Beyond graphene oxides:Emerging 2D molecular sieve membranes for efficient separation☆

Yi Liu

State Key Laboratory of Fine Chemicals,Dalian University of Technology,Linggong Road 2,Ganjingzi District,Dalian 116024,China

Keywords:Two-dimentional material Molecular sieves Membranes Separation Desalination

A B S T R A C T Recent decades witnessed the significant progress made in the research field of 2D molecular sieve membranes.In comparison with their 3D counterparts,2D molecular sieve membranes possessed several unique advantages like significantly reduced membrane thickness(one atom thick in theory)and diversified molecular sieving mechanisms(in-plane pores within nanosheets&interlayer galleries between nanosheets).M.Tsapatsis first carried out pioneering work on fabrication of lamellar ZSM-5 membrane. Since then, diverse 2D materials typically including graphene oxides(GOs)have been fabricated into membranes showing promising prospects in energy-efficient gas separation,pervaporation,desalination and nanofiltration.In addition to GOs, other emerging 2D materials,including 2D zeolites,2D metal-organic frameworks(MOFs),2D covalent-organic frameworks(COFs),layered double hydroxides(LDHs),transition metal dichalcogenides(TMDCs),MXenes(typically Ti3C2TX),graphitic carbon nitrides(typically g-C3N4),hexagonal boron nitride(h-BN)and montmorillonites(MT)are showing intriguing performance in membrane-based separation process.This article summarized the most recent developments in the field of 2D molecular sieve membranes aside from GOs with particular emphasis on their structure-performance relationship and application prospects in industrial separation.

1.Introduction

As one of the most energy-consuming process, separation process accounts for 45%-55%of total energy consumption of the global industry[1]. It has therefore become increasingly urgent to develop energyefficient separation processes to lift the heavy energy burdens.Membrane-based separation represented a relatively new concept but developed quickly due to the low energy consumption,high efficiency,sustainability and easy operation [2]. As an important sub-branch of membrane materials,molecular sieve-based membranes, which could achieve precise separation of multi-components relying on size-based exclusion and preferential adsorption mechanisms, have received tremendous attention in recent decades due to their permanent porosity,framework robustness and versatile available framework structures.

Conventional molecular sieve membranes contain three-dimensional(3D)framework topologies.Since the pioneering work on lamellar ZSM-5 membranes conducted by M.Tsapatsis[3],construction of high performance molecular sieve membranes from atom-thick two-dimensional(2D) nanosheets has become a frontier research topic. In comparison with their 3D counterparts, molecular sieve membranes constructed from 2D nanosheets could be only one-atom thick,therefore enabling minimization of the transport resistance and maximization of the flux;moreover,they were more mechanically flexible[4,5].Particularly it should be emphasized that guest species enabled to permeate through 2D molecular sieve membranes and realize precise molecular sieving relying on either pores within 2D nanosheets(if any)or interlayer passages between neighboring nanosheets,which provides us with unprecedented freedom for tailoring their pore aperture and functionality(schematically shown in Fig.1)[6,7].All these advantages have made 2D molecular sieve membranes superb candidates of high-performance separation membranes.

Graphene and its derivatives are among the most extensively studied 2D layered materials[8].Ever since the year 2012,great attention has been paid to graphene-based separation membranes [9,10] and several excellent review articles dealing with this topic have been published recently [11,12]. Aside from graphene and its derivatives,other emerging 2D layered materials typically including 2D zeolites,2D MOFs,2D COFs,LDHs,TMDCs,MXenes,g-C3N4,h-BN and MT have been fabricated into membranes with controlled microstructures and shown unprecedented opportunities in microfiltration,nanofiltration,ultrafiltration, reverse osmosis, pervaporation and gas separation[6,7,13-18]. Among them, guest molecules selectively permeated through 2D GO-,LDH-,TMDC-,MXene-,g-C3N4,h-BN and MT membranes via interlayer galleries, while in-plane pore-based molecular sieving represented the dominant separation mechanism for 2D zeolite-,2D MOF-and COF-based membranes.

Fig.1.Schematic illustration of both interlayer gallery-based and in-plane pore-based separation mechanisms for 2D separation membranes.

In this review article,the most recent developments in the field of 2D molecular sieve membranes are summarized with particular concerns over innovative methods,approaches and strategies developed for exerting precise control over their microstructures and functionality at microscopic,mesoscopic and macroscopic scales.

2.2D materials employed in membrane-based separation

2.1.2D zeolite membrane

By analogy with their 3D counterparts,2D zeolites possessed several advantages like framework robustness, high thermal stability and tunable hydrophilicity/hydrophobicity.The preliminary work on platelike MCM-22/silica composite membranes presented unprecedented opportunities of 2D zeolite membranes for facile and energy-efficient separation[19,20].Nevertheless,preparation of high performance 2D zeolite membranes has been hindered by 1)the difficulty in delamination of single-unit-cell thick nanosheets and 2)the rigid framework structure,which inevitably led to the formation of interlayer defects. A breakthrough in 2D zeolite membrane fabrication stemmed from the successful delamination of MFI-type zeolites by designing a di-quaternary ammonium-type surfactant C22H45-N1(CH3)2-C6H12-N1(CH3)2-C6H13(called C22-6-6)[21,22],which led to the formation of single-unit-cell 2 nm thick MFI nanosheets.

Fig.2.SEM images of top view of coated MFI nanosheets on a a)AAO and b)porous α-Al2O3 substrates;c)FIB and d,e)TEM images of the cross section of MFI coatings.Scale bars:a-d),200 nm;(e),20 nm.(Reproduced with permission[3].Copyright 2011,American Association for the Advancement of Science.)

