recent advance of inorganic fillers in mixed matrix membrane for gas separation

Separation and Purification Technology 81 (2011) 243–264

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Separation and Purification Technology

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Review

Recent advances of inorganic fillers in mixed matrix membrane for gas separation

P.S. Goh a, A.F. Ismail a,b,⇑, S.M. Sanip a,b, B.C. Ng a, M. Aziz a,c

a Advanced Membrane Technology Research Centre (AMTEC), Universiti Teknologi Malaysia, 81310 UTM, Skudai, Johor, Malaysiab Faculty of Petroleum and Renewable Energy Engineering, Universiti Teknologi Malaysia, 81310 UTM, Skudai, Johor, Malaysiac Faculty of Science, Universiti Teknologi Malaysia, 81310 UTM, Skudai, Johor, Malaysia

a r t i c l e i n f o

Article history:Received 8 June 2011Received in revised form 26 July 2011Accepted 31 July 2011Available online 6 August 2011

Keywords:Mixed matrix membranesGas separationInorganic fillers

1383-5866/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.seppur.2011.07.042

⇑ Corresponding author at: Advanced Membrane(AMTEC), Universiti Teknologi Malaysia, 81310 UTM,+60 7 5535592; fax: +60 7 5581463.

E-mail address: [email protected] (A.F. Ismail).

a b s t r a c t

Mixed matrix membrane (MMM) is a new class of membrane materials that offers the significant poten-tial in advancing the current membrane-based separation technology. As an attractive material that dem-onstrates outstanding separation properties, MMM has been the subject of worldwide academic studiesconducted by many researchers especially those related to membrane technology. The past decades havewitnessed substantial progress and exciting breakthroughs in both the fundamental and applicationaspect of MMM in various forms of separation, particularly in gas separation. These emerging materialsfor separation have been traditionally accomplished by incorporating conventional inorganic fillers suchas zeolite, carbon molecular sieve and silica nanoparticles in a polymer matrix. The recent advances haveshifted towards the introduction of new and novel materials namely carbon nanotubes, metal organicframework and clay layered silicate as potential fillers in the polymer matrix. The successful implemen-tation of MMM depends greatly on the polymer matrix selection, the inorganic filler as well as the inter-action between the two phases. The selection of suitable types of inorganic filler, the surfacemodification, and the performance of the resulted MMM membranes were discussed and representedthe major contribution in this review. The recent efforts to tackle the underlying problems and the effectsof various kinds of modification that would eventually heighten the performance of membrane applica-tions in gas separations were discussed. Better understanding on the improvement and optimization ofMMM process was provided by considering the possible solutions to overcome the problems encounteredduring MMM preparation. This hybrid system holds significant potential and great promise for furtherinvestigations, development and applications. The future direction and perspective in MMM researchfor gas separation was also briefly outlined to further advance the materials for MMM in gas separation.

� 2011 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2442. Mixed matrix membrane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2453. Inorganic fillers as dispersed phase in MMMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2464. Conventional fillers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247

4.1. Zeolite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2474.2. Carbon molecular sieve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2494.3. Silica. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2504.4. Metal oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252

5. Alternative fillers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253
5.1. Carbon nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2535.2. Layered silicate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2565.3. Metal organic framework . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258
6. New and emerging material for MMM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260
6.1. Graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260
7. Conclusion and future direction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261

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244 P.S. Goh et al. / Separation and Purification Technology 81 (2011) 243–264

Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262

1. Introduction

Membrane technology is an attractive separation approach thathas been studied extensively as a result of the fast and energy effi-cient process without any phase changes [1–3]. Membrane-basedseparation involves the use of membrane as a thin barrier betweenmiscible fluids to separate a mixture. This interface may be molec-ularly homogeneous in which it is completely uniform in composi-tion and structure, or it may be chemically or physicallyheterogeneous in which pores or layered structures are formed.During the separation processes, a suitable force such as concen-tration or pressure differential is applied to allow preferentialtransport of one or more feed component across the membrane.Generally, the permeation and selectivity are the two common ba-sic performance characteristics of a membrane. In a broader con-text of definition, the permeability is the ability of the permeatesto pass through a membrane meanwhile the ratio of permeabilityof the more permeable component to that of the less permeableis known as selectivity of the membrane. Higher permeability de-creases the amount of membrane area required to treat a givenamount of gas, thereby decreasing the capital cost of membraneunits meanwhile higher selectivity will result in a higher puritygas product.

Membrane is a promising candidate for separation due to theadvantages offered by the process which include high stabilityand efficiency, low operating cost and capital, low energy require-ment and also ease of operation [4–7]. Furthermore, it is alsoknown for its reliability to be used in remote locations as mem-brane has no moving parts and thus making it mechanically robust.Since the last few decades, membrane-based separation hasemerged as a promising process for various industrial applications,particularly in gas separation industries for air separation, hydro-gen recovery and CO2 removal [8]. Other well established mem-brane processes in chemical and petrochemical industries arelisted in Table 1. One of the most important characteristics ofmembranes is the ability to control the rate of permeation of differ-ent species. The mechanism of permeation can be described usingtwo models, i.e. solution-diffusion model and pore-flow model [9].Diffusion is the basis of the solution-diffusion model in which thepermeable components dissolve in the membrane materials andthen transport across by a concentration gradient. On the otherhand, the pore-flow model describes the transportation of perme-able components by pressure-driven convection flow through tinypores in the membrane materials. In general, two types of gas sep-aration processes have been encountered, i.e. gas permeation and

able 1arious applications of membrane-based gas separation in industries.

Gas separation Applications

O2/N2 Oxygen enrichment, inert gas generationH2/hydrocarbon Refinery hydrogen recoveryH2/N2 Ammonia purge gasH2/CO Syngas ratio adjustmentCO2/hydrocarbon Acid gas treatment, greenhouse gas captureH2O/hydrocarbon Natural gas dehydrationH2S/hydrocarbon Sour gas treatingHe/hydrocarbon Helium separationHe/N2 Helium recoveryHydrocarbon/air Hydrocarbon recoveryH2O/air Air dehumidification

gas diffusion. The former process is of great industrial interestwhere it is a pressure driven process in which the vapor compo-nents pass through a non-porous membrane by a solution-diffu-sion mechanism. Meanwhile, gas diffusion process can be donefor the microporous membranes that operate under a concentra-tion or partial pressure gradient.

The successful application of membranes in an identified gasseparation process, in both lab and industrial scale, greatly de-pends upon the selection of membrane materials with the appro-priate chemical, mechanical and permeation properties. Themajor issue of current membrane manufacturing to researchers isto develop highly selective and permeable membrane materialswhich also show sufficient resistance towards the feed compo-nents as well as towards the operation condition [10]. Gas separa-tion using polymeric membrane has achieved important success ashigh separation performance membrane since the first commer-cial-scale membrane gas separation system which was producedin the late 1970s [11]. Polymeric membranes, particularly thoseprepared from glassy polymers, have received considerable atten-tion because they possess advantages of mechanical properties,reproducibility and relative economical processing capability[12]. Polymeric membranes can be classified as: porous and non-porous membranes. A porous membrane has rigid, highly voidedstructure with randomly distributed, interconnected pores. Separa-tion in these kinds of membranes is dependent on both the molec-ular size and pore size distribution. Non-porous membranes ordense membranes consist of a dense film in which the permeatemolecules are absorbed and followed by diffusion through themembrane matrix under the driving force of a pressure, concentra-tion, or electrical potential gradient. The diffusivity and solubilityof the permeant molecules in the membrane material play a signif-icant role to determine the mechanism of the gas transport. Amongthe many types of polymers that exhibit gas separation propertiesfor gas mixtures, a few glassy polymers such as polysulfone (PSf),polyethersulfone (PESf), polyetherimide (PEI) and polyimide (PI)have been recognized as promising polymers with respect to theirpermeability and selectivity. Despite their suitability for variousapplications in research and commercialization, polymeric mem-branes are still ineffective in meeting the requirement for the cur-rent advanced membrane technology as these materials havedemonstrated a trade-off between the permeability and selectivity,with an ‘upper-bound’ evident as proposed by Robeson. A correla-tion of membrane separation data offering an analysis of the limitsof polymer permeability and selectivity is often referred to as theupper boundary in which the gas separation properties of the poly-meric membranes follow distinct trade-off relations where morepermeable polymers are generally less selective and vice versa[13,14]. The revisited upper bound relationship by Robeson forO2/N2 membrane separation for polymeric membranes is illus-trated in Fig. 1 [15]. Apart from that, polymeric membranes arealso suffering in their limited solvent and poor chemical and ther-mal resistance, as well as the occurrence of swelling phenomenonthat subsequently alter the membrane separation properties [16].

Due to the susceptibility of polymeric membranes to chemicaldegradation and thermal instability, their applications have beenlimited to separation processes where hot reactive gases are notencountered. In order to combat the inherent limitations of poly-meric membrane, research is underway for alternative membranematerials. In this context, inorganic membranes that are formedfrom metals, ceramic or pyrolyzed carbon has attracted global

Fig. 1. Revisited upper bound data for O2/N2 membrane separation [15].

Dense structure

Inorganic fillers

Support layer Dense skin

(a)

(b)

Fig. 2. Mixed matrix membranes in configuration: (a) symmetric flat dense mixedmatrix membrane. (b) Asymmetric hollow fiber.

P.S. Goh et al. / Separation and Purification Technology 81 (2011) 243–264 245

interest as they offer several advantages over the polymeric mem-branes for many gas separation processes. Inorganic membranesare increasingly being explored due to their attractive characteris-tics and advantages over the polymeric membranes [17–20]. Ingeneral, inorganic membranes can be categorized as dense andporous. Porous inorganic membranes such as zeolite and carbonmolecular sieve (CMS) are favourable, rendered by their excellentselectivity which is significantly higher compared to that of poly-meric membranes. Dense inorganic membranes possess very spe-cific separation behaviours, for example metallic membranes thatare hydrogen or oxygen specific [21]. The ability to withstand hightemperature for long time and the resistance to harsh operationenvironment have shown promise for application in membranereactors for many industrial processes. Besides having these robustcharacteristics, inorganic membranes also have much higherthroughput and longer life span compared to polymeric mem-branes. While attracting considerable attention in many industrialprocesses, some disadvantages exhibited by inorganic membranesare very critical and serious to be addressed. Although some inor-ganic materials display properties well above the trade-off curvefor polymers, such properties are challenging to duplicate inlarge-scale modules containing thousands of square meters ofmembrane areas as it involves expensive capital and repairing cost.Furthermore, the nature of inorganic membrane that is brittle andnormally with low surface-to-volume ratio has also hampered thefull optimisation of this material for industrial applications [17].

Many new attempts have been made to overcome the limitationof polymeric and inorganic membranes. Introduction of new mem-brane materials is still a big challenge to be researched in this fron-tier technology, with most researchers looking at techniques toenhance the separation processes. So called mixed matrix mem-brane (MMM), where inorganic fillers are dispersed at a nanometerlevel in a polymer matrix have been identified to potentially pro-vide a solution to the trade-off issues of the polymeric membrane,as well as solving the inherent brittleness problems found in inor-ganic membranes, is illustrated in Fig. 2. The choice of the polymer,inorganic phase and filler particle loading are some of the impor-tant parameters affecting the morphology and performance ofthe MMM. The successful implementation of the further develop-ment of MMM greatly lies on these parameters. In order to choosethe appropriate continuous polymer phase and dispersed inorganic

phase, the transport mechanisms and the gas component preferen-tially transporting through the membranes should be first takeninto consideration [22]. To obtain remarkably well performingMMM that surpass the intrinsic properties of the polymer matrix,the role of inorganic filler as the molecular sieve that favors onecomponent while hindering other components to pass through itis of importance. Molecular sieves such as zeolite and CMS havebeen conventionally packed in the polymer phase to form a denseregion of mixed matrix layer. These nanoporous materials possessthe shape and size selective nature and hence allow molecularsieving discrimination by permitting smaller sized penetrates todiffuse at higher rate than that of larger sized. Recently, alternativenano-structured materials such as carbon nanotubes (CNTs), clayand metal organic framework (MOF) have also found their excel-lent sieving characteristic in the fabrication of MMM. However, de-spite the promising application of these materials as inorganicfiller, the existence of adequate contact between the polymerphase and dispersed filler to eliminate the interfacial defects is al-ways a major issue to be addressed in order to achieve the desiredas well as improved performance of the resulting MMM.

