Metal-organic framework UiO-66 membranes

Metal-organic frameworks (MOFs) have emerged as a class of promising membrane materials. UiO-66 is a prototypical and stable MOF material with a number of analogues. In this article, we review five approaches for fabricating UiO-66 polycrystalline membranes including in situ synthesis, secondary synthesis, biphase synthesis, gas-phase deposition and electrochemical deposition, as well as their applications in gas separation, pervaporation, nanofiltration and ion separation. On this basis, we propose possible methods for scalable synthesis of UiO-66 membranes and their potential separation applications in the future.


Introduction
A vigorous search for novel membrane materials has been stimulated by the growing demand of energy-efficient separations [1][2][3]. Polymer membranes have been extensively investigated and applied in industrial gas separation [4], reverse osmosis [5], etc. due to their easy processing and mechanical strength. Inorganic membranes, for instance, zeolite membranes, have been successfully used for organic dehydration [6]. However, polymer membranes always suffer from low chemical and thermal stability, while zeolite membranes possess issues of brittleness, limiting their applications. Metal-organic frameworks (MOFs) [7], a class of hybrid materials constructed by coordinating metal-containing units with organic ligands, have received tremendous attention from membrane scientists in virtue of their versatile topologies and customizable chemistry. The teams of Caro [8] and Kapteijn [9] reported the earliest Metal-organic framework (MOF) films in 2007 and 2008, respectively, and later on in 2009, a few MOF membranes were explored for gas separation [10][11][12][13][14][15]. In the past 10 years, targeting to various separations, a booming development of MOF membranes has taken place. MOF-5, HKUST-1, IRMOF, ZIF, MIL and CAU membranes have been studied extensively [6,[16][17][18][19][20][21][22]. However, they were always accompanied by concerns about the hydrothermal and chemical stability [23], which ultimately limited their further application in industries.
Recently, zirconium(IV)-carboxylate MOFs (Zr-MOFs) [24] have emerged as promising membrane materials due to their exceptionally high stability. According to Pearson's hard/soft acid/base principle [25], strong coordination bonds are expected by joining Zr 4+ (hard Lewis acid) and carboxylate based ligands (hard Lewis base) to determine the thermodynamic stability of Zr-MOFs. Besides, tetravalent Zr ions require more ligands to balance their charge and thus highly connected frameworks are formed with sufficient steric hindrance against attacks, which guarantees the structural stability from the kinetic aspect [26].
There are two main types of UiO-66 membranes: supported polycrystalline membranes and mixed matrix  membranes (MMMs) . This review focuses on the study of polycrystalline UiO-66 membranes (shortened to UiO-66 membranes unless otherwise stated). However, their development was hindered. During the growth of UiO-66, the high charge density Zr 4+ polarizes the Zr-O bond to present covalent character, slowing down the ligand exchange rate [24]. In this case, it is unfavourable for defect repair during the crystallization process. Consequently, UiO-66 powders with poor crystallinity are harvested after a relatively long reaction time. Low nucleation density and poor intergrowth were reported in the fabrication of UiO-66 membranes.
The silence of the absence of dense UiO-66 membranes was broken by Liu et al. in 2015 [28]. After a thorough optimization of the preparation parameters (composition of mother solution, duration of synthesis and substrates), well-intergrown UiO-66 membrane was fabricated on α-alumina hollow fibers and applied for water desalination. As stated by Liu et al. [28], high nucleation density of UiO-66 and satisfactory intergrowth could be achieved by adjusting the afore-mentioned preparation parameters. Water in the mother solution played an essential role [85], because the SBU of UiO-66 contains OHions in addition to O 2ions. Subsequently, a few continuous UiO-66 membranes supported on varying substrates were reported [86][87][88][89].
UiO-66 membranes were further developed by using modulated synthesis [59,[90][91][92][93][94][95][96]. The so-called coordination modulation method was initially proposed by Tsuruoka et al. [97] and employed in UiO-66 crystal preparation by Schaate et al. [98]. Modulated synthesis of UiO-66 refers to regulating the coordination equilibrium by introducing modulators (e.g., formic or acetic or benzoic acid) as the organic ligands used to hinder the coordination interaction between Zr 4+ and BDC ligands [98]. As a result, the competitive reaction can adjust the rate of nucleation and crystal growth, improve the reproducibility of synthesis procedures and tune crystal features such as size, morphology, and crystallinity. That is in essence the reason why the fabrication of UiO-66 membranes benefited from modulated synthesis.
In this article, we review five approaches for preparing UiO-66 membranes and films, discuss their applications in gas, liquid and ion separations, and provide future perspectives on the development of their preparation methods and potential applications. Such a review about the specific MOF UiO-66 membranes aims to provide guidance for their in-depth investigation from basic research to practical application.

