Mixed Matrix Membranes with Fillers of Intrinsic Nanopores for Gas

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Review

110th Anniversary: Mixed Matrix Membranes with Fillers of Intrinsic Nanopores for Gas Separation Yanan Wang, Xiaoyao Wang, Jingyuan Guan, Leixin Yang, Yanxiong Ren, Nayab Nasir, Hong Wu, Zan Chen, and Zhongyi Jiang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b01568 • Publication Date (Web): 25 Apr 2019 Downloaded from http://pubs.acs.org on April 28, 2019

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Industrial & Engineering Chemistry Research

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110th Anniversary: Mixed Matrix Membranes with

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Fillers of Intrinsic Nanopores for Gas Separation

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Yanan Wanga,b, Xiaoyao Wanga,b, Jingyuan Guana,b, Leixin Yanga,b, Yanxiong Rena,b,

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Nayab Nasira,b, Hong Wua,b, Zan Chenb,c and Zhongyi Jianga,b,*

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a Collaborative

Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin

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300072, China

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b

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Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China.

Key Laboratory for Green Chemical Technology of Ministry of Education, School of

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c

Key Laboratory of Membrane and Membrane Process CNOOC Tianjin Chemical

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Research &

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Design Institute, Tianjin 300131, China.

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ACS Paragon Plus Environment

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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* Corresponding author: Zhongyi Jiang, [email protected]

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Abstract

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Mixed matrix membranes (MMMs), which are fabricated by incorporating dispersed

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fillers into continuous polymeric matrix, have been broadly utilized in gas separation due

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to their unique hierarchical structures and enhanced separation performance. Fillers of

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Intrinsic Nanopores (FINs) have drawn considerable attention because they not only

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can provide additional molecular transport channels, but also have potential size-sieving

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property. Although many excellent reviews about MMMs have been published, quite few

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of them are specifically designed for MMMs containing FINs. This review focuses on

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three kinds of FINs, covalent organic frameworks (COFs), zeolites and metal-organic

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frameworks (MOFs), as the representatives of organic, inorganic and inorganic-organic

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FINs. The typical physical and chemical features of these three kinds of FINs are

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introduced. Afterwards, methods to prepare FINs-based MMMs are briefly introduced.

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Particularly, solutions to preventing the aggregation and improving the dispersion of

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FINs within polymeric matrix are proposed. Next, this review will discuss the

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applications of FINs-based MMMs in gas separation including carbon dioxide capture


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and alkanes/olefins separation. Finally, the tentative perspective on the research and

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development of FINs-based MMMs will be presented.

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1. Introduction

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Membranes are attractive due to their high energy efficiency, low production cost,

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simplicity, and environmental friendliness compared with conventional separation

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technologies.1 The core of membrane separation is membrane materials, which can be

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categorized as polymeric/organic, inorganic and mixed matrix types. Polymeric

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membranes are dominant because of their excellent processability, easy availability and

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adequate mechanical properties for many gas separations.2 Despite these attractive


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features, factors such as the trade-off limits between membrane permeability and

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selectivity, poor resistance to plasticization and organic solvents and aging effects (for

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high free volume glassy polymer) during long-term utilization limit their broader

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application.3 Inorganic membranes avoid some drawbacks of organic polymers. They

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generally exhibited higher thermal and chemical stability, aging/plasticization resistance

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and superior gas separation performance. However, their large-scale application is

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severely hindered by complicated manufacturing procedures and high comprehensive

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costs.4 Mixed matrix membranes (MMMs), which are fabricated by incorporating micro-

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or nanoscale fillers mostly with the inorganic attributes into polymeric matrix, provide

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benefits from both organic and inorganic materials.5, 6

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Ideally, the fillers have intrinsic molecular sieving ability, good interfacial compatibility

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with the polymeric matrix as well as good dispersibility at moderate to high loadings.7

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Since the first attempt in 1976 by introducing 5A zeolite into polydimethylsiloxane

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(PDMS), many other fillers have been explored.8 Compared with traditional nonporous

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fillers such as silica, metal oxides, clay and graphene, fillers of Intrinsic Nanopores

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(FINs) such as carbon molecular sieves, carbon nanotubes (CNT), zeolites, metal-


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organic framework (MOFs), metal-organic polyhedral (MOP), porous aromatic

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frameworks (PAF) and covalent organic frameworks (COFs) appear especially attractive

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because their intrinsic pores can afford additional transport channels.9-14 Since the

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kinetic diameters of gases are typically much less than 1 nm, FINs with pore size less

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than 1 nm are most desired, thereby rendering molecular sieving ability to improve

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selectivity.15

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The separation performance of FINs-based MMMs can transcend the so-called

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polymer upper bound,

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and homogeneous dispersion of the fillers still exist and need to be solved, as will be

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discussed here. Some reviews related to the interfacial compatibility exist, but few

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reviews address agglomeration prevention to improve the dispersion of fillers in

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MMMs.5,

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undermining selectivity and mechanical properties of the MMMs.24 First, the

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incorporation of FINs changes the fractional free volume of the polymer chains and the

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diffusion pathway of the gas molecules. This effect will be restrained by aggregated

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FINs due to the decrease of surface contact area that can influence the surrounding

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but challenges in robust interfacial fillers-matrix adhesion

Filler agglomeration is more challenging at high filler loading, thereby


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polymer. Second, non-selective voids between the filler cluster and the polymer are

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easily generated if aggregates are formed, leading to the loss of selectivity. Third, the

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function of intrinsic nanopores will also be limited if the FINs adhere together to form

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aggregates. Additionally, inhomogeneous dispersion of FINs will hamper the elucidation

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of the relationship between structure and property of the MMMs. Therefore, improving

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dispersibility of FINs is a key need.

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This review considers COFs, zeolites and MOFs, as representatives of organic,

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inorganic and inorganic-organic FINs. As shown in Figure 1, methods to fabricate FINs-

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based MMMs are briefly summarized. Improving the dispersion of FINs at nanoscale

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within the polymeric matrix is considered first, then specific applications of FINs-based

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MMMs in gas separation and a brief perspective are presented.


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Figure 1. The framework of this review.


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2. Representative FINs

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As shown in Figure 2, FINs which should be selected rationally can be divided into

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organic, inorganic and inorganic-organic hybrid types.1 Some representative FINs are

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briefly introduced below.

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Figure 2. Classification of FINs

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Inorganic FINs such as zeolite and g-C3N4 have good mechanical properties which

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can withstand high temperature and pressure. As pioneering inorganic FINs, zeolites

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belong to crystalline aluminosilicates with unique pore structures that can accommodate

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a large variety of cations such as Ca2+, Mg2+.8, 25 , Hundreds of zeolites with pore-size

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ranging from 0.3 nm to over 1 nm have been explored since the first research on

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zeolite-based MMMs in 1976.26-34 Zeolites hold great promise for the preparation of

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MMMs due to their strong thermal and chemical stability, small pore sizes, narrow pore

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size distributions, high porosity and scalability. However, their application as FINs was

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impeded by the limited tunability in chemical functionality.

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The application of MOFs is a breakthrough in the field of membrane separation.

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MOFs are classified as inorganic-organic hybrid FINs. Their frameworks consist of

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inorganic metal ions or clusters coordinated to organic ligands.35 Since the first

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application of MOFs in 2004, numerous researches have been accomplished to explore

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their potential in membrane separation.36 Compared with inorganic FINs, MOFs

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exhibited chemical diversity of organic ligands and tunability in size, shape and chemical


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microenvironment, which can be utilized to facilitate stronger interaction with the

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polymeric matrix, reduce the interfacial microvoids and achieve better dispersion.

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Organic fillers have experienced an unprecedented explosion recently due to their

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potentially superior chemical compatibility with the polymeric matrix brought by the high

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tunability of organic properties. COFs, as an emerging organic FINs, are crystalline

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materials with highly ordered porous structures formed by the reversible polymerization

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of organic building blocks.37-39 By selecting monomers rationally, COFs with different

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pore diameters, diverse functional groups and high thermal and chemical stability can

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be synthesized through strong covalent bonds. Characteristics such as permanent high

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porosity, relatively low density, superior thermal stability, high aspect ratios and tunable

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functionality provide COFs with unique advantages over traditional FINs such as

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zeolites and MOFs.40-42 Since the first attempt of using COFs as FINs in 2016, extensive

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researches on this FINs have been performed.7 However, limited by their complicated

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synthesis procedures and overlarge pore size, only a few studies related to COF-based

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MMMs have been reported so far.

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3. The Fabrication of FINs-based MMMs

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In general, an ideal method of fabricating MMMs should be versatile, controllable and

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ensure the good dispersion of fillers within the polymer matrix. In the past two decades,

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a variety of methods have been explored as briefly introduced below.