The current contribution reviews recent scientific and techno-logically advances in the application of different types of inorganicfiller materials in the fabrication of MMM. As a review of the state-of-the-art in MMM using various kinds of fillers, the progress thathas already been made so far will be explored. The membraneproperties, particularly gas separation performance of the MMMusing both conventional and alternative materials as the inorganicphase are highlighted and discussed. This review article is meant tostimulate the interest and enthusiasm towards the currently well-identified as well as the under exploration research on MMMamong the membrane research community. Aside, it is also aimedto impart greater understanding and hence assess the effectivenessof each class of materials in tuning the intrinsic properties of thepolymer phase to form a MMM with improved and desired separa-tion performance. The conclusion and future outlook summarizethe research activities and present future research directions.

2. Mixed matrix membrane

Since the pioneering literature by Zimmerman et al. [23], sev-eral significant review papers on the prospects of MMM as wellas the research reports on the capability of MMM as alternativemembrane materials for separation processes have been well pub-lished [22,24–27]. Much research into potential new materials for


gas separation membranes is driven by the limitation and disad-vantages which is in existence in the current available polymericand inorganic membranes. Few approaches have been carried outto optimize the structure of the polymer chains in order to improvethe gas diffusivity by increasing the volume fraction with restric-tive or selective channels communicating the voids. In additionto that, attempts have also been made to increase the solubilityof permeate molecules by introducing certain chemical affinityfor a gas to allow adsorption of certain moieties of the polymer[28]. As the consequences of the efforts made, MMM, a heteroge-neous membrane consisting of inorganic filler embedded in a poly-mer matrix, have emerged as an alternative approach in the state-of-art membrane based separation technology.

MMMs are polymer composites that involve phase separatedmembrane separation system. In this structurally engineered hy-brid membrane, inorganic fillers act to create preferential perme-ation pathways for selective permeability while posing a barrierfor undesired permeation in order to improve separation perfor-mance [29]. The superior permeability and selectivity of inorganicmembranes with the processability of polymeric membranes arecombined to achieve synergistic separation performance, in whichthe rigid adsorptive porous type inorganic phase provides superiorseparation properties and the polymeric phase enables the idealmembrane forming hence solving the problem of fragility inherentfound in the inorganic membranes. A wide range of commercial-ized polymer matrix choices exist within the so-called triangularcross hatched region formed by the upper boundary such as Udel,Matrimid and Ultem have been well known for their practicalapplications and are readily available for the incorporation ofhighly selective or permeable dispersed phase to further enhancethe current separation performances [23]. The incorporation of fill-ers with molecular sieving properties in the polymer matrix is ex-pected to lead to higher permeability, higher selectivity, or bothcompared to the polymeric membranes. The incorporation of inor-ganic fillers to the polymer matrix may involve (i) spherical or lowaspect ratio fillers or (ii) filler thin platelets with very high ratio[30]. The latter case is of recent interest as it offers improved sep-aration properties over the conventional case [14]. Although inor-ganic fillers are used in all cases of preparation, the resultingmorphology and membrane separation performance can be variedgreatly, primarily arise from different capability of these molecularsieves, either based on shape or size, to discriminate between dif-ferent molecules present in the feed mixture.

Typically, MMMs are easy to process and manufacture as com-pared with the inorganic membranes. They are normally preparedby casting a solution of polymer and dispersed inorganic phase,and then evaporating the solvent in a controlled environment toobtain a dense membrane. The critical reviews on the recent devel-opment and application of MMM have revealed the viability ofthese materials as an alternative route for the currently existingpolymeric and inorganic membranes in industrial applications[22,23,27]. However, new fabrication approach is still in urgentneed to reduce materials cost and provide greater permeabilityfor further modifications and improvements [31]. The significantbreakthrough on MMM fabrication in useful configurations isundoubtedly essential. Besides that, the comprehensive researchon MMM should also be devoted to the successful integration ofthe inorganic fillers to the polymeric phase [32].

3. Inorganic fillers as dispersed phase in MMMs

The incorporation of inorganic fillers into polymer phase cansignificantly alter the transport properties of one or several gasesthrough the MMM when compared to that of neat polymeric ma-trix. The gas transport properties can be enhanced in several ways.

First, the incorporated filler particles could modify the propertiesof the neighbour polymer phase, which will favor the overall trans-port properties. This effect is especially significant for particles thatare well distributed within the polymer matrix, where the frac-tional free volume can be greatly modified [33]. It is also wellknown that the presence of filler particles may alter the packing,dynamics or conformation of polymer chain near its surface, andimpart great effect on the transport of large penetrates relativeto small ones. Therefore, the noticeable effect on selectivity ofthe gas pair involving gases with significant size difference likeCO2/He can be observed. Apart from that, according to Zimmermanand Koros [23], the interface characteristics of the interface be-tween the inorganic particles and the polymer matrix could deter-mine the path to be passed through by one of the gases over theothers. This characteristic allows the selective transport of certaingases hence improved selectivity is expected, or in some cases, itcan also result in increased permeability by reducing the pathlength of the permeate molecules.

Cages with molecular dimensions open a room for separation totake place, in which the shape and size of these voids can controlsorptive selectivity of the favoured permeating species. The bestestablished example of porous materials with these sieving charac-teristics is the micro-scaled alumino-silicate zeolites and carbonmolecular sieve that have been traditionally featured by theirintrinsically high separation capacities. In fact, the formation andgas separation properties of MMM using these conventional inor-ganic fillers have been well documented [22,34]. On the otherhand, another category of nano-scaled fillers, such as silica, CNTsand layered silicate clay are normally featured by intrinsicallylow separation capacities. Nevertheless, a larger interfacial area be-tween the fillers and the polymer matrix per unit volume of thefillers can be achieved and have recently attracted considerableattention due to their superior permeation properties [35]. Gener-ally speaking, the variation in the structures and features of thesefillers may subsequently produce a difference in the separationperformance behavior of the resulting MMM.

One of the main challenges encountered during the preparationof MMM is to control the chemical structure, surface chemistry aswell as the particle types of the inorganic phase that would poten-tially affect the membrane performance. The preparation of MMMusing polymer continuous phase has been reported to be compli-cated due to the particle agglomeration during solution prepara-tion in which mixing techniques are necessary to break upparticle agglomerates prior to the membrane processing. Whileglassy polymers possess a much attractive separation property rel-ative to rubbery polymers due to their high free volume, the poorpolymer chain mobility during the membrane formation may re-sult in a weak interaction between the filler particles and the con-tinuous polymer phase. The interaction between the polymer andfiller particles is of concern, as the undesirable channels may becreated between both phases if the polymer chains do not com-pletely interact with the filler particles. Classification, which is de-fined as fillers separating out from the membrane matrix due toincompatibility and forming separate filler phases or layers duringthe formation of MMM may deteriorate the gas separation perfor-mance of the membranes [36]. Fig. 3 illustrates the void formationat the sieve-matrix interphase. The formation of these non-selec-tive voids at the interface allows bypassing of gases hence deteri-orates the selectivity of the MMM [37–41]. To overcome thisproblem, the presence of a bridging agent to facilitate good inter-facial interaction is of crucial importance (Fig. 3c). In addition, ap-proaches involving modification of surface hydrophobicity,creating surface roughness as well as the varying of particle com-position and pore architecture have also been investigated[38,42]. Another challenge that could be faced during the prepara-tion of MMM is the partial blockage of the filler pore by polymer

(a) (b) (c)

Fig. 3. Interphase properties of MMMs: (a) void formation between the phases and (b) bridging of the filler and polymer matrix upon surface modification.


chains rendering the role of the incorporated filler [43]. In order tominimize the adverse effect and to deal with the abovementionedchallenges, the well dispersion and incorporation of the inorganicparticles as individuals in the polymer matrix is one of the mostimportant tasks to be addressed. In such circumstance, surfacechemistry of the particles may need to be altered using varioustreatment approaches.

4. Conventional fillers

Over the past decades, in order to establish MMM with highergas separation performance relative to the neat polymeric andinorganic membranes, various inorganic materials have been ex-plored and numerous have been identified as potential filler inmembrane application. The most heavily researched type inorganicfillers are probably zeolite, CMSs and silica nanoparticles whichhave been traditionally incorporated into the polymer matrix.These fillers are known for their versatility from the point of viewof chemical composition, particle shape and possibility of chemicaladaptation when embedded in polymer matrix. Particularly forzeolite and CMS, the potentials are granted by their defined porestructure that may contribute to enhanced gas selectivity. Prepara-tion of MMM using these conventional fillers have received signif-icant attention as they have offered opportunities for advancedseparation that transcend upper bound without increasing the en-ergy and processing cost. However, industry applications of theseconventional fillers in the preparation of MMM is currently atthe bottleneck where the simultaneous increase in permeabilityand selectivity that exceed Robeson’s trade off boundary is hardlyachieved due to the morphological imperfection created in theMMM, in particular the lack of interfacial interaction betweenthe two different phases. To bring the effect of these fillers intoplay, modifications through surface treatment are of urgent inter-est. As mentioned earlier, several detailed reviews on MMM fabri-cated using these conventional fillers have been available. Thus,the following sub-sections will focus on the recent developmentof MMM using conventional zeolite, CMS and silica, with theemphasis given to the surface modification of these fillers. The re-cent work on modification of the fillers and the effects of thesemodified fillers on polymer’s segmental mobility, packing andinterchain interactions as well as the gas transport properties willbe discussed. Insight into the evolutions of the recent developmentof MMM based on these conventional fillers offers a clear pictureon how the attractive properties of the fillers can be further fine-tuned for excellent separation performance.

4.1. Zeolite

Zeolites are crystalline, hydrated aluminosilicate with openedthree-dimensional framework structures, regular intracrystallinecavities and channels of molecular dimension. Zeolites possess

interesting physical and chemical properties and one of the mostremarkable properties, the sorption and diffusion properties ofzeolite, are due to the presence of different size channels and cav-ities that are related to free space or void volume. Since the last fewdecades, the improvement in MMM performance using zeolite asthe dispersed phase has resulted in the commercial alternativeover the polymeric and inorganic membrane. The use of zeolitein the formation of MMM as potential filler for gas separationmembranes has received numerous attentions due to their thermalstability as well as their promising separation and transport prop-erties. Shape selectivity and specific sorption characteristics of zeo-lite can be combined with easy processability of polymer toprovide desired properties in zeolite filled MMM. Numerous num-bers of reports have indicated the favorable effects of employingzeolite as the dispersed phase to improve the permeability andselectivity of various polymers for gas separation of different gaspairs [34,44–52]. The gas transport properties of a zeolite-filledMMM rely strongly on the intrinsic properties of the zeolite parti-cles and the dense polymer phase. The expected improvement ofincorporating zeolite in the polymer matrix is gained from sus-pended zeolite particles that are incorporated into the dense poly-mer matrix. These particles are selective for one permeatecomponent over another. Thus, the undesired components typi-cally travel a more tortuous path around the zeolite particles,thereby decreasing mobility for that component and hence re-sulted in the increase of overall selectivity for the desired species.Early attempts in fabricating MMM using zeolite as dispersedphase focused on the use of common zeolites such as zeolite-A.The proper selection on the type of zeolite is vital to obtain highperformance MMM. Previous researches evidenced that the trans-port properties of MMM is significantly affected by the type of zeo-lite used with pore size ranging from 4 to 10 Å [49,51,53]. Thiswide range of zeolites can separate gas components either by sizediscrimination or selective adsorption, depending on the sievingcharacteristics of the particles. As an example, zeolite 4A, is morefavorable for the separation of O2/N2 gas pair than other zeolitesdue to its appropriate pore size of 3.8 Å to accurately discriminatebetween the size and shape differences of spherocylindrical O2 andN2 molecules [23,37]. Fig. 4 depicts the predicted zeolite 4A-PIMMM performance based on Maxwell’s equation where theshaded boundary represents the commercially attractive regionfor forming a MMM with thin selective layer.

During the early stage of the development of zeolite-MMM,incorporation of zeolite particles into rubbery polymer has shownremarkable progress in the separation performance. The compati-bility between inorganic filler and rubbery polymer is good dueto the high mobility of the polymeric chains capable to entirelyembed the particles [54,55]. Consequently, the promising resultsexhibited have spurred considerable attention for more researchto incorporate zeolite particles into glassy polymer matrix in viewof their mechanical stability and more desirable transport proper-

Fig. 4. Predicted zeolite 4A-PI MMM performance based on Maxwell’s equation.The shaded boundary represents the commercially attractive region for forming aMMM with thin selective layer [37].