In situ synthesis
In situ synthesis is defined as when a porous substrate is immersed in the mother solution without any UiO-66 crystals previously attached to the surface. The nucleation, growth and intergrowth of UiO-66 crystals on the substrate all take place during a single fabrication step.
As exemplified by Liu et al. [86], UiO-66 polycrystalline membranes were fabricated on the prestructured yttriastabilized zirconia hollow fibers (YSZ HF) by an in situ solvothermal approach via a thorough optimization of the heating duration, composition, and temperature of the synthetic mother solutions. As depicted in Fig. 2(a), after 2 h of heating, a very thin amorphous gel layer was formed on the top of the substrate. This was possibly caused by the aggregation of gel particles originating from the mother solution, which were transported to the substrate due to chemical interaction between the ligands and substrate, and Brownian motion. During the consequent synthesis, heterogeneous nucleation occurred probably at the interface of the gel and the solution ( Fig. 2(a)), the only place where both the metal and ligand source were present in abundance. In parallel, further gel settlement could still be proceeding, which buried and disturbed the UiO-66 nuclei. Afterwards, crystals propagated through the gel network and then sank to the substrate by consuming the gel around them. Meanwhile, the aggregation and densification of nanocrystals occurred. With prolonged heating, crystal growth occurred ( Fig. 2(a)) by acquisition of nutrients from the bulk solution, from nearby unreacted amorphous gel and small UiO-66 crystals (Ostwald ripening). A wellintergrown membrane layer ( Fig. 2(a) and (c)) was finally fabricated after continuous heating for 48 h.
As stated by Liu et al. [86], since this membrane was fabricated with simultaneous growth and nucleation, UiO-66 crystals emerging on the surface of the membrane layer were identified in the EDX mapping image (Fig. 2(c)). FTIR-ATR characterization indicated that chemical bonds were established between the UiO-66 ligands and substrate, probably between the carboxyl and zirconium. This chemical interaction provides evidence for disclosing the energy-dispersive X-ray spectroscopy (EDXS) mapping ( Fig. 2(c)). Although no visible UiO-66 crystals were found in the bulk substrate ( Fig. 2(j)), slight intrusion of the C signal (yellow) into the substrate (green) was observed. This might be because the substrate was chemically modified by the BDC ligands during membrane preparation. The chemical interaction could boost the adhesion of the membrane layer to the substrate to a large extent, improving membrane stability.

Secondary growth
Secondary growth is defined as when a porous substrate is immersed in the mother solution with UiO-66 crystals previously attached to the surface. In comparison with the in situ synthesis, the nucleation and growth of polycrystalline membranes can be well-balanced by the secondary growth method.
Modulated synthesis was used in the case of secondary growth of UiO-66 membranes and films. Larger and isolated UiO-66 crystals were always produced with the addition of a modulator, whereas microsized intergrown UiO-66 crystals were yielded without modulation. The addition of monocarboxylic acid modulator could probably form complexes with zirconium cations [98]. Molecular zirconium complexes with different monocarboxylic acids (HO 2 CR, R = t-Bu, C(CH 3 ) 2 Et, etc.) [114,115] and structures similar to that of the SBU in UiO-66 have been described. Such complexes could act as intermediates and the framework construction would then proceed through an exchange between modulator and linker molecules at the coordination sites of the zirconium ion [98]. The application of modulators would decrease the possibility that the linker is connected to the SBU. Therefore, the formation of framework nuclei is disfavored, thus promoting the incubation of larger UiO-66 crystals. Furthermore, modulators can inhibit UiO-66 crystal growth in the (111) direction, leading to the formation of octahedral crystalline configurations rather than the cubic lattices generated from the original synthesis [27].
Friebe et al. [95] reported (002)-oriented UiO-66 membranes employing secondary growth with benzoic acid as a modulator. The growth started from randomly oriented seed crystals until they contacted each other. Afterwards, the crystals grew along the direction with the highest growth rate (i.e., (002)), thus building the top layer of the membrane. The SEM top view in Fig. 3(a) shows the tips of the UiO-66 octahedrons, in good accordance with the model of the van der Drift growth [116]. The crosssection images reveal a 5 mm thick layer with a high orientation and the tilting angle of the octahedrons is around 15° (Fig. 3(a)).
Taking advantage of the uniform size and shape of the octahedral UiO-66 crystals, Lu et al. [117] produced largearea 2D oriented monolayers on a water surface through a liquid-air interfacial assembly technique ( Fig. 3(b)). The obtained monolayers can be further transferred easily to a silicon platform and (111)-oriented UiO-66 films with long-range 3D superlattices can be formed ( Fig. 3(b)). Furthermore, UiO-66 films with preferred (111) orientation were fabricated by repeated solvothermal synthesis (Fig. 3(c)) [105].