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3.1 Membrane Formation without Chemical Reaction

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In most cases to fabricate MMMs, fillers and polymers are prepared individually and

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blended physically in the common solvent to form casting solution. Based on the

8

sequences of adding fillers and polymer, there are three basic methods (Figure 3): (a)

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Polymer solution and FINs suspension are prepared individually and then mixed. (b)

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Polymer solution is prepared first, followed by the addition of FINs. (c) FINs are

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dispersed into the solvent, then the polymer is added. High solubility of the polymer and

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dispersibility of FINs in the solvent is required so that uncontrollable filler aggregation

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and defects in the MMMs can be substantially avoided. The casting solution is stirred

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and sonicated for a certain period (ranging from a few hours to several days) to ensure

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homogeneous dispersion of the fillers and obtain bubble-free mixture, and then is cast


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in flat containers or coated on porous supporting substrates. Self-supported MMMs or

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asymmetric MMMs with dense epidermal layer and macro-porous supporting layer can

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be obtained after removal of the solvent by controlling evaporation in the fume hood or

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placing them into a non-solvent gel bath. Thin MMMs can also be prepared by spin-

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coating of the casting solution. The membranes obtained still need to be put in a

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vacuum oven to remove residual solvents within the pores of fillers, which is extremely

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important for gas separation.

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The greatest advantage of this facial physical method is that various FINs with

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different sizes, porosities, shapes and surface characteristics can be used since the

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fillers are prepared or modified under conditions that are not dependent on the

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fabrication of polymers and membranes. However,


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Figure 3. Schematic of membrane formation without chemical reaction. (a) Polymer

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solution and FINs suspension are prepared individually. (b) Polymer solution is

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prepared first, followed by the addition of FINs. (c) FINs are dispersed into the solvent,

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then polymer is added into the filler solution.

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the pre-synthesized fillers with high surface energy and different physicochemical

8

properties are forcibly dispersed in the polymeric matrix, inducing poor interfacial

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adhesion and filler agglomeration especially at high filler addition.

Besides, the


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enhancement of manufacturing cost is inevitable if this method is applied on a large

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scale since it is time-consuming and labor-intensive.

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3.2 Membrane Formation with Chemical Reaction

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Membrane formation process companied with chemical reaction can overcome some

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limitations of physical mixing method. As shown in Figure 4, chemical reaction that

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occurs in the bulk solution can be divided into two categories. (a) In-situ polymerization:

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polymerization of the polymer monomers occurs in the presence of FINs. The most

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significant merit of this method is that interfacial chemical crosslinking is convenient to

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form if some functional groups that can react with the polymer monomers exist on the

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surface of the FINs. (b) In-situ FINs growth: FINs are generated in the presence of

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polymers or polymer precursors. Then subsequent membrane formation process is

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performed according to steps introduced in section 3.1.


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Figure 4. Schematic of membrane formation with chemical reaction. (a) In situ

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polymerization.

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(b) In-situ FINs growth.

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In this method, FINs are generated within a confined space owing to the isolation

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effect of the polymer network. The drying and re-dispersion of FINs during which

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aggregations are easily formed can be avoided. However, the key point of this method

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is to find a common solvent for membrane casting, crystallization of FINs and/or the


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polymerization of polymer monomers. Also, it is crucial to achieve appropriate

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dissolution of the polymer within the solvent.43 Besides, the reaction rate and extent,

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although can be roughly predicted by the precursor concentration, are difficult to control

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because they are allergic to the content and proportion of reactants. Furthermore, about

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in-situ polymerization, adequate modification of FINs to prevent their sedimentation in

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the monomer dispersion with low viscosity is crucial.


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4. Methods of Improving FINs Dispersion

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FINs prepared with sub-micrometer or even nanometer size absorb a large amount of

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mechanical and thermal energy during their preparation, inducing a relatively unstable

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state with high surface energy. The fine particles tend to aggregate together driven by

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van der Waals attractive interactions between the particles to lower the surface area

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and reach a stable low-energy state.44,

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intermolecular force, the stress between the FINs is significant because of the large

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specific surface area. Moreover, the dispersibility of FINs is also closely related to the

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relative magnitude of the polymer–FINs and inter–FINs interaction forces. The

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aggregation of FINs occurs more easily if the polymer–FINs interaction is not sufficient

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to compete with inter-FINs interaction. With the increase of filler content, the closer

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distance between FINs makes inter-particle interactions predominate over that between

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matrix-particle, resulting in the increase of FINs agglomeration.

45

Although van der Waals force is a weak

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According to the above mechanism, inter-FINs interactions and FINs-polymer

15

interactions should be considered carefully to improve FINs dispersion in polymeric


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matrices. On one hand, the suppression of van der Waals attractive interactions or the

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introduction of forces that can prevent the formation of aggregates such as steric forces

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or repulsive forces is significant. On the other hand, strengthening the attractive

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interaction of FINs-polymer to reduce the relative intensity of inter-FINs interaction, is

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another strategy to control the dispersion of FINs. Also, providing a ‘template’ with

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active functional groups that can act as nucleation sites to immobilize the subsequent

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crystal growth of FINs, solution mixing method and in-situ FINs growth that avoiding the

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additional aggregation arising from the drying process have also been adopted.

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Numerous researches have been conducted to achieve excellent dispersion of FINs so

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that their properties can be fully exploited. Specific methods will be presented in the

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following section.

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4.1 Solution Mixing Method

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In a typical membrane preparation process, it is a great challenge to redisperse the

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dried FINs well into a suspension because of aggregation caused by capillary

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shrinkage: with the solvent evaporates during the drying of FINs solution, capillary

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structures are formed between the particles, and the particles are pressed together into


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clusters tightly by the capillary forces. This effect is especially pronounced for aqueous

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dispersions of hydrophilic FINs. Moreover, some functional groups on the particle

3

surface may form strong covalent bonds between the particles during drying, leading to

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the formation of ‘hard’ agglomerates. Therefore, it is usually difficult to re-disperse them

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well in solvents even with external driving force such as sonication.

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Figure 5. (a) Preparation of PVA/nano-ZIF-8 MMMs from ZIF-8 suspensions with and

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without drying. (b) The transmittance at 650 nm of ZIF-8 suspensions as a function of

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standing time. (Modified and reproduced with permission from ref 47. Copyright 2016,

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Wiley-VCH, Weinheim.)


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Solution mixing is an effective way to prevent filler agglomeration. Wet-state fillers

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without drying-treatment are used. The occurrence of agglomeration during the drying

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process is avoided and the distribution of fillers is improved effectively. Chung and

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coworkers incorporated as-synthesized ZIF-7 nanoparticles without traditional drying

5

treatment into polybenzimidazole (PBI) to minimize particle agglomeration within the

6

fabricated MMMs.46 Strong interactions formed during the drying process were avoided.

7

Thus these nanoparticles were homogeneously distributed within PBI with less

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observable agglomeration. On the contrary, ZIF-7 powder tended to aggregate and

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became un-dispersible in the same solvents due to the formation of strong covalent

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bonds between the nanoparticle surfaces (Zn-im-Zn). Similarly, Wu et al. synthesized

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ZIF-8 nanoparticles with particle size of 60 nm and incorporated dry-fillers and wet-fillers

12

separately into the aqueous


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Figure 6. Cross-section of Matrimid–ZIF-8 composite membranes. (a) Example of poor

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dispersion using dried ZIF-8 nanoparticles (20 wt % loading). (b) Example of good

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dispersion (20 wt % ZIF-8) using as synthesized ZIF-8 nanoparticles. Scale bars are

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500 nm. (Modified and reproduced with permission from ref 48. Copyright 2012, Royal

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Society of Chemistry, London.)

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solution of polyvinylalcohol (PVA) to prepare MMMs (details can be seen in Figure

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5a).47 They found that ZIF-8 suspension without drying treatment still exhibited good

10

stability after standing for 24 hours, while obvious agglomeration occurred to ZIF-8

11

suspension with drying (Figure 5b). The MMMs that were incorporated with wet-state


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fillers showed homogeneously dispersed MOF fillers and better mechanical stability

2

even though the loading of ZIF-8 was very high (up to 39 wt %). Similar work has also

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been reported by Song et al., who mixed the as-prepared colloidal solution of fillers/

4

dried fillers and polymer solution separately.48 As shown in Figure 6, the aggregation of

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ZIF-8 can be observed clearly for the membrane using dried ZIF-8 fillers while no

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clusters or aggregates are observed in SEM of MMMs using pre-synthesized fillers

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although these membranes have the same filler loadings of 20 wt %. Therefore, solution

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mixing method is a facial and efficient method to prevent the formation of aggregations.

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It is appropriate for all kinds of fillers no matter it is organic, inorganic or organic-

10

inorganic types.

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4.2 Manipulate the Interaction of Inter-FINs

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To reduce the agglomeration of FINs, it is necessary to decrease the dominating van

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der Waals attractive force. Addition of suitable polymeric dispersants is widely used to

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confer a polymer-induced stabilization of FINs. On one hand, polymer on the FINs

15

surfaces can serve as a protective layer to screen the attractive van der Waals

16

interaction or can even introduce repulsive forces between the particles. On the other


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hand, the direct contact of the bare FINs surfaces could be prevented (steric force) and

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the interfacial compatibility could also be enhanced due to the existence of the polymer

3

layer.