Fig. 5. ‘Sieve-in-cage’ SEM morphology exhibited by zeolite particles whenincorporated in polymer matrix [47].


ties relative to rubber polymer [51]. In the preparation of MMM, akey factor is represented by the affinity between the two phases in-volved in the final structure. However, unlike the good contact be-tween rubbery polymers and zeolites at their interfaces, initialattempts at fabricating MMM using glassy polymers and zeoliteshave resulted in the presence of voids between the polymer andzeolite. Poor adhesion at the zeolite-polymer interface can resultin ‘‘sieve-in-a-cage’’ morphology that is responsible for the non-selective penetration of gas molecules as depicted in Fig. 5 [47],hence reduce the apparent selectivity of the mixed matrix mem-brane and increase the permeability. While the research duringthe past few decades focused intensively on choosing an appropri-ate molecular sieve with dimensions capable of discriminating twopenetrating gases with precisely similar dimension, the recentdevelopment of zeolite/MMM is looking for effective solution to re-

solve the poor adhesion problem as mentioned above. A widerange of modification involving the surface chemistry of zeolite fill-ers in the formation of MMM has been introduced to allowenhancement in polymer-filler interaction, thus optimization ofthe transport and separation properties.

The early attempts were made through the coating of a dilutedsolution of a highly permeable silicone rubber on the membraneand later on, efforts to eliminate these unselective gaps often fo-cused on the use of a coupling agent, which is normally a lowmolecular weight compound with multifunctional groups, to intro-duce favorable interactions between the polymer and zeolite. Lowmolecular weight organic compound such as TAP has been used asthe tertiary component to link the polymer chain to the zeolitecrystal hence promote better adhesion at the interface. Alterna-tively, the chain characteristics of the polymer matrix can be mod-ified using low molecular weight additive that lead toantiplasticization of polymer matrix [34]. Long aliphatic and poly-aromatic based compounds containing polar atoms, rigid and pla-nar structure are usually used to increase the stiffness of thepolymer matrix due to reduced rate of segmental motions in thepolymer chains which normally resulted in an increased selectivityaccompanied with decreased gas permeabilities [56,57]. Surfacemodification of zeolite using silanes that enables chemical link be-tween zeolite particles and the polymer matrix is also proven to bean effective approach to improve both interfacial adhesion and gasselectivity of the MMM by modifying the surface properties of zeo-lite from hydrophilic to hydrophobic as well as increasing zeoliteaffinity to the functional groups of the polymer matrix [58–60].However, the presence of common silane agent such as aminopro-pyltriethoxysilane (APTES) or 3-aminopropyltrimethoxysilane(APTMS) induces huge number of coupling point on the zeolite sur-face which may result in pore blockage. In such circumstances, theuse of alternative silane agent such as APDMS could be solution toreduce the number to coupling point hence avoid major blockageof the zeolite pores [58]. Another way that has been reported toeliminate the voids and solve the interfacial problem is by addinga plasticizer to increase the flexibility of the polymer matrix[61,62]. Nevertheless, in some cases, the addition of a plasticizermay lower the intrinsic gas separation performance of polymericmaterials. In view of the restrictions arisen from the addition offoreign molecules to the polymer dope, the strategies to create bet-ter adhesion have been suggested to carry out by forming a whis-ker-like structure on the zeolite surface to provide additionalroughness for the interlocking between polymer chains and zeoliteparticles. Table 2 summarizes the recent advances in the surfacemodification to improve interaction between zeolite fillers andpolymer matrix [34,52,63–68].

While intensive efforts are still on-going to improve interactionwith the polymer materials, the recent zeolitic development hasalso switched to structure and surface engineering to fabricatenew kinds of molecular sieve structure with more attractive prop-erties. Hollow zeolite sphere (HZS), a newly developed sphericalmolecular sieve through layer-by-layer assembling, is of interestas it possesses advantages of zeolite as microporous and crystallinemolecular sieve, as well as the properties of spherical filler thatwould minimize agglomeration and hence improve dispersabilityand interaction with the polymer phase [69–71]. The first intentionof using HZS as inorganic filler in MMMs for gas separation hasbeen reported by Zornoza et al. [69], showing that the separationperformance of gas mixtures H2/CH4, CO2/N2 and O2/N2 was signif-icantly improved due to the spherical shape that has contributed togood dispersion and well filler-polymer contact as well as the hol-low nature to allow fast flow and increase the gas permeability.The author also pointed out that, the improvement in the perme-ability was not only attributed by the free volume through the dis-ruption of the polymer chains, but also related to the hollow space

Table 2Recent advances in zeolite/MMMs.

MMM composition Coupling agent/Silaneagent/additive

Gas pair Separation performance Reference

Polymer Filler With modification Without modification

Poly(RTIL) SAPO-34 RTIL [emim][Tf2N] CO2/CH4 a = 24.9, PCO2 ¼ 686 Barrer [63]PES SAPO-34 HMA H2/CH4

H2/CO2

a = 175.8, PH2 ¼ 7:1 Barrera = 4.64, PH2 ¼ 7:06 Barrer

a = 61, PH2 ¼ 0:206 Barrera = 2.45, PH2 ¼ 12:57 Barrer

[64]

PSf Zeolite 3A APTMS H2/CO2 a = 2.4, PH2 ¼ 14:3 GPU a = 1.7, PH2 ¼ 82 GPU [65]PDMC SSZ-13 APDMES CO2/CH4 a = 41.9, PCO2 ¼ 88:6 Barrer – [66]PES Zeolite 4A Dynasylan Ameo CO2/CH4

O2/N2

a = 28.7, PCO2 ¼ 6:7 Barrera = 2.7, PO2 ¼ 4:3 Barrer

a = 2.9, PCO2 ¼ 33:8 Barrera = 2.1, PO2 ¼ 9:0 Barrer

[67]

PEEK Zeolite NaA DEA O2/N2 a = 4.2, PN2 ¼ 0:12 Barrer a = 4.2, PN2 ¼ 0:16 Barrer [52]PC Zeolite 4A p-NA H2/CH4

CO2/CH4



[34]

PMMA Zeolite 4A TMOPMA O2/N2 a = 5.4, PO2 ¼ 0:86 Barrer a = 5.1, PO2 ¼ 2:4 Barrer [68]


present in each HZS particles in which the molecules that selec-tively permeate in the hollow particles are allowed to pass throughmore easily with shorter path, compared to the molecules whosediffusion or adsorption is not favoured by the zeolite fillers. Fig. 6shows the possible route transported by the molecules with differ-ent affinity towards zeolite fillers [69]. In another recent work,Weng et al. [72] has demonstrated the feasibility of using CMS/Al2O3 as substrate to improve gas selectivity of MMM. The pres-ence of the substrate has remarkably increased the roughness inthe channels with the poly(phenylene oxide) and SBA 15 interfacehence resulted in higher permeability of small gases such as H2 andCO2 while interfering the permeability of bigger gases such as N2

and CH4. Momentous improvement of H2/CH4 from 17.6 to 50.9was observed due to the existence of CMS/Al2O3 substrate to facil-itate the aggregation or merging of polymer nodules on the mem-brane surface that is of great importance to obtain MMM with highselectivity.

4.2. Carbon molecular sieve

While zeolites are attracting wide attentions in the fabricationof MMM capitalizing on their size and shape selective capability,the emergence of CMS has also offered an interesting option tobe potentially applied as the dispersed phase in MMM. CMS pro-duced from the pyrolysis of polymer, primarily thermosettingpolymers, are carbonaceous porous solids that contains relativelywide opening with constricted apertures that approach the molec-ular dimensions of the diffusing gas molecules [73] and with thismanner, they are able to effectively separate the gas moleculeswith very similar size. Since decades ago, CMS have been evidentlyproven to be very effective for gas separation in adsorption appli-cation due to their superior adsorptivity for some specific gases[74,75]. Thereafter, the interest in developing CMS membraneshas been encouraging as a result of the outstanding permeabil-ity-selectivity combination compared to that of polymeric mem-branes [8,27,76–79]. The high productivity accompanied withexcellent separation properties that exceed the limit of trade-offboundary are mainly due to the porous nature of CMS that havepromoted their viability as the selective inorganic phase in the fab-rication of MMM. The utilization of CMS particles within polymermatrix is anticipated to offer several advantages over zeolites asthe molecular sieve entities. One of the major benefits expectedfrom the former combination is that CMS particles appear to havebetter affinity to glassy polymers, hence allow good adhesion atthe interface without introducing processing complexities. More-over, the separation performances of the CMS can be tailored bymodifying the pyrolysis protocol to adapt to a specific gas separa-tion [27].

The typical preparation of CMS/MMM involves the pyrolysis ofpolymer to obtain the CMS particles, followed by the incorporationof CMS particles in a polymer to form MMM with desired proper-ties. Previously, the initial attempt to incorporate commercial CMSparticles into rubbery polymer did not give rise to any or too littleimprovement on the separation performance, mainly due to thedead-end porous structure of the particles which was inevitablyinherent in their manufacturing process [80]. Nonetheless, the la-ter prediction by Zimmerman et al. [23] has provided further in-sight into the gas transport properties of ‘permeable’ CMS inMMM. By considering the through porosity in CMS which permitsnot only sorptive cavities for adsorptive processes but also allowsgas permeation, they predicted a promising separation perfor-mance of O2/N2 that were well above the upper bound trade-offcurve when PM800 particles, obtained from the pyrolysis of PI at800 �C under vacuum, were incorporated in Udel without the pres-ence of defects at the molecular sieve/polymer interface. Due to theenormous potential of CMS, continual efforts to develop and im-prove the separation performance of CMS in MMM have beenundertaken, with an ultimate target of achieving simultaneouslyhigh permeability and selectivity while maintaining the superiorprocessibility of typical conventional membrane formation tech-nology [79]. One of the most significant works corresponding tothe incorporation of CMS within polymer matrix to evaluate theirpossibilities and potential for gas separation has been conductedby Vu and coworkers [8,78]. The CMS incorporated Ultem andMatrimid exhibited profound improvement as much as 45% and20% for CO2/CH4 and O2/N2 respectively, indicating that the molec-ular sieving pores and channels of CMS that allow for very high sizeand shape selectivity have without any doubt been responsible forthe remarkable increase in the gas separation performance.

Sharing the commonly found limitations during the processingof zeolite/MMM, the rigid nature of the molecular sieve alwaysgive rise to the difficulty to form continuous and defect free mem-brane. Another critical issue in the fabrication of CMS/MMM is theattainment of good adhesion between the two phases which is re-quired for effective and optimum transport properties. So far, fewapproaches have been adopted to solve the interfacial problemsbetween the phases of CMS and polymer. Through the manipula-tion and control of synthesis method, CMS nanoparticles withwell-defined micropores and are readily dispersed in various sol-vents can be produced [81]. It has also been verified that the occur-rence of the sieve-in-cage morphology can be avoided by usingCMS in polymer dope with relatively high viscosities [8]. Anotherviable route to avoid the sieve in cage morphology is though thesizing or priming of the CMS particles with the polymer matrixor sizing agents [8,82]. In fact, the role of sizing agents is still verymuch debated. Some have considered it as a medium to minimizethe aggregation of CMS particles at high loading and promote com-

Fig. 6. The white arrows represent the faster permeating molecules (H2, CO2 andO2) across 8 wt% HZS-PSF MMMs. While the black arrows represent the moleculesi.e. CH4 and N2 with less interaction with hollow zeolites [69].


patibilization of the sieve with the polymer through the formationof more chemical reactive site and surface area to facilitate betteradhesion without involving complicated chemical reaction andgrafting process, while some claimed that sizing agents are not in-volve in promoting adhesion but is present to prevent the defectsand damage of the particle surfaces [83–85]. Rafizah and Ismail[82] reported the use of PVP K-15 as sizing agent for CMS in thepreparation of CMS/Udel MMM to bridge the sieve and matrix byphysically inducing the molecular interaction and allow the gastransport through the phases without creating additional non-selective and resistive layer in the interphase region hence im-proved the O2/N2 selectivity from 3.69 to 6.05.

