Metal-organic frameworks based mixed matrix membranes for pervaporation

Metal-organic frameworks based mixed matrix membranes for pervaporation

Accepted Manuscript Metal-organic frameworks based mixed matrix membranes for pervaporation Zhiqian Jia, Guorong Wu PII: S1387-1811(16)30335-3 DOI: ...

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Accepted Manuscript Metal-organic frameworks based mixed matrix membranes for pervaporation Zhiqian Jia, Guorong Wu PII:

S1387-1811(16)30335-3

DOI:

10.1016/j.micromeso.2016.08.008

Reference:

MICMAT 7847

To appear in:

Microporous and Mesoporous Materials

Received Date: 29 May 2016 Revised Date:

2 August 2016

Accepted Date: 10 August 2016

Please cite this article as: Z. Jia, G. Wu, Metal-organic frameworks based mixed matrix membranes for pervaporation, Microporous and Mesoporous Materials (2016), doi: 10.1016/j.micromeso.2016.08.008. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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MOFs

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ACCEPTED MANUSCRIPT Metal-organic frameworks based mixed matrix membranes for pervaporation

Zhiqian Jia, * Guorong Wu

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100875, China.

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Lab for Membrane Technology, College of Chemistry, Beijing Normal University, Beijing

Abstract

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Metal-organic frameworks (MOFs)/polymer mixed matrix membranes (MMMs) have great potential in pervaporation separation due to the ease of design and modification of MOFs, along with the compatibility between MOFs and polymer matrix. This article reviews the current status of MOFs MMMs for pervaporation, including

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polymer (hydrophobicity/hydrophilicity,

structure stability), MOFs

(stability,

hydrophobicity/hydrophilicity, surface functional structure, particles morphology and

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pores size), mass transfer, and applications (dehydration of organic solvents, removal of dilute organic compounds from aqueous streams, separation of organic-organic

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mixtures, and membrane reactor). The perspectives and suggestions of MOFs MMMs are given.

Keywords: Metal-organic frameworks; mixed matrix membranes; pervaporation; review

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ACCEPTED MANUSCRIPT 1. Introduction Pervaporation (PV) is considered as one of the most promising technologies for liquid separations in bio-refinery, petrochemical and pharmaceutical industries.

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When a liquid mixture is in contact with one side of the membrane, components are absorbed by the membrane. The absorbents diffuse through the membrane and evaporate as permeates on the other side of the membrane, induced by a vacuum or

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gas purge. The separation of components is achieved by the difference in sorption and

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diffusion. Pervaporation usually deals with the minor components of the liquid mixture, and demonstrates incomparable advantages in the separation of heatsensitive, close-boiling, and azeotropic mixtures due to its mild operating conditions, energy efficiency, ease of operation, eco-friendliness, and high separation efficiency

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[1]. The applications of PV can be divided into four main categories: dehydration of organic solvents, removal of dilute organic compounds from aqueous streams, separation of organic-organic mixtures, and reversible reactions.

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Polymer membranes have been widely employed in the PV process, but the

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swelling of polymer materials usually leads to the reduction of selectivity due to a plasticization effect. Therefore, mixed matrix membranes (MMMs), composed of a polymer and inorganic fillers, were proposed to enhance the PV performance by improving the selective sorption, diffusion and stability via incorporation of appropriate fillers. The fabrication processes of MMMs are based on well-established polymer membrane technologies. However, the weak interfacial bonding, incompatibility and mismatching of the thermal expansion coefficient between the 2

ACCEPTED MANUSCRIPT inorganic fillers and polymeric phases often results in grain boundary defects and then a decrease in the selectivity of MMMs. A trade-off between permeability and selectivity often occurs [2].

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Metal-organic frameworks (MOFs) are crystalline nanoporous materials composed of metal centers and organic linkers. MOFs have received remarkable attention due to their unprecedented properties, such as chemical versatility combined

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with designable framework topologies, high surface area, thermal stability, and

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permanent porosity. MOFs have a number of potential applications in gas storage [3], chemical separations [4], catalysis [5], adsorption [6], chromatography [7], drug delivery [8], and molecular sensing [9], etc. In comparison with inorganic fillers, MOFs have better compatibility with polymer matrix due to the presence of organic

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ligands. [Cu2(bza)4(pyz)n] was the first MOF applied as fillers in PV membranes [10]. Compared with the bare PDMS membrane, a loading of 3 wt% [Cu2(bza)4(pyz)n] showed a separation factor increase of about two times in the PV of 5 wt% of ethanol

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aqueous mixtures at room temperature.

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The MOFs-based MMMs can be divided into two categories: chemically bonded membranes and physically doped membranes. For chemical bonded membranes, MOFs bond with the polymeric matrix. For physically doped membranes, only non-covalent interactions (dispersion, polarity, hydrogen-bonds) exist between MOFs and polymeric matrix. Although there were several excellent review articles focusing on PV, such as polymeric membrane [1], and dehydration [11,12], there is no review about the emerging MOFs-based MMMs for pervaporation. This article 3

ACCEPTED MANUSCRIPT reviews the progress in the MOFs MMMs for PV, including the screening strategies for polymer and MOFs, mass transfer, and applications. It is hoped that this article will provide a comprehensive overview of the current status of MOFs MMMs in PV,

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and open new perspectives toward the development of MOFs MMMs with much improved performances. 2. Polymer