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The polymer coating could be achieved by surface priming technique which is broadly

5

applied in the fabrication of MMMs. A small amount of polymer is added into the filler

6

solution before the drying process. Afterwards, sonication or stir is required so that a

7

thin layer of polymer can be formed at the surface of the FINs. Next, the primed FINs

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were dried and redispersed in the solvent. It was found that the primed FINs exhibited

9

better dispersibility in the solvent than the original fillers, and the particle dispersion

10

within the polymer is improved to a large extent. However, the intensity of sonication

11

during primed FINs dispersal should be considered carefully since intense sonication

12

may result in the delamination of the polymer layer. Priming technique was first

13

attempted by Koros and coauthors, who found that zeolite 4A primed by a small amount

14

of PVAc polymer could achieve uniform dispersion in the casting solution and the

15

resultant MMMs even at a high loading of 40 wt %.49 Surface priming is also commonly

16

used in the fabrication of MOF-containing MMMs. For example, Jiang et al. embedded


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primed ZIF-67 into polymers of intrinsic microporosity (PIM-1). The prepared PIM-1/ZIF-

2

67 MMMs showed homogenous distribution of FINs within the polymer.50 The improved

3

dispersion of fillers by priming technique has also been demonstrated by Musselman,

4

Omidkhah et al.51-54 What is important is that the polymer layer should cover the entire

5

FINs surface so that the interparticle force can be manipulated.55

6

Compared with priming the FINs with electrically neutral polymers, coating them with

7

‘charged polymers’ so that the FINs can be mutually exclusive is a more effective

8

method to prevent aggregations. The charged polymer can be covalently linked or just

9

absorbed on the surface of the FINs.

For example, after coating with poly (ether

10

sulfone) (SPES), zeolite 4A was incorporated into the PES polymeric matrix by Li et al.

11

to prepare MMMs.56 They found that SPES layer could restrain the agglomeration of

12

FINs efficiently for the sulfonic groups on the FINs surface carry negative charges after

13

the dissociation of H+ in the solvent and could provide electrostatic repulsion between

14

the zeolite particles. The effect of sulfonic groups on the dispersion of FINs was further

15

confirmed by PES primed zeolite 4A/ PES MMM, which exhibited pronounced

16

aggregations under the same addition. In another work, polyethyleneimine (PEI) was


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grafted on the surface of ZIF-8 through in-situ synthesis. The resultant PEI grafted ZIF-8

2

showed a highly improved dispersibility than ZIF-8 in the casting solution because of the

3

strong electrostatic repulsive force of the positively charged PEI.57 Similar to solution

4

mixing method, polymer coating is a widely used and practical method that is not limited

5

by the type of FINs. These two methods are usually combined to improve the dispersion

6

of FINs within the matrix further.

7

4.3 Manipulate the Interaction of Polymer-FINs

8

Aggregation occurs more easily when the interaction between filler particles is

9

stronger than that between FINs and polymer. Hence, intensifying the polymer-filler

10

interaction is an effective strategy to prevent aggregation of FINs. With the improvement

11

of the interfacial interaction, the sediment of FINs can also be reduced since the ‘pulling’

12

of the polymer. Methods that can strengthen the interaction of polymer-FINs will be

13

introduced in the following section.

14

4.3.1 Increase the Number of Interaction Sites


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A valid approach to fortify the interfacial interaction of polymer-FINs is to increase the

2

number of interaction sites between the polymeric matrix and the FINs. The number of

3

interaction sites can be derived from the total interfacial surface area, which is positively

4

proportional to the specific surface area of the FINs. In the following part, two major

5

methods to improve the specific surface area will be presented.

6

4.3.1.1 Reduce the Diameter of Spherical FINs

7 8 9

For spherical FINs, the specific surface area (m2 /g) is obtained according to equation (1), Specific surface area 

6000 ρd

(2)

10

in which ρ (g/cm3) and d (μm) are the bulk density and the diameter of the FINs,

11

respectively.58 It is evident that smaller fillers possess higher specific surface area and

12

thus total surface area than larger size fillers at the same loading. Therefore, reducing

13

the diameter of the FINs can significantly increase the number of interaction sites that

14

can interact with the polymeric matrix.


27


Page 28 of 90

1 2

Figure 7. SEM images of dried aggregates of (a) unmodulated UiO-66(100-200nm) (b)

3

water-modulated nanosized UiO-66(20-30nm) MOF nanoparticles.

4

cross-sections of PIM-1/UiO-66 MMM (c) 5 wt % unmodulated UiO-66, (d) 20 wt %

5

unmodulated UiO-66, (e) 20 wt % unmodulated UiO-66 (high magnification), (f) 5 wt %

6

modulated UiO-66, (g) 20 wt % modulated UiO-66, (h) 20 wt % modulated UiO-66 (high

7

magnification). (Modified and reproduced with permission from ref 59. Copyright 2017,

8

Nature Publishing Group, London.)

SEM images of

9


28

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1

So far, several efforts have been devoted to exploring the effects of filler size on the

2

dispersion of fillers. Ghalei et al. fabricated a series of PIM-1/UiO-66 MMMs via physical

3

blending method, aiming to elevate the membrane selectivity through efficient

4

dispersion of fillers.59 As shown in Figure 7, the particle size of UiO-66 was reduced

5

from 100-200nm to 20-30nm by water modulation (Figure 7a-b). There is no perceptible

6

difference between the membranes incorporated with 5 wt % fillers of different particle

7

size. When the filler-loading increases to 20 wt %, aggregation happened to

8

unmodulated MOF (particle size 100-200nm) while modulated UiO-66 (particle size 20-

9

30nm) exhibited much better dispersion within the PIM-1. The separation selectivity for

10

CO2/N2 and CO2/CH4 showed a remarkable improvement owing to the homogeneous

11

dispersion of small-size FINs. Bae and coworkers fabricated 6FDA-DAM/ZIF-90 MMMs

12

and found that the separation performance of the MMMs was enhanced by the

13

incorporation of MOF filler. Meanwhile, the membranes containing ZIF-90A fillers, which

14

is smaller than ZIF-90B, displayed better separation performance.60 Similar work has

15

also been reported by Susilo Japip, who found that MMMs which were embedded with

16

smaller ZIF-71( ≤ 200 nm) exhibited better performance when they were used for gas


29


Page 30 of 90

1

separation.61 Several methods have been used to reduce the size of the filler such as

2

ultrasonication treatment of the as-synthesized fillers and nonsolvent-induced

3

crystallization method, alteration of the synthesis conditions, etc.62

4

4.3.1.2 Change the Morphology of FINs.

5

FINs exhibited different morphologies such as bulk crystals, nanosheets, nanorods,

6

nanoneedles. It is interesting that the interaction of polymer-FINs can be fortified by

7

changing the morphology of FINs. Compared with fillers with other dimensions, 2D

8

nanosheets have intensified interaction with the polymer matrix because of their high

9

aspect ratios, almost full utilization of the FINs and larger interfacial contact areas.

10

Another crucial aspect is that the sheet-like materials can provide longer diffusion paths

11

especially for species with larger molecular dynamic diameter, thus elevating the

12

separation selectivity of the desired permeating species. That’s why 2D FINs have

13

become the most intensively explored materials. Rodenas et al. are the first people who

14

extensively investigated the effect of particle morphology on the structure and

15

performance of the MMMs.63 Either bulk-type, nanosheets or isotropic nanoparticles of


30

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1

CuBDC were incorporated into polymide polymer to prepare MMMs and it was found

2

that the nanostructures of the MMMs were striking different and CuBDC nanosheets

3

showed a better dispersion within the polymer despite the identical filler loading, which

4

was confirmed by the segmented FIB-SEM tomograms of the MMMs. (Figure 8). The

5

better dispersibility mainly arose from the increased interfacial interaction because the

6

surface area of CuBDC nanosheets was ten times larger than that of bulk-type crystals

7

at the same loading. Notably, MMMs with 8 wt % nanosheets even showed 75% to

8

eight times higher in selectivity compared with MMMs with bulk-CuBDC under different

9

operating conditions. The better dispersibility of 2D FINs was further confirmed by Kang

10

et al., who incorporated copper MOF with different morphologies (nanosheets,

11

nanocubes and bulk crystals ) into PBI to prepare MMMs.64 The membrane with

12

nanosheets exhibited the best performance owing to the most homogeneous

13

dispersibility of FINs.


31


Page 32 of 90

1

2

Figure 8. Surface-rendered views of the segmented FIB–SEM tomograms for composite

3

membranes containing bulk-type (a) and nanosheets (b) CuBDC MOF embedded in

4

polyimide(8 wt %). MOF particles are shown in blue, while voids are shown in red.

5

(Modified and reproduced with permission from ref 63. Copyright 2014, Nature

6

Publishing Group, London.)

7 8

Most COFs materials reported so far have 2D layered structures that can be

9

exfoliated into nanosheets.65-71 This property and their organic nature endow them with

10

good dispersibility in the polymeric matrix as promising FINs. For instance, azine-linker

11

COF (ACOF-1) was incorporated into Matrimid® to prepare MMMs.72 The dispersibility


32

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1

was investigated by SEM, which showed that FINs were dispersed uniformly in the

2

polymer even for membranes (Figure 9) with up to 16 wt % ACOF-1. When being

3

applied for the separation of CO2 and CH4, the significantly enhanced permeability

4

(130%) compared with pure polymer was primarily arisen from the large pore size of

5

ACOF-1 while the slightly higher selectivity was attributed to the good dispersion of the

6

nanosheets.