2.1.1.2D MFI zeolite membrane

Based on the above-mentioned study,M.Tsapatsis further conducted a pioneering work on fabrication of 2D zeolite membranes by first exfoliation of multi-lamellar MFI zeolite precursors via melt blending with polystyrene followed by their restacking on porous α-alumina substrates via simple filtration of exfoliated aqueous suspensions(Fig.2)[3].Nevertheless,substantial nanometer-sized gaps existed between neighboring MFI nanosheets so that prepared 2D MFI zeolite membranes did not show any selectivity for p-/o-xylene mixtures.As a result,an additional mild secondary growth step was required to amend gaps between neighboring MFI nanosheets.Later this process was further improved in the following aspects:1)Structure-directing agents(SDAs)were mildly removed with a piranha solution(H2O+H2SO4)leading to an enhanced dispersion in water so that interlayer gaps existing between MFI nanosheets were anticipated to be reduced[23].2D MFI membranes were then prepared by re-self-assembly of uniformly dispersed MFI nanosheets in aqueous solution via simple vacuum filtration,showing an n-/i-butane selectivity of 5.4,which was a clear indication of the dominance of molecular sieving mechanism.2)Floating-particle coating method was employed for facile deposition of single-unit-cell thick monolayer MFI nanosheets, which served as seed layers,on porous substrates[24].After facile secondary growth, an ultra-high p-/o-xylene selectivity (＞10000) was achieved.3)A gel-less secondary growth method was developed for facile elimination of interlayer gaps between sub-100 nm-thick MFI-nanosheet seed layers [25]. Instead of conventional hydrothermal synthesis, in this study MFI-nanosheet seed layers were first impregnated with dilute aqueous TPAOH solution followed by simple thermal treatment under given conditions. Underlying 100 nm-thick silica buffer layers were served as the silica source.This method not only enabled reduction in the manufacturing costs but also enhanced the scale-up reproducibly.

It was noticed that conventionally 2D MFI nanosheets were prepared with the top-down method,which was time-consuming,costly and lowyield.Very recently M.Tsapatsis scored a further breakthrough in fabrication of 2D zeolite nanosheets via the bottom-up approach with improved yield and lower cost (Fig. 3) [26]. In this study, a nanocrystal-seeded growth method triggered by a single rotational intergrowth was developed to produce integrated 5 nm(2.5 single-unit-cell-thick)zeolite nanosheets.These high-aspect-ratio nanosheets allowed facile fabrication of thin defect-free 2D MFI nanosheet seed layers, which were further evolved into high-flux and ultra-selective MFI membranes after secondary growth.Future efforts should be focused on development of facile methods for eliminating interlayer gaps existing between 2D MFI nanosheets.

2.1.2.Other 2D zeolite membrane

AMH-3 is a kind of layered silicate with three-dimensional microporosity(eight-membered rings)within the layers.Because the unique 3D microporosity within layers, AMH-3 has been considered as an ideal nanofiller of MMMs for efficient gas separation. M. Tsapatsis first prepared swollen derivative of AMH-3 by a novel procedure involving sequential intercalation of dodecylamine after ion exchange in the presence of amino acid [27]. Further incorporation of swollen AMH-3 into polybenzimidazole (PBI) matrix leads to a doubling of H2/CO2ideal selectivity(R.T.)than that of pure PBI membranes due to additional molecular sieve properties of swollen AMH-3 nanofillers(Fig.4)[27].In addition,similar ideal selectivity was observed with higher loadings(e.g.,14%)of proton-exchanged AMH-3 particles indicating that diffusion of H2was faster than CO2in AMH-3-derived silicates.Besides PBI,post-modified AMH-3 nanofillers were further incorporated in cellulose acetate(CA)and Polysulfone(PSf)matrices,resulting in significant improvements in gas separation performances[28].

JDF-L1 represents a layered titanosilicate exhibiting with an in-plane pore size ～0.3 nm, making them superb candidates for separating H2-containing gas mixtures.J.Coronas and C.Staudt first prepared JDFL1-copolyimide MMMs with disaggregated plate-like JDF-L1 as nanofillers [29]. Gas permeation results indicated that horizontal dispersion of JDF-L1 particles into the polymer matrix was beneficial for improving the H2/CH4selectivity,owing to the elongated diffusion path for CH4.Further exfoliation of JDF-L1 was achieved by melt compounding commercial PSf together with swollen JDF-L1 processed with PSf[30].Prepared JDF-L1-PSf MMMs showed simultaneous improvement of H2permeability and H2/CH4selectivity,which was attributed to the incorporation of highly dispersed exfoliated JDF-L1 nanosheets.It was noted that in addition to a single JDF-L1 nanofiller,J.Coronas further confirmed that combination of both JDF-L1 and calcined MCM-41 fillers for MMMs gave rise to a synergy effect favoring uniform dispersion of fillers so that both higher H2permeability and H2/CH4selectivity were achieved in comparison with pure polymer membranes[31].

In addition to above-mentioned zeolite framework types,2D MWW zeolite nanosheets with a free internal diameter close to 0.71 nm delimited by 12 MR have been prepared by repeated exfoliation of the corresponding layered precursor ITQ-1[32].Although 2D MWW zeolites may not be ideal for precise molecular sieving due to their relatively large pore aperture,they may serve as high performance catalytic membrane reactors upon incorporation of catalytic active species(like Ti)into the framework.For instance, X. S. Chen prepared dense and well-intergrown 2D Ti-MWW zeolite membranes on porous mullite substrates by secondary hydrothermal synthesis[33].Prepared Ti-MWW zeolite membranes exhibited excellent catalytic activity for the phenol hydroxylation by pervaporation,and the phenol conversion and dihydroxybenzene selectivity reached 22.94%and 98.33%at 323 K,respectively.

2.2.2D MOF membrane

Fig.3.a,b)Top and cross-sectional views of MFI membranes prepared by gel-free intergrowth of MFI nanosheets.Scale bars:2 μm;c)XRD pattern for the MFI membrane;d)p-xylene permeance and selectivity for MFI membrane.(Reproduced with permission[26].Copyright 2017,Nature Publishing Group.)

Owing to the high flexibility in both pore apertures and functionality,MOFs in the form of membranes have offered unprecedented opportunities for energy-efficient gas/liquid separation.Conventionally MOF materials employed in membrane fabrication dominantly possess a 3D framework. In contrast, membranes constructed from 2D layered MOFs have been rarely explored. The difficulty in fabricating high performance 2D MOF membranes lies in the lack of facile methods for exfoliation of bulk MOF materials while maintaining morphological and functional integrity of exfoliated 2D MOF nanosheets.Moreover,facile methods for re-assembly of exfoliated MOF nanosheets into defect-free ultra-thin 2D MOF membranes are yet to be developed.

