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
1. Introduction
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.