Polymeric mixed matrix membranes containing zeolites as a filler for gas separation applications: A review

Journal of Industrial and Engineering Chemistry 19 (2013) 375–393

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Review

Polymeric mixed matrix membranes containing zeolites as a filler for gas separation applications: A review Dariush Bastani *, Nazila Esmaeili, Mahdieh Asadollahi Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran 113658639, Iran

A R T I C L E I N F O

Article history: Received 26 February 2012 Accepted 27 September 2012 Available online 5 October 2012 Keywords: Mixed matrix membranes (MMMs) Gas separation Polymer Zeolite Review

A B S T R A C T

Polymeric membrane technology has received extensive attention in the field of gas separation, recently. However, the tradeoff between permeability and selectivity is one of the biggest problems faced by pure polymer membranes, which greatly limits their further application in the chemical and petrochemical industries. To enhance gas separation performances, recent works have focused on improving polymeric membranes selectivity and permeability by fabricating mixed matrix membranes (MMMs). Inorganic zeolite materials distributed in the organic polymer matrix enhance the separation performance of the membranes well beyond the intrinsic properties of the polymer matrix. This concept combines the advantages of both components: high selectivity of zeolite molecular sieve, and mechanical integrity as well as economical processability of the polymeric materials. In this paper gas permeation mechanism through polymeric and zeolitic membranes, material selection for MMMs and their interaction with each other were reviewed. Also, interfacial morphology between zeolite and polymer in MMMs and modification methods of this interfacial region were discussed. In addition, the effect of different parameters such as zeolite loading, zeolite pore size, zeolite particle size, etc. on gas permeation tests through MMMs was critically reviewed. ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

Contents 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

11.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gas permeation mechanism through polymeric membranes . . . . . Gas permeation and separation mechanism in zeolite membranes Selection of polymer and zeolite for MMMs . . . . . . . . . . . . . . . . . . Zeolite–solvent–polymer interactions in MMMs . . . . . . . . . . . . . . . Flat and hollow fiber membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . Zeolite sedimentation and surface pattern . . . . . . . . . . . . . . . . . . . . Interfacial morphology between zeolite and polymer in MMMs. . . Mechanical properties of MMMs . . . . . . . . . . . . . . . . . . . . . . . . . . . Modification of interfacial region between polymer and zeolite by 10.1. Silanation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2. Low molecular weight materials (LMWMs). . . . . . . . . . . . . . 10.3. Annealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4. Priming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.5. Grignard treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.6. Copolymers and crosslinking of polymers . . . . . . . . . . . . . . . Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1. Effect of zeolite loading on gas permeability. . . . . . . . . . . . . 11.2. Effect of zeolite particle size on gas permeability . . . . . . . . . 11.3. Effect of zeolite pore size on gas permeability . . . . . . . . . . . 11.4. Effect of zeolite shape on gas permeability . . . . . . . . . . . . . .

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* Corresponding author. Tel.: +98 21 66165409; fax: +98 21 66022853. E-mail address: [email protected] (D. Bastani). 1226-086X/$ – see front matter ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2012.09.019

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12. 13.

11.5. Effect of silane modification of zeolite surface on gas separation . . . . . . . 11.6. Effect of low molecular weight materials on gas separation performance 11.7. Effect of Grignard treatment of zeolite surface on gas separation . . . . . . . Future direction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Membrane separation is an energy efficient and economical tool in gas separation applications [1]. Polymeric membranes are currently the dominant materials for gas separation processes such as natural gas sweetening, landfill gas recovery, hydrogen recovery and purification, flue gas separation and air separation, etc., because they have the desired mechanical property and the flexibility to be processed into different modules [2–4]. However, the further development of polymeric membrane separation technology has been constrained by a performance ‘‘upper bound’’ tradeoff curve between the gas permeability and selectivity. To expand the industrial applications of polymeric membranes, it is very necessary to enhance the gas permeability and selectivity by combining the synthesis of high-performance membrane materials with the innovation of membrane fabrication technology [5–7]. Mixed matrix membranes are based on solid–solid system comprised of inorganic dispersed phase inserted in a polymer matrix. These kinds of membranes have the potential to achieve higher selectivity, permeability, or both relative to the existing polymeric membranes, resulting from the addition of the inorganic particles with their inherent superior separation characteristics [8–10]. Schematic of a mixed matrix membrane is shown in Fig. 1. The investigation of MMMs for gas separation was first reported in 1970s with the discovery of a delayed diffusion time lag effect for CO2 and CH4 when adding 5A zeolite into rubbery polymer polydimethyl siloxane (PDMS) [11,12]. Porous and nonporous fillers are the two major inorganic phase materials that have been used for mixed matrix membrane fabrication. If the inorganic filler is porous, it has the effect of molecular sieve, separating gases by their size or shape. The resulting membrane is characterized by higher permeability and selectivity of desired components. In case of pore size significantly larger than the size of molecule, adsorption and selective surface flow mechanism is to be considered as well. Nonporous inorganic fillers cause the decrease of the diffusion of larger molecules and the increase of the matrix tortuous pattern. Presence of small size (nm) inorganic materials may increase void volume by disrupting polymer chains resulting in higher gas diffusion [13]. Among porous materials, zeolites and carbon molecular sieves (CMS) are the most commonly used inorganic fillers for MMM

Fig. 1. Schematic of a mixed matrix membranes (MMMs).

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391 391 391 391 392 392

development [14–19]. Metal organic frameworks (MOF) [20–28], zeolitic imidazolate frameworks (ZIFs) [29–33], activated carbon [15,34], carbon nano tubes [35–39], titanosilicates [40,41], and ordered mesoporous silica [11,42–47] are other types of porous materials that have been applied as the dispersed phase in mixed matrix membrane fabrication. Silica [42,48–53], TiO2 [42,54–57], and fullerene (C60) [58] are the most conventional impermeable inorganic particles used for the MMMs fabrication. CMS are able to separate effectively the gas molecules with very similar size. Since decades ago, CMS have been evidently proven to be very effective for gas separation in adsorption application due to their superior adsorptivity for some specific gases [42,59]. Another class of porous materials that has been employed in MMMs is the metal-organic frameworks (MOFs) comprised of transition metals and transition metal oxides connected by organic ligands to create one-, two-, and three-dimensional microporous structures. MOFs are being studied extensively owing to their exceptionally high surface area, controlled porosity, functionalizable pore walls, affinity for specific gases, and flexible chemical composition due to the presence of strong chemical bonds and modifiable organic linking units [30]. These properties make them promising materials for gas storage, gas separations, and catalysis [24]. For gas storage and separation, MOFs can act as molecular sieves due to their rigid frameworks and finite pore sizes allowing for size exclusion of gas molecules [25,26]. While impressive permeability enhancements for MOF–MMMs were reported, selectivity enhancements were less pronounced [27,28]. Furthermore, the lack of detailed analysis of selectivity enhancements and use of less than optimal experimental methods and data analyses in these pioneering MOF MMM investigations may explain the lack of significant selectivity enhancements rather than inherently low MOF gas separation efficiencies [22]. Zeolitic imidazolate frameworks (ZIFs), comprise a subset of metal–organic frameworks (MOFs) with exceptional thermal and chemical stability [29,31,32]. The framework structure of ZIFs are comprised of transition metal (e.g. Zn, Co) cations bridged by anionic imidazolate linkers. The pore size as well as adsorption properties of ZIFs can be tailored by changing or chemically modifying the anionic imidazolate linker [32]. Recent studies show that ZIFs have similar molecular sieving properties with zeolites [29–33], indicating that they are promising candidates for mixed matrix membrane development. Activated carbon is a good example of large pore size inorganic fillers. Adding rigid materials with large pore sizes (the materials with pore dimensions much larger than the penetrants) into the polymer matrix can induce selective surface flow of special components in the pores of the particles. In this case the more condensable or adsorbable component can adsorb and diffuse selectively through the particles and thus the less adsorbable component permeates more slowly. Therefore, when the feed gas mixture includes condensable components, selective surface flow must be considered [42,60]. Among different fillers, carbon nanotubes are fundamentally new nano-porous material having the potential for overcoming Robeson’s upper bound [35,36]. The properties of polymer nanocomposites containing carbon nanotubes depend on several factors in addition to the polymer: synthetic process used to

D. Bastani et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 375–393

produce nanotubes; nanotube purification process (if any); amount and type of impurities in the nanotubes; diameter, length, and aspect ratio of the nanotubes objects in the composite (isolated, ropes, and/or bundles); nanotubes orientation in the polymer matrix [37–39]. It should however be noted that carbon nanotubes are still very expensive and may have hazardous health effect due to their fast migration speed. Another conventional class of inorganic filler that has received significant attention throughout the development of MMM is silica nanoparticles which can be further categorized into ordered mesoporous silica and non-porous silica. Ordered mesoporous silica with variety of particles size, shape and pore diameter is another form of silica that has been potentially used for the development of new generation of MMM. Mesoporous molecular sieves possess pores large enough (2–50 nm) to readily allow the penetration of polymer chains, resulting in better wetting and dispersion of particles [47]. Ordered mesoporous silica materials have properties such as high mechanic and thermal stability, facility of chemical functionalization, high specific surface areas (>500 m2/g) [11]. The most common ordered mesoporous silica fillers are MCM-41, MCM-48, and SBA-15 [42–46]. In spite of the good adhesion to the polymer matrix, ordered mesoporous materials would offer some limitations concerning gas separation performance due to the gas transport through the inorganic mesoporous membranes commonly follows the Knudsen diffusion model where the permeance is inversely proportional to the square root of molecular weight of the penetrants. The large pores of this material maybe easily blocked by the polymer chains leaving the inner pores inaccessible [11]. Also, the pores of mesoporous materials are too large to achieve size selectivity. Therefore, the pores need to be chemically modified to facilitate selective adsorption [47]. The intensive research in silica/polymer MMM system have shown that the addition of non-porous nanosized fumed silica, which has opposed properties with porous inorganic fillers is of great potential to affect polymer chain packing in glassy and highfree-volume polymers consequently bring about alteration in the gas separation properties. Due to the non-permeability of the nonporous silica particles, the addition of this filler into the polymer matrix does not directly contribute to the change of transport property, but it alters the molecular packing of the polymer chains, resulting in an improvement of the permeation as well as the selectivity [42,53]. Metal oxide nanoparticles such as MgO and TiO2 are emerging materials due to their potential applications for membrane-based separation purposes. The primary particles with diameter in nanoscale and high specific area of these metal oxides allow improvement in particle distribution and prevent non-selective void formation in nanoparticles/polymer matrix interface. Therefore, these nanoparticles are not inherently fused together and have potential to be dispersed individually or in nanoscale aggregates. The incorporation of metal oxides normally exerts similar effect as that of impermeable silica particles in which the alteration of gas transport behaviour in such MMM is the result of chain packing disruption and nanoscale agglomeration of nanoparticle in polymer matrix. Upon the incorporation of TiO2 into the polymer matrix, the dispersion of these nanoparticles was varied depending on the amount of loading [42,57]. Zeolite molecular sieves are excellent materials with significantly higher diffusivity and selectivity than polymeric materials. The accurate size and shape discrimination resulting from the narrow pore distribution ensures superior selectivity. Nevertheless, zeolite membranes have expensive cost and difficulties in forming continuous and defect-free membranes of practical meaning [61–64].