Fig.4.SEM images of a)original and b)swollen AMH-3;c)TEM images of a cross section from 3 wt%swollen AMH-3 nanocomposite.(Reproduced with permission[27].Copyright 2008,Wiley-VCH.)

2.2.1.2D MOF membrane in pure form

Pioneering work on the fabrication of 2D MOF membranes was conducted by W.S.Yang and Y.S.Li who first prepared 1-nm-thick 2D[Zn2(bim)4]nanosheets and constructed ultra-thin(＜10 nm)2D[Zn2(bim)4]membranes with unprecedented H2/CO2separation performance(Fig.5)[34].In the first step,monodispersed[Zn2(bim)4]nanosheets were prepared by exfoliation with a novel soft-physical process involving low speed wet ball-milling and ultra-sonication-assisted exfoliation.Owing to the structural flexibility,theoretical 0.21 nm-sized pores within[Zn2(bim)4]nanosheets permitted fast permeation of H2while other gases with larger kinetic diameters were excluded.In the next step,a hot-drop coating technique was developed for disordered stacking of 2D[Zn2(bim)4]nanosheets on porous α-Al2O3substrates.Under optimized operating conditions,a H2/CO2selectivity ～300 with a H2GPU of ～3000,which had by far exceeded the latest Robeson upper-bound for the H2/CO2gas pair,was achieved.Very recently W.S.Yang further disintegrated bulky[Zn2(Bim)3]into lamellar amphiprotic MOF nanosheets via a modified soft physical exfoliation method[35].Consequently sub-10 nm-thick ultrathin membranes were prepared with the previously developed hot-drop coating method and demonstrated remarkably high H2/CO2separation performance.

In a similar manner,D.Zhao prepared ultra-thin MAMS-1 membranes composed of 2D MAMS-1 nanosheets first by mild exfoliation of layered precursors via freeze-thaw approach in suitable solvent systems,followed by aligning large MAMS-1 nanosheets onto porous AAO substrates via a hot-drop casting method (Fig. 6) [36]. Most notably,membranes derived from well-aligned MAMS-1 nanosheets showed an unprecedented reversed thermos-switchable H2permeability, which was attributed to the structural flexibility of MAMS-1 nanosheets.

Besides exfoliation,H.T.Wang further prepared preferentially b-and c-oriented 2D bulky ZIF-L membranes by conventional solvothermal growth [37]. ZIF-L possessed a cushion-shaped cavity between nanosheets with a dimension of 0.94 nm×0.7 nm×0.53 nm.Owing to their relatively large cavity size and considerable membrane thickness(～5 μm),prepared 2D layered ZIF-L membranes did not show superior separation performance(like H2/CO2and H2/N2) over traditional 3D MOF membranes.To further reduce the membrane thickness,G.L.Zhang and E.Zeng developed a novel gel-vapor deposition(GVD)method,which combined sol-gel coating with vapor deposition,for facile and scalable production of ultra-thin(～17 nm)ZIF-8 membranes[38].Prepared membranes showed considerably high selectivities(3126 and 73.4)for H2/C3H8and C3H6/C3H8mixtures,respectively.

Besides gas separation,2D MOF materials could further serve as ideal candidates for high performance NF membranes.For instance,L.Hong utilized polycationic polymers(i.e.,PEI or PDDA)to regulate the assembling of 2D Zn-TCP(Fe)nanosheets through the cross-linking between polycation coils and the periphery carboxylic groups of 2D Zn-TCP(Fe)nanosheets[39].Owing to the well-aligned channels for efficient water purification,prepared polycation-regulated 48 nm-thick 2D Zn-TCP(Fe)membranes showed ultrahigh permeance of 42430 L·m-2·h-1·MPa-1and excellent rejection rates(＞90%)for organic dye with size larger than 0.8 nm×1.1 nm.

Fig.6.a)The SEM image of layered MAMS-1 crystals.Scale bars:5 mm;b)crystal structure of MAMS-1;c)tilted vertical view of ab plane in MAMS-1 monolayer;d)amplified view of PW1 gated by two pairs of tert-butyl group;e)view along a-axis of MAMS-1 monolayer.(Reproduced with permission[36].Copyright 2017,Nature Publishing Group.)

Fig.5.a)Architecture of layered MOF precursor;b)illustration of the grid-like structure of[Zn2(bim)4]nanosheets;c)space-filling representation of[Zn2(bim)4]nanosheets;d)TEM image of[Zn2(bim)4]nanosheets;e)top and f)cross-section of a[Zn2(bim)4]layer on α-Al2O3 substrate.(Reproduced with permission[34].Copyright 2014,American Association for the Advancement of Science.)

2.2.2.2D MOF membrane in a mixed-phase form

Besides in a pure form,2D layered MOF nanosheets could further integrate with other selective materials(like 3D MOFs,GO and polymers)to form highly compatible 2D MOF-based mixed-phase membranes.For instance,J.Gascon developed a bottom-up synthesis strategy for the fabrication of dispersible[Cu(1,4-bdc)]-polyimide MMMs(Fig.7)[40].Incorporating 5 nm-thick MOF nanosheets into polymer matrices endowed resultant MMMs with outstanding CO2/CH4selectivity(～90). This could be attributed to a superior occupation of the membrane cross-section by MOF nanosheets as compared with bulk crystals,which improved the accuracy of molecular sieving and minimized non-selective permeation pathways.In a similar way D.Zhao prepared MMMs by blending PBI with[Cu2(ndc)2(dabco)]nwith diverse morphologies(bulk crystal,nanocube and nanosheet)[41].Gas permeation tests indicated that the morphology of MOF fillers was closely associated with their separation performance.The best H2/CO2separation performance was achieved in nanosheet-PBI MMMs due to a preferential exposure of the small pore size of the(001)crystal plane(0.37 nm×0.37 nm)towards the gas concentration gradient.In addition to PBI,[Cu2(ndc)2(dabco)]nnanosheets were further incorporated in PIM-1 polymers showing excellent CO2/CH4separation performance[42].Besides gas separation,F.S.Pan further incorporated ZIF-L sheets and ZIF-8 nanocrystals into sodium alginate(SA)matrix for pervaporation dehydration of ethanol [43]. ZIF-L-SA MMMs displayed superior separation performance than ZIF-8-SA MMMs, owing to the ordered water channels for rapid transport of water molecules and the desirable molecular sieving effect.Furthermore,prepared MMMs exhibited excellent swelling resistance and thermal stability.

