mater.scichina.com link.springer.com . . . . . . . . . . . . . . . . . . . . . . Published online 16 April 2018 | https://doi.org/10.1007/s40843-018-9258-4Sci China Mater 2019, 62(1): 25–42

Mass transport through metal organic frameworkmembranesYi Guo and Xinsheng Peng*

ABSTRACT Metal-organic frameworks (MOFs), which arecomposed of metal nodes and organic ligands, possess crystalphase, ordered well-defined porous structure and large surfacearea. Since first reported in 1990, MOFs have attracted ex-tensive attention and the fabrication of MOF membranes hasexpanded their applications and endowed them with a brightfuture in various fields. The mass transportation processthrough MOF membranes is vital during their diverse appli-cations. In this review, the strategies of preparing continuousand well-intergrown MOF membranes are presented firstly.The selective transportation processes of gas molecules, liquidmolecules and ions through MOF membranes are discussed indetail, respectively. The effects of pore entrance size, interac-tion, functional groups decorating on the ligands and guestcomponents on mass transportation have been summarized inthis review as well. In addition, MOF membranes with selec-tive transportation performance demonstrate potential in se-paration, catalysis, energy transformation and storage devices,and so on.

Keywords: mass transportation, metal-organic framework(MOF), membranes

INTRODUCTIONMetal-organic frameworks (MOFs) are a kind of organic-inorganic hybrid solids composed of organic ligands andinorganic metal (also called metal-containing cluster ornodes) with infinite, uniform framework structure [1–8].Notably, various exciting properties of MOFs have beenobserved, distinguishing them from traditional poroussolids. MOF has high degree of designability and ad-justability in their structure and functions [9–11]. Aspecific framework can be produced by designing thecombination of metal nodes and organic ligands due tothe fixed coordination geometries. The structure andproperties can be tuned by presynthesis [12–17] or

postsynthesis [18–23] design of ligands and secondarybuilding unit (SBU). More recently, many studies haveshown that the properties of MOFs could also be mod-ified by introducing guest components into their cavities[24–30]. The hybrid materials present multifunction in-heriting from both the framework and the guest com-ponents. Thus, MOFs demonstrate great potentialapplications in various fields. Owing to the decades’ ef-forts of researchers, thousands of structures of MOFshave been discovered and applied in various fields, suchas separation [31–38], adsorption [20,39–42], optics [43–50], catalysis [19,51–56], sensors [57–63], etc. However,most of these studies are based on MOFs powders.

It is worthy to note the fabrication of MOF films ormembranes is of great importance for many thin-film-based devices, especially for separation membranes [64–66]. Theoretically, MOFs are capable of mitigating thetrade-off phenomenon between permeability and se-lectivity due to the crystal structure and porosity [67,68].It is accepted that the MOF-based mixed matrix mem-branes (MMMs) [69–73], which refer to the compositemembranes with MOF powder fillers dispersed in poly-mer matrix, possess most promise for practical separationapplications. But their permeability is lower compared tothe pure MOF membranes caused by the nonporouspolymer matrix [74–76]. Therefore, the continuous MOFmembranes become the promising candidates whichcould achieve both high permeability and excellent se-lectivity. Apart from separation, they could also be em-ployed in diverse fields in recent years, such as secondarybatteries [77,78] and fuel cells [79].

However, greater challenges lie in the fabrication ofcontinuous MOF membranes. It is necessary for MOFmembranes to be well-intergrown, since the pinholes,grain boundary and cracks can significantly reduce theselectivity. In addition, MOF membranes are generally

State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China* Corresponding author (email: pengxinsheng@zju.edu.cn)

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required to be synthesized on the top of substrates [80–83], contrast to the free-standing MOF-based MMMs.Thus, the fabrication of MOF membranes needs to bewell-designed, although MOF powders can be preparedeasily by hydrothermal or solventhermal method. Up tonow, there are several reviews summarizing the variousstrategies for MOF membranes synthesis in detail[66,84,85]. Here in this review, hence, only the membranesynthesis methods employed in the last three years will bediscussed.

Meanwhile, the compact crack-free morphology guar-antees that the channels of MOF membranes are the mostlikely pathway for mass transport over cross. The masstransportation through MOF membranes during separa-tion will be discussed in detail in this review. The sizesieving effect and the interaction between molecules andMOF play a crucial role. Due to the tunablity of MOF,they can be adjusted easily by ligands exchange anddecoration, as well as the encapsulation of guest mole-cules. In addition, ions transportation through MOFmembranes is also included in this review. The iontransport mechanism and the applications of ionic con-ductive MOF membranes will be present as well. Despitebeing in its infancy, the progress in this domain has al-ready shown the MOF membranes are of great promise. Itis needed to point out that our discussion focuses on theprocess of mass transport in MOFs membranes, especiallythose with well-intergrown continuous morphology. Thepowder MOFs and MOF mixed matrix membranes arenot included. Readers can find it in other reviews [68,85–88].

FABRICATION OF MOF MEMBRANESThe first MOF film was successfully prepared by Fischer’sgroup in 2005 [89]. But it is until 2009 that the gas se-paration of MOF membranes was reported by Lai andJeong et al. [90]. This means the requirements for MOFfilms and membranes are obviously different. The con-tinuous and crack-free morphology is crucial for MOFmembranes to realize separation applications. Thereafter,tremendous efforts have been devoted to the fabricationof MOF membranes and many creative methods havebeen used. In the following section, the methods forfabricating MOF membranes are discussed in detail.

Direct growthThe direct growth is defined as the nucleation, growthand intergrowth of crystals all happen on the substrate bydirectly immersing the substrate in the growth solution,which contains metal ions and organic ligands as solutes,

without any seeding process [91].UiO-66 has drawn much attention attributed to its

ultrahigh stability. It can be used in various applicationseven in harsh chemical environment. Recently, Li et al.[92] prepared a UiO-66 membrane on yttria-stabilizedzirconia (YSZ) hollow fiber (HF) by an in situ sol-vothermal approach. The YSZHF was placed vertically inthe mother solution. Then the mother solution was sealedand heated to 120°C for 48 h to allow the UiO-66membrane growing on the outer surface. After coolingand flushed, the continuous crack-free polycrystallineUiO-66 membrane on YSZ support was successfully ob-tained.