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Therefore, mixed matrix membranes may have the advantages of both materials: high gas separation properties of the molecular sieves, the desirable mechanical properties and economical processing capabilities of the polymers [65]. Realization of the mixed matrix paradigm has presented practical challenges due to the need to understand and control complex issues surrounding the organic–inorganic interfaces within these materials. However, their performance suffers from defects caused by poor contact at the molecular sieve/polymer interface. Several methods have been proposed to improve the polymer–zeolite interaction; hence to avoid non-selective voids. Polymer chain flexibility was maintained during membrane preparation either by annealing the membranes above glass transition temperature of polymer [66– 69] or by adding a plasticizer into the membrane formulation [67]. Alternatively, external surface of zeolites was modified by silanecoupling agents or surface-initiated polymerization with preformed particles to promote the adhesion between zeolite and glassy polymer [65,69,70]. Also, the low molecular-weight additives (LMWAs) were employed to prepare glassy polymer/ LMWAs blend membranes [71–73]. The permeability of a membrane respect gases is a function of membrane properties (physical and chemical structure), the nature of the permeant species (size, shape, and polarity), and the interaction between membrane and permeant species [8,74,75]. The first two, membrane properties and the nature of the permeant species, determines the diffusional characteristics of a particular gas through a given membrane. The third property, interaction between membrane and permeant, refers to the sorptivity or solubility of the gas in the membrane. The permeability coefficient (or permeability), PA, of a permeant ‘‘A’’ is the product of the solubility coefficient (thermodynamic parameter), SA, and the diffusion coefficient (kinetic parameter), DA [76]: PA ¼ SA  DA

(1)

Permeability, PA, through the membrane could be calculated from the following measured parameters: PA ¼

Q Al

DPA

(2)

where QA is the volumetric flow rate of gas ‘A’ at standard temperature and pressure, DP is the trans-membrane pressure drop, l is the effective thickness of the membrane and A is the surface area of the membrane. Permeabilities are customarily given in Barrers, where 1 Barrer is equal 1 1010 (cm3 STP. cm cm2 cmHg1 s1). The selectivity of the membrane, the second important parameter, for specific gas molecules is subject to the ability of the molecules to diffuse through the membrane. The selectivity (or permselectivity) describes the permeability of penetrant ‘A’ divided by the permeability of a second permeant ‘B’:

aAB ¼

PA PB

(3)

In this paper, gas separation through mixed matrix membranes is reviewed and the methods to modify interfacial region between zeolite and polymer were discussed. Also, effect of different parameters such as zeolite properties and zeolite loading on performance of MMMs were summarized. 2. Gas permeation mechanism through polymeric membranes Two types of polymeric membranes are widely used commercially for gas separations: glassy and rubbery polymers. Glassy polymers are rigid and glass-like and operate below their glass transition temperatures (Tg). They have low chain intrasegmental

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mobility and long relaxation times. On the other hand, rubbery polymers are flexible and soft, and they operate above their Tg. They exhibit the opposite characteristics, namely high intrasegmental mobility and short relaxation times [8]. Mostly, rubbery polymers show a high permeability, but a low selectivity, whereas glassy polymers exhibit a low permeability but a high selectivity. Glassy polymeric membranes dominate industrial membrane separations because of their high gas selectivities, along with good mechanical properties. There are not many rubbery polymers other than silicone polymers, particularly poly (dimethylsiloxane) (PDMS), which can be used in gas separations. Glassy polymers such as polyacetylenes, poly[1-(trimethylsilyl)-1-propyne] (PTMSP), polyimides, polyamides, polyarylates, polycarbonates, polysulfones, cellulose acetate, poly(phenylene oxide), and cardotype polymers are extensively studied as polymeric materials for gas separations [77]. Polyimide and polyethersulfone are the most extensively investigated polymer materials for membranes, because most of them exhibit higher gas selectivity as well as higher gas permeability compared to many other glassy polymers. They impart good mechanical properties along with higher chemical and thermal stability in resulting membranes. Structural modifications are required to enhance the permeation properties of polyimides [8,78]. Gas transport in polymeric membranes is affected by several polymer properties, such as morphology, free volume content, intersegmental chain spacing (d-spacing), orientation, crosslinking, polymer polarity, defects, thermal processing history, glass transition temperature, average molecular weight, molecular weight distribution, composition, degree of crystallization, types of crystallites, etc. The presence of crystalline domains in a polymer adds a tortuosity factor to gas diffusion; thus, it makes gas transport more complicated. The free volume present in polymers can be visualized as micro voids or holes dispersed in the polymeric matrix. The penetrant condensability can be calculated from the critical temperature, the boiling point, and the Lennard– Jones potential force constant of the penetrants. Chemical affinity can be defined as the interaction between the gas and polymeric matrix. Stronger interactions between a gas and the functional groups of a polymer result in higher solubility of that gas in the polymer. Therefore, CO2, which has a quadrupolar moment, is highly soluble in polar polymers. Condensability also plays an important role in gas permeation through polymeric membranes. Table 1 gives the condensability and other physical properties of CO2, CH4, N2, O2 and H2 [8,77].

Table 1 Physical properties of CO2, CH4, N2, O2, and H2 [8]. Physical properties

CH4

N2

O2

c d e

At 20 8C. At 4.4 8C. At 37.8 8C. At 0 8C. At 15 8C.

Zeolite has received much attention due to the fact that it has a wide range of structure having different chemical composition and physicochemical properties. Zeolites are extensively used as catalysts, adsorbents, and ion-exchange media [63,80]. Molecules transport through zeolite membranes begin by first adsorbing in the pores, then diffusing along the pore surface, and finally desorbing into the permeate [63,81]. Both adsorption and diffusion affect gas permeation through zeolite membranes. Gases usually physically adsorb in zeolite, and this is a nonactivated, exothermic and reversible process [63]. Molecules adsorb into zeolite pores because of intermolecular attractive forces between the adsorbent and adsorbate. All other properties being equal, heats of adsorption are higher for molecules with larger dipole moments [63,82]. Therefore, CO2 adsorbs stronger on zeolites than other light gases such as H2, CH4, and N2 because CO2 has a higher electrostatic quadrupole moment and a higher molecular weight. The higher CO2 heat of adsorption has been observed on LTA, FAU, CHA, and MOR zeolites in various forms [63,80]. Once molecules adsorb into the zeolite pores, a chemical potential gradient in the membrane drives surface diffusion [63,81,83]. Surface diffusion takes place in any pore size, but its contribution is smaller in pores large enough to allow molecules to escape from the influence of the potential field of the lattice. Diffusion rates of different sized molecules in zeolite pores can differ by orders of magnitude, especially when one molecule is approximately the same size as or larger than the pores. If a mixture contains some molecules that fit into zeolite pores and other molecules that cannot, the membrane separates the mixture by molecular sieving. The structure properties of the some commonly applied zeolites are summarized in Table 2 [63].

H2

Molecular weight 44.01 16.04 28.01 31.99 2.02 Kinetic diameter (A˚) 3.3 3.8 3.64 3.46 2.89 a Density 0.72 1.25 1.429 0.0899 1.977 (at 0 8C, 1 atm., g/L) 82.1 147.1 118.6 240.2 Critical 31 temperature (8C) 45.8 33.5 49.77 12.8 72.9 Critical pressure (atm.) 0.162 0.311 0.436 0.031 Critical 0.468 density (g/mL) b d 0.0106 0.017 0.019 0.0087e Viscosity 0.0148 (at 21 8C, 1 atm., cp) 0.0116c 0.0174 b

3. Gas permeation and separation mechanism in zeolite membranes

Gas molecules CO2

a

The transport of gases in dense, nonporous polymers obeys a solution diffusion mechanism. Under the driving force of a pressure difference across a membrane, penetrant molecules dissolve in the upstream (or high pressure) face of a membrane, diffuse across the membrane, and desorb from the downstream (or low pressure) face of the membrane. Diffusion is the ratecontrolling step in penetrant permeation. The rate-controlling process in diffusion is the creation of gaps in the polymer matrix sufficiently large to accommodate penetrant molecules by thermally stimulated, random local segmental polymer dynamics [1,79].

Table 2 Properties of major zeolite types [84]. Zeolite

Structural type

Structural dimension

Pore size (A˚)

4A 5A ITQ-29 13X NaY ZSM-2 L Beta Silicalite-1 ZSM-5 SSZ-13 SAPO-34

LTA LTA LTA Faujasite Faujasite Faujasite LTL BEA MFI MFI CHA CHA

3D 3D 3D 3D 3D 3D 2D 3D 2D 2D 3D 3D

3.8 4.3 4 7.4 7.4 7.4 7.1 (5.5  5.5) and (6.4  7.6) (5.1  5.5) and (5.3  5.6) (5.1  5.5) and (5.3  5.6) 3.8 3.8

D. Bastani et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 375–393

4. Selection of polymer and zeolite for MMMs Material selection for both matrix and sieve phases is a key aspect in the development of mixed-matrix membranes [9,66,85,86]. This has been demonstrated by a number of researchers who attempted to combine a variety of zeolites with rubbery and glassy polymers. Their studies yielded membranes that failed to exhibit selectivity enhancement even in the absence of defects [87,88]. When designing a mixed matrix system for separating a certain gas pair, the molecular sieving phase must provide precise size and shape discrimination ability to distinguish the molecules. Moreover, zeolites with three-dimensional networks are generally preferred for gas separation since they offer less restricted diffusion paths. Matrix polymer selection fixes minimum membrane performance in the absence of defects. Although rubbery polymers conform more readily to zeolites than glassy polymers, they typically exhibit high permeabilities and low selectivities and therefore push the overall performance of mixed matrix membranes considerably below the upper bond trade-off curve. As a result, the attractive polymer matrix materials are generally glassy with relatively lower permeability and much higher selectivities. Indeed, addition of zeolites or another highly selective media would only improve the already industrially acceptable properties, if defects can be eliminated. Last but not the least, the matrix polymers shall be easily spun into asymmetric hollow fibers to achieve the lab-scale to industrial scale transition. Aside from selection of the individual components for a composite system, the combined properties of polymer and sieve must also be taken into account. It was shown that a maximum exists in the expected selectivity increase [9]. This concept can be interpreted in a qualitative way. If the permeability of the polymeric phase is substantially larger than the sieving phase, gas molecules are likely to transport through the polymers by taking the least-resistance path. As a result, the sieves are ‘starved’ and unable to contribute to selectivity improvement. Nonetheless, it is unnecessary to choose a polymer significantly less permeable than the zeolite because it restricts the productivity of the membrane. An optimal exists when the permeability of the fast gas is slightly lower in the polymer phase than the sieves. In this case, the fast gas can permeate through either phase without strong inclination while the slow gas experiences a longer path because it has to avoid the zeolites. Essentially, the mixed matrix effect is to make fast gas transport faster and slow gas even slower. By properly selecting material combination, optimal performances can be achieved for various separation tasks. It is noteworthy that the intrinsic affinity between polymer and sieve is another important factor in achieving a good mixed matrix membrane. Nevertheless, even without strong intrinsic interaction, it is still probable to create desirable composite membranes by properly tailoring the interface to promote the compatibility [89].