Besides polymers,2D MOF nanosheets could also easily couple with other MOF materials or GOs to form 2D MOF-based composite separation membranes.For instance,Kang developed a novel method involving the employment of 2D layered ZIF-L as seed crystals for the epitaxial growth of continuous ZIF-8 layers which ultimately led to the formation of ZIF-LZIF-8 composite membranes[44].This approach guaranteed effective incorporation of ZIF-L within the ZIF-8 matrix with a high volume fraction of ZIF-L (～28%). Compared with pure ZIF-8 membranes, ZIF-L-ZIF-8 composited membranes exhibited a 3-fold enhancement in H2permeability as well as a decent increase in H2/CO2selectivity.The superior separation performance of ZIF-L-ZIF-8 composite membranes was attributed to the intrinsically high diffusivity of ZIF-L for H2.

2.3.2D COF membrane

2D COFs represent a novel class of crystalline nanoporous materials with highly ordered honeycomb networks. Recent studies indicated that bulk 2D COFs could be further exfoliated into ultrathin 2D COF nanosheets,therefore providing unprecedented opportunities for industrial applications in diverse occasions.

C.L.Zhong first proposed the application of a monolayer triazinebased CTF-0 membrane for effective separation of He and H2purification based on first-principle calculations [45]. Relying on diffusion barrier calculations, it was demonstrated that a monolayer CTF-0 membrane exhibited exceptionally high He and H2selectivity over Ne,CO2, Ar, N2, CO, and CH4with superior He and H2permeabilities. In addition to CTF-0,C.L.Zhong further conducted a systematic study on the CO2/N2separation performance of a series of designed few-layered ultrathin triazine-based CTF-1 membranes[46].Simulation results indicated that regulating the size of interlayer passages between stacked 2D COF nanosheets led to the formation of diverse ultra-thin 2D COF membranes with distinct separation performances.Moreover,controlling the stacking mode between 2D COF nanosheets enabled to construct a favorable energetic microenvironment for achieving superior separation performance.

G.Li and T.Tsuru successfully fabricated uniform and ultra-thin 2D COF-1 membranes with exfoliated microporous COF-1 nanosheets as building blocks(Fig.8)[47].Precise manipulation of the membrane microstructure resulted in formation of ultra-permeable COF-1 membranes due to the ultrafast diffusion of guest molecules through interlayer gaps between assembled COF nanosheets as well as outstanding thermal stability due to the robust covalent bonds in the COF-1 framework.

D.Zhao successfully synthesized water-stable 2D NUS-2 and NUS-3 with excellent thermal stability (～300°C) and resistance towards hydrolysis under both neutral and acidic conditions[48].Bulky NUS-2 particles were further exfoliated into monodispersed nanosheets under sonication and stirring, and then blended with Ultem and PBI into MMMs with excellent compatibility and mechanical properties.Compared with the pure polymer membranes, prepared 20 wt%NUS-2@PBI MMMs exhibited enhanced H2gas permeability and H2/CO2selectivity surpassing the 2008 Robeson upper bound.

Fig.7.a)Schematic illustration of the spatial arrangement of different liquid layers during the synthesis of CuBDC MOF nanosheets;b)the SEM image of CuBDC MOF nanosheets;c)surface-rendered views of segmented FIB-SEM tomograms for nanosheet CuBDC MOF-polyimide MMMs.(Reproduced with permission[40].Copyright 2015,Nature Publishing Group.)

In addition to 2D COF-based MMMs,C.L.Zhong further proposed a GO-assisted layer-by-layer restacking method to fabricate ultrathin GO-CTF-1 composite membranes for gas separation [49]. Favorable functional group interactions between GO and CTF-1 nanosheets led to the formation of 100 nm-thick GO-CTF-1 composite membranes exhibiting high H2permeance as well as competitive H2/CO2selectivity.

Besides gas separation,very recently H.Meng and J.Caro prepared defect-free 2D imine-linked 400 nm-thick COF-LZU1 membranes by in situ solvothermal growth [50]. Prepared membranes exhibited unprecedented water permeance(～760 L·m-2·h-1·MPa-1)as well as considerable rejection rates (＞90%) for dyes larger than 1.2 nm.Long-time operation further confirmed the outstanding water stability of prepared membranes.

Fig.8.Schematic illustration of the preparation of a COF-1 membrane via the assembly of exfoliated COF-1 nanosheets.(Reproduced with permission[47].Copyright 2017,American Chemical Society.)

2.4.2D LDH membrane

LDHs are representative of anionic clays composed of positively charged brucite-like 2D sheets,charge compensating anions and solvation molecules located in interlayer galleries.Compositional flexibility and gallery height adjustability of LDHs give rise to a functional diversity.Furthermore, microstructures of LDHs can be easily tuned by changing the synthetic strategy, therefore making them superb candidates of highperformance separation membranes and complementary buffer layers.

2.4.1.LDH-based separation membrane

By employing a vacuum-suction approach, Tsotsis took an initial step in fabrication of MgAl-CO3LDH-based separation membranes showing an ideal CO2/N2selectivity of 35 after repairing cracks and defects by silicone coating[51,52].In the next step,an electrophoretic deposition(EPD)technique was employed to deposit plate-like LDHs on porous α-Al2O3substrates. Prepared LDH membranes showed preferred selectivity towards CO2[53].

Y.Liu and J.Caro further extended the utilization of LDH membranes in H2purification.Well-intergrown NiAl-CO3LDH membranes could be successfully prepared on mesoporous γ-Al2O3substrates by facile in situ hydrothermal growth (Fig. 9) [54]. XRD pattern further indicated a uniform gallery height of 0.31 nm, which was smaller than kinetic diameters of most gas molecules.Correspondingly, the H2/CH4SF of prepared NiAl-CO3LDH membranes reached ～80.In a further study,it was found that the concentration of CO2dissolved in the precursor solution played a critical role in controlling the preferred orientation of LDH membranes: Trace amount of CO2induced formation of ab-oriented 0.6 μm-thick LDH membranes, whereas over-saturated CO2gave rise to the formation of randomly oriented 5 μm-thick LDH membranes[55].Although both LDH membranes showed clear sizebased selectivity,randomly oriented LDH membranes showed much higher H2selectivity possibly due to reduced mesoscopic defects.Compared with conventional exfoliation-restacking process,nucleation of LDH crystallites,their attachment to substrates and final formation of compact LDH membranes could be completed in one step.