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Polymer matrix is the main part of MMMs. The polymer properties

of MMMs. 2.1 Hydrophobicity/hydrophilicity

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(hydrophobicity/hydrophilicity, structure stability) are crucial for the PV performance

In the case of solvent dehydration, the polymers usually need to have high

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sorption centers (e.g., charged sites) for water, as well as structural rigidity and regularity for selective diffusion [13]. Hydrophilic membranes such as poly(vinyl alcohol) (PVA), chitosan, and polyacrylonitrile (PAN) are often used because they

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preferentially allow water to permeate. Hydrophilic membranes interact with water by

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dipole-dipole interactions, hydrogen bonding and/or ion-dipole interactions (in the case of a polyelectrolyte). PVA is an effective membranes material in dehydration due to its outstanding membrane forming ability, ease of processing, and abundant availability [14]. Chitosan, which is easily modified and chemical stable, is another hydrophilic material. Sodium alginate is a polysaccharide and shows excellent affinity for water. Polyimides are a group of polymers with high resistance to heat, chemicals, and wear, and have excellent mechanical properties. Polyelectrolyte is another 4

ACCEPTED MANUSCRIPT important candidate for PV membranes, and the polymeric anions (such as Nafion) and cations can be self-assembled layer-by-layer in fabrication of membranes [15]. Nevertheless, highly hydrophilic polymeric materials may have stability issues when

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the feed contains high water concentrations. Cross-linking or blending can improve the mechanical strength and stability in aqueous solutions of hydrophilic membranes. For the removal of dilute organic compounds from aqueous streams,

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hydrophobic membranes are typically used. Polydimethysiloxane (PDMS), which is

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biocompatible, flexible, hydrophobic, and stable, has been widely used for organophilic pervaporation. Polyether-block-amide (PEBA), comprising flexible polyether and rigid polyamide segments, is simple to prepare because there is no need of crosslinking; it has been used to remove alcohols from water [16, 17], and the

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permeation flux of butanol is even higher than that of PDMS membranes. Polybenzimidazole (PBI), possessing remarkable resistance to high temperatures and superior compression strength, is another major hydrophobic polymer [18].

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2.2 Structure stability

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Swelling of membranes can enhance the flux while reducing the separation factor. By crosslinking, blending, interpenetrating polymer networks (IPN’s) [19], or manipulating the ratio of hydrophilic to hydrophobic moieties in a polymer, the polymer swelling can be inhibited. Crosslinking makes the polymer chains in the amorphous regions more compact and suppresses the plasticization effects, resulting in less space for species to permeate through, raising of transportation resistance and changing of hydrophilicity. As an example, PVA molecular weight, hydrolysis degree, 5

ACCEPTED MANUSCRIPT crosslinkers, and crosslinking degree have important effects on membrane performance [20]. The PVA membranes crosslinked by glutaraldehyde are less hydrophilic due to the consumption of OH groups, and the membranes are more rigid, to

greater

obstruction

to

diffusion.

In

contrast,

when

3-

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leading

aminopropyltriethoxysilane (less than 5%) was used for crosslinking PVA, the hydrophilicity of the membranes increased, and the permeation rose remarkably while

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the selectivity increased at the same time, thus breaking the trade-off effect [21]. It

with a lowering of crystallinity.

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was claimed that there were strong hydrogen bonds and covalent bonds formed, along

The glass transition temperature (Tg) of a polymer is another important parameter because the polymer properties are quite different both above and below

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this temperature [12]. Modifying the Tg of a material by blending is often used to improve the mechanical characteristics of a polymer so they are more suitable for a particular process.

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3. MOFs

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The structure and properties of MOFs determine the PV performance of MMMs

to

a

great

degree.

In

the

screening

of

MOFs,

the

stability,

hydrophobicity/hydrophilicity, surface functional structure, particle morphology and pores size should be considered. Fig.1 gives some typical structure of MOFs.

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MIL-101(Cr)

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ZIF-8

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HKUST-1

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MIL-53

UiO-66

MIL-100(Fe)

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Fig.1 Structure of some typical MOFs used in pervaporation MMMs.

3.1 Stability

The stability of MOFs in water is important for PV of solutions containing

water. The thermodynamic stability of MOFs in water is governed by the free energy of hydrolysis reaction, and is affected by the following aspects [22]. (1) Metal species. The properties of the metal species (metal oxidation state, ionic radius, polarizability, etc.) play an important role in determining the metal−ligand bond strength. For 7

ACCEPTED MANUSCRIPT example, MOFs containing the group IV metals Ti, Zr, and Hf in the +4 oxidation state correlate with high chemical stability [23]. An inert metal cluster renders it unfavorable for an irreversible hydrolysis reaction. (2) Ligands. Using ligands with

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high pKa values is helpful for higher water tolerances. (3) Polarizability of metal and ligands. Similarity between the metal and ligand’s polarizability (as a hard or soft acid and base) results in a strongly binding coordination complex [24].