7

8

9

Figure 9. SEM images of a) ACOF-1 and b) the cross-section of a 16 wt % ACOF-1@

10

Matrimid® MMMs. (Modified and reproduced with permission from ref 72. Copyright

11

2016, Wiley-VCH, Weinheim.)

12 13

4.3.2 Physical Crosslinking


33


Page 34 of 90

1

Physical crosslinking refers to relatively weak and non-covalent interactions

2

(hydrogen bonds, π-π interaction, etc.) between FINs and polymer chains. Most of its

3

intensity lies between 10 and 70 kJ/mol, which is stronger than a typical van der Waals

4

force (~0.4-4 kJ/mol).73 By introducing physical crosslinking into the MMMs, the

5

interaction between the polymer and FINs can be fortified so that the FINs can be

6

homogeneously dispersed within the MMMs.

7

4.3.2.1 Hydrogen Bond

8

The most common way to form interfacial hydrogen bonds is incorporating functional

9

groups such as amino and amidoxime groups, aminosilane, silane coupling agent,

10

hydroxyl groups, sulfonic acid groups on the surface of fillers or/and the polymer

11

chains.74

12

Silane coupling agents, which can react with the hydroxyl groups on the surface of

13

zeolite, are widely used to improve the interfacial interaction between zeolite and

14

polymeric

15

aminopropyl(diethoxy)methylsilane (APDEMS), zeolite Y showed a better distribution

matrix.

For

example,

after

functionalized

with

3-


34

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1

within cellulose acetate matrix owing to the formation of hydrogen bonds between the

2

NH groups of the amino silane and the carbonyl and oxycarbonyl groups of cellulose

3

acetate.75 The reduction of agglomeration was also verified by the smooth surface of

4

modified zeolite-containing MMMs. Similar phenomenons hav also been found by

5

Kaliaguine and coworkers.76

6

The formation of hydrogen bonds can also be achieved by the functionalization of

7

FINs and/or polymer. As shown in Figure 10a-b, PIM-based MMMs were prepared

8

using amidoxime-functionalized PIMs (PAO-PIM-1) as the polymeric matrix and amine-

9

functionalized MOF (NH2-UiO-66) as fillers.77 According to SEM images shown in Figure

10

10, NH2-UiO-66 had better dispersibility in the PAO-PIM-1 polymer with no particle

11

aggregations observed compared to UiO-66 in PAO-PIM-1 and NH2-UiO-66 in PIM-1,

12

since more hydrogen bonds between the

13


35


Page 36 of 90

1

2

Figure 10. (a) Synthetic scheme of PAO-PIM-1. (b) Schematic illustration of a hydrogen

3

bond network guided interface design of the hybrid membrane. Cross-sectional SEM

4

images with different magnifications and the corresponding optical images of the

5

membranes of pure PAO-PIM-1 (c, d and e), PAO-PIM-1/NH2-UiO-66 (30%) (f, g and h),

6

PAO-PIM-1/UiO-66 (30%) (i, j and k), and PIM-1/NH2-UiO-66 (30%) hybrid membranes

7

(l, m and n). (Modified and reproduced with permission from ref 77. Copyright 2017,

8

Royal Society of Chemistry, London.)

9 10

amidoxime and amine groups are formed at the interface. The interfacial voids are

11

eliminated and the well-defined membrane exhibits higher separation selectivity.


36

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1

Moreover, the dispersion of FINs can also be observed according to the transparency of

2

the MMMs with the same filler loading (30 wt %). The PAO-PIM-1/ NH2-UiO-66 MMMs

3

displayed similar transparency with pure PAO-PIM-1 membrane, indicating the well-

4

dispersed FINs within the polymeric matrix. While the PAO-PIM-1/UiO-66 and PIM-

5

1/NH2-UiO-66 MMMs were semitransparent due to poor dispersion of the FINs.

6

7

8

Figure 11. (a) Structure of copolyimide precursor FDH-xy. SEM micrographs of

9

hydroxyl-copolyimide FDH-11 based MMMs as illustration for particle distribution at

10

loadings of (b) 10 wt %, (c) 15 wt % and (d) 20 wt % NH2-MIL-53. (e) Illustration for


37


Page 38 of 90

1

particle agglomeration in a 6FDA-DAM-based MMM containing 10 wt % NH2-MIL-53.

2

(Modified and reproduced with permission from ref 74. Copyright 2015, Royal Society of

3

Chemistry, London.)

4

In another study, Tien-Binh et al. incorporated HAB with hydroxyl groups into the

5

6FDA–DAM polyimide backbone to prepare 6FDA–(DAM)x–(HAB)y copolyimide

6

(referred to FDH-xy).74 The structure of FDH-xy can be seen in Figure 11a. MMMs were

7

fabricated by using NH2-MIL-53(Al) as FINs and FDH with different x:y ratio as the

8

polymeric matrix. The excellent dispersion of fillers which can be seen from the SEM

9

image in Figure 11b-d, was attributed to the improved polymer-filler interfacial

10

interaction originating from the hydrogen bonds between the amine groups on the MOF

11

surface and the hydroxyl groups in HAB moieties. The same group also combined NH2-

12

MIL-53 with 6FDA-DAM to prepare MMMs, the clusters (about 500 nm) formed by

13

MOFs agglomeration can be easily observed by SEM (Figure 11e), which further

14

verifies the important role of hydrogen bonds in enhancing the dispersion of fillers.

15

Moreover, the membrane prepared with hydroxyl-copolyimide and NH2-MIL-53 was


38

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1

preferred for CO2/CH4 separation because it possessed more excellent selectivity with a

2

minor loss in permeability compared with the membrane without hydroxyl groups in the

3

polymer.

4

Covering FINs with a layer of polymer that can form hydrogen bonds with the

5

polymeric matrix is also widely adopted to improve the interfacial interaction. Inspired by

6

marine mussels which can tether its organism tightly on the surface of different

7

substrates, the adhesive property of polydopamine (PD) has been widely exploited to

8

tune interfacial interaction. For example, Wang and coworkers covered ZIF-8 with an

9

ultrathin PD layer because there are abundant secondary or primary amine groups on

10

PD molecules.78 The introduction of PD remarkably enhanced the interfacial interaction

11

between the polyimide (PI) matrix and ZIF-8 fillers because of the formation of hydrogen

12

bonds between amine groups in PD molecules and the tertiary amine groups in PI

13

polymer. PD-coated ZIF-8 nanoparticles were wrapped more uniformly by PI polymer

14

compared with ZIF-8 in the same matrix. This demonstrated that hydrogen bonds

15

contribute to the better dispersibility of FINs. Homogeneous dispersion of FINs within


39


Page 40 of 90

1

the polymeric matrix has also been achieved by decorating FINs with polyethylenimine

2

(PEI) or polyethylene glycol (PEG).79, 80

3

4.3.2.2 π-π Interaction or Chelating Effect

4

π-π interactions between aromatic rings can be manipulated to improve interfacial

5

interaction and filler dispersibility. For example, NH2-UiO-66 was functionalized by

6

Venna et al. with phenyl acetyl so that the interfacial interaction can be enhanced

7

through π-π stacking.81 The phenyl acetyl functionalized FINs were well dispersed

8

within the polymeric matrix because of the synergistic effect of π-π conjugation between

9

the aromatic ring of the phenyl acetyl group and the aromatic groups in the polymer

10

backbone and hydrogen bonds between the amide linkages and the imide groups.

11

(Figure 12a). The resultant membranes displayed outstanding gas separation

12

performance with CO2 permeability increased by 2 times and CO2/N2 ideal selectivity

13

increased by 25%.

14


40

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1

2

Figure 12. Scheme demonstrating the favorable interactions of (a) π-π interactions

3

between the Matrimid® polymer and IPA based on surface functionality. (b) Chelating

4

effect between cross-linked poly(ethylene oxide) and NH2-ZIF-7. (Modified and

5

reproduced with permission from refs 81 and 82. Copyright 2015, Royal Society of

6

Chemistry, London, and 2017, Wiley-VCH, Weinheim.)

7 8

Chelation formed between ions/molecules and metal ions can also benefit the fortified

9

interaction between chelating ligands and metal ions. For example, Xiang et al. added

10

NH2-ZIF-7 FINs into cross-linked poly(ethylene oxide) to prepare MMMs.82, Solution

11

mixing method was also adopted and chelating effect between Zn2+ in MOF fillers and

12

ester group in XLPEO was designed (as shown in Figure 12b), which contributed to the


41


Page 42 of 90

1

good interfacial interaction and therefore the uniform distribution of FINs within the

2

polymeric matrix.

3 4

4.3.3 Chemical Crosslinking

5 6

Figure 13. The grafting reaction between APDEMS and zeolite surface, and also the

7

reaction between Matrimid® and the surface modified zeolite. (Modified and reproduced

8

with permission from ref 83. Copyright 2015, Elsevier, Amsterdam.)


42

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1

Chemical crosslinking refers to the formation of covalent bonds (200-800 kJ/mol)

2

between the polymer and the FINs. Stronger interfacial interaction has been proved as

3

an effective way to improve filler dispersion.