It was noted that besides NiAl-CO3LDH membranes,with a similar method,Y.Liu and J.Caro further prepared well-intergrown ZnAl-NO3LDH membranes with the gallery height of 0.42 nm,which was larger than H2,CO2,N2and CH4[55].Therefore,SF of H2/CO2,H2/N2and H2/CH4gas pairs on ZnAl-NO3LDH membranes was considerably lower than that of NiAl-CO3LDH membranes.

Fig.9.a)Schematic elucidation of the concept of selective molecular sieving through their interlayer galleries;b)permeances of the single gases through NiAl-CO3 LDH membranes.(Reproduced with permission[54].Copyright 2014,The Royal Society of Chemistry.)

Besides in pure form,Y.Liu and J.Caro further successfully prepared ZIF-8-ZnAl-NO3LDH composite membranes in one step by partial conversion of ZnAl-NO3LDH precursor membranes(Fig.10)[56].Partial dissolution of ZnAl-NO3LDHs led to reduced framework density and therefore enhanced H2permeance. Moreover, since there existed both ZIF-8 and ZnAl-NO3LDH selective layers, prepared composite membranes showed significantly enhanced H2/CH4selectivity.

In addition to interlayer gallery-based separation mechanism,relying on the unique“memory effect”of LDHs,recently S.C.Wang further proposed a calcination-reconstruction method to prepare MgAl-CO3LDH-based facilitated transport membranes by designing high-speed CO2facilitated transport channels[57].Prepared membranes displayed excellent CO2/N2permselectivity(268)and ultra-high CO2permeance(5693 GPU)by in situ reconstruction of calcined MgAl-CO3LDH fillers as high-speed facilitated transport channels.In addition,both ZnAl-CO3and NiAl-CO3LDH fillers also enabled to improve the CO2/N2selectivity to a certain extent.

2.4.2.LDH-based complementary buffer layer

LDHs could also serve competent complementary buffer layers for fabrication of high performance MOF membranes,relying on the high length-to-width ratio, rich surface functionality and unique thermal properties.For instance,with a simple in situ hydrothermal method,vertically aligned MgAl-CO3LDH networks could be introduced on α-Al2O3substrates[58].Consequently,ZIF-8 seeds were uniformly embedded into the LDH network by dip-coating.Owing to the hindrance from adjacent LDH walls,trapped ZIF-8 seeds were firmly attached to substrates and would not easily peel off.Finally,well-intergrown ZIF-8 membranes exhibiting excellent H2permselectivity were prepared after epitaxial growth.

In addition to MgAl-CO3LDHs,it was found that vertically aligned ZnAl-CO3LDH buffer layers could be further in situ synthesized on γ-Al2O3substrates(Fig.11)[59].The high length-to-width ratio and rich surface functionality of LDH grains led to the formation of well-intergrown ZIF-8 membranes by simple in situ solvothermal growth.To sum up,LDH complementary buffer layers have guaranteed facile formation of well-intergrown MOF membranes with ideal gas selectivity.

Fig.10.a)Schematic illustration of partial self-conversion of a ZnAl-NO3 LDH precursor layer into a ZnAl-NO3-ZIF-8 membrane;b)structural evolution of the ZnAl-NO3 LDH membrane after the solvothermal treatment with 2-methylimidazole.(Reproduced with permission[56].Copyright 2015,Wiley-VCH.)

Fig.11.Schematic illustration of in situ solvothermal growth of ZIF-8 membrane on a ZnAl-LDH buffer layer-modified γ-Al2O3 substrate.(Reproduced with permission[59].Copyright 2014,American Chemical Society.)

The unique thermal behavior of LDHs enabled to improve the separation performance of MOF membranes further.Prior to solvothermal treatment with the ligand 2-methylimidazole (2-mIm), H2-selective ZnAl-NO3LDH precursor membranes were calcined at 350°C,generating nano-sized ZnO buffer layers serving as sole metal sources of ZIF-8.H2/CH4selectivity of prepared ZIF-8 membranes reached 83.1,which was by far the highest value ever reported for pure ZIF-8 membranes possibly due to the framework rigidization caused by underlying nano-sized ZnO buffer layers[60].

2.5.TMDC membrane

TMDCs are a representative of layered compounds with the formula MX2showing a“sandwich”structure with two chalcogen atomic layers separated by a transition metal atomic layer [61]. The weak van der Waals interlayer interaction permits facile intercalation of foreign molecules into the interlayer gallery.Recent decades witnessed significant attention paid to TMDC materials due to their unique layered structure and the presence of d-electrons in transition metals.Being analogous to LDHs,the existence of open controllable interlayer spaces enabled selective permeation of guest molecules through TMDC-based separation membranes. Among diverse TMDC materials, MoS2and WS2-based separation membranes have been most extensively studied and shown great potential for efficient gas separation,pervaporation and desalination.In addition,drilling in-plane nanopores with controllable structure and chemical environment,although still in its infancy,has shown great promise in membrane-based separation.

Taking MoS2,one of the most extensively studied TMDC materials as an example,although there existed few in-plane nanopores within a single MoS2nanosheet,results from molecular dynamics simulations have indicated that even a single-layer MoS2nanosheet may exhibit unprecedented performance in gas separation and desalination upon assuming that in-plane nanopores with controlled shape, size and functionality could be drilled in a large scale[62-64].

Fig.12.a)Cross-sectional,b)top,and c)high-magnification cross-sectional SEM images of prepared MoS2 membranes;(d)XRD pattern recorded from the sample.(Reproduced with permission[65].Copyright 2013,The Royal Society of Chemistry.)

Fig.13.Schematic illustration of gas permeation pathway across MoS2 membranes before and after heating at 160°C.(Reproduced with permission[67].Copyright 2016,The Royal Society of Chemistry.)