4.3. Silica

Another conventional class of inorganic filler that has receivedsignificant attention throughout the development of MMM is silicananoparticles which can be further categorized into non-poroussilica and ordered mesoporous silica. These fillers are in generalintroduced in the polymeric matrix to form an heterogenous mem-brane through sol-gel reaction in special conditions in order to de-velop nanoscale particles of the inorganic oxide in a polymermatrix. In this reaction, the silica precursors are hydrolyzed andcondensed into the dispersed nanoparticles in the polymer matrix.Since the dispersion is at the molecular or nano-scale level, theinteractions between the silica and the organic part can be tailoredin order to manipulate the morphological structure at the interfaceof the two phases [86,87]. This interesting finding is in contradic-tion to the Maxwell model prediction of deterioration of the sepa-ration performance by the addition of nonporous fillers to apolymer matrix due to the loss of polymer volume available forsorption and decrease in diffusitivity attributed to the increasedpenetrate diffusion pathway length [88,89]. The intensive researchin silica/polymer MMM system have shown that the addition ofnon-porous nanosized fumed silica, which has opposed properties

with porous inorganic fillers is of great potential to affect polymerchain packing in glassy and high-free-volume polymers conse-quently bring about alteration in the gas separation properties.Due to the non-permeability of the nonporous silica particles, theaddition of this filler into the polymer matrix does not directly con-tribute to the change of transport property, but it alters the molec-ular packing of the polymer chains, resulting in an improvement ofthe permeation as well as the selectivity [90]. Particularly, theincorporation of nonporous silica into polymer matrix can give riseto two outcomes [91]: (i) increase of polymer free volume withoutcreating non-selective voids which in turn results in increased gaspermeation properties and (ii) formation of free volume elementsthat are large enough to permit non-selective Knudsen transporthence resulting in a decrease in selectivity.

The separation performance of silica/polymer system in theearly development did not show significant improvement thattranscends Robeson’s upper bound. In spite of that, stimulatingimprovement in permeability upon the addition of silica particleswithin the polymer matrix has been widely reported [92–96].The preliminary investigation of MMM incorporating silica parti-cles, has proposed that the increased gas permeability was the re-sult of the interaction between the residual silanol groups of thesilica domain and polar gases such as CO2 hence resulting in im-proved gas solubility due to the increase of the Henry’s law coeffi-cient and the Langmuir sorption constants in the MMM.Additionally, the presence of silica particles have induced the mor-phology change at the interface resulting in the increased amor-phous region of the MMM as well as prompting the increase inthe mean distance between the polymer chains through the reduc-tion of the polymer chain packing density at the interface betweenthe two phases. In such case, the polymer structure stiffness as-cribed to the increased tortuosity and restricted segmental motionhas resulted in higher diffusivity and diffusivity selectivity by dis-rupting inter-chained packing. An increasing polymer backbonestiffness, also plays a more determining role in the polymer sepa-ration performance in comparison with the solubility factor[93,97]. The nature of the interface morphology, corresponding tochanges in the free volume concentration, free volume cage sizeas well as the total frictional free volume of the MMM is stronglyinfluenced by the type of silica, i.e. methyl phenyl or silanol groupsthat may interact with the polymer matrix [98]. Besides that, thesilica particle loading also significantly affects the permeability ofthe gases as at high loading of the nanoparticles and with an in-creased aggregate size, may also give rise to the nonlinear expan-sion of the free volume [99]. In spite of very little improvementin terms of the gas selectivity [100], the permeation enhancementupon the addition of silica nanoparticles is still an interesting topicas this characteristic becomes significant in low-permeable poly-mers like Matrimid 5218 and polybenzimidazole (PBI) that are sus-ceptible to permeability increasing effect, thus pushes theperformance of these polymers to transcend the upper bound.

For MMM derived from the incorporation of high loading of sil-ica nanoparticles, the permeation behaviour of the composites de-pends greatly on the chemical compatibility of the silica particleswith the polymer matrix. Recent studies have shown that the weakinteraction between the silica particles and polymer matrix mayinduce void formation during film fabrication, which has a signifi-cant effect on the physical properties, as well as the gas transportperformance of the hybrid membrane [88,101–104]. Paul andTakahashi [101] revealed that dispersion of fumed silica nanopar-ticles into PEI has resulted in small but very influential voids or de-fects formation at silica/polymer interface or within theaggregates. However, by controlling the interaction between thesetwo phases, the amount of voids at the interface can be controlledand the presence of extra voids volume is believed to contribute toimproved gas permeability. Therefore, the reduction in selectivity

Fig. 8. The transport properties i.e. (a) permeability and (b) permselectivity of the gases through neat PBI and PBI silica hybrid membranes versus weight fraction of silica inthe polymer [87].


regardless of gas pair indicated the voids created a parallel path-way through the polymer matrix, as illustrated in Fig. 7, henceallowing unselective transport of all gases when considering thatthere is no effect of the nanoparticles on the local matrix proper-ties. In the consecutive study where the silica particles were chem-ically modified with silane coupling agent with unique bifunctionalgroups to create greater compatibility with the polymer matrix,they reported the reduction of the gas permeability for all gasesdue to the elimination of parts of the void contents [105–107]. Ithas been suggested that by using a coupling agent, the hydroxylgroups on the surface of the silica particles could be replaced bythe organofunctional groups of the coupling agent, hence facilitatebetter reaction with the polymer chains [108].

While the exploratory research on silica/polymer MMM empha-sized the ability of this hybrid system in boosting the gas perme-ability and justified the suitability of silica particles in highlyselective but low permeability polymeric membrane to result ingas separation performance that surpasses the trade off curve, re-cent investigations have focused on the efforts to improve both

Fig. 7. The possible distribution of spherical silica fillers in MMMs that are partiallyfused together (a) dispersed filler and voids and (b) agglomerated filler and voids[101].

permeability and selectivity in order to make this material moreattractive for industry applications. Sadeghi et al. reported a simul-taneous enhancement in all gas permeability and as much as 20times increase in CO2/N2 and CO2/CH4 gas selectivity in MMM pre-pared by incorporating high loading of silica nanoparticles withinethylene vinyl acetate (EVA) [109] and PBI [87] via sol–gel method.Fig. 8 shows the transport properties of gases through neat PBI andPBI-silica MMM. The relative high content of silica accompanied bythe increased number of active OH group sites has given rise to anincrease in solubility and a corresponding decrease in the diffusiv-ity of the gases through the MMMs that eventually resulted in theenhanced gas solution of condensable CO2 gases in the polymermatrix and reduction in the permeability of the non-condensableN2 gas, hence changing the dominant gas permeation mechanismfrom the diffusion to the solution diffusion. The authors alsopointed out that, the reduced diffusivity of the gases can be relatedto the restricted motion of the gas molecules in the polymer phaseand formation of pathways with more tortuosity in the polymerupon the addition of silica particles. Very recently, Xing and Ho[110] have investigated the separation performance of MMM pre-pared by incorporating fumed silica into crosslinked polyvinylalco-hol–polysiloxane. The addition of silica particles has resulted insignificant effect on the separation performance in which the silicacontent is the most influential factor to manifest simultaneouslyhigh CO2 permeability and CO2/H2 selectivity, which can be attrib-uted to the decrease of H2 transport within the MMM as the pack-ing density of nonporous silica increased.

Ordered mesoporous silica with variety of particles size, shapeand pore diameter is another form of silica that has been poten-tially used for the development of new generation of MMM. Priorto their employment in MMM, these mesoporous materials havealready received worldwide attention as viable absorbent andmembranes in many applications [111,112]. Two of the most com-mon members, MCM-41 and MCM 48 are distinguished from theirpore structure in which the former has a one-dimensional porechannel structure while the latter possesses three-dimensionalinterconnected cubic pore structure. Since the discovery of theirfavorable effects of increasing the permeability of MMM withoutdecreasing its selectivity due to its good compatibility with thepolymer matrix [38], the potential application of these orderedmesoporous materials have been expanded as inorganic filler inthe fabrication of MMM. Unlike the role of other size selectivemolecular sieves, the incorporation of these mesoporous materialsin MMM may result in a selective membrane through the improve-ment of the filler-polymer interaction [113]. Since the cross sec-tional areas per chain of the most selective synthetic polymer are


around 1 nm2 or less and considering the typical ordered mesopor-ous silica has relative large pores ranging from 2 to 50 nm, thepolymer chains are able to penetrate into the mesoporosity ofthe filler to give rise to a real homogenous nanoporous composite,hence render the good adhesion between the fillers and polymermatrix. Another interesting feature of these ordered mesoporoussilicas when incorporated in polymer matrix is mainly due to thelarge surface area with abundant reactive silanol groups, whichcan bridge the polymer chains through hydrogen bonding.

The increase in gas permeability as compared to those in neatPSF was observed for MCM-48/PSF [114] and MCM-41 /PSF [115]MMM by Kim et. al. The double increase in permeability of theMMM was associated to an increase in diffusivity as well as solu-bility. When the loading of mesoporous silica in the polymer ma-trix is high enough to lead to the formation of a connected silicainorganic phase, the transport properties of the composite can beruled by the inorganic phase [116]. The increase in diffusivity canbe accounted to the presence of high diffusivity tunnels and redis-tribution of the rigid polymer chains near the pore entrance. Thecontinuous pathways present in the polymer matrix incorporatedwith MCM-48 as well as MCM-41 silica have allowed the diffusionof gas molecules solely through the molecular sieve phase. Theconcurrent increase of the gas solubility was attributed to the highcoverage of silanol groups on the silica surface that has resulted inhigher adsorption capacity of the mesoporous silica relative to thatof PSF.

However, studies also revealed that the hydrogen-bondinginteractions between the surface silanol groups on the surfaces ofthe mesoporous silica and the aryl ether groups of PSF can be det-rimental to the permeability of gases as the silanol groups maycause blockage near the pore entrance, leaving the inner poresinaccessible [38]. Moreover, this material suffers from some limita-tions concerning gas separation performance as the gas transportcan be dictated by the inorganic mesoporous and the mechanismcommonly follows the Knudsen diffusion model where the silicafillers has higher permeability than the polymer phase.

Recently, Zornoza et al. [94,113] investigated the spherical mes-oporous MCM-41 with 2–4 lm pore size by taking advantage ofthe narrow size distribution that facilitate the formation of homo-geneous MMM through the minimization of agglomeration of thesilica particles. Increase in both permeability and selectivity forH2/CH4 and CO2/N2 at optimized loading of 8 wt% has been re-ported, due to the bimodal pore distribution that favors the gas dif-fusivity as well as the disruption of polymer chain packing andlinking due to the presence of silica fillers. The results displayed

Fig. 9. Results for (a) H2/CH4 and (b) CO2/N2 at different pressures and temperat

in Fig. 9 indicates that with the optimum loading and testing con-dition (temperature and pressure), the relationship between per-meability and selectivity for the mixture are located within theRobeson upper bound region. The study also revealed that the dif-ferent approach use for the removal of structural agent from the bi-modal pores of the ordered mesoporous silica spheres has anaffecting role in the separation performance where the selectivityof chemically extracted mesoporous silica was slightly lower thanthat of the calcined ones, while exhibiting relatively higher perme-ability. The authors concluded that these chemically extractedmesoporous silica spheres could be an energetically cheaper alter-native for the fabrication of silica/MMM [94].

4.4. Metal oxide

The preparation of MMM using impermeable particles has beendominated by the use of fumed silica as the dispersed phase.Although some grades of fumed silica contain primary particlesas small as 13 nm in diameter, these particles are chemically fusedtogether so that it is not possible to disperse the primary particlesindividually or in nanoscale aggregates. Metal oxide nanoparticlessuch as MgO and TiO2 are emerging materials due to their potentialapplications for membrane-based separation purposes. The pri-mary particles with diameter in nano-scale and high specific areaof these metal oxides allow improvement in particle distributionand prevent non-selective void formation in nanoparticles/poly-mer matrix interface. Therefore, these nanoparticles are not inher-ently fused together and have potential to be dispersedindividually or in nanoscale aggregates. The incorporation of metaloxides normally exerts similar effect as that of impermeable silicaparticles in which the alteration of gas transport behaviour in suchMMM is the result of chain packing disruption and nanoscaleagglomeration of nanoparticle in polymer matrix.