However, it is usually difficult to fabricate well-inter-grown MOF membranes via a direct solvothermal ap-proach since the heterogeneous nucleation of MOF onthe surface of the substrate is not efficient. Notably, thequality of MOF membranes heavily depends on themembrane substrate bonding. Chemical modification ofthe substrate is an effective strategy to enhance thebonding. Huang et al. [93] applied dopamine to treat thetop surface of porous α-Al2O3 disks, resulting in a poly-dopamine layer on the disk surface. The ZIF-8 membranewas next fabricated on the polydopamine modified α-Al2O3 disk via the direct growth. The polydopamine layeris favorable for anchoring the ZIF-8 nutrients and pro-moting the nucleation and growth of compact ZIF-8membrane on the substrate, attributed to its high covalentreaction ability. Similarly, 3-aminopropyltriethoxysilane(APTES) modified α-Al2O3 disks were applied for ZIF-9nucleation and formation of continuous membranes viadirect solvothermal approach as shown in Fig. 1a, b [94].

Some other novel direct growth methods were em-ployed to the fabrication of MOF membranes, since thenucleation happened not only on the surface of thesubstrate but also in the mother solution of in situ sol-vothemal methods mentioned above, resulting in hugewaste.

The interfacial microfluidic process was carried out byNair et al. [95,96] to prepare ZIF-8 membranes on theinner surface of hollow fiber. The fabrication process andthe SEM images are shown in Fig. 1c–f. The hollow fiberwas soaking in a 2-methylimidazole (2-mIm) aqueous,while the Zn(NO3)2 organic solution was injected into thelumen of the hollow fiber with continuous flow, and 2-mIm diffused over cross the wall to contact Zn2+ at theinner surface of the hollow fiber. Then the nucleationhappened on the inner surface and the ZIF-8 grew intocontinuous membranes. The continuous flow of Zn2+

solution is of significance, since insufficient Zn2+ in the

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lumen under the static growth conditions would lead tononcontinuous membranes. Another kind of microfluidicprocess was designed by Coronas et al. [97] to synthesizethe ZIF-9 membrane on the inner surface of the Nihollow fiber. Metal and ligand solution were alternatelyinjected into the hollow fiber lumen by microfluidicpumps. The MOF membrane was obtained by repeatingthe injections of metal and ligand solutions 6 times. Fi-nally, polydimethylsiloxane (PDMS) was employed to sealthe outer and inner surfaces of the hollow fiber, beforeseparation test.

Secondary growthSecondary growth, in contrast to direct growth, refers tothe membrane fabrication on substrates modified byMOF crystal seeds. The secondary growth demonstratesadvantage over direct growth, since it is relatively easy toobtain continuous well-intergrown MOF membranes onseeds modified substrates. The seeding procedure is vitalfor this growth method. Several strategies have been

widely used recently including dip coating, spinningcoating, rubbing, wiping, in situ growth, etc.

The seeding procedure can be classified into two cate-gories. One refers to the method in which the MOF seedsare synthesized solely and then modified onto the sub-strates. Recently, dip-coating method was employed toimplant the MOF seeds onto substrates [98,99], which isshown in Fig. 2a, b. The pre-synthesized seeds were firstdispersed in the corresponding solvents. After dip-coat-ing and drying process, the seeds were firmly anchoredonto the surface of substrates and ready for the secondarygrowth. Vacuum filtration is also a feasible strategy to laydown MOF seeds onto the porous substrates. As reportedby Liu et al. [13], UiO-66 seeds were filtrated onto theporous Ni sheet disk, and the seeds loading can be ad-justed easily by the amount of seeds solution to be fil-trated.

Other seeding procedure refers to MOF seeds beingfabricated directly onto the surface of substrates in situ. Itis generally called reactive seeding approach. ZIF-68 seedswere obtained by soaking the ZnO substrate in ligandssolution (2-nitroimidazole and benzimidazole) and after asolvothermal process, in which the Zn2+ of substratesreacted with ligands in solution to form ZIF-68 crystals[100]. Similarly, MIL-96(Al) seeds were prepared throughthe same method on α-Al2O3 substrates in H3BTC solu-tion [101]. In addition, the reactive seeds can also beprepared by the conversion of metal source for MOFmodified on the surface of substrates. For example, theZnO can be laid down to substrates via dip-coating [102],in situ deposition [102], manual-rubbing [98], etc.Thereafter, ZnO reacted with ligands to form MOF seedson the surface of the substrate. Moreover, copper hy-droxide nanostrands (CHNs) [103] and zinc hydroxidenanostrands (ZHNs) [104] were filtrated onto the poroussubstrates to act as the metal source for seeds fabrication,as reported by our group recently [105]. Due to the highactivity, CHNs and ZHNs can convert to HKUST-1 andZIF-8 seeds in reacting with H3BTC and 2-mIm, respec-tively, at room temperature (Fig. 2c, d). More interest-ingly, the MIL-110 nanorod arrays on porous aluminumoxide (AAO) substrates obtained through in situ growthcould serve as hetero-seeds for HKUST-1 membranes.

Filtration assembling of two-dimensional (2D) MOFmembranesNotably, recent studies have demonstrated that the 2DMOF nanosheets were also favorable to be designed asmembranes. A 2D MOF made of iron porphyrin complex(TCP(Fe)) was obtained through a soft-template assisted

Figure 1 The illustrations of direct growth of MOF membranes. (a) Thedirect solvothermal approach for fabricating ZIF-9 membranes onAPTES-modified α-Al2O3 disk, and (b) the corresponding SEM image ofthe ZIF-9 membrane. Reproduced from Ref. [94] with permission fromthe American Chemical Society, Copyright 2016. (c, e) The interfacialmicrofluidic process for fabrication of ZIF-8 membranes on the innerside of hollow fibers, and (d, f) the corresponding SEM images of theZIF-8 membrane. (c, d) Reproduced from Ref. [95] with permissionfrom John Wiley and Sons, Copyright 2016. (e, f) Reproduced from Ref.[96] with permission from the American Association for the Advance-ment of Science, Copyright 2014.