At present interlayer gallery-based separation remained the dominant separation mechanism for MoS2-based membranes. For instance, X. S.Peng fabricated 1.7 μm-thick MoS2membranes by self-assembling atomthick MoS2nanosheets via vacuum filtration (Fig. 12). Prepared MoS2membranes exhibited a water permeance 3-5 times higher than GO membranes without sacrificing the rejection ratio(89%)for Evans blue[65].

In addition to desalination,J.Jin further prepared AAO-supported 17 nm-thick MoS2membranes by simple vacuum filtration for gas separation[66].Nevertheless, prepared membranes exhibited poor H2/CO2selectivity(～3),implying that substantial non-selective defects existed.Therefore,facile methods for elimination of interlayer defects were yet to be developed.In a further study conducted by M.Eswaramoorthy,500 nm-thick MoS2membranes were prepared via vacuum filtration of 1T MoS2solution (Fig. 13) [67]. Surprisingly, prepared 1T MoS2membranes exhibit unprecedented H2permeability (1175 barrer) as well as H2/CO2selectivity(8.29).Moreover,a significant increase in gas permeability was observed at higher operating temperature(＞160°C)due to the phase transition of MoS2from 1T to the 2H.

In contrast to the pursuit of sub-micrometer thickness,H.G.Park prepared freestanding 20 μm-thick stacked MoS2-platelet membranes by simple vacuum filtration[68].A clear size-based selectivity for organic vapor molecules was observed, which could be interpreted as mechanistic interplay of steric hindrance,compatibility with the 2D hydrogen bonding network of water and electrostatic interaction.To further associate the desalination performance of MoS2membranes with their nanochannel size,X.M.Bao discovered that fully hydrated MoS2membranes maintained a 0.9 nm gallery height,and the low hydraulic resistance of smooth and rigid nanochannels significantly contributed to the high water flux; in contrast, completely dehydrated MoS2membranes maintained a 0.3 nm free spacing which was almost impermeable to water[69].

Post-modification represented an effective method to achieve further performance enhancement of MoS2membranes.M.A.Bissett produced 5 μm-thick MoS2membranes with high rejection rate(～99%),high water fluxes(5 times higher than GO membranes with similar thickness)and long-term operation stability by simple immersion of MoS2precursor membranes in dye solutions under deliberately controlled conditions[70].Post-modification of MoS2nanosheets with charged dye molecules led to a change in the surface charge near the membrane interface,which unambiguously contributed to the enhanced desalination performance. In addition to charged dyes, X. S. Peng further reported postmodification of 2D channels within MoS2-laminated membranes with an ionic liquid, [BMIM][BF4] via the infiltration process (Fig. 14) [71].Compared with the bulk[BMIM][BF4],MoS2-supported ionic liquid membrane exhibits superior CO2permeance and CO2/N2permselectivity,which was attributed to the nanoconfined [BMIM][BF4] enabling preferential diffusion of CO2molecules.

Besides in pure form,delaminated MoS2nanosheets could be further incorporated into polymer matrices for construction of high performance MMMs.Y.T.Zhang first used MoS2nanosheets as an inorganic nanofiller of Pebax 1657 matrix for CO2/N2separation[72].A simple drop-coatingevaporation approach was employed to prepare the MoS2-Pebax MMMs.Both permeability(64 barrer)and selectivity(93)of the MMMs had far exceeded pristine Pebax membranes. It was assumed that stronger adsorption energy of MoS2nanosheets to CO2than N2contributed to the enhancement of CO2/N2selectivity. In addition to gas separation,Z.Y.Jiang further prepared delaminated MoS2-Pebax MMMs for removal of thiophene from the n-octane/thiophene mixture,relying on the proper binding energy of thiophene molecule on the basal plane of MoS2nanosheets[73].Under optimized conditions,prepared MMMs exhibited a permeation flux of 11.42 kg·m-2·h-1with an enrichment factor of 9.11,which were both higher than that of pure Pebax membranes.

Fig.14.Illustration of fabrication process of IL-impregnated MoS2 membranes via infiltration.(Reproduced with permission[71].Copyright 2017,American Chemical Society.)

Being analogous to MoS2membranes,X.S.Peng further prepared WS2membranes composed of exfoliated WS2nanosheets via vacuum filtration[74]. Prepared membranes showed 2-fold enhancement in water permeance of MoS2laminar membranes with similar rejection rate.To enhance the water flux further, ultra-thin Cu(OH)2nanostrands were employed as hard templates to generate more fluidic channel networks,resulting in a 2-fold increase of the water flux without significantly degrading the rejection rate.

Besides in pure form, V. Bruggen further fabricated PES-WS2flat-sheet MMMs for Human Acid removal from aqueous solution via non-solvent induced phase separation (NIPS) [75]. A significant enhancement in the permeability was observed as a result of the incorporation of ultra-low fractions of nano-WS2.Simultaneously,bother fouling resistance and solute rejection rate were remarkably enhanced.

2.6.MXene membrane

MXenes are a family of early transition metal carbides receiving increasing attention since their first discovery.Ti3C2TXas a representative of MXenes[76],was conventionally prepared from Ti3AlC2precursors through a HF etching process. Owing to the framework flexibility,extraordinary structural stability, rich functionality,intrinsic surface hydrophilicity and excellent exfoliation capability,Ti3C2TXhas offered unprecedented opportunities for membrane-based separation.

2.6.1.Pure MXene membrane

Y.Gogotsi conducted a pioneering study on MXene-based separation membranes [77]. Exfoliated 2D Ti3C2TXsheets were assembled into freestanding or supported membranes therefore demonstrating ultra-fast water flux and molecular sieving capability towards salts depending on both the hydration radius and charge of ions.Moreover,different from GO membranes, the ultra-fast water flow through MXene membranes was attributed to the presence of one or more H2O molecule layers between Ti3C2Txlayers in the wet state.To enhance the water flux further, H. H. Wang further prepared AAO-supported MXene membranes by filtration deposition during which colloidal Fe(OH)3nanocrystals were employed the as distance holder (Fig. 15)[78]. Prepared MXene membranes exhibited unprecedented water permeance(＞10000 L·m-2·h-1·MPa-1)while maintaining a high rejection rate(90%)for guest molecules(＞2.5 nm).Such high water permeance was interpreted as the following two aspects: First,nanometer-sized MXene sheets were employed instead of the traditional micrometer-sized MXene sheets so that transport pathways were greatly reduced.Second,intercalated Fe3O4nanoparticles,which were subsequently removed by mild acid etching,were used as pillars to achieve larger interlayer distance and create more nanochannels.To improve the salt rejection ratio further,W.Q.Jin prepared ultra-thin(～60 nm) MXene membranes for pervaporation desalination [79].Owing to unique 2D interlayer channels as well as intrinsic hydrophilicity, prepared MXene membranes showed excellent NaCl rejection capacity(99.5%),high water flux and excellent long-term stability.