379

polymer than the solvent, while the polymer has a stronger affinity for the zeolite surface than the solvent [85]. 6. Flat and hollow fiber membranes Flat dense MMMs have been actively pursued in industry and academia for gas separation in the last 20 years; however, they have thick dense selective layer and much lower gas permeation flux [14]. In Table 3 different kinds of zeolite–polymer MMMs, separation tasks and operating conditions are summarized. In addition, in Table 4 the performance of zeolite–polymer MMMs are given. To further expand the application of the promising MMMs, a more effective membranes structure, the asymmetric hollow fiber membranes, should be explored. Miller et al. [90], Ekiner and Kulkarni [91], and Koros et al. [92] initially carried out work on mixed matrix hollow fibers for gas and hydrocarbon separations. Compared with flat membranes, hollow fiber is more favored due to the following advantages: (1) a larger membrane area per volume and (2) good flexibility and easy handling in the module fabrication [19,93]. There are many factors in synthesis of hollow fiber membranes. The most important issues in MMMs hollow fibers are (1) how to make mixed matrix layer thickness as thin as possible; (2) how to reduce defects in the selective skin and (3) how to really take advantage of the high-selective nature of zeolite molecular sieves. For high rates of production, the dense separating layer of the membrane must be as thin as possible, yet strong enough to withstand considerable transmembrane pressure differential driving forces [94]. Such an arrangement is ideally achieved with asymmetric hollow fibers, which consist of a thin dense layer (skin) and a porous support layer [93]. Dual hollow fibers [19,20,34,35] are applied to reduce material cost, because they require only one of the two materials to be deemed expensive and of high performance as the selective layer [95]. The schematic of dual hollow fiber membrane is shown in Fig. 2. To minimize the voids induced by the unfavorable interaction between polymer and zeolite phases in the dual-layer hollow fibers, silicon rubber coating and heat treatment were employed. Jiang et al. [96] demonstrated that the heat treatment is an effective method to narrow voids to the range that can be cured by silicon rubber coating. The heat-treatment induces the relaxation of the stresses imposed in hollow fibers as spinning, thus leading to higher packing density of polymer chains and reducing the outer surface defects. Regrettably, although the heat-treatment method really narrowed voids between polymer and zeolite phases to the range that can be cured by silicon rubber coating, it also greatly decreased the gas permeation flux due to densifying the whole mixed matrix outer layer [19,74,97–99]. 7. Zeolite sedimentation and surface pattern

5. Zeolite–solvent–polymer interactions in MMMs The balance of three interactions between polymer–solvent, polymer–molecular sieve, and solvent–molecular sieve determines the extent of polymer adsorption from solution (which, in turn, determines the polymer sieve adhesion). A polymer molecule is more solvated in a good solvent and has a larger coil size than if it is in a poor solvent. Therefore, as the solvent power decreases, the dimension of the polymer molecule in solution decreases and the amount of adsorbed polymer molecule increases, because the coil surface area occupied decreases. The solvent–sieve interaction is important because it is desirable for the solvent to desorb from the sieve surface when the polymer segments approach. Thus, an ideal system would be one where the sieve has a stronger affinity for the

During the fabrication of a MMM, one factor of great importance is particle agglomeration due to sedimentation or surface pattern (migration to the surface). Due to the very different physical properties and difference in density between zeolite and polymers, precipitation of zeolite may occur during the MMM preparation [11]. High zeolite loading could lead to more sedimentation of zeolite particles and bigger chance of voids formation during membrane fabrication. It is obvious that the viscosity of the dope increases with increasing of zeolite loading. At low zeolite loading, zeolite particles could not be uniformly dispersed in the cast film due to the low viscosity of the casting solution. Therefore, it is important to optimize the zeolite loading in order to achieve high gas separation performance [68].

380

Table 3 Different kinds of zeolite–polymer MMMs, separation tasks and operating conditions. Zeolite

Zeolite particle Size

Zeolite loading

Solventc

Polymer concentration (wt. %)

Additived

Type of effect of additive

PC PDMS

Average size: 3 mm 1.7 mm, 2.3, 4, 1.5

Z/P b: 5–30% (w/w) Z/Z + P: 15,30,50 wt. %

DCM DCM

P/S: 12% (w/v) P/S: 1:10 wt.%

pNA –

LMWA –

PDMS

4A Silicalite-1, NaX, NaA, Graphite Silicalite

20 and 40 wt.%

–

–

–

Crosslinked PDMS PEBA

SSZ-13 ZSM-5

0.1, 0.4, 0.7, 0.8, 1.5, 8 mm – 1–5 mm

– P/S: 15 wt.%

APDMES –

Silanation –

PEEK-WC PES PES

4A 3A,4A,5A 4A

15 wt. % 30 wt.% –

APDEMS, DEA APDEMS –

Silanation Silanation –

PES PES

NaA, AgA 4A

PES PES

3A, 4A,5A SAPO-34

PES/PI (=20/80 wt.%) PES/PI PES/PI (=50/50 wt.%) PI

4A BEA 4A Nu-6(2)

PI

4A

PI PI PI

4A, 5A, 13X, NaY L Sodalite

PI

ZSM-5

PI Fluorinated PI Poly(imide siloxane) PI, PSF

FAU/EMT ZSM-2 L Silicalite-1

PI/PSF (0/100, 30/70, 50/50, 70/30, 100/0 wt.%) Crosslinkable PI-PDMC Crosslinkable PI

ZSM-5

500–800 nm – – Hollow zeolite sphere: 4 mm; Crystals: 0.3 mm  1 mm  2.0 mm –

(SM) SSZ-13 FAU/EMT

Crosslinkable PI PMMA PPO PPZ PSF

SSZ-13 4A SBA-15 SAPO-34, ALPO 4A

3 mm 1–2 mm Nano size 4A: 50–140 nm, commercial 4A: 1–5 mm 1–2 mm Average size: 100 nm 1–5 mm 0.5–1 mm

Z/Z + P: 30 wt.% 10, 20, 30, 40, 50 wt.% Z/P: 20 wt.%

Iso-octane, Chloroform – n-Butanol and n-propanol (4:1 wt.%) DCM, DMA NMP NMP

20, 30, 40, 50 wt.% 20 wt.%

NMP NMP

30 wt.% 30 wt.%

– –

– –

20 wt.% 20% (w/w)

30 wt.% 20% (w/v)

– HMA

– LMWM

Less than 2 mm 0.1–0.3 mm – 60 nm  1000 nm  1000 nm –

Z/(Z + P): 20 wt. % 10, 20, 30 wt. % 10–50 wt.% 4, 8, 15 wt.%

NMP Dimethyl sulfoxide NMP NMP NMP DCM

P/S: 25/75 wt.% – 20 wt.% P/S: 88 wt.%

– – – –

– – – –

Z/total materials: 15% vol.%

–

–

Plasticizer

0.64–4.33 mm

43 wt.%

DMSO

P/S: 1/(5.71) wt. %

1. RDP Fyroflex; 2. Di-Butyl Phthalate; 3. 4-Hydroxy Benzophenone TAP

LMWM

250–300 nm Average size: 105 nm 200 nm

Z/P: 20/80 wt.% Z/P: 15, 25, 35 wt.%

THF DMF

P/S: 24 g/l –

APTES ADMS

Silanation Silanation

Z/P + S: 10, 20, 30% (w/w) 25 wt.% 20 wt.% 0–20 wt.% 4, 8, 16 wt.%

TCE

10% (w/w) solution

–

–

NMP THF THF Chloroform

P/S: 10 wt.% – – 90 wt.% of solvent, 10 wt.% polymer + zeolite

APDEMS APTES – –

Silanation Silanation – –

0, 10, 20 wt.% of total

DCM

–

–

– 700–900 nm

25 wt.% 25 wt.%.