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method [106]. Subsequently, this 2D Zn-TCP(Fe) MOFwas mixed with polycationic polymer solutions. Fig. 3apresents that the 2D MOF membrane was successfullyprepared by vacuum filtration of the mix solution ontothe ultrafiltration support. Another method for mem-brane fabrication of 2D MOF nanosheets was employedby Yang et al. [107]. A modified soft-physical exfoliationtechnology was used to prepare 2D [Zn2(benzimidazole)3

(OH)(H2O)]n (Zn2(Bim)3) nanosheets from layered pre-cursors. Finally, the membrane was assembled onto α-Al2O3 porous substrates by a facile hot-drop coatingmethod, as shown in Fig. 3.

Self-confined solid conversionIn recent years, a novel self-confined solid conversionmethod was developed to fabricate MOF membranes onporous substrates by us [105,108–114]. The copper hy-droxide nanostrands (CHNs) [105,108–111], copperoxide nanosheets (CuO NSs) [112] and zinc hydroxidenanostrands (ZHNs) [113,114] were employed as metalsources in MOF membranes fabricating process, asshown in Fig. 4. Notably, these metal hydroxide or metaloxide nanostructures are highly positively charged, re-

sulting in high reactivity. The solution of these nanos-tructures was filtrated onto porous substrates to preparemetal precursor films. After soaking in ligand solution,the precursor films reacted with ligands and the nuclea-tion took place at the top surface of metal precursor film.With the crystal growing, the solid conversion proceededfrom top to down until all the precursor film converted toMOF. Thus, the continuous well-intergrown MOFmembranes were obtained. Through this method,HKUST-1, MOF-5, ZIF-8 membranes with high qualitywere successfully synthesized by using corresponding li-gand solutions, such as BTC, BDC and 2-mIm.

On the other hand, an appealing advantage of the solidconversion method lies in the facility of introducing guestcomponents into MOF membranes in situ, as shown inFig. 5 [79,115,116]. As mentioned above, the CHNs, CuONSs and ZHNs are highly positively charged, whichmeans the negatively charged components would as-semble on them via electrostatic attraction. Therefore, thehybrid precursor films containing guest components arequite easy to be prepared. And the continuous MOF layerfirstly formed on the most top of the hybrid film whensoaking them into ligand solution. This MOF layer played

Figure 2 The illustrations of secondary growth or seeding procedure of MOF membranes. (a) The illustration of reactive seeds prepared by theconversion of metal compounds, and (b) the corresponding SEM image of the ZIF-8 membrane. (a, b) Reproduced from Ref. [104] with permissionfrom the Royal Society of Chemistry, Copyright 2015. (c) The schematic diagram of dip-coating method for preparing MOF seeds onto the substrates,and (d) the corresponding SEM image of the Ni-MB membrane. (c, d) Reproduced from Ref. [99] with permission from John Wiley and Sons,Copyright 2016.

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a significant role in blocking guest components to escapeto solutions. Thus, the guest components were well en-capsulated into the MOF crystals, and nice guest-com-ponent@MOF membranes were obtained. In the last fewyears, we have introduced ions, proteins, linear polymers,DNA, carbon nanotubes, nanoparticles, polymer spheresinto MOF membranes [79,115,116] to obtain multi-functionlized MOFs membranes. This solid conversionmethod would pave a new way to fabricate hybrid MOFmembranes with determined multifunctions.

GAS MOLECULES TRANSPORT THROUGHMOF MEMBRANESAttributed to the tunable pore size and chemical tailor-ability, MOF has attracted great interests and presentspotential in separation. Diverse MOF structures can betried to find one suitable for the separation of differentmolecule size. Since the first MOF membrane was re-ported in 2009, tremendous studies on gas separation ofMOF membranes were reported [85,88,91,117]. Herein,the gas molecule transport process through MOF mem-branes is discussed. Generally, the gas molecule transportcan be affected by the MOF pore size, the interaction

between MOF and gas molecules and existence of guestcomponents in the MOF cavity.

Size sieving based gas transportationThe size sieving effect plays a significant role especiallywhen the MOF membranes are applied to separate mixedgases, in which some molecules are larger than the poresize while the others are smaller than the pore size. And itis worthy to note the entrance size and pore size of MOFare not always the same. The entrance size is usuallysmaller than pore size, such as HKUST-1 [118,119] andZIF-8 [120]. Several ZIF membranes have been reportedfor gas separation due to their high chemical stability[95,96,120–122]. The entrance of ZIF-8 is 3.4 Å (Fig. 6a).But MOF structure often has lattice flexibility to someextent, which means the entrance size is in a small range.Thus, in the permeance test, some molecules with a largekinetic diameter like CH4 (3.8 Å) [123], C3H6 (4 Å) [124]and C3H8 (4.3 Å) [123] can also pass through the ZIF-8membrane. But, the larger C3H8 demonstrated muchslower transport than smaller ones. As reported, the H2

(2.9 Å) [123], CO2 (3.3 Å) [123], N2 (3.6 Å) [123], CH4

and C3H6 presented fast transport over cross the mem-brane. The permeances of them are 2–3 orders higherthan C3H8. Moreover, molecules like i-C4H10 (5 Å) [123]and SF6 (5.5 Å) [123] can be blocked efficiently by sizesieving. Therefore, the cutoff region of ZIF-8 lies in thesize range of 4–4.3 Å. Meanwhile, the ZIF-8 lattice can bestiffened and the flexibility can be reduced upon electricfield polarization, and the framework transformed fromcubic to monoclinic and triclinic polymorphs [125]. It ismore difficult for gas molecules to transport through thestiffened lattice than in the flexible one, resulting in thelower gas permeances. The entrance of the frameworkchanged from 3.4 to 3.6 Å with the space group trans-forming from I-43m to Cm upon the electric field, leadingto the improved C3H6/C3H8 separation. These indicatethat the flexibility of framework yields a significant effecton gas permeation. Nevertheless, the ZIF-7 membranewith smaller entrance size of 3 Å can be applied to se-parate small molecules like H2 and CO2 [120]. The nar-rowed channels of ZIF-7 (Fig. 6b) may put extrahinderance to CO2 over H2, leading to much lower per-meance of CO2 than H2.