Very recently the application of MXene membrane was further extended to gas separation. H.H.Wang fabricated Ti3C2TXmembranes with uniformly aligned sub-nanometer interlayer galleries by using abundant functional groups on MXene nanosheets(Fig.16)[80].Both H2permeability and H2/CO2selectivity of prepared Ti3C2TXmembranes were superior to the state-of-the-art membranes. Moreover, MD simulations were further carried out to investigate the mechanism of selective gas diffusion in MXene membranes.Results showed that the MXene membrane structure significantly influenced gas diffusion behaviors and therefore,the gas separation performance[81].In a recent study, W. Q. Jin reported the fabrication of 20 nm-thick MXene membranes with the vacuum-assisted method [82]. Prepared ultrathin MXene membranes showed outstanding molecular sieving property towards separating H2from mixed H2/CO2gas pair.Moreover,it was found that interlayer spacing of MXene membranes could be precisely controlled by pillaring MXene nanosheets with borate and PEI molecules,showing excellent CO2separation performance transcending the upper bound of state-of-the-art membranes.

2.6.2.MXene-based composite membrane and MMMs

Fig.15.Schematic illustration of the MXene membrane fabrication process.(Reproduced with permission[78].Copyright 2017,Wiley-VCH.)

Besides in pure form, H. T. Jung further prepared 90 nm thick Ti3C2Tx-GO composite membranes for water treatment and molecular separation[83].GO nanosheets were mixed with Ti3C2Txto minimize the undesired penetration of guest molecules through nonselective channels (like inter-edge defects or voids). Prepared composite membranes effectively rejected dye molecules with hydrated radii above 0.5 nm as well as positively charged dye molecules.In the case of MXene-based MMMs,J.T.Wang further prepared Ti3C2Tx-PEI and Ti3C2Tx-PDMS MMMs for purification of alcohol-based mixtures[84].Alcohol fluxes were found to be enhanced significantly due to a considerable promotion on the transfer of alcohol molecules along the nanosheet surface. Simultaneously, the rejection ratio was dramatically improved with MWCOs below 200 Da,owing to the blockage of solute transfer by nanosheets.

Post-modification represented an effective method for improving the separation performance of MXene membranes.Relying on abundant surface groups on Ti3C2Txsheets,diverse functional groups,including--NH2,--COOR,--C6H6and--C12H26,were grafted on their surface,respectively[85]. Interestingly, all functional groups grafted on Ti3C2Txsheets enhanced the flux for isopropanol,ethyl acetate,toluene and n-heptane,irrespective of hydrophilicity/hydrophobicity of the polymer matrix.

Fig.16.a)Single-gas permeability through MXene membranes with different thicknesses;H2/CO2 separation performance of MXene membranes as a function of b)temperature and c)time;d)H2/CO2 separation performance of the MXene membranes compared with state-of-the-art gas separation membranes.(Reproduced with permission[80].Copyright 2018,Nature Publishing Group.)

2.7.2D g-C3N4 membrane

Recently great attention has been paid to 2D g-C3N4materials composed of g-C3N4sheets stacked together via weak van der Waals interactions[86].Layered g-C3N4sheets consists of tri-s-triazine units interconnected with tertiary amino groups forming regularly distributed triangular 0.311 nm-sized nanopores.It was predicted that these nanopores may allow preferential permeation of small-sized molecules(like H2O,He and H2)through g-C3N4-based membranes.In addition,unique physicochemical property,outstanding thermal and chemical stability as well as the low production cost have made g-C3N4materials ideal candidates for membrane-based separation.

2.7.1.Pure g-C3N4membrane

Relying on first-principle calculations, M. W. Zhao theoretically demonstrated that g-C3N4membranes held great potential in He separation from other gas molecules[87].The He/H2selectivity at R.T.was calculated to be ～107.Moreover,theoretically g-C3N4membranes may also serve as competent quantum sieving membranes for 3 He/4He separation with a selectivity of 18 at 49 K.

H.H.Wang fabricated AAO-supported 2D g-C3N4membranes with artificial nanopores and self-supporting spacers by vacuum filtration(Fig. 17) [88]. Prepared 160 nm-thick g-C3N4membranes showed a water flux of 290 L·m-2·h-1·MPa-1with a rejection rate of 87%for 3 nm-sized molecules.Moreover,permeation experiments and molecular dynamic simulations further indicated the dominance of a nanofluidic phenomenon.In a follow-up study,polyacrylic acid(PAA)was introduced to regulate nanochannels formed between neighboring g-C3N4nanosheets[89].Prepared g-C3N4-PAA hybrid membranes possessed higher water flux without sacrificing much rejection rate.Nevertheless,noticeable fouling was observed upon the addition of PAA.

2.7.2.g-C3N4-based MMMs

Improving the dispersity of g-C3N4in the polymer matrix represented an effective approach for enhancing the separation performance of g-C3N4-based MMMs.To achieve this purpose,Y.M.Li synthesized acidified g-C3N4with smaller size and higher solubility[90].Compared with pure polyamide (PA) membranes, acidified g-C3N4-PA MMMs exhibited a 79.3%increase in permeate flux while maintaining a quite high NaCl rejection ratio as well as an improved antifouling performance.

Fig.17.a)A cross-sectional view of g-C3N4 membranes as a stack of nanosheets supported on porous AAO substrates and b)separation performance of g-C3N4 membranes as a function of the diameter of the largest cross-section of rejected molecules.(Reproduced with permission[88].Copyright 2017,Wiley-VCH.)