Biphase synthesis
As claimed in the modulated synthesis, isolated UiO-66 crystals were always produced instead of intergrown ones. An interpretation was provided by Shan et al. [96] that the partially deprotonated ligand caused by the accumulated protons in the reaction solution is the key factor preventing the intergrowth of the UiO-66 crystals (Fig. 4). With the addition of a deprotonating agent, trimethylamine (TEA), in an in situ biphase solvothermal reaction (Fig. 4), wellintergrown UiO-66 membranes and films were fabricated with tunable (200) and (111) orientations. As shown in  (2) of 2D monolayer UiO-66 on a water surface and a silicon platform, respectively; cross-sectional SEM images (3) and the corresponding X-ray diffraction (XRD) patterns (4) of silicon platform-supported UiO-66 films comprising one, two, and three monolayers of microcrystals prepared by repetition of the transfer process using self-assembly; (c) SEM images (1) surface; (2) cross section) of UiO-66 film prepared by three repeated solvothermal syntheses and the corresponding XRD patterns (3). Reproduced from [95,105,117] 4(b), a hexane-dimethylformamide (DMF) biphase system was designed. TEA was initially dissolved in the hexane phase, and metal and ligand sources were charged in the DMF phase together with the modulator. Since the TEA could diffuse from the hexane phase to DMF phase and act as a deprotonating agent, the quantity of partially deprotonated ligands were efficiently reduced. Finally, the intergrowth between UiO-66 crystals was facilitated, affording dense membranes.

Gas-phase deposition
Atomic layer deposition (ALD) [118][119][120][121][122] in a mode also known as molecular layer deposition (MLD) is a technique where two or more precursors are individually pulsed into a reaction chamber in the gas phase and left to react with and saturate the surface of a substrate. When the surface is saturated by the first precursor, excess precursor is carried away by purging with an inert gas, and then the second precursor is applied in the same way. A thin-film is constructed with a thickness of one atomic layer or one molecular layer at a time by reiterating these steps in a cyclic process.
Lausund et al. [106] deposited UiO-66 thin films in an all-gas-phase process by the aid of ALD. Sequential reactions of ZrCl 4 and 1,4-benzenedicarboxylic acid formed amorphous organic-inorganic hybrid films that are crystallized to the UiO-66 structure after the treatment in acetic acid vapour (Fig. 5(a) and Table 2). The stoichiometry between metal clusters and organic linkers was well controlled by modulation of the ALD growth with additional acetic acid pulses. Unlike other fabrication methods, which rely on solvothermal nucleation and growth, the all-gas-phase method is based on scalable, solvent-free, seed-free, thickness-controllable, a wellestablished material processing technology to coat irregular substrates.
By applying vapor-assisted conversion (VAC) [108], highly oriented thin films of UiO-66 and UiO-66(NH 2 ), were produced on a variety of surfacesbare gold, gold surfaces modified with thiol SAMs, and bare silicon ( Fig. 5(b)). The obtained MOF films are well intergrown and possess a high degree of crystallinity and crystal orientation extending to large areas. The relationship between the rate of crystallization and formation of the oriented MOF film was revealed by adjusting the parameters including modulator equivalents, precursor concentration, temperature, and reaction duration.