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Kinetic stability of MOFs relies on the activation energy barrier, which

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depends on the product and reactant states as well as the specific reaction pathway and transition states involved. The kinetic factors such as hydrophobicity and ligand sterics can increase the activation energy for hydrolysis. (1) Hydrophobicity. Hydrophobicity can prevent water from adsorbing into the pores or clustering around

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the metal center [25, 26]. Incorporating hydrophobic fluorinated and alkyl functional groups on the ligands can improve the stability under humid conditions [27]. (2) Steric hindrance. High metal coordination numbers create a crowding effect that

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prevents the formation of water clusters near the metal center [23]. The

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interpenetration of individual frameworks can be used to obtain more water-stable MOFs [28]. The distinction between thermodynamically stability and kinetic stability is that the former can withstand exposure to liquid water conditions whereas the latter are stable with exposure to water vapor only [22]. Variations of the MIL, ZIF, and UiO materials have been studied extensively in PV due to their high chemical stability properties. MIL-100 is formed from octahedral of M3+ trimers (M = Fe, Al, Cr) connected by oxygen atoms from the 1, 3, 8

ACCEPTED MANUSCRIPT 5-benzenetricarboxylate (BTC) ligand under harsh conditions (pH=0.6~1, high temperature) [29]. The highly charged trivalent metals lead to a strong metal−ligand bond, resulting in excellent stability. MIL-100(Cr) was absolutely stable when

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soaking in water for 12 months. In the MIL-101(Cr) structure [30], the supertetrahedral unit is made up of three chromium trimers that are connected through terephthalate linkers (BDC). MIL-101(Cr) is highly stable and even maintains its BET

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surface area and PXRD pattern after immersion in boiling water for 1 week [31].

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Zeolitic imidazolate frameworks (ZIFs) are a class of MOFs that share similar pore topologies with zeolites along with excellent chemical and thermal stability. ZIF-8 possesses ZnN4 tetrahedra connected by 2-methylimidazolate linkers [32], and the structure is stable in water for 3 months [33]. ZIF-7 and ZIF-90 (imidazole-2-

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carboxaldehyde) show good hydrothermal stability in the presence of steam due to their hydrophobic nature. MIL-125-NH2 is synthesized from titanium(IV) isopropoxide, and displays high stability and reproducible adsorption and desorption

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isotherms after many water isobar cycles [34]. UiO-66, synthesized by ZrCl4 and

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BDC, possesses 12-coordinated zirconium-oxo clusters and high stability in water [35]. Some of the thermodynamically stable MOFs in the presence of water are given in Table 1.

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ACCEPTED MANUSCRIPT Table 1 Some of the thermodynamically stable MOFs

Common name

Metal

Ligands

Formula unit

center Fe, Al, Cr

BTC

Cr3OX(BTC)2, X = OH, F

MIL-101

Cr

BDC

Cr3F(H2O) 2O(BDC)3

MIL-101-SO3H [36]

Cr

BDC-SO3H

Cr3F(H2O) 2O(BDC-SO3H)3

ZIF-8

Zn

2-methylimidazolate

Zn(MEIM)2

Bio-MOF-14 [37]

Co

adenine

Co2(AD)2(C4H9CO2)2

MIL-96(Al) [38]

Al

H3BTC

Al12O(OH)18(H2O)3(Al2(OH)

Window size

(Å)

(Å)

25, 29

5.5, 8.6

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MIL-100

Cage diameter

29, 34

12, 16

-

-

11.6

3.4

15.85, 22.35

3.6

14, 14, 31

2.5-3.3

4)(BTC)6

Al

H2TCPP

TCPP(AlOH)2

31.9, 6.6, 16.9,

6, 11

JUC-110 [40]

Cd

H2THIPC

Cd(HTHIPC)2

8.19, 8.30

4.5

MONT1 [41]

Cu

Pcp, 4,40-bipy

Cu2(PCP)2BIPY

MIL-125-NH2

Ti

NH2-BDC

Ti8O8(OH)4(BDC-NH2)6

UiO-66

Zr

BDC

Zr6O6(BDC)12

42.3, 42.3, 9.6

5-7

20.7

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3.2 Hydrophobicity/hydrophilicity

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The hydrophobicity/hydrophilicity of MOFs is mainly determined by the

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ligands. Hydrophobic MOFs exhibit substantial adsorption selectivity for organic solvents over water [42]. For example, the hydrophobicity of ZIF-8 results in no adsorption of water before the onset of capillary condensation. ZIF-71, possessing a RHO topology with lager cages (1.68 nm) interconnected through pore windows of 0.48 nm, is even more hydrophobic than ZIF-8. FMOF-1, prepared by Ag+ and 3, 5bis(trifluoromethyl)-1,2,4-triazolate, is highly hydrophobic and does not adsorb water while adsorbs appreciable amounts of alkanes and aromatics [43]. MAF-2, made from

10

10

12.5, 6.1

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Al-PMOF [39]

ACCEPTED MANUSCRIPT Cu2+ and 3, 5-diethyl-1, 2, 4-triazole, is hydrophobic and does not adsorb water even at 100% RH, but it readily adsorbs alcohols (loadings of 0.2 g g-1 for ethanol below 0.2 P/P0) [44]. Mg-MOF-74 (Mg2+/2, 5-dihydroxybenzene carboxylic acid) [45] and are relatively hydrophilic.

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ZIF-93 (Zn2+/4-methyl-5imidazolecarboxaldehyde) [46]

The hydrophobicity of pores can be determined by experimental and computational studies based on the heat of water adsorption [47, 48]. The water adsorption loadings

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at low pressure can also be used to determine relative hydrophobicity of adsorbents

3.3 Surface functional structure

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[49].

The surface functional structure of MOFs determines the interactions between MOFs and the components as well as the polymer. The former is responsible for the

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sorption. For instances, the uptake of methanol on ZIF-90 is higher than that on ZIF-8 due to H-bonding of methanol with the carbonyl group of ZIF-90 [50]. The hydrogen

adsorption.