Z.Y.Jiang prepared g-C3N4-PIM MMMs by simple physical blending[91].It was found that incorporation 2D g-C3N4sheets in PIM-1 matrices influenced the packing of PIM-1 polymer chains and created additional diffusion pathways at the interface,resulting in an increment of permeability coefficients;moreover,periodic in-plane triangular nanopores of g-C3N4sheets facilitated preferential permeation of H2so that H2/CH4and H2/N2selectivities were improved without any compromise in gas permeability.In the same manner,F.S.Pan further prepared g-C3N4-sodium alginate (SA) MMMs for pervaporation dehydration of water/ethanol mixtures[92].Additional interfacial diffusion paths and regularly arranged in-plane nanopores jointly contributed to excellent water permeability and water/ethanol selectivity for g-C3N4-SA MMMs. In addition, prepared MMMs exhibited excellent long-term stability,owing to the strong interfacial forces.Y.J.Zhao further incorporated g-C3N4nanosheets into a poly(vinyl alcohol) (PVA) matrix,showing improved water flux and separation factor for H2O/ethanol mixtures as well as enhanced swelling resistance,mechanical stability and heat-resistance properties[93].

T.W.Xu further modified porous polyethersulfone(PES)substrates with r-GO/g-C3N4for preparation of high performance thin film composite (TFC) polyamide forward osmosis (FO) membranes [94]. Results indicated that incorporation of r-GO/g-C3N4resulted in ～20%increase of water flux,which could be attributed to the optimization of pore structure,enhanced wettability and the lower structure parameter.

X.Quan prepared g-C3N4/r-GO/CA composite photocatalytic membranes by assembling g-C3N4/r-GO photocatalysts on CA supports via vacuum filtration and N2pressing[95].Owing to the high visible-light photocatalytic activity of g-C3N4and photogenerated charge separation resulting from hetero-structure between g-C3N4and r-GO,prepared composite membranes exhibited superior performance in water treatment (like Rhodamine B, CODMn, TOC, UV254and bacteria removal)under visible light irradiation than membrane filtration alone.

2.8.Other emerging 2D separation membranes

Similar to graphite,h-BN also possesses a layered structure in which B and N atoms are bound by strong covalent bonds,whereas single h-BN nanosheets are held together by weak van der Waals interactions.Since h-BN is extremely stable under harsh conditions (such as acidic and basic solutions), h-BN-based separation membranes have become very attractive for organic solvent and water separation applications.Nevertheless,it has proven quite challenging to fabricate high quality h-BN separation membranes due to the poor water dispersibility and intrinsic hydrophobicity of h-BN nanosheets.To alleviate this problem,W.W.Lei developed amino-functionalized boron nitride(FBN)membranes by vacuum filtration of water-dispersible FBN flakes on porous Nylon substrates[96].Prepared membranes showed not only excellent resistance to water and organic solution,but also considerable acidic,alkaline,and oxidative media stability.Moreover,prepared FBN-based membrane exhibited considerable separation performances in both aqueous and organic solvents due to the small size of FBN flakes and stable nanochannels between neighboring FBN flakes,showing promising perspectives in industrial separation and purification applications.

MT, which have the general formula (Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2·nH2O, are representative of cationic clays composed of two tetrahedral sheets of silica sandwiching a central octahedral sheet of alumina.Compositional flexibility in both the negatively charged layers and charge-balancing cations gives rise to a functional diversity.The uniform and highly adjustable interlayer spacing of MT makes them ideal candidates for membrane-based separations.In a recent study,Z.Wang and M.D.Guiver prepared MT/PSf MMMs containing aligned MT interlayer galleries as high-speed CO2transport channels[97].MT layers were bonded and aligned onto porous PSf substrates with the chain stretching orientation of polyvinylamine acid.Resulting MT/PSf membranes exhibited high CO2permeance and mixed-gas selectivity,making them potentially useful for CO2capture from flue gas or CO2removal from natural gas and syngas.

3.Conclusion and Perspective

To summarize,in the past decades a major breakthrough has been made in the field of 2D separation membranes,relying on the theoretic atomic-level thickness,diversified permeation paths and mechanical flexibility.Besides graphene and its derivatives(like GOs),other emerging 2D layered materials, like 2D zeolites, 2D MOFs, 2D COFs, LDHs,TMDCs, MXenes, g-C3N4, h-BN and MT, have shown unprecedented performances in microfiltration,nanofiltration,ultrafiltration,reverse osmosis,pervaporation and gas separation.Considering their structural and functional diversity,it is believed that 2D layered materials as building blocks will continue to contribute significantly to the performance enhancement of molecular sieve membranes and their widespread applications in industry.

Nevertheless,several critical issues are yet to be solved in further commercialization of 2D membranes.

1) Facile methods for bottom-up synthesis of 2D nanosheet building blocks.At present 2D nanosheets were obtained mainly through controlled exfoliation of their bulk layered counterparts.Nevertheless,it's still a big challenge to obtain highly monodispersed nanosheet building blocks with few in-plane defects and considerable productivity.In contrast,although still in its infancy,the bottom-up approach was considered very attractive and promising for bulk production of high quality 2D nanosheets as has been confirmed by recent studies[98-100].

2) Uniform deposition or distribution of nanosheet building blocks.As mentioned above,microstructures of 2D membranes exerted great influence on their separation performances.For instance,uniform spatial arrangement of 2D nanosheet building blocks was considered advantageous for elimination of non-selective diffusion paths of guest molecules in the case of both 2D-material-based pure membranes and MMMs.It is therefore anticipated that an innovative process allowing regular deposition or distribution of monodispersed 2D nanosheets with controlled microstructures could be developed.

3) Creation of uniform in-plane nanopores on non-porous nanosheets.First-principle MD simulations have demonstrated the great potential of graphene,TMDC and MXene nanosheets in membrane-based separation(like gas separation and desalination)in case uniform in-plane nanopores with controlled shape,size,functionality and density were created.At present,diverse techniques like ion beam irradiation[101],oxygen plasma etching [102] and oxidative etching [103] have demonstrated their competence in drilling nanopores in non-porous nanosheets;nevertheless,these techniques are still far from mature.Therefore,reliable methods allowing precise microstructural manipulation of drilled in-plane nanopores in a large scale were urgently required.