Electrochemical deposition
Electrochemical MOF deposition [123] has been proposed as a promising method for in situ deposition and patterning on conductive surfaces on the basis of two different mechanisms corresponding to anodic and cathodic deposition. In anodic deposition, MOF film formation occurs on a metal anode in contact with a ligand solution in virtue of the release of a critical concentration of metal ions by anodic dissolution [124]. On the other side (cathodic deposition), a solution containing metal ions, ligands, and a so-called probase is put in contact with a cathodic surface. Film deposition in this case relies on an increase in pH near the cathodic surface, where electrochemical reduction of the probase leads to local base generation and subsequent ligand deprotonation, enabling MOF formation [125].
As demonstrated by Stassen et al. [110], electrochemical deposition of the UiO-66 and its isoreticular analogues has been identified and elucidated. The crystallite size, film morphology, together with the deposition mechanism were rationalized through synthesis modulation. Whereas anodic deposition results in superior adhesion of the MOF layer onto the metallic zirconium substrate, which is attributed to the formation of an oxide bridging layer (Fig. 6), cathodic deposition has the merit of broad substrate flexibility.
Electrophoretic deposition (EPD) was used for the patterned growth of UiO-66 thin films on conductive glasses [109]. EPD is a well-established technique for depositing thin films, especially from nanoparticulate building blocks. The application of a DC electric field to a suspension composed of charged particles and nonpolar solvent can result in particle transport and deposition onto a conductive substrate [109]. During the synthesis of UiO-66, some surface defects are present (possibly due to missing metal nodes), which will give rise to partially negative charges on its surface. During the EPD process, those negative charges drive the particles toward the positively charged electrode and fabricate films.

Applications of UiO-66 membranes
Applications of UiO-66 membranes were predominately located in the separation field. The effective aperture size and functional groups of the UiO-66 type MOFs determine the membrane separation capability as predicted by molecular sieving and adsorption-diffusion mechanism. The flexibility of framework, missing ligand defects and substitutes on the ligands redefine the aperture size of UiO-66 rather than the 0.6 nm as estimated from crystallographic data. The functional groups of UiO-66 type MOFs are abundant. The OH groups from SBU, as well as the phenyl and the substituent groups from the ligands provide versatile adsorption sites. Herein, we discuss four categories of application based on membrane processes (Tables 1 and 2): gas separation, pervaporation, nanofiltration and electrochemical ion separation.

Gas separation
Liu et al. [28] applied the UiO-66 membranes constructed by in situ synthesis to gas separation. The gas permeance did not follow the kinetic diameters of the gases because of the larger aperture size of UiO-66 (~6.0 Å) ( Fig. 7(a)). Figure 7(b) shows the kinetic diameters of the studied gases. The H 2 permeance was ca. 7.2Â 10 -7 mol$m -2 $s -1 $Pa -1 , with a high H 2 /N 2 ideal selectivity of 22.4. Owing to the effect of preferential CO 2 adsorption, the permeance of CO 2 (9.5 Â 10 -7 mol$m -2 $s -1 $Pa -1 ) is higher than that of all the other studied gases, leading to a satisfactory CO 2 /N 2 separating selectivity (29.7). As claimed, UiO-66 was a good membrane material for the purpose of H 2 purification and CO 2 capture. The similar order of gas permeation was recently confirmed by Wu et al. [126].
Gas separation was also performed by Friebe et al. [95] using (002) orientated UiO-66 membranes fabricated by secondary growth with modulated synthesis. Different from the above observation, the permeance of CO 2 was lower than that of H 2 and N 2 ( Fig. 7(b)). The permeance of H 2 is the highest compared with that of the other gases (N 2 , CO 2 , CH 4 , C 2 H 6 , C 3 H 8 ), and the permeance decreased significantly with kinetic gas diameter, which seems to be Fig. 6 Schematic illustration of the anodic and cathodic electrochemical deposition mechanisms for UiO-66 films. Reproduced from [110] with permission, copyright American Chemical Society, 2015.  Not reported [117] in good accordance with the concept of molecular sieving.