– Chloroform

15 wt.% polymer blend, 85 wt.% DCM – P/S: 10–12 wt.%

– Silanation

500 nm – 2–3 mm ALPO: 10–20 nm Average size: 100 nm

25 wt.% 20, 33 wt.% 0,1,5, 10, 15 wt.% Z/P: 25%, 22% wt.% Z/P: 0, 15, 25, 30, 35 wt.%

THF THF Chloroform THF NMP

– 15 wt. % P/S: 10 wt.% – P/S: 8 wt.%

– APTES, APMDES, APDMES – TMOPMA – – –

15 wt.% Z/P: 10, 30 wt.%

– Silanation – – –

D. Bastani et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 375–393

Polymera

ITQ-29 Nu-6(2)

2.5 mm 5–10 mm

PSF-Ac

3A, 5A

–

Z/P: 4, 8, 12 wt.% Z/P + S + Z: 0.47, 0.87, 1.92 wt.% 40 wt.%

PVA PVA

4A 4A, SSZ-13

2–2.5 mm –

Z/P: 15, 25, 40 vol. % Z/P: 15 vol.%

PVA PVA, Ultem PVA/Ultem1 1000 polyetherimide Crosslinked PVA/PEG: 36/64 wt.% Glycerinedimethacrylaturethanetriethoxysilane (GUS)-based ORMOCER SEBS-29S

4A 4A 4A

0.5–1.5 mm 5 mm 5 mm

5A

3–5 mm

50 vol.% 15, 30, 40 wt.%. 15, 30, 40 wt.% sieve loading film Z/P: 18.7, 33.2, 58 wt.%

BEA

200 nm–1 mm

20–40 wt.%

Deionized (DI) water Ethyl acetate

BEA

10 wt.% of the dry polymer

Teflon AF 1600

Silicalite-1

Average size: 0.62 mm 0.35, 0.080 mm

PES PES

4A Outer layer: BEA Inner layer: Al2O3

2 mm BEA: 300 nm; Al2O3: 0.2 mm

350 nm: 29 wt.%, 80 nm: 30.0, 40.2 wt.% 20 wt.% of zeolite in total solid Outer layer: 20 wt% of solution; Inner layer: 0–60 wt.%

Outer layer: PES; Inner layer: PI

Outer layer: BEA; Inner layer: – Outer layer: BEA

0.3 mm

Outer layer: PSF; Inner layer: PI PI Outer layer: PSF; Inner layer: PI Ultem1 1000 polyetherimide Ultem, Matrimid

BEA Outer layer: BEA; Inner layer: – Hssz-13 MFI

DCM, THF DCM

– –

– –

– –

TCM

Addetive/P: 25 wt.%, P/S: 6.5 wt.% P + Z: 20–25 wt. % (Z + P)/solvent: 1:4 wt.%

APTMS

Silanation

– –

– –

25 wt.% P/S: 20 wt.% P/S:20 wt.%

– – –

– – –

–

–

–

Irgacure-184 (1-hydroxycyclohexyl-phenyl-ketone)

–

Toluene

2 g of ORMOCER resin was added into the solvent (0.5 g of ethyl acetate) 20 wt.% in toluene

Various organosilanee

Silanation

Galden HT 110

–

–

–

NMP NMP

P/S: 25/75 wt.% Outer layer: PES/NMP/EtOH: 35/50/1 wt.%; Inner layer: PES/NMP/EtOH: 25/61/14 wt.%

Dynasylan Ameo (DA) –

Silanation –

Z/(Z + P): 20 wt.%

NMP

–

–

BEA: 0.4 mm

Z/(Z + P): 10, 20, 30 wt.%

p-Xylenediamine/ methanol: 2.5/100 (w/v)

Hydrogen bonding between Z&P

0.3–0.5 mm Average size: 0.4 mm

Z/P: 20% wt.% Z/P: 20 wt.%

20 wt.% Outer layer: P/S:30 wt.%; Inner lyer: P/S: 23 wt.%

– –

– –

–

Z/S: 10 wt.%

26–30 wt.%

APDMES

Silanation

2,5 mm; 100, 300 nm

20,30, 35 wt.%

NMP, EtOH as a solvent and nonsolvent NMP NMP and EtOH as the solvent and Nonsolvent NMP THF lithium nitrate –

Outer layer: PES/NMP/EtOH: 35/50/15 wt.%; Inner layar: PI/NMP/EtOH: 20/67/13 wt.% Outer layer: 30 wt.%; Inner layer: 23 wt.%

–

–

–

Toluene, DCM 4A: DCM SSZ-13: Isopropanol Toluene DCM, Toluene DCM or toluene

Separation task

Configuration

Thickness (mm)

Upstream pressure

Downstream pressure

Temperature (8C)

Ref.

Pure gases H2, CO2, O2, N2, CH4 Pure gases N2, O2, CH4, He, CO2

Flat Flat

3.7 bar Atmospheric

– Vacuum: 102 mbar

25 15–65

[73] [103]

Pure gases O2, N2, CO2 Pure gases CO2, CH4; Mixed gas CH4/CO2 Pure gases O2, N2, CO2 Pure gases H2,He,CO2, O2, CH4, N2 Pure gases He, H2, O2, N2, CH4, CO2 Pure gases He,H2, O2, N2, CO2, CH4

Flat Flat

35–70 For each filler, MMM with 3 thickness: 100, 150, 500 400–600 –

– 65 psia

– –

28 35

[86] [104]

Flat Flat Flat Flat

55–100 – 60–90 50–80

Carrier gas H2 Vacuum: 102 mbar Atmospheric Variable – pressure constant – volume

25 25–65 35 35

[13] [69] [102] [152]

Flat Flat Flat Flat Flat

60–70 50–60 60–100 100–150 –

100 kPa Atmospheric H2 gas: 3.5 atm other gases: 10 atm He, H2 gases: 2 atm O2, N2, CO2, CH4: 10 atm Pure gas: 10 atm; Mixed gas: 20 atm For H2: 3.5 atm; other gases: 10 atm H2: 3.5 atm, O2, N2: 10 atm 2 bar Pressure difference: 1 bar

Variable Variable Variable Variable –

35 35 35 35 Room temp.

[146] [105] [67] [72] [151]

Pure Pure Pure Pure Pure

and mixed gases CO2, CH4 gases He,H2, O2, N2, CO2, CH4 gases H2, O2, N2 gases H2, CO2, CH4 gases of O2 and N2

– – – –

pressure pressure pressure pressure

constant constant constant constant

– – – –

volume volume volume volume

D. Bastani et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 375–393

PSF PSF

381

382

Table 3 (Continued ) Configuration

Thickness (mm)

Upstream pressure

Downstream pressure

Temperature (8C)

Ref.

Flat

He: 2 atm, other gases:10 atm

Variable – pressure constant – volume

35

[141]

Pure gases O2, N2 Mixed gases H2/CH4

Flat Flat

PI/BEA: 20–40; PES/BEA: 30–50 60–80 75–100

Feed pressure of 1 bar 276 kPa

Variable – pressure constant – volume Permeate side swept by Ar at atmospheric pressure

Room temp. 30

[68] [142]

Pure gases O2, N2 Pure gases He, CO2, O2, N2, CH4 Pure gases He, O2, N2, CH4, CO2

Flat Flat Flat

– – 110–130

Pure gases H2, N2 Pure gases H2, O2, N2, O2, CH4; Mixed gases H2/CO2, CO2/CH4, CH4/N2 Pure gases CO2, CH2; Mixed gases CO2/CH4 Pure gases He, O2, N2, CH4, CO2 Pure gases He, O2, N2, CH4, CO2 50/50 by volume H2/CH4, CO2/N2, and O2/N2 mixtures Pure gases O2, N2, CH4, CO2 Mixed gases CO2/CH4, Low concentration of Toluene as a contaminant (500 or 1000 ppm) Pure gases CH4, CO2; Mixed gases CO2/CH4 Pure gases CH4, CO2; Mixed gases CO2/CH4 Pure gases O2, N2 Pure gases H2, N2, CH4, CO2 Pure gases CO2, H2, N2, CH4

Flat Flat

Variable – pressure constant – volume Atmospheric

35 35 35

[66] [71] [158]

50 –

40–90 psia 1 atm 4 atm, In addition to this CH4 and CO2 at 8 and 12 atm – 1100 Torr with a single gas

Variable – pressure constant – volume Variable – pressure constant – volume

25, 60, 100 35

[115] [47]

Flat

–

150 Psi

Variable – pressure constant – volume

35

[108]

Flat Flat Flat

62 152 –

– 4 atm 275 kPa

– 35 35

[159] [136] [143]

Flat Flat

150–250 –

2–5 bars 500 ppm toluene:700 Psi, 1000 ppm: 500 Psi

Variable – pressure constant – volume Variable – pressure constant – volume Swept with a 1 cm3(STP)/min stream of Ar at atm pressure Variable – pressure constant – volume Variable – pressure constant –volume

35 35

[125] [138]

Flat

35–50

150 psia

Variable – pressure constant – volume

35

[126]

Flat

–

Constant – volume

35

[127]

Flat Flat Flat

Pure gases: 65 psia; Mixed gases: 700 psi – 2 kg/cm2 51.8 Psia

– Variable – pressure constant – volume –

35 26 22

[70] [147] [156]

Swept by helium at 1 atm He used as the carrier gas, except for H2/CH4, Ar was employed Atmospheric pressure at the permeate side sweep stream of Ar Variable – pressure constant – volume

Room temp. 35

[144] [149]

25

[148]

25–55

[109]

Air Mixed gases H2/CH4, O2/N2, CO2/N2 Mixed gases H2/CH4

Flat Flat Flat

– 30–50 PPZ-SAPO-34: 20-30; PPZALPO: 10–20 60 108  41 for DCM; 144  23 for THF 75–175

Pure gases H2, CO2; Mixed gases H2/CO2 Pure gases O2, N2 Pure gases O2, N2 Mixed gases CO2/CH4

Flat

–

Flat Flat Flat

– – –

Pure gases CH4, CO2, O2, N2 Pure gases O2, N2, CH4, CO2 Mixed gases CO2/CH4 Pure gases H2, He, N2 Pure gases O2, CO2, ethylene

Flat Flat Flat Flat Flat

Pure gases CO2, CH4, O2, N2, H2, He Pure gases O2, N2, CO2, CH4 Pure gases O2, N2

1 atm 50 psia 300, 425, 538 kPa

Variable – pressure constant – volume Variable – pressure constant – volume Variable-pressure constant-volume

35 35 35

[85] [162] [163]

62.5 62.5 20–45 100 50–70

Mixed gases: Up to 12 bar, pure gas: 12 bar 40–90 psia 65 psia If CO2/CH4 = 10/90: 40 psia; If CO2/CH4 = 50/50: 440 psia 4.5 atm 4.5 atm 100–460 Psi 0.5–2.0 bar Atmospheric

– – 1 atm Variable – pressure constant – volume N2 used as the carrier gas

[129] [164] [140] [145] [160]

Flat

75

1 bar

Variable – pressure constant – volume

35 35 16.4–40 Room temp. CO2, O2: 23 Ethylene: ambient temp. (25–27) 25–85

Hollow fiber Hollow fiber

Active layer: 0.28 Selective layer: 0.81–5.32

10 bar All gases: 100 psi, except CO2: 20 psi

Atmospheric Variable – pressure constant – volume

Room temp. 25

[65] [107]

[165]

D. Bastani et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 375–393

Separation task Pure gases He, N2, O2, CO2, CH4

35–70 – Flat Flat

Hollow fiber Hollow fiber

Pure gases He, O2, N2 Pure gases He, O2, N2, CH4, CO2; Mixed gases CH4/CO2 Pure gases H2, CO2, O2, N2, CH4 Pure gases CO2, O2, N2, CH4

a PC: polycarbonate; PDMS: poly(dimethylsiloxane); PEBA: poly (ether-block-amide); PEEK-WC: polyetheretherketone; PEG: polyethyleneglycol; PES: Polyethersulfone; PI: Polyimide; PMMA: Poly(methyl methacrylate); PPO: poly(phenylene oxide); PPZ: substituted polyphosphazene; PSF: polysulfone; PSF-Ac: polysulfone-acrylate; PVA: poly(vinyl acetate); SEBS-29S: Elastomeric block copolymer of styrene-b-(ethylene-ran-butylene)-b-styrene (Kraton G1652, Kraton Polymers). b P: Polymer; S: solvent; Z: zeolite. c DCM: Dichloromethane; DMA: dimethylacetamide; NMP: N-Methyl-2-pyrrolidone; DMSO: dimethylsulfoxide; THF: Tetrahydrofuran; DMF: Dimethylformamide; TCE: 1,1,2,2-tetrachloroethane. d pNA: p-nitroaniline; APDEMS: (3-aminopropyl)-diethoxymethyl silane; (APMDES): 3-aminopropylmethyldiethoxysilane; (APDMES): 3-aminopropyldimethylethoxysilane; DEA: diethanolamine; HMA: 2-hydroxy 5-methyl aniline; TAP: 2,4,6-triaminopyrimidine; APTES: Aminopropyltriethoxysiliane; ADMS: 3-aminopropyl(diethoxy) methylsilane; TMOPMA: 3-(trimethoxysilyl)propylmethacrylate; Dynasylan Ameo (DA): 3-aminopropyltriethoxysilane (Dynasylan1 Ameo). e Various organosilanes [R(CH3)nSiX(3n), where X is chloro or alkoxy group with n = 0 and 2, and R is an alkyl chain varying from CH3 to C18H37.