The internal rotation of ligands also affects the entrancesize. For example, the triangular pore entrance size ofUiO-66 fluctuating between 3.7 and 9.2 Å results fromthe 3 orientable benzene rings performing flips aroundthe C2 symmetry axis, as shown in Fig. 7a [126]. In ad-dition, the entrance sizes and the channel morphology of

Figure 3 (a) The 2D Zn-TCP(Fe) MOF membrane prepared by vacuumfiltration. Reproduced from Ref. [106] with permission from AmericanChemical Society, Copyright 2017. (b) The 2D Zn2(Bim)3 membranefabricated through hot-drop coating method. Reproduced from Ref.[107] with permission from John Wiley and Sons, Copyright 2017.

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MOF in different orientation are not the same due to itsanisotropy. In MIL-96(Al) system, the gas transport islimited to a 2D “zigzag” pathway (Fig. 7b) [101]. Thepermeance tests show the MIL-96(Al) membrane with{0k0} equivalent facets exposed allows an obvious fastertransport. The diffusion in the a-direction and b-direc-tion are easier than that in c-direction. The effect ofstructural orientation can extend to other MOF. As forHKUST-1, the entrance in {100} facets is 9 Å and theentrance in {111} is 4.6 Å. Recently, we prepared HKUST-1 membranes with different facets exposed. The mem-brane with {100} facets exposed demonstrates high CO2

permeantion than that with {111} facets (Fig. 7c) [110].The pore size of MOF can also be tuned by modifica-

tion of groups on the ligands and introduction of guestcomponents into its cavities. There are plenty of studieson this aspect about MOF powders, while few are re-ported about MOF membranes. The groups and guestcomponents can not only minimize the pore size but alsointeract with the gas molecules passing through the MOFmembrane. Here, we focus on the pore size of MOFmembranes and the interaction will be discussed in detailin the following section. Very recently, Heinke et al. [127]decorated the ligands with photosensitive groups (Fig.8a). The azobenzene side of the ligand F2AzoBDC can

switch from trans to cis by irradiation with green light(530 nm) and from cis to trans with violet light (400 nm).The pore size was much minimized when the azobenenzeaside was in trans isomer. The permeances of ethyleneand propene decreased by about 25% and 30%, respec-tively, while the permeance of H2 was barely affected bythe trans/cis swithing. In the meantime, Caro et al. [128]introduced azobenzene into UiO-67 membranes (Fig. 8b).The entrance of UiO-67 is 8 Å, and the azobenzene lengthchanges from 5.5 to 9 Å during cis/trans switching. Theazobenzene molecules occupied the cavities of UiO-67,narrowing the channel for gas molecules to transport.Therefore, the CO2 permeance was higher when azo-benzene was in cis isomer than that in trans isomer,mainly attributed to the gating effects.

The gas transport in 2D MOF nanosheets fabricatedmembranes is slightly different. The Zn2(Bim)3 na-nosheets have ordered honeycomb-like apertures with adiameter of 2.9 Å [107]. H2 molecules, as shown in Fig. 9,can pass through the apertures, while CO2 and a bit of H2

transport through the tortuous interlayer galleries. Thiskind of transport process has resulted in different pathlengths for H2 and CO2, leading to the higher H2 per-meance than CO2 over cross the membrane.

In a word, the size sieving effect is of great significance

Figure 4 The illustrations of solid conversion method of preparing MOF membranes. (a) The HKUST-1 membranes prepared by conversion of CuONSs film; (b) the corresponding SEM image of the HKUST-1 membrane. (a, b) Reproduced from Ref. [112] with permission from John Wiley andSons, Copyright 2016. (c) The ZIF-8 membranes prepared by conversion of ZHNs film, and (d) the corresponding SEM image of the ZIF-8membrane. (c, d) Reproduced from Ref. [114] with permission from the Royal Society of Chemistry, Copyright 2014.

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in gas molecule transport in MOF membranes. Themolecules which are smaller than the pore size can easilydiffuse into the cavity of MOF and pass through themembrane fast, while the larger ones than the pore sizewill be blocked. Notably, the lattice flexibility leads to theentrance and pore size floating in a small range. Theentrance size of MOF can also be affected by the internalrotation of ligands. Due to the anisotropy, the entrancesize and transport path of different orientation are not thesame. In addition, the pore size of MOF can even tunedby decorating side groups onto ligands and introducing

guest components into its cavities. In the transport pro-cess, gas molecules are more likely to collide to the fra-mework than each other, since the molecular free path ismuch larger than MOF pore size. As reported, the gasmolecule with smaller kinetic diameter presented higherpermeance. And the permeances had a liner relationshipwith the inverse of the square root of their molecularmass. That means the gas permeation behaviors mainlyfollow Kundsen diffusion [97,129–131].

Interaction determined gas transportationDuring the gas molecules transport through the channelsof MOF, the interaction between them takes a significantpart. Generally, the stronger interaction results in thelower gas permeance due to the extra hinderance on gasdiffusion. The interaction is regarded from the polariza-tion of gas molecules, which has been particularly in-vestigated in gas adsorption by MOF [40].

In some MOFs, metal sites are coordinately un-saturated, which are called open metal sites. And the openmetal sites demonstrate high reactivity. It is reported that

Figure 6 The crystal structures of (a) ZIF-8 and (b) ZIF-7. The entrancesize of ZIF-7 is 3 Å and the entrance size of ZIF-8 is 3.4 Å.

Figure 5 The schematic diagram of MOF membranes with introduction of various components through solid conversion method. (a) The growingprocess of MOF membranes with encapsulation of functional species via solid conversion method. (b) The HKUST-1 membranes with encapsulationof PSS, and (c) the corresponding SEM image of the PSS@HKUST-1 membranes. (b, c) Reproduced from Ref. [116] with permission from John Wileyand Sons, Copyright 2016. (d) The ZIF-8 membranes with encapsulation of DNA; (e) the corresponding SEM image of the ZIF-8 membranes. (d, e)Reproducd from Ref. [79] with permission from John Wiley and Sons, Copyright 2018.