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bonding is not strong enough to cause significant distortion of the framework during

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The MOFs-polymer interaction determines their compatibility and interfacial defects or gaps. The components preferentially diffuse through the gaps because of lower diffusion resistance, leading to the enhancement of permeation fluxes. Hydrophobic MOFs in hydrophilic membranes may lead to high permeability and low selectivity due to the existence of interfacial defects. For example, the ZIF-8/PVA interaction is limited, and the ZIF-8/PVA membranes with 5 wt% loadings display a five-fold increase in permeation flux along with decreased separation factor [51]. At 11

ACCEPTED MANUSCRIPT higher loadings of ZIF-8, the separation factor decreases even more significantly due to the agglomeration of nanoparticles in the PVA matrix. The use of MOFs that contain organic linkers similar to elements of polymer units or MOFs that interact

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strongly with polymer may eliminate interface defects and improve selectivity. For examples, ZIFs have better affinity and interaction with polybenzimidazole (PBI) [52]. ZIF-71/PEBA has strong interactions, and ZIF-71/PEBA MMMs with 20 wt%

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loading shows a simultaneous enhancement in both separation factor (18.8) and flux

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(520.2 g m-2 h-1) at 37 ℃in the biobutanol recovery from acetone–butanol–ethanol (ABE) fermentation broth [53]. Assumption that the linker–polymer interactions dominate the MOF–polymer interactions, Hansen solubility parameters (HSP) of polymers and MOFs would be helpful for selecting the best MOF for desired polymer

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material [54]. A limitation of HSP application is related to insufficient availability of HSP data especially for MOFs [55].

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To achieve MOF functionalization, two strategies have been employed [56]: (1) Use of a functionalized ligand as the organic building block. However, the

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functional groups may interfere with the MOFs formation, or not be compatible with the MOF synthetic conditions. (2) Postsynthetic modification (PSM) of MOFs. A given MOF structure can be modified with different reagents, thereby generating topologically identical, but functionally diverse MOFs. Multiple functional units can also be introduced into a single framework. The purification and isolation of modified products are facile. For instance, MIL-53(Al)-NH2 nanocrystals were modified with formic acid, valeric anhydride and heptanoic anhydride, and then incorporated into 12

ACCEPTED MANUSCRIPT PVA matrix for use in the pervaporation of 92.5 wt% ethanol [57]. It was found that, with the increased hydrophobic constants of the surface substitutes (-NHCOH, NHCOC4H9 and -NHCOC6H11), the ethanol permeance rises while the selectivity drop

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through the surface modification of MIL-53(Al)-NH2.

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(Fig.2). The PV performances were successfully tuned from trade-off to anti-trade off

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Fig.2 Effects of surface groups of MIL-53(Al) on permeance and selectivity of MMMs in dehydration of 92.5 w% ethanol at 40 °C. A. pristine PVA B. MIL-53 C. MIL-53-NH2 D. MIL-

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53-NHCOH E. MIL-53-NHCOC4H9 F. MIL-53-NHCOC6H13. The MOFs loading in the MMMs is 4 wt% (Reproduced from ref. 57with permissions from Elsevier).

3.4 Dispersion of MOFs in polymer

The dispersion of MOFs in polymer affects the interfacial defects and membrane performance. In the preparation of MOFs, MOFs particles may agglomerate in the conventional drying procedure due to strong capillary force among 13

ACCEPTED MANUSCRIPT particles and high surface energy of particles. To solve the problem, several methods were reported. (1) Use wet MOF. After synthesis, MOFs are separated by centrifugation from the suspension, washed, re-dispersed in solvent and then mixed

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with polymer solution. (2) Surface modification and pre-priming of MOFs to improve the MOFs/polymer compatibility. Wet MOFs were primed with a small amount of polymer solution to introduce a thin coating, and then the remaining polymer is added

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and stirred [58]. By this method, the particles can be distributed homogeneously in

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polymer without agglomeration. (3) Enhancing repulsion forces of MOFs particles. After synthesis, the washing of MOFs particles may have important effects on the dispersion stability. It was found that positive surface potentials of newly prepared ZIF-8 particles decreased with washing times, resulting in increased aggregate size

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[59]. The ZIF-8 particles without washing with methanol can be dispersed well in water, and there was no settlement even upon centrifugation at 8000 rpm for 6 minutes. (4) Spray self-assembly. To obtain a maximum loading of MOFs while

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maintaining uniform dispersion in PDMS, simultaneous spray self-assembly of a ZIF-

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8–PDMS suspension and a cross-linker/catalyst solution was employed, and a 40% (w/w) ZIF-8 loading was obtained [60] (Fig.3). The MMMs showed a high total flux (4846.2 g m-2 h-1) and a separation factor of 81.6 in the recovery of n-butanol from 1.0 wt% aqueous solution at 80°C. (5) Increasing particles size. It was reported that the micron particle size avoids agglomeration of particles [61].