[93] [132] 25 35 – –

[18] [94] 35 35

1–5 atm 1All pure and mixed gases: 114.7 psi, except CO2: 23 psia 3.7 bar O2, N2: 4.5 atm; CH4, CO2: 2 atm

Permeate side was vacuumed –

[14] [95] 35 35 Variable – pressure constant – volume Permeate side was vacuumed

1.5–3 D outer–D inner: 182–420 Outer layer: 1.5–12 20, 30 Hollow fiber Hollow fiber

5 atm 1–5 atm

Pure gas: 35; Mixed gas: room temp. 1.8–7

Pure gases O2, N2, CH4, CO2; Mixed gases O2/N2, CO2/CH4 Pure gases O2, N2, CH4, CO2 Pure gases O2, N2

Hollow fiber

Pure gas: 2–10 atm; Mixed gas: 200 psig

–

D. Bastani et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 375–393

383

In contrast to sedimentation, particles may move to the membrane surface and agglomerate. This phenomenon often occurs when the membranes are formed at high temperatures. There were alternating areas with aggregations of zeolites and areas with no zeolites at all. This is extremely undesirable because aggregation of zeolites causes a dramatic decline in the transport performance of the membrane [66]. It is believed that surface pattern is the result of convection cells that form during casting of films. The formation of convection cells in liquids that are heated or cooled can be due to instabilities driven by buoyancy or surface tension [100]. The schematic for the formation of a pattern at the surface is shown in Fig. 3. Increasing casting solution viscosity, decreasing the membrane thickness, and heating the membrane from the top side may efficiently prevent the surface pattern from propagating [11,66]. 8. Interfacial morphology between zeolite and polymer in MMMs It is clear that modifying the interface region between the bulk polymer and zeolite surface is necessary to achieve performance increase above those of the pure polymer. Although the interphases on zeolite occupy an extremely small volume fraction (i.e., less than 10%), it appears to have a significant effect on the separation performance of mixed matrix membranes [65]. Fig. 4 summarizes the schematic diagram of various structures at the polymer/zeolite interface region [101]. Case 1 shows a homogenous blend of polymer and sieve as ideal interphase morphology. Case 2 corresponds to a region in the external polymer phase as a result of shrinkage stresses generated during solvent removal which named polymer chains rigidification. Case 3 indicates poor compatibility between molecular sieve and polymer matrix or ‘‘sieve-in-a-cage’’ morphology, which cause the formation of voids at the interfacial region. Case 4 represents a situation in which the surface pores of the zeolites have been partially sealed by the rigidified polymer chains. Two factors seem to be critical to the formation of the interphase: the nature of the polymer–sieve interaction, and the stress encountered during material preparation. The interaction between the polymer and sieve is a fundamental property of the chemical nature of the polymer and sieve surfaces, and can be attractive, repulsive, or neutral [66]. The second factor cited above is stresses generated during removal of solvent. To better understanding how the flexibility of the matrix can affect the final mixed matrix material morphology, one can consider a simple example. For simplicity, consider a sieve with a neutral interaction, i.e., neither attractive nor repulsive with two different polymers that differ in their flexibility. If one polymer is extremely flexible, while the other is very rigid the formation of a mixed matrix film using conventional solution casting techniques can be expected to provide rather different outcomes. When the film is cast, one can envision a polymer solution in intimate contact with the sieve. As the solvent evaporates, the overall film will shrink due to solvent loss, but in both cases, the highly solvent swollen polymers are expected to maintain adequate contact with the sieve. When the remaining solvent leaves the system, further shrinkage can induce huge stresses in the rigid polymer and cause a tendency to detach from the sieve surface. Even when all the solvent has left the intrinsically flexible polymer, it can conform to the sieve surface, and any residual stresses can be relieved, thereby suppressing a defectively packed ‘‘interphase’’ [66]. The revealing of submicron gaps around the sieves may be formed due to poor adhesion between the polymer and zeolite particle creating channels between the interphases or cages surrounding the filler [65].

384

Table 4 Performance of zeolite–polymer mixed matrix membranes. Permeability (Barrer)

Selectivity

Ref.

H2

N2

O2

CO2

CH4

H2/N2

O2/N2

H2/CH4

CO2/CH4

– 5.8–7.2 –

8.1–14.1

0.8–1.79 6.1–6.9 370–1000

0.074–0.266 6.25–6.9 –

53–135

–

5.8–8.8 1.8–2.2 2–2.5

–

–

7–450

10.5–400

3.23–8.4 7.75–8.2 2000–5000 56.5–88.6 100–450

52–76.6

–

0.115–0.249 5.2–6.8 180–350

–

–

0.92–1.93

–

31.6–44.1 2.6–3.3 CO2/O2: 4.5–6 41–43.8 –

– 3.5–12.5 5.23–10.4

– 3–10.5 4.94–8.3

0.16–90 0.03–0.09 0.058–0.0907

– 0.3–0.7 0.363–0.583

– 1.25–2.5 1.56–2.32

– 0.025–0.07 0.0501–0.0743

– 85–120 85.17–91.51

0.94–4.2 6.1–7.7 6.26–6.43

– – –

– 28–46 31.14–31.22

– 6.67

– 5.82 3–6.5 7.06–12.57

– 0.0640–503 0.00208–0.0825

– 0.406–471 0.00308–0.77

1.1–1.4 1.75

0.03–0.05 0.0541

– 90.9 78–90

– 0.936–6.34 5.8–6.8

– –

28–37 32.3

CO2/O2: 1.11–9.39 CO2/N2: 1.03–17.69 CO2/N2: 0.87–21 He/N2: 95–135 He/N2: 90.17–114.66 He/CO2: 3.35–4.48 H2/CO2: 3.17–3.58 – He/N2: 104.2

1.53–5.12

0.041–0.206

61–175

24.9–37.4

H2/CO2: 2.45–4.64

0.322–0.972

12.13–74.72 2.25–4.16

9.42–16.4

0.278–0.882

18.6–33.9

CO2/N2: 16.9–29.3 He/N2: 52.8–93.8 He/O2: 11.8–13.4

12–24

4.2–16

–

– 0.00181–1.35 2.3 0.029–0.11 0.14–0.62

4.0 0.033–6.58 10.1 – 1.35–3.17

1.2 3.64–17.57

5.73 10.78–44.19 2.3–2.8 0.7275–1.3537 – – – 0.6–2.2

– 0.185–33.4 18.3–20.1 – 8.27–15.41 16.1–70.76 15.96 59.04–199.41 7.2–18.7 1.3524–2.2074 85–128 13.7–41.2 148–153 – 55–80

– 0.23–0.25 – 0.0003–4.87 0.3–0.7 – 0.12–0.56 0.22–6.4 0.66 4.98–41.61

30.2–51.3

– – 2.32–53.5 97.6 – –

– 27–49.4 –

7.4–13.1 19.56–36.31

30.98 47.11–75.78 15.4–38.4

1.12–5.92 4.28–6.99

–

0.65–4.5

–

7.2 4.2–18.2

113–281 58.8–142.6

– 5–10.35

– 117–206 –

– 64.8–169.8

4.78 6.9–8.5 1.8501–3.0275 – – – 5.1–5.38

– – – – 100–152

0.2403–0.7317 – – – 0.16–0.4 2.5–5.5

–

–

–

–

17–64

–

– – –

0.22–0.23 – –

1.3–1.8 – –

– – –

5.9–7.7 – –

– –

0.28–.45 0.28–0.33

–

–

–

– 8.4–69.3 – 512–7180

– 10.7–57.3 – 1450–5590

– 0.04–0.77 – 29.8–1140

0.35–0.6 0.36–0.99 –

– – 0.088–0.284 –

– –

–

– – – 4.22–48.23 – – 4.33–11.5 1.6–3 1.7–3.4 44–260

– – 0.09–0.61

– –

– 4.01–21.9 35.8–47.9 14.25–82 – –

915–1385 32.2–2050

1835–5736 176–3720

– – – –

0.2872–0.9446 – 0.19–1.18 – – 2–5

–

– – – – 30–40

CH4/He: 1.6–2.2 CO2/N2: 10–14

– – 2.23–617 – 27.49–67.19 11–80 24.18

80.3–180 – – – – 27–50

Others

CO2/N2: 20.6–102

H2/CO2: 2.30–2.57

CO2/N2: 39.8–41.7 2.3369–4.4051 47–53 15.4–80.2 34.7–38.9 – 17–28

– – –

17.5–57.7

H2/CO2: 1.5–2 CO2/N2: 17–20 CO2/He: 19 CO2/N2: 55 CO2/H2: 12

– 38–118 73–398

– – –

–

7.3–10.4 –

– –

9.3–12 7.2–12.5 –

– –

– – 40.6–49.4 48–60 34–75 18–35

– 1.1–2.1

–

–

H2/CO2: 1.67–3.57

3.5–13 – 46.1–1080

– 74–267 – 4.6–5.1

– – – He/N2: 70–246 CO2/N2: 3.1–5.9 He/N2: 6.3–17

[73] [103] [86] [104] [13] [69] [102] [152]

[146]c [105] [67]c [72] [151]a [141]d

[68] [142] [66] [71] [158] [115] [47] [108] [159] [136] [143] [125] [138] [126] [127] [70] [147] [156]

[144] [149] [148] [109]a [85] [162]c [163] [129] [164] [140] [145] [160]b [165]

D. Bastani et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 375–393

He

D. Bastani et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 375–393

He/N2: 2.1–81 He/N2: 170–231

– – –

[65]a [107]a [14]a [95]a [18]a [94]a [93]a [132]

There are mainly four approaches to reduce or eliminate the voids, especially for glassy polymers. The first one is to promote the adhesion between polymer matrix and molecular sieve phases by modifying the zeolite surface with silane coupling agents [87,102]. The second one is to introduce low molecular weight materials to fill the voids between polymer and molecular sieve phases [71]. The third one is to apply high processing temperatures close to Tg of polymeric materials to maintain the polymer chain flexibility during the membrane formation [14,66,67,102]. The fourth one is to prime the surface of zeolites by polymer [69,73,85,103–105].