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Mg-MOF-74 presented highest CO2 adsorption capcacitydue to the open Mg sites (Fig. 10a) [132]. Therefore, theMg-MOF-74 provided high hinderance on the CO2 dif-fusion caused by the interaction between open Mg sitesand CO2. The reduced CO2 mobility in Mg-MOF-74membranes results in the low permeance. As a result, theCO2 permeance was only a half of CH4 whose polarity wasweaker under the same condition. What’s more, Mg-MOF-74 membranes can be post-modified by ethylene-diamine as reported by Caro et al. (Fig. 10b) [133]. Theethylenediamine would anchor to the open Mg sites, notonly minimizing the pore size but also providing stronger

interaction with CO2. CO2 was adsorbed to amine groupsof ethylenediamine in Mg-MOF-74 membranes, leadingto further decreasing CO2 mobility [134]. Consequently,CO2 transported much more slowly in amine-modifiedMg-MOF-74 membranes. The tests show that the CO2

permeance decreases to just 1/5 of CH4.It is more common of the interaction between gas

molecules and organic ligands, since quite a lot of MOFsdo not have open metal sites and the ligands present moretunability. In ZIF-8 membranes, Zn saturated coordinatesto N of 2-mIm, leaving no open metal sites. CO2 is fa-vorable to be adsorbed attributed to the polarization of 2-mIm [98,123,135,136]. In the permeation test of H2 andCO2, the size sieve has little influence since the diameterof CO2 and H2 is smaller than the entrance size of ZIF-8.As expected, H2 transported faster than CO2 because ofthe smaller diameter. But the ideal separation factor of

Figure 9 The schematic diagram of H2 and CO2 molecules transportthrough the 2D Zn2(Bim)3 membranes. Reproduced from Ref. [107] withpermission from John Wiley and Sons, Copyright 2017.

Figure 8 (a) The decoration of azobenzene group on the ligand of theF2AzoBDC. Reproduced from Ref. [127] with permission from JohnWiley and Sons, Copyright 2017. (b) The encapsulation of azobenzenemolecules in the cavities of UiO-67. Reproduced from Ref. [128] withpermission from the American Chemical Society, Copyright 2017.

Figure 7 (a) The pore entrance fluctuation of UiO-66 by the 3 orientable benzene rings performing flips around the C2 symmetry axis. Reproducedfrom Ref. [126] with permission from American Chemical Society, Copyright 2017. (b) The zig-zag pathway of gas transport in MIL-96(Al)membranes. Reproduced from Ref. [101] with permission from American Chemical Society, Copyright 2016. (c) The illustration of different poreentrance sizes of different facets and corresponding gas permeances of membranes with different morphologies. Reproduced from Ref. [110] withpermission from American Chemical Society, Copyright 2014.

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H2/CO2 exceeds the Kundsen selectivity [98,135]. Thiscan be explained as the interaction between ligands andCO2 putting extra hinderance on gas diffusion. In themeantime, light hydrocarbons, CH4, C2H4 and C2H6,presented minor anomalies [123]. Although they arelarger than the entrance size of ZIF-8, they transportedfaster than N2 which is smaller than the entrance size. Inthis case, the adsorption of these hydrocarbons to ZIF-8aided it to diffuse into ZIF-8 channels and to transportthrough the membrane, which makes them higher per-meances than N2. In addition, the interaction can betuned by exchanging the ligands. ZIF-9 has a similarstructure to ZIF-8 [94]. But benzimidazole, the ligand ofZIF-9, demonstrates stronger polarization than 2-mIm.The strong adsorption of CO2 to benzimidazole furtherdecreased the permeance and resulted in higher idealseparation factor of H2/CO2.

The interaction can be enhanced by decorating groupsonto ligands. Meng et al. [137] decorated –NH2 to theligands of MIL-53. CO2 is favorable to anchor onto the–NH2 groups. The NH2-MIL-53 membrane exhibitedhigh H2 permeance but low CO2 permeance, thus de-monstrating excellent H2/CO2 ideal separation factor be-cause of the adsorption effect. On the contrary, thefunction groups of ligands can also enhance CO2 trans-port. As reported by Qian et al. [12], the carboxylic acidgroup modified MIL-100(In) exhibited higher transportrate of CO2 than N2. It is also easy to design the ligands tomake the pore walls of MOF hydrophobic. For example,there were few H2O adsorbed in the hydrophobic UiO-66-CH3 [13]. The adsorption of CO2 to MOF was reducedas a result of absent of H2O. Therefore, the hydrophobicMOFs generally demonstrated high CO2 permeance andwere employed as membranes for CO2 separation fromN2.

It is more complicated in the transport process of

mixed gases [138,139]. When it was in the low or mod-erate CO2 partial pressures, the diffusion rate of CO2

decreased due to the adsorption, while the other gaspermeance demonstrated inconspicuous decrease. Whenunder high CO2 partial pressures, the CO2 layer adsorbedon the MOF pore walls can block the other gas, such as N2

or CH4, transport through the MOF channels. Conse-quently, CO2 showed higher permeance than the othergas, leading to efficient CO2 separation. The transportmechanism is demonstrated in Fig. 11. This phenomenonwas predicted theoretically by Keskin and Sholl [140].

Guest components in MOFs affect gas transportationIn the last few years, diverse components have been en-capsulated into MOF to realize its multifunction. Theordered pore structures and large surface areas makeMOFs appropriate hosts for nano-components [24]. Theactive sites of MOFs can work synergistically with theencapsulated components so that the hybrid materials areable to exhibit novel properties that differ from those oftheir constituents. This has led to the development ofmultifunctional MOFs for various applications.