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Fig.3 Fabrication of ZIF-8/PDMS membranes by spray self-assembly technique (Reproduced from ref.60 with permissions from Wiley). 3.5 Particles morphology and pores sizes

MOFs particles morphology plays an important role in MMMs. Reducing the

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size of MOFs particles is highly desirable to enhance the external specific surface and the accessibility to the interior surface as well as the reduction of membrane thickness. Submicrometer-sized ZIF-71 filled MMMs showed significant improvement in

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ethanol recovery than the micrometer-sized ones [62]. Use of selective 2D porous

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fillers might result in an enhanced selectivity due to a much longer diffusion path for the non-permeating species [63]. Core-shell particle with hydrophilic core and MOF shell can be a solution to enhance water flux and compatibility with polymer. The MOFs pores size is an important factor for PV processes. When the pore

size is located between the molecular kinetic diameters of two components, the smaller molecule can diffuse into the pores, whereas the larger molecule is excluded, displaying a molecular sieving effect. If the pore size is slightly larger than the kinetic 15

ACCEPTED MANUSCRIPT diameter of the larger molecule, a kinetic separation based on the difference of diffusion rates is achieved. When the pore size is obviously larger than both of the molecules, the two molecules may be separated mainly by the differences in their

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equilibrium adsorption. ZIF-71 has a window size of 0.48 nm, which is larger than the kinetic diameter of water (0.27 nm) and ethanol (0.45 nm). Thus these molecules can diffuse through the ZIF-71, and the selectivity is thermodynamically dependant [64].

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However, unlike traditional inorganic zeolites with rigid frameworks, MOFs

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are generally structurally flexible, which explains the absence of a clear cut-off and the diffusivities of molecules with kinetic diameters larger than the pore size [65, 66]. For example, methanol and ethanol, with respective kinetic diameters of 3.6 Å and 4.5 Å [67, 68], can diffuse through ZIF-8 with large cavities and small windows (3.4 Å),

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exhibiting diffusion selectivity for methanol over ethanol [50]. The flexible MOFs can also exhibit high selectivity for guest inclusion by adapting their framework structure accordingly. ZIF-8 shows an exceptionally high adsorption capacity for isobutanol

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(kinetic diameter of 0.50 nm). Molecular simulations showed that each sodalite cage

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of ZIF-8 can accommodate six isobutanol molecules at 3.5 kPa. In contrast, the ZIF-7 nanoparticles show insignificant adsorption of isobutanol, possibly because of the smaller aperture size (0.30 nm) and the much more rigid framework [69]. 4. Mass transportation process The mass transportation process in pervaporation can be described by the solution-diffusion model. The solubility and diffusivity of the components in a membrane determine the membrane performance. 16

ACCEPTED MANUSCRIPT In general, HSP are used to predict the solubility [70, 71]. HSP consists of three components: the contribution of hydrogen bonding interaction (δh), the contribution of polar interaction (δp), and the contribution of dispersion interaction

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(δd).The big difference between water and organics in hydrogen-bonding interaction suggests that dehydration of organic solvents and removal of organics (especially hydrocarbons and chlorinated hydrocarbons) from water are feasible for

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pervaporation. The dispersion interactions of organics (e.g. isomers) are comparable

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or similar, suggesting that separation of organics is out of the reach of the thermodynamic discriminating capability of membranes. In this situation, kinetic discriminating based on shape and size of the organics should be taken, and diffusionselective membranes can be used [72].

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The free-volume theory is employed to predict the diffusion coefficient of components in polymer because free-volume parameters for many kinds of polymer and solvents have already been determined [73]. Suppression of the fractional free

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volume of membranes can effectively improve diffusion selectivity. Glassy polymers

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are generally more shape and size selective, and display diffusion selectivity. For diffusion in MOFs particles [74], if the system is statistically uniform, the simple Fickian model can be directly applied. Inhomogeneities such as surface or internal barriers require some adjustments to the model. For hierarchical pore systems, when there is rapid exchange between the different regions, the diffusivity corresponds to the mean of the diffusivities in the different regions. In contrast, when the condition of rapid exchange is not fulfilled, a dual resistance Fickian model can be used for simple 17

ACCEPTED MANUSCRIPT hierarchical pore structures such as the micropore/macropore system, while Monte Carlo or MD simulations offer the only realistic approach for more complex hierarchical pore structures. The techniques of interference microscopy (IFM) and IR

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microscopy (IRM) have been employed for obtaining intracrystalline diffusivities under both equilibrium and non-equilibrium conditions [75, 76].

Pervaporation data are usually reported as fluxes (Ji) and separation factor (βij).

Cip / C jp

(1)

Cif / C jf

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ij 

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βij is expressed as,

where Cip and Cjp are the molar concentrations of components i and j on the membrane surface of permeate side, Cif and Cjf are that of feed side. The fluxes and

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separation factors are not only functions of membranes’ intrinsic properties, but also of operating conditions (feed concentration, permeate pressure, temperature). Therefore, comparison of fluxes and separation factors obtained under different

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operating conditions is difficult. Membrane permeability (Pi), permeances (Pi/l) and

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selectivity (αij) are related to the intrinsic properties of membranes, and are thus more useful parameters in the PV process [77].