39.6–43.9 39–45

2.86–46.28 – 29.8–37.1 24–32

9. Mechanical properties of MMMs

2.13–7.26 0.87–6.89 0.92–7.16 1–7.4 0.91–0.96 0.93–6.5 7.7–8.2 7.8–8.5

– – – –

Obviously, the addition of zeolite particles enhanced the mechanical strength of the MMMs adsorbents within certain range [106]. In order to investigate the effects of inorganic fillers on the mechanical properties of membrane, stress–strain tests could be carried out for the membranes. The complexities of the mechanical properties of the composite membranes could be explained from two phenomena. On one hand, the mechanical properties of composite membranes were highly dependent on the polymer/ particle interface properties. A poor compatibility between the filler particles and the polymer matrix would result in the failure of the transfer of the external force from the polymer matrix to the inorganic fillers. As the filler content increased, some particles were prone to form aggregates, resulting in weaker compatibility between the polymer matrix and the filler particles, which could reduce the modulus and tensile strength of the composite membranes. On the other hand, the incorporation of the inorganic fillers increased the crystallinity of the composite membranes, which could enhance the mechanical properties of the membranes. In other words, due to its semicrystalline property, the composite membranes were not only influenced by the polymer/particle interface properties, but also the variation of the crystallinity of the composite membranes [45]. Zhang et al. [106] observed that the Young’s moduli (E) of the mixed-matrix membranes increase with the mesoporous ZSM-5 content and reach amaximum at 20% loading. Both the tensile strength and high strain decrease with the mesoporous ZSM-5 content. The increase in Young’s modulus suggests that the interfacial adhesion between the mesoporous ZSM-5 particles and polymer chains must be good. Widjojo et al. [107] reported that the mechanical strength of PES–beta zeolite/PES–Al2O3 dual-layer mixed-matrix hollow fiber membranes was improved and its shrinkage in the radial direction was reduced during heat treatment.

d

c

b

0.044–0.78 0.041–0.044

The unit of permeability is GPU. The unit of permeability is (ml mm)/m2 day. The permeability of zeolite 4A MMMs was only reported. The permeability of PI MMMs was only reported.

1.48–1967 48–69.3

a

6.23–11.3 2–31

– – – – 0.035–11.81 – 0.00561–.0562 – 1.62–33.77 – 0.183–2.08 1.5–3

0.574–8.94 0.0046–5.71 0.0572–1173 0.3–8.5 136–6504 0.102–1075 1.7–3.26 0.35 0.079–4.19 0.0026–5.88 0.00855–1269 – 143–6954 0.0183–1156 – – – – – – – –

CH4/N2: 0.82–1.6

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10. Modification of interfacial region between polymer and zeolite by different ways 10.1. Silanation The use of different silane coupling agents such as gaminopropyl-triethoxy silane, N-b-(aminoethyl)-g-aminopropyltrimethoxy silane, N-b-(aminoethyl)-g-aminopropyltrimethoxy silane, g-glycidyloxy-propyltrimethoxy silane, (3-aminopropyl)dimethylethoxy silane, g-aminopropyl-diethoxymethyl silane, and g-aminopropyl-dimethylethoxy silane (APDEMS), allows to modify the zeolite surface in order to improve compatibility of the inorganic filler with the matrix [69,93,102,108–110]. It was shown from the silanation study of zeolites that the hydroxyl groups of zeolites could make hydrogen bonds with the amino silane agent [71]. Further, silanes with a second reactive end group (amine) could be used to bond polymer chains to the zeolite thus promoting adhesion between the zeolite and the bulk polymer

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Fig. 2. Schematic cross-section morphology of the dual-layer hollow fiber with a zeolite–polymer mixed matrix skin [19].

phase in the membrane [93]. Fig. 5 shows a schematic silanation of zeolite surface with APDMES coupling agent. A very destructive force affecting adhesion between the hydrophilic zeolites and polymer is the migration of water to the hydrophilic surface of zeolite. Water attacks the interface,

destroying the bond between the polymer and zeolite, but fundamentally, amino silane will create a water-resistant bond at the interface between the zeolite and polymer. These unique chemical and physical properties of amino silane will enhance the bond strength, thus it provides a stable bond between two materials [65]. Three analysis methods are used to determine whether the silane modification of zeolite surface had taken place. The first one is elementary analysis, the second analysis method is X-ray photoelectron spectroscopy (XPS) and the third method is FTIR [70,102]. Matsumoto et al. [111] have identified that there are three types of silanol groups on the surface of zeolite, i.e., single ((SiO)3Si–OH); hydrogen-bonded ((SiO)3Si–OH OH–Si(SiO)3) and geminal ((SiO)2Si(OH)2). Among these silanol groups, free SiOH groups are the most reactive groups and may provide the sites for the physical and chemical adsorption of silane coupling agents. With appropriate silane coupling agents, one may not only modify surface properties of zeolite from hydrophilic to hydrophobic, but also increase zeolite affinity to the functional groups of the polymer matrix as illustrated in references [102,112–114].

Fig. 3. Development of the instability in films cast at elevated temperature [11].

Fig. 4. Schematic diagram of various structures at the polymer/zeolite interface region [101].

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Fig. 5. Schematic of the envisioned coupling reaction [93].

Li et al. [115] are fabricated MMM from sodalite nanocrystals (Sod-N) dispersed in polyimide. The structure of silanated sodalite–polyimide interface is illustrated in Fig. 6(a). Sodalite nanocrystals are composed of a crystalline sodalite core and a thin amorphous aluminosilicate shell with amino-groups (=Si– (CH3)(CH2)3NH2). The thickness of the amorphous aluminosilicate shell is roughly estimated to be around 2 nm assuming that all amino-groups are contained in the shell. The high-quality bonding between the sodalite nanocrystals and the polymer matrix is realized by forming covalent linkers via the imidization reaction of the amino- groups with the polyimide monomers (Fig. 6(b)) [115]. 10.2. Low molecular weight materials (LMWMs) Filling the space between zeolite particles and polymer chains would be more convenient and effective than surface treatment of zeolites. Low molecular weight materials (LMWM) could play this role if they would be able to interact simultaneously with zeolites

and polymers. One could think of many low molecular weight materials, which could induce a hydrogen bond with hydroxyl and carbonyl moiety. Besides making hydrogen bond, they must be soluble in the solvent used to make the polymer dope solution. They also must be solid at room temperature. Otherwise, they will evaporate during membrane fabrication, and thereby will lose their ability of illuminating interfacial voids [71]. Long aliphatic and polyaromatic based compounds containing polar atoms, rigid and planar structure are usually used as LMWAs in MMMs such as p-nitroaniline(pNA) [73], 2,4,6-triaminopyrimidine (TAP) [71] and 2-hydroxy 5-methylaniline (HMA) [72,116–118]. Fig. 7 shows the feasible hydrogen bonding mechanisms of TAP with zeolites and polymer (polyimide) [71]. The influence of LMWMs on membrane performances was explained by antiplasticization of polymer matrix, which was described as the increasing the stiffness of the polymer matrix due to reduced rate of segmental motions in the polymer chains [73]. Variations of free volume have been used to explain the effects of antiplasticization in polymeric materials. In that case, the inclusion

Fig. 6. Schematic representation of sodalite–polyimide interfacial structure (a) and covalent linker between Sod-N and polyamide (b) [115].

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Fig. 7. Interaction model between zeolite, 2,4,6-triaminopyrimidine and polyimide [71].

of small molecules into a polymer raises the rigidity of the material with a subsequent suppression of motions in the polymer chain. Because of that, there are changes in physical properties of the polymeric materials [119,120]. Antiplasticization effect of LMWMs additives, which decreased both the glass transition temperature and gas permeabilities of the polymer. The extent of antiplasticization depended on the degree of interaction between the additive and the polymer as well as the concentration of additive in the matrix [121–123]. The threshold amount of LMWMs needed to eliminate the interfacial voids should be related to some surface properties of the zeolites. However, it was found to be related with none of the characteristic properties of zeolites. Among zeolites having the same crystal structure, a zeolite of a smaller particle had a larger external surface area per given weight. This can explain why smaller zeolites need more LMWMs [71]. Although Yong et al. [71] was used the TAP as an additive, the content of TAP to polymer was at least 21% of polymer. At this high concentration, the compatibilizer becomes as a main material instead of being an additive. S¸en et al. [73] used p-nitroaniline(pNA) as a compatibilizer between polycarbonate(PC) and zeolite 4A. They observed that even at very small concentrations like 1–2%w/w pNA behaved as a compatibilizer and significantly improved the gas separation performance. As a result, studies showed that incorporation of low molecular weight additives, even at very low concentrations, increase the separation performance of polymer/zeolite MMMs significantly by creating a synergistic combination. The performances of ternary mixed matrix membranes move through the upper bound curve with selectivity improvement thus these membranes can be a promising tool to surpass the upper bound limit [117,118].