An ultrathin MOF membrane with well-aligned me-sporous GO (MGO) nanosheets fabricated by a nacre-mimetic “assembly-and-intergrowth” approach is re-ported recently [141]. The MGO nanosheets were bene-ficial to the formation of the intergrown MOFmembranes, whereas did not block the permeance path-way of gas molecules since they can transport through thepores of MGO (Fig. 12a). The thickness of the membranewas only 430 nm, but the membrane demonstrated ex-cellent H2/C3H8, C3H6/C3H8 separation properties withhigh gas permeances. Otherwise, little molecules like 1,2-bis(4-pyridyl)ethylene (bpe), which possess small size,were feasible to be encapsulated into the cavities of MOF

Figure 11 Gas transport mechanism through MOF membranes (a)under low CO2 partial pressure and (b) under high CO2 partial pressure.Reproduced from Ref. [139] with permission from John Wiley and Sons,Copyright 2016.

Figure 10 The crystal structures of (a) Mg-MOF-74 and (b) that afterpost-modified by ethylenediamine. Reproduced from Ref. [133] withpermission from Elsevier, Copyright 2015.

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[142]. The [Ni(L-asp)2(bpe)]·(G) (L-asp=L-aspartic acid,G=guest) was successfully constructed and applied on gasseparation. The randomly dispersed bpe molecules in thechannels of [Ni(L-asp)2(bpe)]·(G) along the a axis lead tothe formation of narrow pores in the structure (Fig. 12b).Consequently, the gas permeance slightly decreased, butthe ideal separation factor of H2/CO2 enhanced, due tothe strong interaction between CO2 and the guest bpemolecules, which makes it more difficult for CO2 than H2

to pass through the narrow pores.As mentioned above, the self-confined solid conversion

method exhibits great advantage especially in the for-mation of hybrid MOF membranes [79,115,116]. Aunanoparticles (NPs) were successfully encapsulated intothe HKUST-1 membrane via the solid conversion methodin spite of the larger size of Au NPs than the HKUST-1cavities [115]. This hybrid membrane took advantages ofboth the size sieving effect of HKUST-1 and catalyticperformance of Au NPs, thus demonstrating size selectivecatalysis. The larger olefin, cis-stilbene, was efficientlyblocked by HKUST-1, while the smaller one, like n-hex-ane, diffused into the membrane, and both of them ex-hibited complete hydrogenation with bare Au NPs. Thenthe n-hexane in HKUST-1 would be hydrogenated by thecatalysis of Au NPs embedded in the HKUST-1 mem-brane, as shown in Fig. 12c. The bi-functional Au NPs/HKUST-1 membrane can be applied to simultaneous gasseparation and catalysis.

LIQUID MOLECULES TRANSPORTTHROUGH MOF MEMBRANESWith the years’ dedication on the improvement of MOFstability, MOF membranes have been employed for liquidseparation in recent years. The liquid molecules transportin the MOF membrane differs from gas molecules. Thediffusion resistance mainly comes from the collision be-tween the liquid molecules. And the transport process canbe understood by using the solubility-diffusivity concept[93]. The liquid transport is driven by the pressure dif-ference between the two sides of MOF membranes and itconsists of two important steps, the adsorption-dissolu-tion on the surface of MOF membranes and diffusion inthe channels of MOF membranes. Hence, apart from themolecular sieving effect, the affinity of the surface forliquid molecules and the interaction between MOFmembranes has a significant influence on the transportprocess.

The property of the surface of MOF membranes isespecially crucial for pervaporation tests, in which theaffinity of membranes for one species over another wouldresult in the faster transport of that species cross themembrane. The well-intergrown UiO-66 membrane ex-hibited promising performance on the separation ofethanol/water mixtures (Fig. 13a) [92]. Ethanol was morefavorable than water to adsorb to the UiO-66 membrane

Figure 12 (a) Gas molecules transport through the ZIF-8/MGO mem-brane. Reproduced from Ref. [141] with permission from the RoyalSociety of Chemistry, Copyright 2017. (b) The structure of [Ni(L-asp)2

(bpe)]·(G) with bpe moelcules in the cavities. Reproduced from Ref.[142] with permission from the Royal Society of Chemistry, Copyright2017. (c) The gas transport and catalysis process through the Au NPs/HKUST-1 membranes. Reproduced from Ref. [115] with permissionfrom Springer Nature, Copyright 2014.

Figure 13 The illustrations of pervaporation of MOF membranes. (a)The separation of water and organics by UiO-66 membranes. Re-produced from Ref. [92] with permission from John Wiley and Sons,Copyright 2017. (b) The seawater desalination process of ZIF-8 mem-branes. Reproduced from Ref. [93] with permission from Elsevier,Copyright 2016.

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and diffuse through the channels of MOF membranes,hence the ethanol-selective permeation was demon-strated. In the meantime, smaller molecules, like metha-nol, obtained higher flux; and larger ones, like p-/m-xylene (0.67/0.71 nm) [143] obtained lower flux. This canbe explained by the molecular sieving effect of UiO-66membranes. Surface hydrophobic modification is anotherstrategy to evaluate the ethanol-selective permeation ofMOF membranes. As reported, PDMS was vapor de-posited onto the hydrophilic Ni2(L-asp)2bipy (bipy=4,4'-bipyridine) membrane to switch it to hydrophobic [144].Consequently, the Ni2(L-asp)2bipy@PDMS membraneachieved lower water flux and increased ethanol flux.Ethanol even became the selective permeating componentunder certain condition.

On the other hand, the interaction between liquidmolecules and MOF channels benefits to liquid permea-tion. As we all know, UiO-66 is composed of Zr6O4(OH)4

nodes and BDC ligands, and the hydrophilic adsorptionsites, like hydroxyl groups, were supposed to be favorablefor water diffusion [92]. At the early stage of permeation,considerable n-butanol molecules could pass through themembrane together with water, possibly due to the hy-droxyl groups being partly hindered by some guest mo-lecules. After removing the guest molecules by the watermolecules, the hydrophilic adsorption sites were openand available to preferentially adsorb water against n-butanol. Thus the membrane demonstrated enhancedwater flux after the activation and excellent separation ofwater/n-butanol [92]. The interaction can come from li-gands as well. As pointed by Lin et al. [100], the phenylgroups of the benzimidazole ligands as well as point de-fects within the Zn4N tetrahedral of ZIF-68 membraneswere potential to form π-complexes with p-xylene mole-cules. On the contrary, the interaction between p-xyleneand ZIF-68 decreases the flux.