Pi 

Jil xi i pi*  y i p

(2)

wherein Pi is the permeability of component i, which is a product of solubility and diffusivity; l is the membrane thickness; Ji, γi, and pi* are the flux, activity coefficient, and saturated vapor pressure of component i, respectively; xi and yi are the respective mole fractions of component i in the feed and permeate; p is the total pressure at 18

ACCEPTED MANUSCRIPT permeate side. γi and pi* were determined by the Wilson equation and the Antoine equation, respectively. The selectivity αij, defined as the ratio of permeabilities or permeances of components i and j, Pi Pj

(3)

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i 

membrane performance [78], PD  2 PC  2 D ( PC  PD ) PD  2 PC   D ( PC  PD )

(4)

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Peff  PC

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For MMMs, the Maxwell model has been widely used for the prediction of

where Peff is the effective permeability, PC and PD represent the permeability of continuous phase and dispersed phase respectively, and ΦD is the volume fraction of dispersed phase. The Maxwell model describes the permeation of a pure gas through a

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membrane, and is applicable for low filler loadings since it assumes that the streamlines associated with diffusive mass transport around filler are not affected by the presence of nearby filler [79]. It should be noted that the classic solution-diffusion

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theory is valid for permeation through essentially non-swollen membranes. When

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appreciable membrane swelling occurs, both the solubility and diffusion coefficient become concentration-dependent, and the classic solution-diffusion theory should be modified.

The concentration polarization also occurs in PV, especially in the removal of

dilute organic compounds using rubbery membranes with high permeation flux and selectivity [80]. In this case, the component in the feed is depleted much faster than provided, and a concentration-polarized layer is induced in the vicinity of the 19

ACCEPTED MANUSCRIPT membrane surface, leading to decreased membrane selectivity and permeation flux. The use of turbulence-promoting spacers, Dean vortices (Fig.4), two-phase feeding [81], and vibrating modules [82] can cope with the challenge. An increase in the

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permeation flux by a factor of 2–3 was reported for the Dean flow technique [83]. To reduce mass transfer resistance, asymmetric membranes and composite membranes are preferred. In composite membranes, the active skin layer is supported

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on a porous substrate and the resistance to mass transport is usually small [8,9].

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Notably, the swelling of the active skin layer and substrate should be coordinated to prevent interfacial stress and disintegration of the composite structure. To enhance the compatibility between the skin layer and substrate, the substrate modification (e.g. by

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plasma grafting [84] ) is sometimes needed.

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Fig.4 Schematic representation of the dean flow in the bore of the spiral-wound hollow fiber

(Reproduced from ref.83 with permissions from Wiley)..

5. Application

The MOFs MMMs have found viable applications in pervaporation in the following areas: (1) dehydration of solvents; (2) removal of dilute organic compounds from aqueous streams; (3) organic–organic mixtures separation; (4) membrane reactor. Table 2 gives some typical applications of MOFs MMMs in PV processes. 20

ACCEPTED MANUSCRIPT 5.1 Dehydration of solvents Dehydration of organic solvents, e.g. alcohols, ethers, esters, acids, etc. is usually performed using hydrophilic membranes pervaporation. The separation results

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from the synergic effect: water is both preferentially dissolved and transported in the hydrophilic MMMs due to its much smaller molecular size. Generally, with increasing temperature and water content in the feed, the fluxes increase while the

SC

separation factor reduces due to increased plasticization; with increased MOFs

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loading, the fluxes increase due to the enhanced fractional free volume. ZIF-8 nanoparticles with sizes less than 50 nm were added in PBI for the dehydration of ethanol, isopropanol (IPA) and butanol [85]. The water permeability of ZIF-8/PBI (1:1) MMMs was about one order of magnitude higher than that of a

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pristine membrane (14 000–22 000 vs. 1200–2300 Barrer) without much decrease in selectivity. ZIF-90 nanoparticles with an average particle size of 55 nm were embedded in P84 for the dehydration of IPA [58]. The flux of MMMs increases as

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ZIF-90 loading increases, while the separation factor maintains at 5432 when the ZIF-

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90 loading was less than 20 wt%, but reduces to 385 when the ZIF-90 loading was 30 wt%. The application of sulfonated polyethersulfone (SPES) as a primer to ZIF-90 nanoparticles before fabricating 30 wt% ZIF-90 MMM resulted in an increase in the separation factor from 385 to 5668 due to enhanced affinity between ZIF-90 particles and P84, as well as preferential sorption of water over IPA. ZIF-7/CS membranes were used for dehydration of ethanol at 25 °C [86]. The separation efficiency of MMMs with 5 wt% ZIF-7 incorporation was 19 times higher than that of pristine 21

ACCEPTED MANUSCRIPT membranes because of the rigidified polymer chain of MMMs. In dehydration of acetic acid, the NH2-MIL-125(Ti)/NaAlg MMMs with 6 wt% loading showed significant improvement in flux and selectivity [87]. Core-shell particles made of

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MCM-41 mesoporous silica spheres coated with ZIF-8 crystals was employed in the dehydration of ethanol using polyimide (PI) Matrimid® 5218 as polymeric matrix [88].However, it was found that the PV flux decreased because the hydrophobic

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character of ZIF-8 shell inhibits water molecules contacting with the silica core. The

solvents (such as THF, dioxane).