It is in common consent that, the incorporation of metal oxidenanoparticles in the optimum condition would result in signifi-cantly improved permeability while maintaining selectivity or incertain cases, diminishing the overall diffusivity selectivity of theMMM due to the inability of the porous structure of nanoparticlesto selectively distinguish the gas molecules based on their size dif-ference. Several research work related to MgO embedded in MMMhave been reported [117–119]. According to Hosseini et al. [117],the enhanced gas permeability was ascribed to the effect of nano-particles on the gas diffusivity due to the incorporation of highlyporous MgO nanoparticles which have substituted some portionsof the dense structure of polymer chains in the membrane struc-

ures for mesoporous silica sphere incorporated into Matrimid and Udel [94].


ture. The increase in permeability was also seen to be the result ofthe presence of microvoids at the polymer–particle interface aswell as the inherent transport properties of porous particles MgOnanoparticles that encompass pore size that is larger than the ki-netic diameter of gases in which all the gas molecules, regardlessof size, can pass through the particles without any resistanceagainst their transport. In principle, the presence of affinity andinteraction between MgO surface and some gas species, CO2 forexample, has also prompted the enhancement in the transport ofthe molecules. For instance, Matheucci [119] reported that CO2

permeability increased from 52 barrer in the neat poly(butadiene)to 650 barrer in MMM containing 27 vol% (nominal) MgO, ascribedto the surface properties of MgO nanoparticles that are basic innature also show high affinity of physisorption towards acidicCO2 molecules hence adsorbed large concentration of CO2 even atlow pressure and consequently enhanced the gas permeability.

Matteucci et al. have conducted systematic study to investigatethe influence of TiO2 particle surface chemistry on the gas trans-port properties of the MMM based on both glassy [120] and rub-bery polymer [121]. Upon the incorporation of TiO2 into thepolymer matrix, the dispersion of these nanoparticles was varieddepending on the amount of loading. The MMMs were presumedto be defect free at low particle loading in which the nanoparticleswere dispersed individually and in nanoscale aggregates whereaswhen high loading of TiO2 were filled into the polymer, some nano-particles formed micron-sized aggregates, indicating the presenceof transmembrane defects at such high filler concentration. Besidesdemonstrating great influence on the dispersion properties, theloading of TiO2 was also found to manipulate the gas transportproperties of the MMMs. The diffusivity selectivity of gas pairs con-sisted of CO2 and nonpolar gas appeared to increase with increas-ing particle loading. Similar trend was observed in a recent studyperformed by Moghadam et al. on the effect of TiO2 when embed-ded in glassy polymer [122]. The void formation in nanoparticles–polymer matrix interface as well as agglomeration of particles andweak interaction between polymer–nanoparticles at interface, par-ticularly at high loading has led to greater permeability comparedto that predicted by Bruggeman’s model. In general, reduction inMMMs selectivities relative to pure Matrimid has been observed,due to the decrease in diffusivity selectivity. In the case of CO2,the permeability enhancement of Matrimid containing 20 vol%TiO2 was 2.45 times higher than neat Matrimid, while CO2/CH4

selectivity decreased by 33%, revealing that the use of TiO2 nano-particles improved membrane performance in CO2/CH4 separationyet presented a trade-off line with relatively similar slope with re-spect to Robeson upper bound. This tendency has arisen from theincrease in free volume and voids, which in turn decrease thesize-selective nature of the membrane, so that permeabilityenhancement of large molecules such as nitrogen and methanewas more significant than that of smaller one.

5. Alternative fillers

Despite the rapid advancement in research focusing on conven-tional fillers that have been traditionally applied in the preparationof MMM, the efforts to create ideal membrane with desired sepa-ration properties are still hampered by many undesired andunavoidable obstacles, as identified in the above mentioned stud-ies. For instance, the sieving characteristic of zeolites and CMS isattractive however their size and aspect ratio is less favourablefor the production of asymmetric membrane that utilizes a thinpermselective layer. Furthermore, the rigid pore size with limitednumber of possible structure and composition in the convention-ally used zeolite and CMS are insufficient to effectively discrimi-nate the gas molecules of similar sizes. A filler material with a

controllable cage dimension is of greater interest as this featuremay facilitate better sieving properties to the resulting MMM. An-other point of concern is that the external surface modification ofthe filler particles, particularly with silane coupling agents, hasunavoidably give rise to the pore blockage and limit the transportof the gas molecules [39]. Apparently, the developments of MMMusing conventional fillers are significant yet inadequate to fit theincreasing expectation for the practical application in industries.The challenges faced by this MMM have urged extensive re-searched on the use of other promising alternative materials tohelp mitigate the existing problems in order to meet the desiredmembrane separation performance.

It is only very recent since attention was driven to this newclass of alternative potential fillers. Nowadays, the advancementin science and technology has allowed the synthesis of nanomate-rials with great control in respect to their composition. While thereare still much work devoted to the basic synthesis and character-ization of the building blocks, the current challenge seems to trans-fer the nano-scale properties of these materials into macro-scalestructures. The remarkable versatility of these alternative nano-structured fillers upon the incorporation in MMM has been recog-nized as one of the greatest achievements in the area of membranebased gas separation processes as this mixed matrix system hasshown tremendous promising separation results. The followingsections review and provide further details on how these recentlyemerging materials, i.e. carbon nanotubes (CNT), clay and metal or-ganic framework (MOF) have offered new and huge opportunitiesto the development of MMM endowed with attractive and desiredcharacteristics that are comparable or even triumph over the con-ventional inorganic fillers.

5.1. Carbon nanotubes

In recent years there have been tremendous research advancesmade for carbon nanotubes (CNTs) due to their unprecedentedphysical and chemical properties. The remarkable increase in thenumber of papers published in the fundamental research as wellas in the advanced engineering has indicated the great and unlim-ited potential of this material to generate huge research activity inmost areas of science and engineering. CNTs are a novel and inter-esting graphitic carbon material that is made up of graphite sheetsthat have been rolled into a tube. This hollow cylinder, usuallycapped at least at one end, is considered as nearly one-dimensionalstructures according to the high length to diameter ratio where thediameter of a nanotube ranges from a few to tens of nanometers,while its length can reach up to several millimeters. Generally, asdepicted in Fig. 10 [123], CNTs can be synthesized as singular tubesknown as single walled carbon nanotubes (SWCNTs) or as a seriesof shells of different diameters spaced around a common axiscalled multiwalled carbon nanotubes (MWCNTs). Since the discov-ery of this relatively new class of carbon material, researchers haveenvisaged by taking advantage of their extraordinary electronic,mechanical, thermal and optical properties that have sparked greatinterest in their use in a wide spectrum of promising applications.While the research in CNTs is continuing to discern the utility ofthis nanostructured material, more possible applications are cur-rently being explored and developed. One of the most significantapplications of CNTs is probably as an ideal reinforcing filler inpolymer composites that are poised to exhibit exceptional and de-sired properties through the careful and detailed control of theprocessing and fabrication parameters. The initial interest of incor-porating CNT in polymer lay in the possibility offered by the nano-tubes to result in superb mechanical and electrical properties ofthe fabricated polymer nanocomposites [124–129]. In the contextof application in polymer composite, MWCNTs are of particularinterest over SWCNTs in view of their relatively low cost and avail-

Fig. 10. (a) Different structures of CNTs and (b) TEM of aligned CNTs [123].


ability in larger quantities as a result of their more advanced stagein commercial production [130].

Since the last decade, considerable research, as summarized inTable 3, has been focused on the incorporation of carbon nano-tubes into polymers to prepare MMM [36,131–137]. The intensiveresearch conducted are primarily inspired by the theoretical workby Sholl and co-workers [138,139] that have reported about thetransport rate of small gases such as H2 and CH4 in close packedbundles of SWCNTs being extremely rapid and in orders of magni-tude faster than in zeolite and any other known inorganic nanopor-ous materials. The pioneering work of incorporating alignedMWCNTs into polymer matrix for transport studies conducted byHind et al. [140] has provided a clear indication of the potentialof the nanotube inner cores to act as a feasible channel for thetransport of N2 and proposed the potential application of CNTs inchemical separation and sensing. Several atomic simulation stud-ies have suggested that CNTs can be served as an ideal candidatefor gas adsorption and separation purposes as a result of theirsuperior selectivity and permeability for the transport of lightgases [141,142]. The exceptional high self and transport diffusivityis attributed to the extraordinary inherent smoothness of the po-tential-energy surface defined as when the nanotubes are well dis-persed in the polymeric matrix and all gas molecules pass throughthe tube channels resulting in high gas permeability [136]. In thecase where the CNTs are vertically aligned to the membrane sur-face, they would behave like pinholes that allow rapid transportof the gases passing through the channel of the nanotubes, leadingto high gas permeability without selectivity [36]. The adsorptionselectivity of the gas pair is strongly driven by the interactionpotentials of the molecules with the graphitic CNT walls. The dif-ference in the sorption interaction between the permeating gas

Table 3Recent advanced in CNT/MMMs.

Polymer Functionalization/modification

Optimum loading Gas pair

PES H2SO4/HNO3 5 wt% MWCNTs CO2/N2

PES H2SO4/HNO3

Ru metal5 wt% MWCNTs CO2/N2

PEI Triton-X100 1 wt% MWCNTs O2/N2

PI Chitosan 1 wt% MWCNTs CO2/CH4

PBNPI H2SO4/HNO3 15 wt% MWCNTs H2/CH4

CO2/CH4

BPPOdp HNO3 5 wt% MWCNTs5 wt% SWCNTs

CO2/N2

CO2/N2

PSf H2SO4/HNO3 10 wt% SWCNTs O2/N2

CH4/N2

molecules and CNT walls gives rise to improved separation factorof the gas pair in which it has been evidenced theoretically andexperimentally that CH4 molecules showed preferential sorptioncapacity over other gases like H2, O2 and N2, resulting in enhancedgas selectivity [136].

The expected superior separation properties of the CNTs arehard to be realized due to the difficulties to assess and achievemorphological defect-free CNT/MMM in real experimental studies.In order to obtain ideal membrane structure in which the spacesbetween the CNTs are filled with a continuous polymer film andthe closed ends of CNTs are etched opened, the polymer must havehigh wettability with the nanotubes to allow good interaction be-tween the two phases. Unfortunately, the effort of incorporatingCNTs into polymer matrix share the same challenge as other inor-ganic fillers where the poor adhesion of the different phases elicitsthe presence of an interphase polymer layer near the nanotubes,with properties differing dramatically from the bulk polymer.Apart from the incompatibility of the filler and polymer matrix,the intrinsic properties of CNTs which tend to form stabilized bun-dles in nature due to the presence of van der Waals force are also ahindrance to the well dispersed nanotubes in the polymer matrix[143,144]. The nature of the dispersion problem for CNTs is ratherdifferent from other conventional fillers, such as spherical particlesand carbon fibers, because of the characteristics of CNTs such assmall diameter, high aspect ratio and thus extremely large surfacearea [145]. As a result from the agglomeration and entanglement ofCNTs, the defect sites are created hence drastically weakening theeffect of CNTs as the filler in the MMM. Correspondingly, to fullyutilize the advantages featured by CNTs in a practical manner,the surface modification of CNTs using physical and chemical ap-proaches have been adopted. There are two major issues that need

Separation performance Reference

Neat polymer CNT/polymer

a = 22.5, PN2 ¼ 0:12 Barrer a = 22.5, PN2 ¼ 0:2 Barrer [131]a = 21.8, PN2 ¼ 0:12 Barrer a = 22.1, PN2 ¼ 0:20 Barrer

a = 26.5, PN2 ¼ 0:13 Barrer[132]

a = 21.8, PN2 ¼ 0:12 Barrer a = 21.8, PN2 ¼ 0:12 Barrer [133]a = 19.1, PCO2 ¼ 8:9 Barrer a = 10.0, PCO2 ¼ 14:3 Barrer [134]a = 6.7, PH2 ¼ 4:7 Barrera = 3.7, PCO2 ¼ 2:6 Barrer

a = 6.7, PH2 ¼ 8:0 Barrera = 3.7, PCO2 ¼ 3:4 Barrer

[135]

a = 30, PN2 ¼ 2:6 Barrer a = 31, PN2 ¼ 4:7 Barrera = 30, PN2 ¼ 2:6 Barrer

[36]

a = 5.1, PN2 ¼ 0:17 Barrera = 1.0, PCH4 ¼ 0:17 Barrer

a = 5.4, PN2 ¼ 0:23 Barrera = 1.2, PCO2 ¼ 0:28 Barrer

[136]


to be tackled during the surface modification of CNTs prior to theinsertion into the polymer matrix. First is to debundle the highlyentangled nanotubes to allow improved interfacial interactionand second, the dispersion of the nanotubes to obtain CNTs thatare homogeneously distributed within the MMM to facilitate effec-tive gas transport [146].

