient MCPMs.3, 4However, the complex surface modification methods and poor long-term
stability of the deposited surface layers are drawbacks of MCPMs. In
addition, the ion separation performance of MCPMs is hindered by the
tradeoff between ion permeation and permselectivity. Another feasible
approach for achieving high permeation and permselectivity is the
incorporation of nanopores in membrane matrices to facilitate ion
sieving.5, 6 However, the pores generated using
traditional chemical reactions, such as chemical
crosslinking7and acid-base
reactions8, or
crystallinity
adjustments9 are typically not uniform, and thus,
inhibit the improvements in ion permeation and permselectivity. The
shortcomings of the current MCPMs have dictated the need for more facile
methods for the fabrication of high-performance permselective membranes.
Such methods should facilitate both the size-sieving and ion-charge
separation mechanisms, which are governed by the pore geometry and
electrostatic forces, respectively, for fast ion permeation and high
membrane permselectivity.10-12
Metal–organic frameworks (MOFs), a class of porous crystalline
materials that consist of metal ions or clusters connected with organic
ligands,
present great potential for ion separation owing to their well-ordered
and subnanometer-sized pores.13-15 However, the
applications of MOFs for membranes, and in particular for ion
separation, are limited owing to several challenges, including the
preparation of ultrathin and defect-free MOF
membranes.16, 17 Nevertheless, several researchers
have described the deposition of phase-pure MOFs on inorganic substrates
or porous polymer supports and have reported fabricating membranes with
good ion separation performance.18-22 However, the
complexity of the fabrication process of phase-pure MOF membranes and
quick propagation of cracks owing to their brittleness limit their
large-scale applications.23 Moreover, the poor
compatibility between MOFs and their polymer supports further induced
unavoidable intrinsic instability in MOF-containing
membranes.24-27
Given all the drawbacks of MCPMs and concerns associated with the use of
MOFs for membranes, we hereby proposed a facile method for the
fabrication of efficient and durable MCPMs using
UiO-66(Zr)-NH2 and a polyamide (PA) layer. We selected
UiO-66(Zr)-NH2 owing to its high water stability and
tunable angstrom-scale pore size, which matches the diameters of the
hydrated Li+ (0.76 nm), Na+ (0.72
nm), and Mg2+ (0.86 nm) ions well.28Hydrolyzed polyacrylonitrile (HPAN) was used as the substrate owing to
its negligible ion transport resistance.29 In this
study, UiO-66(Zr)-NH2 nanoparticles were prepared by
reacting zirconium (IV) chloride (ZrCl4) with
2-aminoterephthalic acid (2-NH2-BDC). Subsequently, a
fraction of the
Zr4+ ions in
UiO-66(Zr)-NH2 was replaced with Ti3+ions, which neutralized some of the positive charge and introduced a
negative charge in the porous framework of
UiO-66(Zr)-NH2. The obtained product will hereafter be
denoted as UiO-66(Zr/Ti)-NH2. The facile post-synthetic
method could promote the fast transportation of ions through the pores
of UiO-66(Zr/Ti)-NH2. Following interfacial
polymerization (IP), the acyl chloride groups of trimesoyl chloride
(TMC) reacted with UiO-66(Zr/Ti)-NH2 and
diethylenetriamine (DETA), as illustrated in Scheme 1a (where we used
UiO-66(Zr)-NH2 as an example), and produced a uniform
polyamide layer that contained embedded MOF nanoparticles (Scheme 1b).
Briefly, the prepared ultra-thin (~100 nm) MOF surface
layers that contained ion transfer channels could simultaneously
increase cation permeation and selectivity, and thus, circumvented the
tradeoff between ion permeation and permselectivity. The proposed metal
ion replacement strategy could further guide the membrane design and
facilitate charge regulation for the subnanometer-sized pores of many
MOFs that could be used for MOF-containing membranes for ion separation
purposes. The method proposed for the fabrication of thin-film
nanocomposite (TFN) membranes is described below. Moreover, the
electrochemical properties and separation performance of the membranes
were analyzed in detail, and were further compared with those of the
commercial CSO membrane.