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feasible applications of MOFs MMMs in dehydration can be broadened to other

5.2 Removal of dilute organic compounds

The application of MMMs in the removal of dilute organic compounds

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includes the removal of volatile organic compounds from aqueous streams, and the removal of biofuels from a fermentation broth. Furfural is regarded as an important platform molecule in future biorefineries

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as it can be derived from lignocellulose. A ZIF-8/silicone rubber membrane supported

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on a hierarchically ordered stainless-steel mesh was used for pervaporation of furfural from an aqueous feed. Single compound adsorption isotherms of furfural and water on ZIF-8 powders show an obvious preference for furfural, with 45 wt% adsorption at saturation and nearly no adsorption of water. For the ZIF-8/silicone rubber MMMs, an almost constant selectivity factor of 10 and a permeability of 104 Barrer for furfural were achieved at different feed concentrations (0.5–3 wt%) and 80 °C. Upon

22

ACCEPTED MANUSCRIPT increasing the operating temperature (60–120 °C), both the furfural permeability and selectivity decreased due to a diminished furfural adsorption in the membrane [89]. The bio-fuel processing cost is mainly on the recovery of bio-alcohols from

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the fermentation broth, which contains only approximately 1–5 wt% bio-alcohols (including butanol, ethanol, etc.). An organophilic pervaporation unit coupled to fermentation can recover bio-alcohols in situ, as well as reduce the product inhibition

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effect. In the recovery of bio-alcohols, the separation factor of ZIF-71/PDMS

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membrane (PDMS: ZIF-71=10: 2) is nearly double that of a pristine membrane, and the flux is also improved [90]. ZIF-8 possesses hydrophobic channels and displays a reversible gate-opening effect upon variation of isobutanol pressure or temperature. The ZIF-8/PMPS membrane (WZIF-8/WPMPS = 0.10) showed a separation factor of

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34.9–40.1 and flux of 6400 g m-2 h-1 in the recovery of isobutanol (1.0 wt%) from water at 80 °C [91, 92]. The challenge in bioethanol recovery is to develop ethanol selective membranes that allow for increasing the ethanol concentration from 2 –5%

EP

(typically) to 30 –40%. If this is possible, the first distillation column in conventional

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processes can be economically replaced by a pervaporation unit [93].MIL-53/PDMS MMMs with 40% loading were used for ethanol permselective pervaporation. Compared with a pristine membrane, the flux of the MMMs significantly increased from 1667 to 5467 g m-2 h-1, while the separation factor remained 11.1. The increased flux was attributed to the water-repellency surface and ethanol-affinity channels of MIL-53 [94]. 5.3 organic–organic mixtures separation 23

ACCEPTED MANUSCRIPT Organic–organic mixtures separation, such as methyl tert-butyl ether (MTBE)/methanol(MeOH),

dimethyl

carbonate

(DMC)/methanol,

benzene/cyclohexane, benzene/hexane, ethylbenzene/xylene, desulfurization of

Cu3(BTC)2/PVA

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gasoline, etc., is crucial and challenging in chemical industries. (BTC=benzene-1,3,5-tricarboxylate)

membranes

were

fabricated on a ceramic tubular substrate by a pressure-driven assembly method, and

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then used in separating 50 wt% toluene/n-heptane mixtures. Compared with a pristine

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PVA membrane, the separation factor and permeate flux improved from 8.9 and 14 g m-2 h-1 to 17.9 and 133 g m-2 h-1 respectively due to enhanced affinity between toluene and Cu3(BTC)2 particles [95]. [Cu2(bdc)2(bpy)]n was incorporated into sulfonated polyarylethersulfone with cardo (SPES-C) for the separation of MeOH/MTBE

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mixtures. [Cu2(bdc)2(bpy)]n preferentially adsorbs MeOH over MTBE, and both the sorption selectivity and diffusion selectivity increase with the addition of [Cu2(bdc)2(bpy)]n, and the flux attains to 0.288 kg m−2 h−1 at 20 wt% loading [96]. membranes

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MIL-101(Cr)/PDMS

with

6%

loading

supported

on

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polyvinylidene fluoride (PVDF) were used for the desulfurization of model gasoline (thiophene and n-octane mixture), and a flux of 5.2 kg m−2 h−1 (increased by 136% compared with the pristine membrane) and an enrichment factor of 5.6 (increased by 38%) were achieved [97]. The reason is that the packing of PDMS chains was interrupted by MIL-101(Cr) and thus the fractional free volume was increased, resulting in increased permeation flux. In PV removal of thiophene from model gasoline, Cu3(BTC)2/PDMS membranes with 8% loading were applied, and the 24

ACCEPTED MANUSCRIPT permeation flux increased by 100% and the enrichment factor increased by 75% in comparison with PDMS control membranes [98]. In the organic–organic separation, the selectivities below 10 are to be expected, with typical values of 3 –4 being more

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common [93]. Table 2 Typical applications of MOFs MMMs in PV processes.

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5.4 Membrane reactors

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PV combined with a chemical reactor is promising for removing by-product (e.g. water) from the reversible reactions to break the thermodynamic equilibrium limitations and avoid catalyst deactivation. Cu3(BTC)2 (HKUST-1)/polyimide (PI) Matrimids 5218 MMMs have been applied in the esterification of acetic acid with

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ethanol. At 70 °C, the membrane reactor reached a conversion of 70%, higher than the equilibrium value (66%) [99]. MIL-101(Cr)/polyimide Matrimid® MMMs were also applied in the same reaction, and the MMMs provided a higher permeability than the

EP

bare PI membrane. Furthermore, MIL-101(Cr)/PI MMMs exhibited a higher stability

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to the reaction medium than HKUST-1/PI MMMs, maintaining constant molar fractions in permeate, conversion and permeation flux for more than 3 days [100]. In addition to classical esterification reactions, other reactions (e.g. synthesis of n-butyl acetate by trans-esterification) should also be considered in the future research. 6. Conclusions and future perspectives MOFs/polymer mixed matrix membranes have great potential to achieve outstanding pervaporation performances due to the ease of design and modification of 25