Aside from the relatively smooth CNT surfaces that render weakinterfacial bonding, the nanotubes present large area density ofgraphitic walls as well as core entrances that can be functionalizedby molecules of length, hydrophobicity or chemical functionality,depending on the application of the modified CNTs. Generally,the surface modification of CNTs can be conducted in two manners,i.e. by physical and chemical means. The most conventionally andcommonly used functionalized method is through the surfacetreatment with strong inorganic acids [147–149]. During the acidtreatment, functional groups such as hydroxyl and carboxyl groupsare introduced to the surface of the CNTs to improve their compat-ibility with the polymer matrix. During the processing, the func-tionalized CNTs are soluble in a wide range of organic solvents asthe hydrophobic nature of CNTs has been altered to hydrophilicdue to the attachment of polar functional groups. As the enhance-ment in gas permeability of MMM is mainly ascribe to the dis-turbed polymer chain packing by the fillers, the intact interactionof the polymer matrix and functionalized CNTs is expected to im-part more intimate insertion of the nanotubes into the polymerchains hence more effectively increase the gas permeability [36].As mentioned earlier, CNTs show high tendency to entangle asropes or in bundles. Acid treatment has been known as a cuttingprocess to disentangle and disintegrate the nanotubes for betterdispersion in polymer matrix, as well as to open up the closedend of the CNTs to facilitate greater diffusitivity of gases molecules[137,150].

Weng et al. [135] studied the effect of high MWCNT loading inPBNBI on the gas separation performance. H2SO4/HNO3 mixed acidtreatment resulted in the disintegration of the nanotubes intosmall fragments that showed high dispersion in the PBNPI matrix,even at high loading of 15 wt%. They observed a maximum increasein the diffusitivities of CO2 and CH4 at 15 wt% MWCNT due to thepresence of high diffusitivity tunnels in the CNTs within the poly-mer matrix as well as the introduction of more free volume of poly-mer chains into the MMM. This result is in contradiction with theprevious observation where the high loading of CNTs may lead toagglomerations and create nanogaps between the two phases inwhich the polymer chains become discontinuous and the tortuos-ity around the agglomerated CNTs limits further increase in the gaspermeability [36,137]. Apart from contributing to improved inter-facial interaction, the presence of functional group from the acidtreatment has been proven to interact with the penetrating polargases to improve the solubility of the gas molecules in the mem-branes [151]. Very recently, Ge et al. [131,132] reported the in-crease of CO2/N2 gas selectivity due to the greater affinity of thecarboxyl groups on MWCNTs with polar CO2, which can increase

Fig. 11. Proposed gas transport pathway through MMMs with different loading of MWCchannels provided smooth flow and penetration of gas through the polymer matrix [15

the solubility of the polar gas while hindering that of non polarN2 gas. This feature is of great interest as the higher solubility ofCO2 can compensate the selectivity loss caused by the increase offree volume upon the introduction of CNTs. Similar effect has beenobserved for CNTs that were modified with rubidium metal (Ru)[132]. When the Ru modification site was well controlled and theRU particles were mainly deposited on the external surface ofCNTs, greater gas adsorption difference between CO2 and N2 onthe outer surface was induced hence altering the solubility and to-tal gas selectivity. It was found that Ru-modified sites can increasediffusion resistance of N2 gas due to the stronger adsorption of thegas molecule on the Ru impregnated external wall, and conse-quently facilitated the enhancement of the CO2/N2 selectivity.

While chemical modification via acid treatment has demon-strated viable route to modify the surface of CNTs, many effortshave been put forward to developing alternative physical approachthrough non-covalent functionalization to overcome the draw-backs of chemical modification particularly the damage to theCNT structures that may eventually lead to deterioration of theintrinsic properties. The most commonly applied non-covalentfunctionalization to tailor the interfacial characteristics of CNTsare polymer wrapping and surfactant dispersion. The former ap-proach is achieved through the van der Waals interaction and p–p stacking between CNTs and polymer chains containing aromaticrings to form supermolecular complexes of CNTs [134] meanwhilethe latter involves the dispersion of CNTs in the presence of surfac-tants with different charge to exert physical adsorption of the sur-factant molecules on the surface of CNTs hence lowers the surfacetension to effectively prevent the agglomeration and facilitate thedispersion of CNTs [133]. Aroon et al. [134] developed MMM incor-porated with 1 wt% of chitosan wrapped CNTs. A significantimprovement of CO2 and CH4 permeability of 10.47 and 2.26 Bar-rer, respectively and increase in CO2/CH4 selectivity as much as51% compared to neat PI were observed. The enhancement of theseparation performance was observed due to the wrapping ofchitosan molecules around the MWCNTs hence promoting verygood segmental bridging of the nanotubes and PI matrix to preventformation of voids and therefore improved the selectivity. On theother hand, the open-ended chitosan functionalized CNTs actedas tunnels within the polymer matrix to render high gas perme-ation. Similarly, Sanip et al. [152] employed beta cyclodextrin, atoroid with an inner cavity of several angstroms in diameters, aswrapping polymer to non-covalently functionalize MWCNTs. Theattached hydroxyl groups upon the grinding of beta cyclodextrinwith the nanotubes played a significant role towards the enhance-ment of the CO2 gas permeability by increasing the solubility of thegas molecules through the strong interaction while passingthrough the channels of CNTs. The proposed transport pathwayof the gases through MMMs with different cyclodextrin treatedMWCNT loading is depicted in Fig. 11. At high loading, it is foundthat regardless of the role of cyclodextrin as the dispersing agent,the agglomeration and saturation of CNT particles in the polymer

NTs: (a) 0.5%, (b) 0.7% and (c) 1.0%. At optimum loading of 0.7%, the oriented CNT2].


matrix hindered the fast transport of gases due to increased resis-tance through the nanotube cavities. Another non-covalent ap-proach of CNT surface modification prior to the incorporation inMMMs has been developed recently through surfactant dispersion[133]. The presence of non-ionic surfactant, Triton X100 on the sur-face of MWCNTs has reduced the agglomeration hence improvedthe distribution and dispersion of the nanotubes within the PEImatrix. As such, the Triton X100 treated MWCNT/PEI MMM hasexhibited increase in permeance of O2 and N2 as much as 88%and 120% respectively without much sacrificing the selectivity.

The exploration of CNTs has undoubtedly given a vivid impres-sion on the potential of this nanostructured material as promisingfiller in MMM. Therefore, CNT/MMM is expected to serve as a valu-able mean for upgrading the separation performance of the cur-rently existing polymeric membrane. Presently, research effortshave been actively pursued to utilize the CNTs filled polymer ma-trix in the field of membrane separation, particularly in gas separa-tion. The main interest of CNT/polymer system concerns thesmoothness of their interior channels that is the main characteris-tics for the rapid transport of gases within the nanotubes. Despitethe viability and merits demonstrated by CNT, one point of concernis that the success implementation of the hybrid matrix still reliesvastly on the characteristics of the CNTs as well as the interactionwith the polymer matrix. For instance, CNTs as filler has been se-verely restricted due to the difficulties with the dispersion ofentangled CNTs during processing and poor interfacial interactionbetween the nanotubes and polymer matrix. Additionally, it is alsonoted that the dispersion of the agglomerates is not complete dueto a stronger interaction between the primary nanoparticles thanwith the polymer. The state of the CNTs dispersion in a polymermatrix plays a decisive role for the further application of theMMM. Therefore, a greater understanding of these issues is inwanting to further boost the separation performance to a higher le-vel. Another drawback of CNTs as a nanofiller is their higher pro-duction cost [153]. Therefore, the mass production of CNT-basedfunctional composite materials becomes very challenging.

5.2. Layered silicate

As illustrated in Fig. 12, clay is a naturally occurring layered alu-minium silicate that present itself in the form of perfectly crystal-

Fig. 12. Structure of lay

line aluminium octahedral and silica tetrahedral sheets of one to afew nanometers thick and hundreds to thousands nanometers inextent, depending on the particular layered silicate [154]. The claymineral, which can be further categorized into five main groups,namely smectite, illite, kaolinite, chlorite or sepiolite, is a memberof the 2:1 phyllosilicates structural family, in which the centraloctahedral alumina sheet is sandwiched between two tetrahedralsilica to form sheets that are piled in parallel to each other andare bonded by local van der Waals and electrostatic forces [155].The effort of incorporating layered silicates into polymer matrixhas been known for few decades stimulated by pronouncedimprovement in the mechanical, dimensional, thermal, and barrierperformance properties, even at very low silicate layer loading[156,157]. One of the attractive properties is the ability of the sili-cate particles to disperse into individual layers to obtain an aspectratio as high as 1000 for fully dispersed individual layers, which ishard to be realized in other poorly dispersed particles [156]. It isgenerally known that layered silicates can exist in two types ofstructure: tetrahedral-substituted and octahedral-substituted.The polymer matrix can interact more readily with these than withoctahedrally-substituted material as for the latter structure, thenegative charge is located on the surface of the silicate layers.

Montmorillonite, a member of the smectite group, consists oftwo fused silica tetrahedral sheets sandwiching an edge-sharedoctahedral sheet of aluminium is probably the most commonlyused among the available clay minerals. Depending on the polarityof the filler, the silicate layers can be coupled through relativelyweak dipolar and van der Waals forces or in some cases, by strongionic forces [158]. Montmorillonite is applied in the formation ofhybrid membrane due to their attractive properties of relativelyhigh cationic exchange capacity, high aspect ratio and ease ofexpansion, which could facilitate the intercalation of this materialinto a wide range of organic species, besides presenting relativecompetitive low cost and natural abundance [157,159,160]. Alongwith montmorillonite, hectorite and saponite are the layered sili-cates that have attracted great attention in the fabrication of nano-composites [161,162]. Upon the addition of layered silicate fillerinto the polymer matrix, two idealised and somewhat simplifiedpolymer layered silicate nanoscale morphologies are anticipated:(i) the intercalated structure resulted from one or two extendedpolymer chains that are inserted into interlayer spaces of the lay-

ered silicates [154].

Fig. 13. Possible layered silicate distribution arising from the interaction of the filler and polymers: (a) phase-separated, (b) interacted and (c) exfoliated [170].


ered clay and (ii) the exfoliated structure in which clay layers aredispersed into the polymer matrix. In both cases, clay fillers areable to create a stable layer to the polymer and exhibit uniqueproperties not seen for conventionally filled polymers [163,164].

The resulting unique properties possessed by the layered sili-cates render the nanocomposites ideal materials for products par-ticularly for high-barrier packaging for food and beveragepackaging Generally low oxygen permeability is required for oxy-gen sensitive food product, while for some food products certainamount of oxygen is required [158,168]. The incorporation of lay-ered silicate into polymer matrix exhibited remarkable molecularbarrier properties as a result of the hindered diffusion pathwaythrough the nanocomposite membranes. The studies on gas trans-port properties of clay/polymer revealed that an appreciable reduc-tion of both diffusion and solubility coefficients are responsible forthe permeability reduction of O2 through the membrane, accred-ited to the tortuous path towards the diffusing gas molecules andthe reduced molecular mobility of polymer chains due in the pres-ence of layered silicate filler particles. The aspect ratio of exfoliatedsilicate platelets also has a critical role in controlling the micro-structure of polymer–clay nanocomposites and their gas barrierperformances [169]. The presence of fillers decreases the availablefree volume in the polymer and the interactions along the clay andpolymer matrix interface thus affecting the sorption process ofmore condensable gases in the polymer membrane which will pro-vide less sorption sites for gas molecules [158]. Fig. 13 depicts thetypes of mixed matrix that can be obtained when clay is embeddedin the polymer matrix. Depending on the interaction between thetwo phases, three types of composites can be typically obtained,i.e. (i) phase separation, (ii) intercalated and (iii) exfoliated [170].

Recently, a new application for clay minerals as the filler inMMM has been demonstrated. The interesting findings of separa-tion performance have spurred considerate interest in the system-atic transport study of these polymer–layered silicatenanocomposite materials. The easily tailored pore size and compo-sitional variability available of the layered silicate when embeddedinto polymer network have shed light in applications that spanfrom material development to membrane separations [160].Defontaine et al. [171] reported a drastic increment up to 145%

for CO2/CH4 selectivity by sacrificing the gas permeability com-pared to the neat PDMS membrane using MMM incorporated withSepiolite clay minerals. Sepeolite, a layered silicate with tunnel-like structure of approximately 3.7–10.6 Å cavity size character-ized by the inversion of one every six Si–O–Si bonds in their tetra-hedral layer to form interrupted octahedral layers. These layerswere found to be covalently embedded in the polymer networksthrough the formation of Si–O–Si bonds between the Sepiolite par-ticles and the polymer chains due to the presence of abundant edgesilanol groups in the Sepiolite structure thus producing a hybridpolymeric material with inorganic and organic parts.