The size sieve effect generally dominates in the filtra-tion and desalination process, in which the solvent mo-lecules can be pushed through the MOF membrane whilethe solute was blocked by the small pore structure. The2D MOF membrane which consists of copper ions and tri(β-diketone) ligands possesses a pore size of ~0.75 nm[145]. It exhibited efficient blocking of gold nanoparticleswith diameter larger than 2.4 nm with high water fluxattributed to the size sieve. ZIF-8 membranes have alsobeen applied to the rejection of dye molecules [113,146],such as Rose Bengal and Rhodamine B with molecularsizes larger than the entrance of ZIF-8. The dye moleculeswere efficiently blocked by the framework while watermolecules can easily pass through the membrane driven

by the pressure. Attributed to the 3D channel structure ofZIF-8, the membrane demonstrated quite high water flux.For dye molecules rejection, not only the size sieve effectplayed a crucial role but also the electrostatic effect. Theopposite charged attractive forces between the membraneand solute would drive the solute to slip the channels ofthe membrane. As proved by Hong et al. [106], the 2DZn-TCP(Fe) membrane with negative surface chargeachieved higher rejection for negatively charged or zeronormal charged molecules, like methyl red, methyl or-ange and brilliant blue than positively charged moleculesmethylene blue (Fig. 14a). This mechanism can be alsoapplied to desalination process. The hydrated diametersof ions in seawater are generally larger than the entrancesize of ZIF-8 (Fig. 13b) [93]. Therefore, the well-inter-grown ZIF-8 membrane possessed high ions rejectionover 99.8% with fast water transport through the mem-brane. Similarly, the continuous ultra-stable UiO-66membrane with entrance size of 6 Å achieved improvedwater flux as well as the excellent rejection due to the sizesieve effect (Fig. 14b) [147].

IONS TRANSPORT THROUGH MOFMEMBRANESThe ions transport process differs from the aforemen-tioned molecule transport through MOF membranes,

Figure 14 (a) The separation of water and dye molecules by the 2D Zn-TCP(Fe) membrane. Reproduced from Ref. [106] with permission fromJohn Wiley and Sons, Copyright 2017. (b) The separation of water andions by UiO-66 membranes. Reproduced from Ref. [147] with permis-sion from American Chemical Society, Copyright 2015.

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following the different mechanism and exhibiting differ-ent characteristics. Diverse applications of MOF mem-branes refer to the ions transport process, especially asmembranes for secondary batteries and fuel cells. TheMOF membrane exhibits efficient ion-selective transportin the system of batteries or fuel cells, which is able toenhance the over performance of the devices.

There are two general mechanisms related to thetransport of ions, which are known as the Grotthuss andthe Vehicular mechanisms [148]. The Grotthuss me-chanism or hopping mechanism refers to the transpor-tation of ions within a hydrogen-bonded network ofwater molecules. The ions transport occurs upon severingof ion-water bonds, transport of the ion and subsequentrearrangement between nearby water molecules. Ions“hop” along the conduction pathway within hydrogen-bond network of water. The Vehicular mechanism ad-dresses the transport of ions through self-diffusion of ion-carriers. These transport mechanisms can applied to ions,like Li+, Na+, K+, as well as protons. Generally speaking,the cleavage of ion-water bonds requires less energycontribution than the diffusion of the ion carriers. Ac-cordingly, the Grotthuss mechanism achieves activationenergy (Eact) < 0.4 eV [149–151], and the process with Eact

> 0.4 eV refers to the Vehicular mechanism [152–154].The proton conduction of MOF has been studied for

about a decade but limited to pressure pelleted powders[148,155]. As is well known, MOF is generally poor ionconductor. Hydrophilic function groups are required tobe introduced into the framework to improve its con-ductivity through the modification of ligands or the en-capsulation of guest molecules. Up to now, theconductivity of modified MOF reaches up to ca.10−1 S cm−1. However, the ion conductive MOF powderscannot satisfy the requirement of devices. And there isstill great hindrance on the fabrication of MOF mem-branes with high ion conductivity

Very recently, well-intergrown polystyrenesulfonate(PSS) threaded HKUST-1 (PSS@HKUST-1) membranesand DNA threaded ZIF-8 (DNA@ZIF-8) membraneswere successfully fabricated by our group via a solidconversion process [79,116]. The ample sulfonate groupsof PSS were favorable to coordinate to H2O and aid tobuild a 3D hydrogen-network along PSS inside thechannels of HKUST-1 membranes [116]. Notably, notonly the size sieve effect but also the binding affinity ofsulfonate groups to ions had influence on the ionstransport. Higher binding affinity resulted in easier con-densation of cation-sulfonate pairs and more difficulty fordissociation for fast transportation. The Eact of the

transport of Li+ is 0.21 eV, corresponding to the Grot-thuss mechanism. Hydrated Li+ with diameter of 0.76 nmand least binding affinity can diffuse into the HKUST-1(0.9 nm) channels and fast transport along the pathway ofthe hydrogen-bonded network. Both the size sieving ef-fect and the binding affinity contributed to the significantselective-transport of Li+ over Na+, K+ and Mg2+, as shownin Fig. 15a. The Li+ conductivity of PSS@HKSUT-1membranes reached up to 5.53×10−4 S cm−1 at 25°C and1.89×10−3 S cm−1 at 70°C, much higher than that of pris-tine HKUST-1 membranes. Compared to HKUST-1, ZIF-8 possesses smaller entrance size of 0.34 nm. The narrowchannels lead to a more complicated proton transportprocess. The proton conductivity of ZIF-8 membranes isquite low since the hydrophobic pore walls are not fa-vorable for proton transport. DNA molecules were en-capsulated into the channels of ZIF-8 membranes to buildthe proton transport pathway since the hydrophilicgroups of DNA, like amidogen and phosphate groups,coordinate an amount of water molecules into mem-branes (Fig. 15b) [79]. The proton conductivity improvedfrom 1.37×10−5 S cm−1 of ZIF-8 membranes to 0.17 S cm−1

of DNA@ZIF-8 membranes at 75°C, 97% RH, as well asthe Eact increased from 0.4 eV to 0.86 eV. Nevertheless,the Eact of DNA@HKUST-1 membranes was just 0.36 eV.It indicates that the narrow channel structure of ZIF-8puts extra energy barrier to the proton conduction incontrast to HKUST-1. It was easy to be understood thatthe Eact of DNA@ZIF-8 membranes elevated with theincreasing of DNA content since the DNA moleculestogether with water molecules further narrowed down thesizes of the pore entrance. Therefore, in spite of the Eact ofthe DNA@ZIF-8 membrane higher than 0.4 eV, theprotons should transport through the membrane by the