4

In many studies, amino silanes can work as a “linker” between the FINs and the

5

polymer since they can react with hydroxyl groups on the FINs surface (especially

6

zeolites) and functional groups in the polymer at the same time. As shown in Figure 13,

7

Zeolite Y was grafted with amino silane (APDEMS), which can further react with the

8

imide groups in Matrimid® chains.83 The crosslinking network was formed with the aid of

9

this linker.

10


43


Page 44 of 90

1

Figure 14. (a) Post synthetic modification of UiO‐66‐NH2 with methacrylic anhydride and

2

subsequent polymerization with butyl methacrylate (BMA) by irradiation with UV light.

3

SEM images of the surfaces of (b) a PSP-derived membrane with 20 wt % MOF loading

4

and (c) a membrane of a UiO-66-NH2/ PBMA blend with 20 wt % MOF loading.

5

(Modified and reproduced with permission from ref 87. Copyright 2015, Wiley-VCH,

6

Weinheim.)

7 8

Therefore, sedimentation of fillers was prevented, interfacial defects were eliminated

9

and better dispersion of nanoparticle was achieved in the Matrimid® 5218/zeolite Y

10

MMMs.

11

Post-synthetic polymerization (PSP) is an emerging method to construct covalent

12

bonds between the FINs and the polymeric matrix. The prerequisite of this method is

13

that there are some polymerizable functional groups on the surface of the FINs. Then

14

the FINs are mixed with the organic monomer or oligomers to produce MMMs with FINs

15

covalently linked to the polymeric matrix.84 The chemical crosslinking can be achieved

16

directly if some inherent polymerizable functional groups exist in the FINs. For example,


44

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1

the chemical crosslinking between novel MOF fillers (Mg-MOF-74) and PIM-1 would

2

take place because the hydroxyl functional groups on the surface of FINs can react with

3

the fluoride chains ends of PIM-1 monomer.85 The covalent bonds which formed during

4

the polymerization of the monomers improved the polymer-filler interaction observably.

5

The resultant MMMs displayed great filler dispersion because of the fortified

6

interactions.

7

However, in most cases, chemical modification to introduce polymerizable functional

8

groups on the surface of FINs is the prior condition of this method. For example, UiO-66

9

was modified with amino groups, which can react with the monomer (DCTB) of PIM-1.

10

Then monomers and catalyst required for PIM-1 polymerization were added into UiO-

11

66-NH2 solution. Therefore, the crosslinking was achieved during the synthesis of PIM-1

12

and FINs were anchored to the polymer chains tightly. The resultant MMMs acquired

13

exceptional interfacial adhesion and filler dispersibility.86 In another case (as shown in

14

Figure 14a), Zhang et al.87 modified UiO-66-NH2 with methacrylic anhydride functional

15

groups that can copolymerize with butyl methacrylate (BMA) to obtain UiO-66-NH-Met.

16

The modified FINs were mixed with BMA and photoinitiator


45


Page 46 of 90

1 2

Figure 15. Chemically Cross-linked Membrane Based on UiO-66-IL–ClO4 NPs and the

3

Polyurethane Oligomer. (Modified and reproduced with permission from ref 88.

4

Copyright 2017, American Chemical Society, Washington.)

5 6

phenylbis (2,4,6-trimethylbenzoyl) phosphine oxide, then copolymerization of modified

7

FINs and acrylate monomers occurred under the irradiation of UV light. Thus,

8

standalone MMMs with chemical crosslinked MOF-polymer were prepared. UiO-66-NH-

9

Met was uniformly distributed within the polymeric matrix and no aggregations and

10

clusters were observed in the MMMs even at a high loading of 20 wt % owing to the

11

introduction of interfacial chemical interactions (Figure 14b). The key role of crosslinking

12

was further verified by the SEM image of MMMs fabricated by simply blending of PBMA


46

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1

and UiO-66-NH2, in which the formation of conglomerates could be easily observed at

2

the same loading (Figure 14c).

3

Ionic liquid has also been used to functionalize FINs so that covalent bonds can form

4

between the FINs and polymer. As shown in Figure 15, Yao et al. reported a novel

5

MMMs, fabricated via polymerization of ionic liquid modified UiO-66 and the

6

polyurethane oligomer.88 In this way, the fillers and polymer were linked by covalent

7

bonds and MOF aggregation in the polymer was effectively avoided. Compared with

8

pure polymer membrane, the resultant MMMs with 50 wt % MOF loading exhibited

9

permeance increases of 5.9, 1 and 2 times for CO2, N2 and CH4 respectively and thus

10

largely enhanced selectivity for the separation of CO2/N2 and CO2/CH4.

11

Although chemical crosslinking possesses great potential in solving aggregation of

12

FINs, it has inherent limitations. Chemical crosslinking, especially the emerging PSP

13

method only works for FINs with functional groups that can participate in the

14

polymerization reaction. Thus, the application of FINs with relatively poor chemical

15

mutability such as zeolites and some MOFs is limited. It is worth mentioning that COFs


47


Page 48 of 90

1

may have great potential to form chemical crosslinking with the polymer considering

2

their excellent functionality although there is no relevant research so far.

3

4.4 Synergistic Effect of Composite FINs

4

Combining different fillers represents an efficient strategy to prevent the aggregation

5

of FINs because of the synergistic effect of FINs with different surface characteristics.

6

For example, 1D or 2D materials can serve as a template in fillers synthesis. In this

7

method, suitable surface functional groups such as carboxyl and carbonyl functional

8

groups on the ‘template’ are essential since they can serve as nucleation sites to

9

immobilize the subsequent crystal growth of MOFs.


48

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1 2

Figure 16. (a) Schematic diagram of a 6FDA-durene MMM containing ZIF-8 decorated

3

CNTs.

4

Filler volume variation at different depths of (b) 15 wt % ZC/6FDA-durene MMM (c) 15

5

wt % ZIF-8/6FDA-durene MMM, which is deriving from the image of FIB-SEM

6

tomogram. (Modified and reproduced with permission from ref 94. Copyright 2016,

7

Royal Society of Chemistry, London.)

8


49


Page 50 of 90

1

89, 90

The location and dispersion of FINs crystals are thus directed and controlled by

2

the ‘template’. In turn, MOF particles on the surface of these materials can serve as a

3

steric barrier to suppress the agglomeration of the ‘template’. The composite fillers can

4

also present in another form: inorganic fillers such as zeolite are coated with a layer of

5

MOFs or MOFs are coated with organic COFs to form core-shell structure so that the

6

inorganic properties of the FINs can be improved by the organic shells. A lot of

7

composite fillers have been prepared up to now, such as MOF@GO, MOF@CNT,

8

MOF@silica, COF@MOF and [email protected]

9

Lin et al. fabricated MMMs using CNT/ZIF-8(refer to ZC) as fillers.94 As shown in

10

Figure 16a, the growth of ZIF-8 was confined to the surface of CNT thanks to the

11

existence of carboxyl groups, leading to an even distribution of ZIF-8. Moreover, the

12

composite ZC possess higher surface area (1100–2000 m2/g) relative to ZIF-8 or CNT.

13

The excellent ZC dispersibility was proved by the less fluctuation of the filler volume

14

variation at different depths (Figure 16b) owing to the CNT skeleton. On the contrary,

15

the volume distribution of ZIF-8 exhibited great fluctuation over depth because of filler

16

aggregation (Figure 16c) in 6FDA-durene / ZIF-8 MMM. Compared with ZIF-8


50

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1

containing MMMs, ZC-based MMMs displayed higher selectivity for propylene/propane

2

separation since the better dispersion of FINs reduced the non-selective interfacial

3

voids greatly.

4

NH2-UiO-66 was coated with TpPa-1 layer by Cheng et al. to construct composite

5

fillers.97 Pure organic COF coating layer endowed the membrane with strong polymer-

6

filler interaction, which was proved by the increased glass transition temperature of

7

MMMs incorporated with 5% composite fillers. Homogenous dispersion of the

8

composite FINs can be easily confirmed by SEM images and energy-dispersive X-ray

9

mapping. To further confirm the effect of the COF coating, pure MOF/ PSf membranes

10

were prepared as a comparison. MOF fillers tended to aggregate into clusters as

11

expected. When being applied to CO2/CH4 separation, the membrane with composite

12

FINs displayed remarkable enhancement in selectivity while membranes with single

13

filler manifested the opposite effect because of the formation of voids brought by the

14

aggregation of FINs. Similarly, the MOF-silica composite fillers were well dispersed

15

within the polymer owing to the increased compatibility resulting from the MOF shell.96


51


Page 52 of 90

1

Therefore, composite filler is another alternative way to design MMMs with better filler

2

dispersion and separation performance.

3

4.5 In-situ FINs Growth

4

5

Figure 17. Preparation of the ZIF-8 mixed matrix membrane. a) Assembly of Zn2+ on the

6

substrate. b) Assembly of PSS and formation of ZIF-8 particles. c) Proposed structure of

7

the membrane. d) Cross-section SEM image of the resulting membrane (two layers).


52

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1

(Modified and reproduced with permission from ref 100. Copyright 2014, Wiley-VCH,

2

Weinheim.)

3

In situ FINs growth has been actively explored to improve the dispersion of fillers. The

4

interfacial incompatibility can also be improved to some extent. More details about this

5

method can be seen in section 3.2.