Despite the potential exhibited by these layered silicate mate-rial for the application in MMM, layered silicates are usually hydro-philic and their interactions with non-polar polymers are notfavourable. As a result, these silicate compounds must be compat-ibilized with the polymer matrix through various forms of surfacemodifications prior to the incorporation to improve the miscibilityof clay with the polymer, thereby achieving good dispersion ofclays in the polymer matrix. Another urgent interest for the surfacemodification is to expand the interlayer space of the clay, in orderto allow large polymer molecules to enter into the interlayer space[167]. Modifications of the nanolayer surface are generallyachieved either by surfactant cation exchange where the hydratedcations within the clay galleries have been replaced by proper cat-ionic surfactants (e.g. alkylammonium) by a cation exchange reac-tion. The structure and extent of modification are found dependinggreatly on the type and amount of surfactant used. For instance,the size of the cationic head group of the surfactant, the lengthand the number of alkyl chains present in the surfactant moleculeshave been previously reported to play a role in the degree of exfo-liation and intercalation [172,173]. However, these determiningparameters might be varied from one system to another dependingon the type of materials as well as the processing methods, thusunambiguous relationship could not be established among them[174]. One of the leading demonstrations of the gas transport prop-erties of clay/polymer MMM has been conducted on surfactantmodified Montmorillonite KSF embedded in polyamide wherethe increasing amounts of clay filler has induced depletion of O2

and N2 permeability but increase of the O2/N2 selectivity. This is


possibly due to the barrier properties of the dispersed nanolayer inthe polyamide nanocomposite membrane that resulted in a tortu-ous diffusion pathway for the O2 and N2 [175].

While surfactants have been widely used to render the clay sur-face hydrophobic, this modification is accompanied with an unde-sired shortcoming where it generally reduces or impedesdelamination of the clay mineral particles into thinner or singlelayers [176]. Furthermore, it has also been pointed out that onlythe ionic part of the surfactant could interact in a favorable waywith the charged surface of the sheet-like clay particles whereasthe long alkyl tail only shows limited compatibility with the poly-mer chains [177]. As a consequent, the research direction hasmoved towards seeking for a better compatibilization that is ex-pected interact with the solid particle surface and with the matrixpolymer and thus cater for more promising interfacial propertiesbetween the phases. The edge modifications through the introduc-tion of coupling agent can be accomplished by anion exchange orsilane treatment [165,166]. A later study conducted by Hashemif-ard et al. [178] has demonstrated more interesting CO2/CH4 sepa-ration performance for both permeability and selectivity. Theincorporation of organic modified montmorillonite, Cloisite 15Ainto PEI polymer matrix was found to hinder the formation of voidsby providing suitable adhesion between the clay filler and polymermatrix. The increased selectivity of 28% relative to the neat PEIexhibited by the clay/MMM was attributed to the high aspect ratioof clay particles combined with the intercalation due to the pres-ence of tallow in Cloisite 15A to render an increase of tortuosityalong the gas penetrant path across the dense skin layer. In anotherstudy, the authors reported the 0.5% loading of silylated-halloysitenanotubes (HCT) has resulted in the 27% CO2 permeability incre-ment and 8% enhancement in the CO2/CH4 selectivity [179]. Hal-loysite, a naturally occurring hydrated polymorph of kaolinitewith tubular nanostructure, was incorporated into PEI with theaid of N-b-(aminoethyl)-c-aminopropyltrimethoxy silane(AEAPTMS) to facilitate the tubular dispersion throughout theMMMs and enhancing the polymer-filler adhesion. The functional-ization of clay minerals with organosilane has been explored as away to improve clay dispersion in the polymer matrix, thusincreasing the intrinsic properties of the resulting clay/polymernanocomposites. Aside from higher degree of tortuosity that is be-lieved to yield higher selectivity, the increase in selectivity of HNT/PEI was also ascribed to the presence of silane agent attached tothe HNTs in the polymer matrix. Surface modification using silaneagents with amine groups is able to effectively enhance the solubil-ity of CO2 solubility which in turn leads to increase in CO2 perme-ability and selectivity. Therefore, in this study, the selectivity ispossibly contributed by the shape or size discrimination passingbetween the intercalated clay layers. Another attractive layeredmaterial that has been recently explored is JDF-L1 sheet particleswith promising characteristic for separation of H2 containing gaspair based on selective sieving pores size across the silicate layers[180]. The pore size of approximately 3 Å has effectively offered afavourable transport for H2 while rendered an important barrieraction for CH4, O2 and N2 with relatively larger kinetic diameter(>3 Å), particularly when a parallel preferential orientation of theJDF-L1 sheet particles the polymer matrix is expected.

Layered silicate clay mineral has been long used as the reinforc-ing phase due to its easy availability, low cost and more impor-tantly environmentally friendly. However, the application of claymineral is still new in MMM. The transport properties of clay/poly-mer MMM have been scarcely reported and only a few scientificpublications are available. Therefore, the corresponding knowledgeabout this process is still incomplete. However, the potential of thismaterial is undeniable and this predestines the clay minerals aspromising filler in MMM, which demand a simultaneous improve-ment in both selectivity and permeability for gas separation.

Remarkably, these primary results have suggested that there liesgreat potential for further improvement through more feasiblestructural modification and optimization of the clay mineral. Nev-ertheless, more research remains to be carried out in order to fullyunderstand the relationship between the clay structures and trans-port interaction.

5.3. Metal organic framework

Metal organic framework (MOF), a hybrid of organic and inor-ganic porous materials, is essentially coordination polymer formedin the most elementary sense by connecting metal ions with organ-ic linkers to obtain fascinating tuneable pore geometries and flex-ible frameworks that opens a new avenue to be efficiently appliedin many potential applications [181]. Generally, MOFs are built upof inorganic sub-units in the forms of clusters, chains, layers orthree dimensional arrangements that are connected to organiclinkers possessing complexing groups such as carboxylates, phos-phonates, N-containing compounds by strong ionocovalent or da-tive bonds [182]. MOFs can be made up from dimers, trimers,tetramers or polyhedral chains in which the linkers are readilymodified with various organic groups while maintaining the samestructure type in order to preserve the desired properties. Fig. 14illustrates a few commons MOFs with different structure architec-tures that could lead to various forms of frameworks and porosity[182]. MOFs are generally classified into rigid and dynamic. Theformer do not exhibit any flexibility behaviour with small changesin volume reflecting the content of their pores while the latter dis-plays great flexibility and their applications depend not only on thesize of the aperture, but also on adsorption conditions. The pores inMOFs have a very uniform distribution and unlike the spherical orslit-shaped pores that are usually observed in zeolites or activatedcarbons, MOFs incorporate pores with crystallographically well-defined shapes including square which may lead to different siev-ing properties [183–186]. Like other porous molecular sieves, theselective gas adsorption in MOFs is achieved mainly based on thefollowing principle mechanisms which are capable of workingindependently as well as cooperatively in both rigid and flexibleMOFs: (i) adsorbate–surface interactions that involves the chemi-cal and/or physical interaction between the adsorbent and theadsorbate and (ii) size-exclusion that depends on the dimensionand shape of the framework pores [187].

The motivation for incorporating MOFs in MMM could benumerous. Some of the exhibited characteristics are high surfaceareas which contribute to high sorption capacity, controlled poros-ity, high affinity for certain gases and high affinity with the poly-mer chains due to the presence of the organic linkers in MOFs, aswell as the high flexibility in terms of chemical composition whichallow the changes with systematically varying pore size, function-alities and chemical properties of the MOFs through tailored syn-thesis approaches in order to facilitate the selective binding oradsorption of target gases [188,189]. MOFs with ultra large pores(>1 nm) might allow penetration of polymer chains to facilitatethe adhesion of polymer chain to the embedded MOF particles.Molecular dynamics simulations conducted on self-diffusion andtransport diffusion of small molecules in MOFs pointed out thatdiffusivities in MOFs are similar to those in zeolites and the knowl-edge of molecular diffusivities in MOFs might be used to screenMOFs for use in an identified process [190]. It is found that descrip-tion based on Knudsen diffusion is insufficient to describe diffusionin MOFs with relative larger pores as the large open volume insideMOFs’ frameworks are only filled with molecules at very high pres-sure. Thus, it is more appropriate to conclude that diffusion of mol-ecules in MOFs is dominated by motions where the adsorbedspecies remain in close contact with the surfaces defined by thepore structure throughout their diffusion.

Fig. 14. Few commons MOFs with different structure architectures that could lead to various forms of frameworks and porosity: (a) HKUST-1, (b) MOF5, (c) Sodalite-ZMOFand (d) zeolite-rho ZMOF [182].

Fig. 15. Prediction of different combinations of MOF and polymer to achieve CO2/CH4 separation with high gas selectivity and permeability [191].


Aside from the preliminary understanding towards the charac-teristic and transport behavior of MOFs, the study and the interestfor these materials are still at an initial stage and answers arehighly desirable for many unanswered questions. One of funda-mental challenges to fabricate MOF/polymer MMMs that are tech-nologically attractive is to select appropriate MOF/polymer pairs aswell as the MOF that are selective towards certain gases. These cri-teria appear essential for high performance MMM as there areenormous numbers of MOFs with a broad range of sieving proper-ties that could potentially be used as the dispersed filler phase in apolymer matrix. Keskin and Sholl [191] adopted Maxwell model topredict mixture permeation for CO2/CH4 mixtures in MOF/Matri-mid MMM using molecular simulations and mixing theories. Theprediction showed that the incorporation of MOF which is eithera permeable but unselective MOF like MOF-5 or the high selectiveone such as Cu(hfipbb)(H2hfipbb)0.5 can impart very influential im-pact on the separation performance of the resulting MOF/MMMs.Specifically, for polymer that is very permeable but less selective,identity of high selective MOF as filler is of great importance to ob-tain a MMM that surpass the trade-off curve for a relevant gas sep-aration. The trend shown in Fig. 15 provides further prediction ofdifferent combinations of MOF and polymer that would offer indis-pensable new knowledge towards the rational identification of themost appropriate MOF/polymer pairs to achieve CO2/CH4 separa-tion with high gas selectivity and permeability.

The pioneer attempt of incorporating MOF copper (II) biphenyldicarboxylate-triethylenediamine as filler in MMMs to facilitatemethane transport was reported by Yehia et al. [192] in whichthe CH4 selectivity has shown encouraging improvement that fur-

ther implied that MOFs can be served as a suitable candidate as thedisperse phase for gas separation due to their unique sieving prop-erties. MOF-5, a cubic three-dimensional structure made fromZn4O clusters linked by three 1,4-benzenedicarboxylate moleculeshave been assembled into thin film membrane to promote its


usage in gas applications and proven to offer unique opportunitieswith a potential to achieve superior performance [193,194]. Perezet al. [43] studied the gas separation performance of MOF-5/Matri-mid and observed increased permeability up to 120% while theideal selectivity remained constant compared to that of neat poly-mer at 30% MOF-5 loading. Nevertheless, Matrimid incorporatedwith MOF-5 has exhibited a remarkable increase in selectivity forCH4 in gas mixture of CO2/CH4 and N2/CH4 due to the coupling ef-fect of favourable MOF-5 affinity towards CH4 and larger solubilityof CO2 and N2 in Matrimid. The transport mechanism is not solelybased on solubility and diffusitivity in the polymer but also theKnudsen and surface diffusion as facilitated by the MOF-5 porosityas well as by the uniformity of the surface of its wall. Another typeof MOF that shows high affinity towards CH4 and favours the per-meation of CH4 is Cu-4,40-bipyridine-hexafluorosilicate (Cu-BPY-HFS) which is built up from a square network of copper bipyridylcomplexes that are pillared by SiF6

2� ions [195]. Zhang et al.[196] reported a profound increment of CH4/N2 gas mixture selec-tivity from 0.95 to 1.7 attributed to the high surface area and affin-ity of Cu-BPY-HFS in the MMM towards CH4 that resulted in theincreased solubility and hence selectivity toward CH4.