10.3. Annealing An initial attempt to heal poor contact between polymer and zeolite defects was tried by annealing already formed mixed matrix membranes above the glass transition temperature (Tg) [66]. Tg provided a qualitative estimation of the flexibility of polymer chains, and was very useful to compare the polymer chain rigidity of mixed matrix membranes at different zeolite types and loadings with that of pure polymer membrane [67]. When annealing process was performed at temperature above the Tg, a stronger bond between polymer matrix and zeolite was formed compared to that of annealed at temperature lower than Tg. The increase in the polymer chain flexibility at higher temperature helped the polymer chain to be better attached to the zeolite surface [68,69]. However, annealing did not lead the defective membranes to any significant improvement in the morphology. The experiment indicated that once the ‘‘sieve-in-a-cage’’ morphology is formed, it is extremely hard to create a material with good contact between the polymer and the sieve [66]. For polymers with high Tg, one may consider decreasing Tg by means of the incorporation of a plasticizer into the polymer matrix because it is very difficult to find a non-volatile solvent with an enough high boiling point to match the temperature requirement during the membrane formation. However, the addition of a plasticizer may lower the intrinsic gas separation performance of polymeric materials. Therefore, choosing glassy polymers with an intermediate Tg should be a more appropriate alternative for this solution [67]. Vacuum degassing can effectively avoid the formation of voids between polymer and zeolite phases resulting from the air adsorbed on the surface of zeolite particles. With the development of membrane fabrication technology, another important factor

D. Bastani et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 375–393

influencing gas separation performance has been identified, that is the cooling protocol after annealing (i.e., immediate quenching or natural cooling down). Immediate quenching after annealing membranes above Tg may make polymer chains being frozen quickly as polymer chains are still in the random status. Therefore, the resultant membrane has a higher free volume in the polymer matrix and subsequently higher gas permeability without the loss of selectivity. This quench method has been applied widely in the membrane fabrication process [67,124]. However, if applying this method to form the mixed matrix membranes, polymer chains may be detached from the zeolite surface as polymer chains are suddenly frozen, resulting in the formation of voids between polymer and zeolite phases because of their different thermal coefficients of expansion. The purpose of applying the natural cooling after annealing is to make polymer chains harden and shrink much slowly and gradually with the decrease of temperature. The zeolite is less affected by the decrease of temperature due to its inorganic properties; therefore, polymer chains may better adhere on the zeolite surface [67]. 10.4. Priming To date, most of MMM studies have used rather large inorganic particles 0.3–5  106 m (0.3–5 mm). This is mainly due to the fact that more severe agglomeration occurs in the polymer matrix when smaller particles are used, especially at high particle loadings. The agglomeration of nanoparticles with a size of less than 1  107 m (100 nm) has significantly impeded their application in developing the practical mixed matrix membranes. This is because the agglomeration is thought to be responsible for defects among the particles and/or between the polymer matrix and particle phases [73,105]. Therefore, zeolite particles were then primed by adding approximately 5– 15 wt.% of total amount of Polymer. Prior to addition of remaining bulk polymer, thorough mixing between the priming polymer and the suspension of zeolites ensures that the polymer effectively coats the zeolite particles [104]. The purpose of priming is to increase the compatibility between zeolite and polymer in MMMs, and to minimize the aggregation of zeolite particles [69,73,85,99,105,108,125–127]. 10.5. Grignard treatment Recently, a reliable method of mixed matrix formation has been established. This method employs a Grignard treatment (GT) method to modify the zeolite surface [128–130] and to improve the interfacial adhesion between the zeolite and polymer, the most widespread limitation facing mixed matrix application. The Grignard treatment involves growing Mg(OH)2 whiskers on the surface, consisting of two steps: (i) a crystal seeding step and (ii)

389

the crystal growth step [131,132]. Fig. 8 shows the formation mechanism of the distinctive crystal morphology of Mg(OH)2 on the zeolite 4A surfaces. The GT method created roughened surface morphologies composed of whisker- and platelet-shaped nanocrystals. The highly roughened zeolite surfaces are thought to promote adhesion at the polymer particle interface via thermodynamically-induced adsorption and physical entanglement of polymer chains in the whisker structures by minimizing the entropy penalty, and yielded defectfree composite membranes with enhanced gas separation efficiency. An entropic force drives the polymer chains away from the filler surface, while favorable interactions with the surface can potentially overcome these configurational limitations. Hence, the interplay of energetic and entropic factors determines the final structure of the composite. It has been proposed that nanoroughened structures may stabilize polymer chain adsorption at such interfaces, relative to flat surfaces. It is known that the improved particle polymer interfacial adhesion facilitates particle dispersion in the composites [133–135]. The Grignard process was originally developed using an aluminosilicate such as zeolite 4A. A dealumination step provides nanocrystal seeds on the surface [130]. As noted above, this treatment was originally limited to aluminosilicate zeolite, because the whisker crystal seeds were formed during the dealumination step. Therefore, Grignard treatment is also effective on high silica zeolites if an aluminumcontaining zeolite, like 4A is used as an aluminum source. It has been observed that the Grignard treatment is ineffective on silica or high silica zeolite particles because of the lack of appropriate crystal seeds, on which Mg(OH)2 whiskers can grow [89,128,131]. 10.6. Copolymers and crosslinking of polymers Another way to prepare mixed matrix membranes which does not require the modification of zeolite surface is the employment of block copolymer containing both rigid and flexible chains [69,136,137]. The flexible component is a rubbery polymer, which provides flexibility and good contact with the zeolite surface, thus removing the need to use coupling agents, plasticizers or chemical reactions to prevent voids between the polymer and zeolite. The rigid component is a glassy polymer that has intrinsically higher selectivity than the rubbery polymer component. Also, the crosslinking of binary polymers can improve gas separation performance not found in a single polymer. This can also promote mechanical strength, chemical resistance, and thermal stability [138,139], thus stabilizing the gas transport properties. After crosslinking, the molecules of two polymers are covalently connected together to form a new polymer chain network via acetal linkages. Moreover, the combination of the crosslinking technique with the mixed matrix approach is

Fig. 8. Illustration of the formation mechanism of the distinctive crystal morphology of Mg(OH)2 on the zeolite 4A surfaces [89].

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11. Results and discussion

the PES-N-A membrane. A comparison of the gas permeation results for the PES-N-A and PES-M-A membranes shows that the nanostructured material has better performance in both of the gas permeabilities and the gas pair selectivities [152].

11.1. Effect of zeolite loading on gas permeability

11.3. Effect of zeolite pore size on gas permeability

Zeolite loading in the continuous polymer phase is an important factor, which affects the MMMs performance. In some cases by increasing zeolite loading the permeability was increased [13,47,68,71,108,109,115,125,140–145] and in other MMMs, the permeability was decreased [67,69,70,73,102,136,146–149]. Two possible hypotheses are considered for the decline in permeability. One is due to the inhibition of polymer chain mobility near the polymer–zeolite interface or rigidification of polymeric chains. The other hypothesis for the decline in permeability is possibly due to the partial pore blockage of zeolites by polymer chains. Even though polymer chains can hardly enter into the zeolite pores, they may obstruct a part of pores because of attaching to the zeolite surfaces. The increasing trend of selectivity is easily understandable due to the molecular sieving mechanism [67]. An increase in the permeability is due to the increase in void volume, which resulted in poor polymer–sieve contact. The formation of void spaces is due to the inability of the polymer chains to adhere properly to zeolite particles. Similar observation was also reported elsewhere [68,88,113,150]. When the zeolite loading was increased, the Tg of MMMs was increased up. This was most likely due to the increase in regularity of the macromolecules present around the zeolite particles. The presence of dispersed particles could affect the mechanical properties of the continuous phase. The zeolite restricts the movement of the polymer chains through the formation of hydrogen bonding (O–H) between the zeolite particles and the polymer, which increases the Tg. Li et al. [67] concluded that the increase in Tg with an increase in zeolite loading might indicate a fair interfacial interaction between polymer and zeolite [47,67,68,72,151].

The pore size of zeolites is another important factor, which influences the gas separation performance of mixed matrix membranes. It is obtained that permeability almost increases with an increase in zeolite pore size. This phenomenon implies that using large pore-size zeolite for MMMs would potentially offset the negative effects of partial pore blockage and polymer chain rigidification on permeability. Therefore, the gas molecules transport easier through the pores of zeolites. However, the selectivity decreases by zeolite pore size [67,71,102,109].

expected to simultaneously increase both the membrane stability and CO2 permeability [140].

11.2. Effect of zeolite particle size on gas permeability To date, the most of studies on polymer/inorganic MMMs use large particles, with sizes in the micron range. Tantekin-Ersolmaz et al. [86] indicated that the permeabilities of the silicalite–PDMS mixed matrix membranes increase with increasing particle size of the silicalite crystallites. This behavior may originate from difficulties in permeation due to the enhanced area and number of the zeolite–polymer interfaces in the cases employing relatively smaller particle sizes. It is most probable that mass transfer resistances occur at the zeolite–polymer interfaces. Both the higher total external area values (signifying a larger interface area) and the enhanced number of zeolite particles (signifying a higher number of interface crossing) obtained when smaller zeolite particles are employed may lead to an increase in the severity of the mass transfer resistances. The ideal selectivities, on the other hand, generally seem to be less affected by the changes made in the particle size [86]. On the other hand, Huang et al. [152] fabricated the polymer– zeolite mixed matrix membranes by incorporating nanosized or microsized zeolite 4A into polyethersulfone (PES-N-A and PES-MA, respectively). They showed that the diffusion of gas molecules through zeolites is generally dependent on the zeolite particle size and the Si/Al ratio. The higher Si/Al ratio, i.e., the lower cation content, will usually render the adsorbate having much higher diffusivity. The microsized zeolite 4A has lower Si/Al molar ratio than the nanosized counterpart, which may have rendered the PES-M-A membrane having higher gas diffusion hindrance than

11.4. Effect of zeolite shape on gas permeability Even though it has been recognized that the size and the shape of fillers have significant effects on the MMM performance, there is hardly any work of thorough investigation that can be found in the literature. As well, the data are very scarce for the intrinsic permeation properties of the inorganic particles, which is hampering the use of transport models such as the Maxwell equation to predict the MMM performance [15]. Barrier films which offer resistance to gas diffusion by filling the polymer with an impermeable flake appropriately oriented perpendicular to the diffusion have been reported [153,154]. The impermeable flakes have a high aspect ratio, i.e. the ratio of second largest dimension to the shortest dimension, the largest dimension being considered infinite in comparison to the other two. This results in reduction of the permeability of gases through the film due to increased diffusive path length. The concept could be used in improving gas separation efficiencies by incorporating a selective flake, which could be an exfoliated layered zeolite, into the polymer matrix. In two-dimensional molecular sieve materials the layers are bound together by hydrogen bonding with the template. Chemical modification to break apart the layers of these molecular sieve materials using a surfactant to form a selective flake with a high aspect ratio is termed as exfoliation. The transport of the less interactive gas would be reduced due to an increase in the diffusive path length whereas the interactive gas should be able to go through both the flake and polymer phase. This concept has been reported in literature for improving separation efficiencies of CO2 over N2 and CH4 [155]. The mixed-matrix membrane was formed using a polyimide and modified 2D layered aluminophosphate [155]. Jha et al. [156] reported that the increase in the CO2/H2 and CO2/He ideal separation factors refers to a probable tortuous path for the low sorbing gases due to flakes with high aspect ratios in the polymer matrix. Lately, it has been shown that spherical particles of ordered mesoporous silica MCM-41 with narrow particle size distribution (in the 2–4 mm diameter range) would facilitate the preparation of highly homogeneous MMMs. This spherical filler would minimize agglomeration and hence improve dispersability and interaction with the polymer for two reasons: (i) the spherical shape limits the contact between silica particles, and (ii) the micrometer size spherical particles provide a lower external surface area to volume ratio than that used in other reports (for instance, with approximately 80 nm MCM-41 particles [16]). Alternative approach to obtain MMMs would consist in the use of hollow zeolite sphere (HZS) particles. The obtained HZS-MMMs would benefit from the advantages of zeolites, as microporous and crystalline molecular sieves, (as well as the properties of spherical filler that would minimize agglomeration and hence improve dispersability