Pervaporation
In 2017, Liu et al. [28] reported UiO-66 membranes for organic dehydration. The membrane was activated onstream and remained robust after being treated with boiling benzene and water. No discernible degradation of membrane performance was recognized in the following 200 hours' stability test for water/n-butanol and water/ furfural separation even sulfuric acid was introduced ( Fig. 8(a)). At higher temperature (e.g., 80°C), the membranes exhibited a very high flux of up to ca. 6.0 kg$m -2 $h -1 and great separation factor ( > 45000). This performance, in terms of separation factor, is 10-100 times of commercially available silica and polymeric membranes with equivalent flux. The resistance against harsh environments was superior to commercial zeolite NaA membranes. Under the same in situ synthesis condition, Huang et al. [87] fabricated UiO-66 membranes on micropatterned YSZ substrates. The resulting membranes displayed a 100% improvement in the apparent water permeation flux over conventional flat UiO-66 membranes without compromising the separation factor of water over n-butanol. Wu et al. [90] synthesized UiO-66-NH 2 membranes for pervaporative desulfurization with an optimum permeation flux of 2.16 kg$m -2 $h -1 and a separation factor of 17.86 under 40°C for 1312 ppm thiophene/n-octane mixtures. The separation factor is higher than polymer-based membranes in the literature (Fig. 8(b)). As evidenced, the preferential adsorption of thiophene is an important contribution to the selectivity. Moreover, the studies from Wan et al. [89], Miyamoto et al. [91] and Wu et al. [93] extended the applications of pervaporation to other systems.

Electrochemical ion separation
Zhang et al. [88] reported UiO-66 membranes for ultrafast selective transport of alkali metal ions. The resulting membranes can preferentially transport Li + over other alkali metal ions following unhydrated size exclusion mechanism, with the ion transport rate order of Li + > Na + > K + > Rb + . The LiCl/RbCl selectivity is of~1.8, which outperforms the LiCl/RbCl selectivity of 0.6-0.8 evaluated in traditional membranes ( Fig. 10(a)). This study may potentially open up a new avenue for efficient ion separations in the future.
Cyclic voltammetry (CV) experiments were conducted to assess the molecular sieving capability of UiO-66 films supported on FTO using redox-active species (including Ru(NH 3 ) 6 3+ (diameter ca. 0.55 nm) and Fe(phen) 3 2+ (diameter ca. 1.3 nm)) as probes [99]. The UiO-66 coated electrodes showed moderate CV signals for Ru(NH 3 ) 6 3+ but were not responsive to Fe(phen) 3 2+ , verifying their size-selective accessibility to these two species, which is in line with the fact that the pore aperture of UiO-66 (0.60 nm, estimated from crystallographic data) is between the diameter of Ru(NH 3 ) 6 3+ and Fe(phen) 3 2+ . The ion discrimination of UiO-66 film (healing with polydimethylsiloxane (PDMS)) was further evidenced by the electrochemical study in a mixed solution of Ru(NH 3 ) 6 3+ and Fe(phen) 3 2+ , where well-defined redox peaks were observed only for the former species ( Fig. 10(b)).

Conclusions, remarks and perspectives
With adequate members of the UiO-66 family and exceptionally high stability, UiO-66 based membranes stand out from MOF membranes as well as novel porous material membranes for organic purification under harsh conditions.
Regarding synthetic protocols, in situ synthesis is a facile method for fabricating UiO-66 membranes. Electrochemical deposition will be a promising method for coating the membranes on devices. Although secondary growth is the benchmark method for large-scale production of polycrystalline zeolite membranes, gas assistant deposition [119] and interfacial synthesis [131] may have opportunities in scaled-up synthesis of UiO-66 membranes.
Precise separation is one of the future directions for membrane-based separation. The author speculates that there would be some optimal preparation conditions where the UiO-66 membranes have the opportunity for (i) separating isomers of hydrocarbons; furthermore, (ii) purification of organics under harsh conditions may offer the other position for UiO-66 membranes.
New membrane materials are always accompanied by challenges. (i) In line with the principle of green chemistry, water [132,133] is more welcome than DMF as an alternative solvent, which reduces the cost of UiO-66 membranes. Consequently, systemic optimization of synthetic variables is required. (ii) Novel zirconium sources are desired because the usual metal source ZrCl 4 requires careful storage to avoid deliquescence. (iii) Scalable synthesis requires a clear understanding of the membrane reproducibility and substrates.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.