ACCEPTED MANUSCRIPT MOFs, along with the compatibility between MOFs and polymer matrix. The following issues should be considered in the future research: (1) Tailoring the topology structure and surface functional groups of MOFs to adjust the

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MOFs/polymer and MOFs/component interactions. Core-shell particle with hydrophilic core to promote water flux and MOF shell can be another solution for enhanced compatibility with polymer. (2) Adjusting polymer micro-structure

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including crystallinity and fractional free volume. (3) Exploiting the synergistic

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effects of MOFs and polymer by choosing the proper MOF-polymer couple. (4) Optimizing membrane structure such as fabrication of supported ultra-thin membranes to reduce mass-transfer resistance. (5) Exploiting organic solvent-resistant membrane materials, and assessing the membranes chemical stability and long-term

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stability. (6) Developing theoretical predictions method of MMMs performance. (7) Investigating the flexibility of MOFs structure (breathing or gate-opening) on MMMs performance. (8) Paying more attention to organic/organic separation due to huge

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markets in the chemical and petrochemical industries. Although there is still a long

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road ahead for the application of MOFs MMMs in industrial pervaporation, the works published to date are an excellent basis for future developments. We believe that we will witness a rapid development of MOFs MMMs with examples of industrial applications in the near future.

Acknowledgement

26

ACCEPTED MANUSCRIPT The authors gratefully acknowledge the support from the China Scholarship Council (No. 201308110020 ). The authors thank Mark Anson (Columbus, Ohio, US) for his help in editing of the paper.

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Corresponding author. Email address: [email protected] (Z. Jia)

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Renew. Energy 88 (2016) 12-19.

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ACCEPTED MANUSCRIPT

Table 2 Typical applications of MOFs MMMs in PV processes.

Matrix

T (°C) 25

Ethanol/water (90/10)

Chitosan (CS)

18

MIL-53, 40

Ethanol/water (95/5)

Polydimethylsilo xane (PDMS)

3

HKUST-1, 40

Ethanol/water (90/10)

Polyimide (PI)

56

ZIF-71, 29

Ethanol/water (5/95)

PDMS

5

ZIF-8, 58.7

Isopropanol/water (85/15)

Polybenzimidazo les (PBI)

Ethanol/water (92.5/7.5)

MIL-53-NHCOC4H9, 7.5 ZIF-8, 5

Ethanol/water (92.5/7.5) Isopropanol/water (90/10)

ZIF-90, 30

Isopropanol/water (85/15)

ZIF-8, 5

n-butanol//water (5/95)

Poly(vinyl alcohol) (PVA) PVA PVA

PG/l ( GPU)*

Selectivity

PG/l (GPU)*

selectivity

Increase in PG/l (%)

Increase in selectivity (%)

1865.6

19.1

1055.2

363.7

-43.4

1804.2

251

8.4

826

12.5

229.1

48.8

42

332

39

589

29.7

77.4

-23.8

50

426

9

727

15.4

70.7

71.1

26

0.6

261

1.4

903.8

133.3

29

3463.5

547

214.7

1786.2

-93.8

30

1199.5

588

167.5

1860.0

-86.0

60

Reference

[79] [86] [80] [85]

[78]

3.8

40

5605

10

17157

30

206

200

[53]

3.8 76

40 30

5605 20099

10 129.8

24641 3191

27 160.6

340 -84.1

170 23.7

[53]

P84

20

60

255

268

248

3945.6

-2.7

1372.2

PDMS

1.8

80

2158

476.4

2751

1048

27.5

120.0

AC C

MIL-53-NHCOH, 4

EP

n-butanol/water (85/15)

50

MMMs

70

TE D

ZIF-7, 5

Ethanol/water (85/15)

Pristine membrane

Membrane thickness (μm)

SC

Feed composition (wt%/wt%)

M AN U

Filler and Loading (wt%)

[49] [56] [92]

ACCEPTED MANUSCRIPT

20

40

3783

Cu3(BTC)2, 0.75

n-heptane/tolene (50/50)

PVA

6

40

1444

MIL-101-Cr, 6

n-octane/thiophene (99.9/0.1)

PDMS

16

30

3353

NH2-MIL-125(Ti), 6

HAc/H2O (90/10)

NaAlg

7.8

30

8397

(Cu2(bdc)2(bpy))n, 30

Methanol/Methyl tertbutyl ether (15/85)

Sulfonated polyarylethersulf one with cardo (SPES-C)

24

15122.1

40

225

4619

RI PT

Polyether-blockamide (PEBA)

560.1

22.1

-96.3

13

3055

22.5

111.6

73.1

0.4

10600

0.62

216.1

55.0

8417

18040.9

0.2

311.0

389

2665

72.9

70.8

SC

n-butanol/water (1/99)

4389.9

M AN U

Zn(BDC)(TED)0.5, 10

1560

[93]

[87]

[90] [81]

[89]

AC C

EP

TE D

* PG/l is the permeance of the minor component. In order to compare the instrinc properties of MMMs, we calculated the PG/l of MMMs using Aspen Plus 7.2 software.

ACCEPTED MANUSCRIPT

►Metal-organic frameworks/based mixed matrix membranes for pervaporation are reviewed. ► Progress in polymer, MOFs, mass transfer, and applications are given.

AC C

EP

TE D

M AN U

SC

RI PT

► Perspectives and suggestions of mixed matrix membranes are provided.