6

As shown in Figure 17, Li and coworkers employed Zn(NO3)2 and Hmim as the

7

building block of the ZIF-8 (filler phase) and PSS as the polymer to prepare MMMs by

8

coordination-driven in situ self-assembly.100 This method allowed the fabrication of

9

MMMs containing well-dispersed FINs within one step. Particles aggregation that often

10

occurred in the process of synthesis, drying and activation was avoided. The resultant

11

ZIF-8/PSS MMMs outperformed those MMMs prepared by physical blending method.

12


53


Page 54 of 90

1

Figure 18. SEM images of pure Matrimid and MMMs fabricated using (a) in situ MOF

2

growth and (b) physical mixing. (Modified and reproduced with permission from ref 101.

3

Copyright 2018, American Chemical Society, Washington.)

4 5

The better dispersibility of FINs within UiO-66/ Matrimid® MMMs fabricated by in-situ

6

FINs growth was also verified by Marti et al.101 The influence of fabrication methods on

7

FINs dispersibility was shown in SEM images of different MMMs. For MMMs fabricated

8

by in-situ synthesis (Figure 18a), UiO-66 was uniformly distributed within MMMs at filler

9

loading of 2 wt% and 5 wt%. When the FINs loading was increased to 11 wt%, white

10

clumps of MOF agglomeration began to emerge. However, for MMMs fabricated using

11

physical mixing, white clumps could be observed in all prepared MMMs (Figure 18b).

12

Different methods to improve the dispersion of FINs have been discussed according

13

to the generality of various methods. When MMMs are designed, researchers may

14

follow the following procedures. If a common solvent with appropriate dissolution of

15

polymers can be found for the crystallization of FINs and membrane casting, in situ

16

FINs growth can be tried first. Similarly, if a solvent can be used for both membrane


54

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1

casting and polymer synthesis, in situ polymerization will be a good choice. Especially,

2

by modifying FINs with functional groups which can react with the polymer monomers,

3

the covalent bonds may form between the FINs and the polymers during polymerization.

4

If this common solvent cannot be found, a ‘linker’ that can react with polymers and FINs

5

simultaneously should also be considered. If chemical crosslinking is difficult to happen,

6

it should be taken into consideration whether the FINs and/or the polymers can be

7

functionalized to form physical crosslinking structure at the interface. At the same time,

8

it still needs to be considered whether the FINs can be synthesized with a smaller size

9

and delaminated into two-dimensional sheets with high aspect ratio. Moreover, the

10

synergistic effect of composite FINs may also be utilized. If none of the above methods

11

are feasible, simple ‘polymer coating’ and ‘solution mixing’ would be helpful to achieve

12

better dispersion of the FINs.

13

14

5. Application of FINs-based MMMs in Gas Separation


55


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1

Gas separation processes such as CO2 capture from the natural gas or biogas, H2

2

purification and the separation of ethylene and ethane, propylene and propane, etc. are

3

crucial operations in chemical industries. Researchers have been exploring a high

4

energy-efficiency method to replace the traditional separation techniques such as amine

5

scrubbing or cryogenic distillation102 since these processes are extremely energy

6

intensive. Gas separation membranes especially MMMs composed of FINs provides an

7

alternative method to separate gas mixtures with low energy consumption.

8

The classic solution-diffusion mechanism of MMMs can be described as equation (2)

9

and equation (3), in which P, D, and S is the permeability, diffusivity, and solubility

10

coefficients, respectively; 𝛼𝑎/𝑏 is the ideal selectivity while 𝑃𝑎 and 𝑃𝑏 is the permeability

11

coefficients of gas components "a" and "b" respectively.

12

13

P=S  D

 a /b 

(3)

Pa Sa  Da Sa Da    (4) Pb Sb  Db Sb Db

14

Therefore, the permeability of a specific component can be enhanced by increasing

15

its diffusivity and solubility coefficients. Diffusivity coefficient increases with the


56

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1

decrease of penetrant size and the rise of chain flexibility and fractional free volume of

2

the polymer. While the solubility coefficient can be increased by increasing the

3

interactions between the polymer and the penetrant and decreasing temperature.

4

Selectivity reflects the separation capability of a membrane. The selectivity of the gas

5

pairs can be increased by increasing diffusivity-based selectivity (

6

solubility-based selectivity (

7

arises from the difference in condensability and affinity with the membrane for different

8

gas molecules. Diffusivity-based selectivity depends mainly on the difference in the

9

molecular size of the gas.

Da ) or/and the Db

Sa ) of components a and b. Solubility-based selectivity Sb

10

Size-sieving is another mechanism existed in FINs-based MMMs. Sieving occurs

11

when the effective size of molecular transport channels falls between the dimensions of

12

the separated mixture. Substances with a molecular size smaller than that of transport

13

channels can pass through the channel, while substances with bigger size are rejected.

14

The mixture can therefore be separated effectively with extremely high selectivity. For

15

FINs- based MMMs, the aperture size of FINs can be utilized to achieve gas separation


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1

with high efficiency. For example, ZIF-8 with a pore size of 0.34nm should display

2

excellent separation performance by allowing the pass of CO2 (0.33nm) molecules and

3

blocking the pass of CH4 (0.38nm) molecules. However, the framework of FINs with

4

organic moieties is flexible. Larger gas molecules CH4 can also pass through the

5

intrinsic nanopores of ZIF-8. Although strict sieving is challenging to achieve in practical

6

application, the aperture size of FINs can still contribute to prior diffusion of small gas,

7

which is meaningful to the improvement of selectivity.

8

Facilitated transport mechanism also exists in FINs-based MMMs. The facilitation

9

effect is owing to the introduction of some carriers that can react with a specific

10

component of the mixture reversibly into the membrane. The rapid transport of this

11

component is achieved by being bonded quickly and being released facilely by the

12

carriers. Therefore, the intensity of the reverse reaction should be considered carefully

13

to achieve high facilitated transport efficiency. The transport of CO2 can be facilitated by

14

Bronsted bases such as OH-, CO32-, COO-, -NH2, etc. through reversible nucleophilic

15

addition reaction. These carriers can be introduced by the side chains of polymeric

16

backbone or the functional moieties of FINs. Facilitated transport of unsaturated


58

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1

molecules such as olefins and CO2 can also be achieved through π complexation with

2

transition metal ions such as Ag+, Cu2+, Au+, Pd2+, etc. The strength of π complexation,

3

which is mainly determined by the electronegativity of metal ions, is closely related to

4

the efficiency of facilitated transport. Proper electronegativity of metal is critical since the

5

complexation will be difficult to occur if the electronegativity is too low. On the contrary,

6

if the electronegativity is too high, the reverse reaction will be impeded, and CO2/olefin

7

will be hard to release from the carriers. Among metal ion carriers, Zn2+ with

8

electronegativity 1.65 is a potential candidate as CO2 facilitated transport carrier. It can

9

serve as a benchmark for the rational selection of metal ions. For the facilitated

10

transport of olefin, metal cations with electronegativity ranging from 1.6 to 2.3 are

11

preferred.103

12

For the separation of CO2/N2, CO2/CH4 and alkanes/olefins, membranes based on

13

classic solution–diffusion mechanism are insufficient to obtain a product with high purity.

14

The introduction of multiple transport mechanisms is a promising way to obtain MMMs

15

with outstanding separation performance, which is a noteworthy development trend.

16

The performance improvement of MMMs is mainly relying on the construction of


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1

physical and chemical microenvironment that can promote the transport of a specific

2

component. Size sieving effect from the FINs can also contribute to the enhancement of

3

selectivity. Their intrinsic pores and the polymer-FINs interfacial channel provide

4

external pathways for gas molecules to elevate permeability. Moreover, metal building

5

blocks or the functionalized organic moieties of the FINs or polymeric backbones can

6

facilitate the transport of a specific component. The following section will focus on

7

summarizing the application of FINs-based MMMs in carbon dioxide capture and the

8

separation of alkanes and olefins since these are regarded as two of seven chemical

9

separations to change the world.104

10

5.1 CO2 Capture

11

The increase of anthropogenic emissions of CO2 and other hydrocarbons poses an

12

environmental burden on our society. Capturing these gases from power plants, refinery

13

exhausts or air economically and environmentally is critical to realize sustainable

14

development of society. FINs-based MMM is a cheaper alternative method with minimal

15

energy cost.