Recently, a new class of MOF with exceptional chemical stabil-ity, zeolitic imidazolate framework (ZIF) with tetrahedral networkand sodalite cagelike structure that resemble the structure type ofzeolite has been identified as an attractive molecular sieve forsmall gas molecules such as H2 and CO2 [67,182,197]. The additionof ZIF in MMM is suitable to accomplish such purpose as the porewindow size of these MOFs can be precisely tuned for a desired gaspair separation. ZIF-8 and ZIF 90, with pore apertures of 3.4 and3.5 Å respectively can readily exclude gas molecules with largersize from small H2 and CO2 hence demonstrating promising resultsin H2/CH4 and CO2/CH4 gas pair separation. In addition, the pres-ence of imidazole linker in these ZIF members is also a contributingfactor to the enhanced selectivity as this ligand has been found tofavorably interact chemically with CO2 through non-covalentbonding [198]. Bae et al. [188] have reported the performance ofZIF 90/6FDA-DAM MMM that surpassed the trade-off of polymericmembrane performance in which the enhancement in both perme-ability and selectivity was mainly due to the selective sorption anddiffusion of CO2 in the ZIF-90 crystal. In another similar study con-ducted by Ordonez et. al. [197] using ZIF-8/Matrimid MMM, signif-icant increase in the gas permeability was observed at ZIF-8loading lower than 50 w/w% due to the presence of MOF fillers thathas increased the distance between polymer chains and disruptedchain packing in the polymer thus showing an increase in the freevolume. A remarkably enhanced selectivity for gas pairs containingsmall gas molecules was reported at 50% ZIF loading. The incre-ment of 213% and 290% for CO2/CH4 and H2/CH4 respectively wasascribed to the molecular sieving effect of the small pore apertureof ZIF-8 that favors the diffusion of small gas while hindering theeasy diffusion of large gas molecules like CH4. As such, ZIFs facili-tate the transport of small molecules through the aperture and intothe pore, competitive adsorption for gas pair contains both smallmolecules cannot be realized by these ZIFs. In such circumstanceswhere separation of small molecules like H2/CO2 is desired, ZIFwith narrower pore size of about 3 Å, which is between the kineticdiameter of H2 (2.9 Å) and CO2 (3.3 Å), is of particular interest as itis expected to show a clear cut-off between these two gases andthus can be served as an attractive filler candidate for MOF basedMMMs. Li et al. [67] investigated the potential application of ZIF-7 incorporated polyetherimide MMM for H2 separation and notice-ably improved separation performance that transcends the trade-off curve was achieved primarily due to the precise size-selectivemolecular sieving abilities exhibited by ZIF-7.

The gas transport properties of MOF/MMMs have so far beenfound to be technologically attractive. MOFs are very promising

candidates that stands out for selective gas adsorption, whichcan lead to gas separation due to its remarkable selectivity andtuneable properties. Furthermore, MOFs can be easily synthesizedand variation of MOF compositions and structures including highaspect ratio MOFs is achievable. Organic linkages provide usefulplatform for chemical reaction to take place that may improveadhesion. Unfavourably, the main constraints of MOF whenembedded in polymer matrix are lower thermal and chemical sta-bility due to the presence of the organic linkers. In this aspect, theyare still far to compete with traditional fillers such as zeolites andCMS. Furthermore, thermal stability of the polymers can be ob-tained when high temperature is applied to the matrix during sol-vent removal. These drawbacks must be balanced against therational design process for tailoring structures to specific proper-ties. However, one should realize that, the true strength of MOFsprings directly from the modular process by which they are con-structed as they allow greater chemical alteration on a periodicscale, since the methodology for organic transformations is well-established [199]. So far, the studies performed have underlinedthe key role played by the structure of MOFs that are able to offerselective separation based on their sorption capability and porewindow size. Such studies have also added further insight intothe dependency of the separation performance on the structurearchitecture in terms of the pore size and rigidity where emphasiscan be given during the construction of MOF based MMMs.

6. New and emerging material for MMM

6.1. Graphene

Graphene, as new class of carbon nanomaterials, is found to beeconomical and has novel properties similar to CNTs. Fig. 16 com-pares the existing low-dimension carbon allotrope i.e. fullerene,CNT and grapheme [200]. Since the early 1940s, the experimentalinvestigation of graphene properties, as a standalone object, hasbeen almost inexistent until recent years due the difficulty to iden-tify and univocally characterize the single-atom thick sheet. The di-rect observations of isolated graphene monolayer recently havesparked an exponentially growing interest in this two-dimensionalmaterial. Recent studies showed that graphene could be used as aviable and inexpensive filler substitute for CNTs in nanocompositesowing to the excellent in-plane mechanical, structural, thermal andelectrical properties of graphite [201]. Nowadays, graphene andgraphene-based nanocomposites are being studied in nearly everyfield of science and engineering and possess much importance dueto their exceptional electronic and mechanical properties [202–205]. Recent progress has shown that the graphene based materialscan have a profound impact on the gas transport. The investigationand tailoring of its transport properties from macroscopic to molec-ular scales have now captured the attention of many researchers.

Graphite can be a good candidate as filler in MMMs due to itshigh aspect ratio [206,207]. Permeation rate of gas molecules dif-fusing through membranes can be decreased by embedding highaspect ratio, impermeable particles that provide tortuous pathsand reduce the cross sectional area available for permeation[208]. N2 and He permeation through PC films reinforced withgraphite and functionalized graphene sheets (FGS) were studiedand the results showed that FGS appears to be slightly better forblocking the diffusion of N2 [209]. On the FGS surfaces, there canbe atomistic perforations that CO2 evolution left during pyrolysistreatments, which more likely allow penetration of smaller mole-cules such as He than larger ones [210]. Furthermore, increasedfree volume near the graphene-PC interface may selectively expe-dite transmission of smaller permeants [211]. As such, N2 and Hepermeability could be reduced by incorporation of graphite and

Fig. 16. Comparison of carbon allotropes with different dimension: fullerene (0-dimension), CNT (1-dimension) and graphene (2-dimension) [200].


FGS. Interestingly, FGS turns out to be a more effective barrieragainst diffusing molecules with a larger kinetic diameter implyingits potential for materials for gas separation.

The use of graphene in different applications is rapidly emerg-ing in current scientific research. Cost effective production ofgraphene sheets and the potential in gas transport provides a plat-form for the development of commercially feasible MMMs usingthis novel material as filler. However, the effective use and exper-imental demonstration of pure grapheme in MMM for gas separa-tion still remain a challenge. Like CNT and other nanomaterials, thekey challenge in synthesis and processing of bulk-quantity graph-ene sheets is the formation of aggregates that hinder complete andhomogeneous dispersion of individual graphene sheets in varioussolvents. By solving this issue, the new concepts of graphene filledMMMs for desired gas separation can soon be realized.

7. Conclusion and future direction

At present, the existing membrane materials are inadequate atfully exploiting new opportunities in the gas separation applica-tion. The hybrid system in MMM have the potential to addressthese requirements by synergistically combining the processingversatility of polymers with the separation characteristics of inor-ganic molecular sieves and has been the subject of numerous re-searches. The interplay between basic science and engineeringhas resulted in some of the most exciting accomplishments inthe development of MMM. It is believed that this emerging ap-proach is feasible to attain desired features from both phases andovercome the bottlenecks in each medium.

In the last decades, the studies have asserted the unprecedentedrapid growth in the field of MMM. MMM for gas separation areconventionally comprised of zeolite, CMS and silica particlesembedded in polymer matrix with the ultimate goal of surpassingthe Robeson trade-off boundary by overcoming the drawbacks ofthe existing organic and inorganic membrane materials. The pastdecades have witnessed an intense research effort on the applica-tion of these fillers in various kinds of glassy and rubbery poly-meric materials to perform promising gas separationperformance. Unlike the transport mechanism in polymeric mem-branes which is depending largely on the transient intersegmentalgaps to effectively separate the gas molecules, inorganic fillers

with molecular sieving properties are known to be benefited bythe specific pore diameters that fall in between the kinetic diame-ters of the target gas molecules, resulting in a large diffusive selec-tivity to the overall selectivity. The typical fillers that behave in thisattractive manner are zeolites and CMS that have been unsurpris-ingly granted with tremendous attention since the first explorationof MMM in the area of membrane based separation. Nonporous sil-ica is a class of impermeable filler that have attracted great interestrooted from their great potential to affect polymer chain packing inpolymers to alter the gas separation properties. Metal oxides suchas MgO and TiO2 are nanoparticles that have been studied based ontheir potential to be used as fillers in MMM. These nanoparticlesnormally exhibit high affinity towards certain gas molecules basedon their surface interactions, hence wield the advantages of facili-tated transport for gas separation through the adoption of appro-priate strategy for design and fabrication of the MMM. While theexploration of MMM incorporated with these fillers have broughtabout some forms of delighting breakthrough to be potentiallyused in separation processes, most of the contemporary studieshave concurrently pointed out that there is an obstacle to the suc-cessful introduction of inorganic molecular sieve materials into thepolymer matrix mainly due to poor compatibility between the fill-ers and polymer matrix, especially when rigid glassy polymer isused as the continuous phase. The interfacial region, which is atransition phase between the polymer and inorganic filler, is ofparticular importance to crucially determine the separation perfor-mance of the MMM. Of importance is the issues related to weakinteractions between a glassy polymer matrix and inorganicmolecular sieves that may lead to the formation of nonselectivevoids and resulting in undesired Knudsen flow [39]. Therefore, in-stead of seeking perfect matching of the inorganic fillers and poly-mer matrix to battle the weakness of MMM in term of adhesionand interaction between the two phases as broadly focused duringthe early studies, recent efforts have pinpointed the necessity tomodify the surface of fillers to enhance the interfacial properties.Modification of fillers and polymer matrix has become an expand-ing field of research and different approaches have been proposedto account for the limitations that originate from the poor interac-tions between both polymers and fillers.

Although early studies were mainly focused on the incorpora-tion of conventional inorganic fillers, particularly zeolites with


molecular sieving properties; the research trend in the last fiveyears has shifted towards the utilization of various potential nano-structured filler particles such as CNTs, MOFs and clay. These new-ly explored fillers have shown to be interesting materials as acompetitive alternative for MMM. The need to further explorethe development of MMM by using alternative technologies haspushed these nanomaterials to the forefront of applications dueto their outstanding gas separation properties. On the other hand,graphene, a one-atom-thick planar sheet of sp2-bonded carbonatoms densely packed in a honeycomb crystal lattice have gener-ated enormous interest for its possible implementation in thepreparation of MMM. There is no doubt that graphene has shedsome light for a new research dimension in searching for newmaterials.

Even after decades of extensive research, the potential ofemploying these inorganic fillers have not reached the expectationof current gas separation performance. This may be due to severalreasons, such as membrane defects and related processing issues,as well as the unfavourable orientation of fillers in the MMM. Ithas become clear that improving the interaction between fillersand polymer matrix is the most important criterion towards prac-tical usage of MMM in gas separation. The introduction of func-tional groups can improve dispersion of fillers and changechemical affinities of penetrants in nanocomposite membranes.This enhancement is proposed to originate from the strong interac-tions between the fillers and the polymer matrix. The previousstudies have also revealed that, the transport properties in termsof the permeability and selectivity can be further optimizedthrough the detail governing of some influential key parameters.For this reason, the development of filler materials with controlledproperties through various forms of surface functionalization hasbeen a subject of great research challenge to membrane-based sep-aration processes.

MMM consist of inorganic fillers incorporated into a polymermatrix have the potential to extend or even surpass the separationbenefits exhibited by the polymeric membrane platform. The un-ique behaviour found in this hybrid system will allow us to applyit in new and different applications. The membrane technology re-search community has made great progress in the last decade, yetthe currently gained knowledge in respect to the application po-tential of these fillers in the fabrication of MMM may just the tipof the iceberg. Further understanding of the fundamentals of thismaterial, including structure and dynamics, will allow us to pro-duce materials avoiding the trade-offs of conventional systems.The main concern of future research should be dedicated to the uti-lization of specific molecular interactions to control structure andmorphology and prediction from nanostructural data and molecu-lar dynamics simulations. The future of the field is indeed verybright and breakthroughs in the near future are possible with thecontinual development of these applications. Through the predic-tion of the materials properties or maximum theoretical perfor-mance for different classes of fillers, many new types of fillersare expected to emerge as the research topic becomes more andmore popular and there are still multitudes of structures waitingto be discovered. Hybrid materials with organic–inorganic orbio–inorganic character represent not only a new field of basic re-search but also, via their remarkable new properties and multi-functional nature, offer great prospects for many newapplications in extremely diverse fields.

Acknowledgement

The authors would like to express gratitude to Ministry of Sci-ence, Technology and Innovation Malaysia for the financial supporton the work undertaken in this research centre.

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recent advance of inorganic fillers in mixed matrix membrane for gas separation

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