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and interaction with the polymer phase [57,157].The first intention of using HZS as inorganic filler in MMMs for gas separation has been reported by Zornoza et al. [57], showing that the separation performance of gas mixtures H2/CH4, CO2/N2 and O2/N2 was significantly improved due to the spherical shape that has contributed to good dispersion and well filler-polymer contact as well as the hollow nature to allow fast flow and increase the gas permeability [42,143]. 11.5. Effect of silane modification of zeolite surface on gas separation In most studies amino silane coupling agent has been chosen for chemical modification of the zeolite surface [65,69,70,102, 104,115,126,158–160]. Generally, the three main pathways of gas transport through the mixed matrix membranes are through dense polymer layer of matrix, highly selective zeolite sieve and nonselective gaps or voids between the matrix and sieve particles. Ismail et al. [65] indicated that the trend of permeability of PES mixed matrix hollow fiber membrane as a function of silane concentration and untreated zeolite were similar for all gases studied indicated that similar permeation mechanism for all of the membranes. Based on data, the permeability of all gases decreased with increasing silane concentration in PES modified zeolite while selectivity of these tested gases increased with increasing amino silane concentration. The increase in selectivity may be due to the formation of better contact between polymer and zeolite phases [65,108,109,126]. The silane coupling agents are postulated to impede the passage of the penetrant through adsorbing channels of the zeolites [71]. However, in some cases, surface modification by the silane coupling agents was reported to enhance interfacial adhesion but hardly improve permeability [69,70,102,104,158,160]. Some results indicate the influence of silane modification on the glass transition temperature of MMMs. The Tg of the mixed matrix membranes increased with the increasing of silane concentration on the surface treatment of the zeolite particles. This phenomenon reveals that the silane modification of zeolite affects the mechanical properties of continuous phase as suggested in the previous study [16]. The increase of Tg might be due to that zeolite restricts the movement of the polymer chains by the formation of hydrogen bonding between the zeolite particles and polymer [65,67,72]. 11.6. Effect of low molecular weight materials on gas separation performance Some researchers investigated the effects of LMWM on MMMs performances. Low molecular weight materials show different effects on gas separation processes. Some membranes have shown higher permeability and lower selectivity, while the other membranes have indicated a decrease in the permeability and an increase in the selectivity [71–73].S¸en et al. [73] suggested adding a low molecular-weight organic compound to the membrane formulation as a third component. The permeabilities of all gases through PC/pnitroaniline (pNA)/zeolite 4A membranes were lower than those through pure PC membrane. As the pNA concentration was increased for a given zeolite content, the extent of decrease in permeabilities increased. Similarly, when the zeolite content was increased at a constant pNA concentration, the extent of decrease in the permeabilities increased. The changes in permeabilities can be correlated with the kinetic diameter of the permeating gas. Also, the selectivity increased with increasing pNA and zeolite 4A concentrations in the membrane formulation [73]. 11.7. Effect of Grignard treatment of zeolite surface on gas separation Since transport is very sensitive to subnanometer defects due to the angstrom-sized gas molecules, it is ideally suited to probe the

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interfacial integrity between the inorganic and polymer phases. A new strategy to promote the interfacial adhesion in polymer/ silicate composites by creating nanoscale morphology is Grignard treatment. Remarkable changes have been observed in gas transport properties of composite films using the modified particles with nanostructured surface morphology [129]. Gas permeation measurements demonstrated that Grignard treatment shows impressive enhancements in gas separation efficiency. A thermodynamic argument was proposed to qualitatively explain why such nanoscale structures contribute to improved adhesion at the interfaces. A surface roughening effect has presumably contributed to the enhancement via thermodynamically induced adsorption and physical interlocking in the whisker structure [93,129,132]. 12. Future direction Recent attention is focused on mixed matrix membranes (MMMs) which have the advantage of high permeability together with better selectivity based on the presence of inorganic particles in the bulk of polymeric matrices. However, developments on the fabrication and application of MMMs containing inorganic particles inside polymer for gas separation are still quite low compared those for neat polymeric membranes, providing an opportunity for future developments [11]. The complicated situations existing at the interface connecting the polymer and particles make the formation of MMMs with expected performance difficult. The mechanisms behind these phenomena require intensive investigation. Recent molecular dynamic simulations of mixed matrix materials have shown decreased polymer chain mobility and permeability near an interface [11,59]. The keys to the next-generation MMMs may be to produce nano-size fillers without agglomeration and to obtain their separation properties. Clearly, nano-sized particles would allow for the formation of very thin membrane layers and distribution of nano-sized particles is more uniform compared with the distribution of micro-size particles. Smaller particles provide more polymer/particle interfacial area and enhance polymer–filler interface contact. Also, Nano-sized fillers can easily migrate to the outermost selective skin layer during the hollow fiber spinning [10,11,15]. One factor that is not often discussed is the fact that the shape and morphology of a particle of a given material can change significantly, as the size is reduced to these levels. The result could be a change is properties that will affect the membrane performance [10]. Furthermore, evaluation of performance in real conditions such as high temperature, high pressure and in the present of plasticizing components are necessary in order to search for thermally and chemically robust polymer-sieve pair. Better prediction of membrane performance under industrially relevant feed conditions is also important as the real conditions are usually fluctuated over time and membrane properties is a strong function of these operating parameters [161]. Hollow fibers are one of the most efficient membrane morphologies. Nevertheless, research on fabrication of mixed matrix hollow fibers is quite limited. Regarding the different formation mechanisms of dense films from asymmetric membranes, the following aspects should be considered for the formation of defect-free mixed matrix structure in asymmetric membranes: (1) the effect of the rheological properties of the dope solutions and the spinning conditions on skin layer formation, (2) the effect of particle shape and hydrophilicity on the skin layer micro-scale morphology during phase inversion, and (3) a reduction in the skin layer thickness, which entails the employment of ultrafine particles [11].

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Finally, but also very important, is the processing ability and economic cost. There are many polymers shown on a Robeson plot for a given separation but the number that can be economically processed as membranes is much smaller. The same issue is true for mixed matrix membranes [10]. These issues indicate that MMMs still have a long way to go to fully exploit its potentials. 13. Conclusions The following summary and conclusions can be drawn from this article review:  Mixed matrix membranes with zeolite molecular sieve dispersed phase in a polymer matrix have potentials to provide both high gas superior selectivity of the molecular sieves and the desirable mechanical and economical properties of the polymers. A significant mismatch and incompatibility between the properties of zeolite phase and polymer phase can produce unsatisfactory performance of MMMs.  The poor compatibility between the polymer matrix and sieve particles has been improved through some methods, such as the silane modification and priming on the particle surface, the introduction of a LMWA agent between the polymer matrix and inorganic particles, and the application of high-processing temperatures during the membrane formation.  The vast majority of mixed matrix membrane configurations are flat sheet configurations; however, hollow fiber MMMs hollowfiber membrane has become a favored configuration for gas separation systems due to its many advantages such as larger membrane area per volume, good flexibility and easy handling in the module fabrication.  To date, the most of studies on zeolite–polymer MMMs use large particles, with sizes in the micron range. Smaller particles provide more polymer/particle interfacial area and enhance polymer–filler interface contact. Some results shows that the nano size particles embedded in MMMs has better performance in comparison with micron size particles. However, much research and development are still needed to develop zeolite–polymer mixed matrix membranes for gas separation applications. References [1] R.W. Baker, Membrane Technology and Applications, 2nd ed., John Wiley & Sons, New York, 2006. [2] M. TakhtRavanchi, T. Kaghazchi, A. Kargari, Desalination 235 (2009) 199. [3] C.E. Powell, G.G. Qiao, Journal of Membrane Science 279 (2006) 1. [4] M. Ulbricht, Polymer 47 (2006) 2217. [5] R.D. Noble, R. Agrawal, Industrial & Engineering Chemistry Research 44 (2005) 2887. [6] W.J. Koros, R. Mahajan, Journal of Membrane Science 175 (2000) 181. [7] L.M. Robeson, Journal of Membrane Science 62 (1991) 165. [8] Shekhawat, D., Luebke, D.R., Pennline, H.W., 2003. A review of carbon dioxide selective membranes, A Topical Report, National Energy Technology Laboratory United States Department of Energy December 1, 2003. [9] C.M. Zimmerman, A. Singh, W.J. Koros, Journal of Membrane Science 137 (1997) 145. [10] R.D. Noble, Journal of Membrane Science 378 (2011) 393. [11] T.S. Chung, L.Y. Jiang, Y. Li, S. Kulprathipanja, Progress in Polymer Science 32 (2007) 483. [12] D.R. Paul, D.R. Kemp, Journal of Polymer Science: Polymer Physics 41 (1973) 79. [13] K. Friess, V. Hynek, M. Sˇı´pek, W.M. Kujawski, O. Vopicˇka, M. Zgazˇar, M.W. Kujawski, Separation and Purification Technology 80 (2011) 418. [14] Y. Li, T.S. Chung, Z. Huang, S. Kulprathipanja, Journal of Membrane Science 277 (2006) 28. [15] M.A. Aroon, A.F. Ismail, T. Matsuura, M.M. Montazer-Rahmati, Separation and Purification Technology 75 (2010) 229. [16] D.Q. Vu, W.J. Koros, S.J. Miller, Journal of Membrane Science 211 (2003) 311. [17] D.Q. Vu, W.J. Koros, S.J. Miller, Journal of Membrane Science 211 (2003) 335. [18] L.Y. Jiang, T.S. Chung, C. Cao, Z. Huang, S. Kulprathipanja, Journal of Membrane Science 252 (2005) 89.

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