Figure 15 (a) The schematic diagram of ions transport through thePSS@HKUST-1 membrane. Reproduced from Ref. [116] with permis-sion from John Wiley and Sons, Copyright 2016. (b) The illustration ofproton transport through the DNA@ZIF-8 membrane. Reproducedfrom Ref. 79 with permission from John Wiley and Sons, Copyright2018.

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Grotthuss mechanism instead of the Vehicular mechan-ism. Due to the small entrance of ZIF-8, methanol mo-lecules with the size of 0.43 nm was difficult to diffuseinto the channels of ZIF-8 membranes, leading to a verylow methanol permeability of 1.25×10−8 cm2 s−1 at 75°C.The selective transport of protons makes the DNA@ZIF-8membranes promising to be the proton exchange mem-brane in fuel cells [79].

CONCLUSION AND OUTLOOKIn the last few decades, MOF attracted more and moreattention and became one kind of promising porousmaterials. The increasing number of studies and furtherunderstanding about synthesis, design and applicationbring it a bright future. Up to now, tens of thousands ofMOF structures have been reported and the surface areaof MOF exceeds those of traditional porous materialssuch as zeolites and carbons. Since MOF is kind of crystalmaterials, it possesses ordered pore structure which isfavorable for mass transportation [1–8]. In the recentyears, there is a blooming development of MOF mem-branes for mass separation [85,88,117,156]. The mass,including molecules and ions, transports through thechannels of MOF membranes has been great attractive[68,91,147]. While, more difficulties lie in the fabricationof MOF membranes than powders, and they still need tobe synthesized on the porous substrates. Generally, thesynthesis methods include the direct growth and sec-ondary growth [91,100,143]. In order to obtain well-in-tergrown continuous MOF membranes, the modificationof substrates is necessary as well as some novel methodlike microfluidic process. The self-confined solid con-version method may pave a new way to achieve well-intergrown MOF membranes, which also demonstratesgreat advantage of preparing hybrid MOF membraneswith diverse functional components. As for moleculetransportion, the pore size, channel structure and theinteraction yield a conspicuous effect on the mass trans-portation behavior. Thanks to the high degree of desig-nability and adjustability of MOF, the pore structure andinteraction can be tuned by decorating function groupsonto the ligand or introducing various components intoits cavities. Apart from molecules, ions and protons canalso transport through the channels of MOF after con-structing hydrogen-bond networks inside the MOFmembranes [79,116]. The ions transport follows theGrutthuss mechanism and the pore size of MOF and theaffinity of function groups suggest a selective ion trans-port process of hybrid MOF membranes.

Although there are lots of studies on the transport

process on MOF membranes, it is still in the initial stage.There is a lot of work which needs to be done in thefuture since plenty of applications of MOF membranesrefer to the mass transport process. Firstly, the in-troduction of guest molecules inside the cavities of MOFmembranes endows them multifunction. It suggests thatthe hybrid MOF membranes may realize the fast selectivemass transport energy transformation or catalytic reac-tion at the same time by combining the unique propertiesof guest molecules. Secondly, more attention should bepaid to the design and fabrication of mesoporous MOFmembranes since major efforts have been made for themicroporous MOF membranes for ions or small mole-cules separation in the past. Mesoporous MOF mem-branes hold huge potential for the selective transport ofbiomacromolecules and other functional nanoparticles.Thirdly, MOF membranes should be scalably producedwith predetermined performance for inductrial applica-tions. Last but not least, novel in-situ observation tech-nology and theoretical simulation are required toelucidate the mass transportation behavior through thenanochannels of MOF membranes. MOF membraneswith unique mass transport properties may significantlyimprove the performance of functional devices.

Received 24 January 2018; accepted 21 March 2018;published online 16 April 2018

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Acknowledgements This work was supported by Key Program of

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National Natural Science Foundation of China (51632008), ZhejiangProvincial Natural Science Foundation (LD18E020001) and the NationalNatural Science Foundation of China (21671171).

Author contributions Guo Y searched the references and wrote the

paper. Peng X designed the outlines and modified the manuscript. Bothauthors contributed to the general discussion.

Conflict of interest The authors declare no conflict of interest.

Yi Guo currently is a PhD candidate at the School of Materials Science and Engineering, Zhejiang University. Hisresearch interest mainly focuses on the design and synthesis of MOF membranes with ionic conductivity and theirapplications for energy transformation and storage.

Xinsheng Peng received his PhD in 2003 at the Institute of Solid State Physics, Chinese Academy of Sciences. He becamea full professor at the School of Materials Science and Engineering, Zhejiang University in 2010. His research interestfocuses on the design and synthesis of functional membranes and controlled mass transportation in energy and en-vironmental science.

金属有机框架物薄膜中的传质郭弈, 彭新生*

摘要 金属有机框架物(MOF)是由金属节点和有机配体依靠配位键结合组装而成的晶体材料, 具有规则的孔道结构和巨大的比表面积. 自1990年被提出以来, MOF便引起了广泛关注; 同时MOF薄膜的成功制备扩大了其应用范围, 使其应用于诸多领域. 在MOF薄膜的应用中,跨膜传质过程至关重要. 本文首先综述了近年来MOF薄膜材料的制备方法, 接着分别详细讨论了气体分子、液体分子和离子的选择性跨膜传输. 在传质过程中, MOF的窗口尺寸、配体上修饰的功能基团以及孔道中的客体分子均会对离子传输产生影响. 具有选择性传输特性的MOF薄膜在分离、催化和能量存储和转化领域均有潜在应用.

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