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1

Zeolites are commercially available materials that have been used in industry for

2

more than 50 years. ZSM-5, zeolite 3A, zeolite 4A, silicoaluminophosphate-34 (SAPO-

3

34), silicalite 1, zeolite T, etc. are commonly used as FINs. SAPO-34 is a well-known

4

FINs because of its proper pore size of 0.38nm.105

5

prepared by Oyama and coworkers and incorporated into PEI to prepare MMMs. They

6

found that solvents used during the membrane fabrication had a great effect on the

7

separation performance since pore blockage of FINs occurred when solvent with

8

smaller molecular size (dichloroethane (DCE)) was used, leading to the reduction of gas

9

permeability. On the contrary, the membrane permeability was significantly higher when

10

the larger solvent (N-methyl-2-pyrrolidone (NMP)) was used. Besides, the aggregation

11

problem was also solved by modifying the surface of FINs with amine cations, which

12

can form hydrogen bonds with the polymeric matrix. The MMMs with modified FINs

13

displayed significantly improved selectivity compared with membranes with bare FINs

14

and pure polymer membrane. Zeolite T was also a potential FINs to fabricate MMM that

15

was used in CO2/CH4 separation. Shariff and coworkers incorporated as-synthesized

16

zeolite T nanoparticles into 6FDA-durene polymeric matrix directly and produced MMMs

SAPO-34 nanosheets were


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1

with enhanced gas separation performance.27 According to results of pure gas

2

permeation tests at the pressure of 3.5 bar and temperature of 303K, the prepared

3

MMM displayed enhanced permeability with negligible loss in selectivity. For the Zeolite

4

T/6FDA-durene membrane with 1 wt % zeolite T loading, the permeability of CO2 and

5

CH4 was 843.6, and 44.2 barrer, respectively, with ideal CO2/CH4 separation selectivity

6

of 19.1. CO2 permeability of the MMMs was 80% higher, and CO2/CH4 ideal selectivity

7

was 172% higher than that of the pristine 6FDA-durene, which meant that Zeolite-based

8

MMMs were attractive candidates for natural gas purification. Besides, the MMMs

9

demonstrated a remarkable improvement in plasticization resistance for CO2 at 20 bar,

10

which is three times higher than the pure polymeric membrane. Li et al. modified zeolite

11

surface using a novel silane coupling agent (3-aminopropyl) diethoxymethylsilane

12

(APDEMS).102 The effect of the addition of novel silane coupling agent on the

13

separation performance of MMMs was investigated. It was found that the permeability

14

and selectivity of MMMs made from APDEMS< 20 wt % modified zeolite were both

15

higher than that of unmodified zeolite. Apart from that, they also investigated whether

16

and how this coupling agent affected the reinforcement of polymer chains and partial


62

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1

pore plugging of the zeolite. The presence of APDEMS resulted in the distance of about

2

5-9 Å between the polymer chains and the zeolite surface, reducing the degree of pore

3

plugging caused by the polymer chains.

4

MOFs are more attractive porous materials since their organic ligands can be

5

modified to increase the interaction with the polymer and thus improve the separation

6

performance by increasing the interaction with one component. HKUST-1, ZIF, UiO, MIL

7

etc. are the most commonly studied FINs. For example, Lin et al. prepared HKUST-

8

1/IL/6FDA−durene polyimide MMMs by incorporating HKUST-1, a typical micro-sized

9

MOF that was decorated with IL layer into 6FDA-Durene.106 IL, as a MOF/polymer

10

binder, improved the affinity between the HKUST-1 and polymer significantly and

11

reduced the volume fraction of nonselective interfacial voids. In comparison with the

12

MMM that was incorporated with HKUST-I only, the HKUST-IL MMM thereby exhibited

13

enhanced gas separation performance with CO2 permeability of 1101.6 barrer and

14

CO2/CH4 selectivity of 29.3, which transcended 2008 upper bound. The HKUST-IL

15

MMM stands out among MOFs-based MMMs in the literature, showing great potential

16

for CO2 capture from natural gas or biogas. ZIFs such as ZIF-7, ZIF-8, ZIF-71, ZIF-67


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1

have been extensively investigated as FINs, too.50,

2

was created within the ZIF-8/polyimide MMMs by Kertik et al.109 In-situ thermal

3

treatment up to 350℃ with the existence of oxygen was conducted to form cross-linked

4

polymeric matrix and to improve the polymer-filler interfacial quality by inducing

5

chemical crosslinking between the polymer and the imidazole groups. Notably, full

6

oxidation of the FINs was prevented with the protection of the polymeric matrix under

7

this high temperature, leading to the formation of amorphous structures with better size

8

sieving effect for the separation of CO2/CH4. More importantly, the molecular sieve

9

network endowed ZIF-8/polyimide MMMs with a breakthrough in stability and resistance

10

to plasticization ability (40 bar). The amorphous ZIF-8/polyimide MMMs also showed the

11

highest CO2/CH4 selectivity among commercial polymer membranes, which made it

12

possible to achieve higher selective separation in gas separation. The MIL series MOFs

13

such as MIL-53, MIL-101 have also been studied widely. The separation performance of

14

MIL-53 based MMMs is closely related to the fabrication conditions because of the

15

flexible structure and the ‘breathing effect’. NH2-MIL-53/polyimide MMMs with different

16

FINs loadings and thicknesses were fabricated by Rodenas et al. to investigate the

107-109

A molecular sieve network


64

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1

relationships between structure and performance.110 They found that the final

2

configuration of the NH2-MIL-53 within the MMMs was determined by the solvent

3

evaporation rates. Narrow pore configuration that contributes to the enhancement of

4

separation performance is more likely to appear in thin membranes fabricated under

5

higher operation temperature, which is equal to fast solvent removal. NH2-MIL-53(Al)

6

with three different morphologies were also incorporated into Matrimid® and 6FDA-DAM

7

by the same groups to study the impact of the filler morphology on the gas separation

8

performance.111 The MMMs that were incorporated with 20 wt% nanoparticles showed

9

the most considerable performance improvement that was close to the Robeson limit:

10

permeability up to 660 barrer and separation factor for CO2/CH4 up to 28.

11

Besides zeolite and MOFs, COFs have also been used as FINs to fabricate MMMs

12

with excellent gas separation performance. For example, Jiang and coworkers made

13

PIM-1/SNW-1 mixed matrix membranes to separate CO2/CH4 and CO2/N2.112 They did

14

not find any visible agglomeration or defective voids because of the small sizes of the

15

FINs and the high compatibility between SNW-1 and PIM-1. The MMMs showed 27.4%

16

and 37.6% improvement in selectivity for CO2/CH4 and CO2/N2, respectively. What is


65


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1

worth mentioning is that the permeability of CO2 was increased up to 116%.

2

Furthermore, Biswal et al. made TpPa-1@PBI-Bul and TpBD@PBI-Bul MMMs with

3

highly loaded COFs (≈50%), which is substantially higher than most MOFs-based

4

MMMs.38 The effect of pore sizes of COFs on gas separation performance was

5

investigated. They found that TpBD possessing larger pore size of 1.8nm could elevate

6

the permeability more prominently due to the reduction of diffusion resistance for all

7

kinds of gases. CO2/CH4 selectivity of the MMM is still appreciable at a high loading of

8

50 wt % with an almost seven times elevation in permeability for CO2 compared with

9

pristine PBI-BuI. Besides, NUS-2 and NUS-3, two water stable COFs that were

10

exfoliated into nanosheets of 50-100 nm, were also incorporated into polybenzimidazole

11

(PBI) to fabricate MMMs with loadings up to 30 wt % by Zhao and coworkers.7 In the

12

MMMs with lower loading of COFs (10-20 wt %), the solubility and diffusivity of CO2 and

13

CH4 were both improved because of the affinity between COFs and gas molecules and

14

the increased free volume introduced by the FINs. However, an obvious improvement

15

was observed for CO2 transport, leading to the increase of CO2/CH4 selectivity. Notably,

16

NUS-3 based MMMs exhibited comparable selectivity for CO2/CH4 separation with a


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1

much larger permeability of CO2 thanks to the larger pore size compared with NUS-2.

2

When being applied for H2/CO2 separation, the as-prepared MMMs with 20 wt %

3

loading exhibited significantly enhanced selectivity thanks to the increased diffusivity of

4

H2 and selective CO2 sorption of the COF fillers. The outstanding performance indicates

5

that 2D crystalline COFs are potential candidates to prepare MMMs. Recently, 3D-COF

6

with secondary amine with its backbone has also been designed and incorporated into

7

6FDA-DAM polymide to prepare MMMs for CO2/CH4 separation.113 The membrane with

8

15 wt % loading of COF displayed a significant enhancement in CO2 permeability (up to

9

140%) without any loss of selectivity. Furthermore, severe physical aging of the glassy

10

polymeric matrix was relieved effectively thanks to the immobilization effect of amine

11

functional groups: 97% of the initial membrane performances was maintained after

12

aging for 240 days.

13 14

5.2 Alkanes/olefins separation


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1

The separation of alkanes and olefins is a major branch of chemical separation. The

2

global demand for ethane and propene is as high as 200 million tonnes per year since

3

they are important raw materials to produce plastic. Separating them in industry mainly

4

relies on cryogenic distillation (high pressure, −160℃), which consumes up to 0.3% of

5

global energy use.104 With the rapid development of membrane technology, FINs-based

6

MMMs have been a potential alternative to obtain high purity olefins economically.

7

Up to now, MOFs are widely studied as FINs since their open metal sites can provide

8

binding interaction with olefins through π electron systems, which can increase the

9

solubility coefficients of olefins and accordingly increase the solubility-based selectivity.

10

For example, Bachman et al. incorporated MOF-74 nanocrystals with different metal

11

sites into polyimide to prepare FINs-based MMMs and explored the effect of different

12

metal sites on the membrane performance. MMMs with Co- and Ni-MOF-74 displayed

13

improved permeability and selectivity for the separation of ethylene and ethane while

14

MMMs with Mg- or Mn-MOF-74 exhibited improvement only in permeability. The

15

phenomenon can be ascribed to the increased interaction between the exposed metal

16

sites and the surrounding polymer along with the series Mg < Mn< Co < Ni. Therefore,


68

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1

Co- and Ni-MOF-74 were well dispersed within the polymeric matrix thanks to the

2

additional interaction with the polymer while the other two tended to aggregate and form

3

interfacial voids.114

4

ZIFs is another kind of FINs that have been intensely investigated to improve the

5

alkanes/olefins separation performance of MMMs. Among them, ZIF-8 is the most

6

popular one because its aperture size (0.34nm) can discriminate penetrants selectively,

7

improving the diffusivity-based selectivity.115 It has been demonstrated that the

8

diffusivity of propylene was 125 times faster than that of propane.116 Several works have

9

been reported using ZIF-8 as FINs. For instance, Askari et al. incorporated nano-size

10

ZIF-8 into 6FDA-Durene/DABA (9/1) by solution mixing to fabricate MMMs.115 The

11

prepared membrane showed great potential in industrial C3H6/C3H8 separation with

12

propene permeability of 47.3 barrer and significant improvement in C3H6/C3H8 ideal

13

selectivity from 11.68 to 27.38 at the loading of 40 wt%. Moreover, the membrane also

14

exhibited superior plasticization suppression characteristics due to the crosslinking of

15

the carboxyl acid in the DABA. Similarly, Zhang et al. also reported ZIF-8 based MMMs

16

(48.0 wt % ZIF-8 loading) with significant enhancement in ideal propane/propene


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1

selectivity (31.0) and propene permeability (56.2 barrer), which was 150% and 258 %

2

higher than the pure polymer membrane, respectively.117 Other ZIFs such as ZIF-4 have

3

also been studied yet.118

4

Cu3BTC2 is an emerging kind of FINs in separating Olefin and paraffin since it

5

possesses preferred dissolution of olefin over paraffin due to the interaction between

6

the exposed Cu(II) metal sites with the olefin.119 Therefore, this FINs has great potential

7

to improve the permeability of ethylene. In one study, Cu3BTC2 was incorporated into

8

P84 to fabricate MMMs.120 The resultant MMMs with 20 wt % Cu3BTC2 exhibited

9

increased ethylene/ethane selectivity up to 73% while affording almost unchanged

10

permeability. Results of the sorption experiments showed that the solubility coefficients

11

of ethylene increased with the increasing loading of Cu3BTC2 while the diffusion

12

coefficient of this component exhibited opposite trends, indicating the facilitated

13

dissolution of ethylene owing to the strong interaction with the Cu metal sites and the

14

hindered diffusion of both two components by the immobilized ethylene within the

15

MMMs. This mechanism was further confirmed by the slight reduction of permeability

16

resulting from the feed pressure increase.


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1


6. Conclusion and Outlook

2

Over the past few years, researches on MMMs with high performance have been

3

flourishing with the significant progress on FINs — from inorganic, inorganic-organic to

4

purely organic types. The polymeric matrix provides the MMMs with excellent

5

processability while the intrinsic pores provide MMMs with high permeability and

6

selectivity Although MMMs possess distinct advantages compared with pure

7

polymeric/inorganic membranes, the poor dispersion of FINs within the polymeric matrix

8

still exists due to the limitation of their inherent property, especially for zeolites.

9

This review summarizes the fabrication method for FINs-based MMMs, including

10

membrane formation with and without chemical reaction. Different efforts to promote

11

FINs dispersion have been reviewed. To sum up, a uniform casting solution is the

12

prerequisite to obtain a uniform dispersion of FINs in the MMMs for the conventional

13

preparation method. ‘solution mixing to avoid the drying of FINs’ and ‘reducing the

14

interaction of inter-FINs by polymer coating’ can often achieve well dispersed casting

15

solutions. When preparing MMMs, smaller-size fillers, layered fillers displayed better


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1

dispersibility owing to the larger contact area and more interaction sites with the

2

polymer. Also, the filler and/or the polymer can be modified, functionalized or coated

3

with a layer of polymer, so that hydrogen bonds, π-π stacking interactions or covalent

4

bonds can form between the filler and the polymer. The synergistic effect of composite

5

fillers can also prevent filler aggregation. Moreover, some novel preparation methods

6

such as in-situ FINs growth can overcome obstacles of conventional methods. These

7

ways can be combined to improve the dispersion of fillers further.

8

Despite the great progress of MMMs, further efforts are still required. Some

9

challenges and opportunities are highlighted as follows: (1) Membrane material: Organic

10

FINs such as COFs is a new research direction. Compared with zeolite and MOFs, the

11

enhanced chemical stability arising from the covalent bonds and the excellent

12

compatibility with the polymeric matrix due to the organic structure means that COFs

13

deserve vigorous exploitation. However, the application of COFs is limited by its pore

14

size (from 0.46 nm to 4.7 nm), which is unsuitable for discriminating gas mixtures with

15

different sizes.121-124 Therefore, the development of novel effective methods to

16

synthesize COFs with smaller pore size and reduce the pore size after synthesis is of


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1

top priority so that size-selective channels can be constructed with the MMMs. Besides,

2

some polymers possessing size-sieving effect and high permeability such as polymers

3

of intrinsic microporosity also deserve further research attention. (2) Membrane

4

fabrication: proper matching of polymer and FINs is significant for fabricating MMMs

5

with high performance. A huge variety of polymer materials and FINs make the rational

6

design very difficult. Although some computational methods have been used to simulate

7

the interaction of FINs and polymer and some models125-127 such as Maxwell’s

8

equations and its derivative have been proposed to predict the separation performance

9

of the MMMs, a more rational combination of the FINs and polymer and a deeper

10

understanding of polymer-FINs microenvironment are still needed. Besides, although

11

some novel fabrication methods can avoid the drawbacks encountered in conventional

12

physical blending method, they are usually more complex and their large-scale

13

preparation remains a great challenge. Also, the cost of compression equipment in

14

recycling operation is the largest in most industrial membrane processes, which means

15

high membrane permeance is critical and ultrathin membranes (0.1-1.0 μm) are

16

required. However, most reported MMMs are evaluated as 50-150 μm thick and the


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1

formation of defects occurs easily when the membrane thickness is reduced.128

2

Therefore, how to fabricate thin and defect-free membrane needs to be further explored.

3

(3) Membrane structure: compared with polymeric and inorganic membranes, the

4

structure of FINs-based MMM is the most complicated. With the incorporation of FINs,

5

more transport channels are constructed by the intrinsic nanopores of filler and

6

interfacial area. Although SEM, TEM, positron annihilation lifetime spectroscopy

7

(PALS), tomo-graphic focused ion beam scanning electron microscopy (FIB-SEM), etc.

8

have been widely applied to observe the interfacial morphology and filler dispersion,

9

they are not sufficient to elucidate the interfacial state directly and quantitatively. At

10

present, we often use separation performance or mechanical properties as indirect

11

evidence to determine whether there are defects at the interface. It remains a challenge

12

to clarify the relationship between the structure and performance of the MMMs

13

quantitatively. To promote the development of MMMs, direct characterization technique

14

and judgment basis are highly required. (4) Mechanisms: molecule transport

15

mechanism is critical to design MMMs with superior performance rationally. Molecular

16

transport process within the membrane is influenced by multiple transport mechanisms.


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1

Solution-diffusion mechanism, size-sieving mechanism and facilitated transport

2

mechanism are generally coupled within the membrane. However, analysis of mass

3

transport based on different mechanisms is still qualitative and semi-empirical.

4

Quantitative analysis has not been achieved yet. Therefore, more rational integration

5

and optimization of multiple transport mechanisms are needed.

6

In summary, with the rapid advancement of FINs with diverse categories, functions

7

and structures, FINs-based MMMs have held great promise in gas separation. Further

8

investigations of FINs-based MMMs will be crucial and should be highly encouraged to

9

exploit their potentials so that they can be utilized more broadly and efficiently.

10


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Page 76 of 90

1

ASSOCIATED CONTENT

2

Supporting information

3

The Supporting Information includes separation performance of some representative works using

4

different methods to improve FINs dispersion.

5

AUTHOR INFORMATION

6

Corresponding Author

7

* E-mail: [email protected] (Z.J.)

8

Notes

9

The authors declare no competing financial interest.

10

ORCID

11

Zhongyi Jiang: 0000-0002-2492-4094

12

ACKNOWLEDGMENT

13

This work was supported by the National Key R&D Program of China (No.

14

2017YFB0603400), State Key Laboratory of Organic-Inorganic Composites (oic-

15

201701004), National Key Laboratory of United Laboratory for Chemical Engineering


76

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1

(SKL-ChE-17B01), National Natural Science Foundation of China (No. 21838008,

2

21621004 and 21878215), State Key Laboratory of Separation Membranes and

3

Membrane Processes and Tianjin Polytechnic University (No. M1–201701), State Key

4

Laboratory of Petroleum Pollution Control (No. PPC2017014). National Science and

5

Technology Major Project (2016ZX05025-004-006), Tianjin Key R&D Program Science

6

and Technology Support Key Project (17YFZCGX00310).

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