A review of polymeric composite membranes for gas separation and energy production

Progress in Polymer Science 97 (2019) 101141

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Progress in Polymer Science journal homepage: www.elsevier.com/locate/ppolysci

A review of polymeric composite membranes for gas separation and energy production Can Zeng Liang a , Tai-Shung Chung a,∗ , Juin-Yih Lai b,∗ a b

Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117585, Singapore Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei, Taiwan

a r t i c l e

i n f o

Article history: Received 21 March 2019 Received in revised form 26 May 2019 Accepted 6 June 2019 Available online 21 June 2019 Keywords: Gas/vapor separation Composite membrane Thin film composite Gas transport Phase inversion Aging/plasticization

a b s t r a c t Membrane-based technologies for gas and vapor separations have been industrialized for more than three decades due to (1) the invention of asymmetric composite membranes and (2) their superior energy-efficient and environment-friendly characteristics. However, industries demand more robust and productive composite membranes to meet the harsh conditions in new or emerging applications. The fabrication of thin film composite (TFC) membranes consisting of an ultrathin selective layer and a strong porous substrate will likely play the pivotal role to meet the future demands. In this review, we revisit the basis of composite membranes for gas and vapor separations with the aim to produce next-generation high-performance composite membranes. This review focuses on (1) the fundamentals of gas transport theories through composite membranes, (2) fabrication techniques, and (3) performance characteristics and stability of composite membranes. We examine, elucidate and explore the state-of-the-art techniques for the fabrication of composite membranes. Based on the resistance model, we derive a new generalized equation for gas permeance, which is simple, informative and applicable to composite membranes. We also outline the directions, challenges and key areas for future R&D in the field of gas/vapor-separation composite membranes. © 2019 Elsevier B.V. All rights reserved.

Contents 1. 2.

3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Theory of gas transport through membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. Gas permeation and electrical circuit analogy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2. Composite membranes comprising a single dense-selective layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.3. Composite membranes with two defect-free layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.4. Composite membranes with n defect-free layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.5. Resistance of the substrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.6. The general gas permeance equation for TFC membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Fabrication of composite membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.1. Preparation of porous membrane substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.2. Fabrication of TFC membranes with porous substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.2.1. Composite membranes with slightly defective substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Abbreviations: ALD, atomic layer deposition; ATRP, atom transfer radical polymerization; CA, cellulose acetate; CN, cellulose nitrate; CVD, chemical vapor deposition; FFV, fractional free volume; GPU, gas permeation units; HT, Henis and Tripodi model; IP, interfacial polymerization; MOF, metal-organic framework; NIPS, non-solvent induced phase separation; PA, polyamide; PAN, polyacrylonitrile; PAS, polyaminosiloxane; PDMS, polydimethylsiloxane; PE, polyethylene; PEBA, polyether block amide; PEG, polyethylene glycol; PEI, polyetherimide; PES, polyether sulfones; PI, polyimide; PIM, polymer of intrinsic microporosity; PPO, polydimethylpheny1ene oxide; PSF, polysulfone; PTFE, polytetrafluoroethylene; PTMSP, polytrimethylsilylpropyne; PVDF, polyvinylidene fluoride; PVP, polyvinylpyridine; PVP, polyvinylpyrrolidone; RM, resistance model; SEM, scanning electron microscopy; SI, international system of units; STP, standard temperature and pressure; TFC, thin film composite; TIPS, thermally induced phase separation; UV, ultraviolet; VIPS, vapor induced phase separation; ZIF, zeolitic imidazolate framework. ∗ Corresponding author. E-mail address: [email protected] (T.-S. Chung). https://doi.org/10.1016/j.progpolymsci.2019.06.001 0079-6700/© 2019 Elsevier B.V. All rights reserved.

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C.Z. Liang et al. / Progress in Polymer Science 97 (2019) 101141

3.2.2. Composite membranes with highly porous substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.3. Other advanced techniques to form TFC membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Gas transport through composite membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Aging and plasticization of TFC membranes for gas separation and energy production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Conclusion and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4. 5. 6.

Nomenclature p A A1 A2 I J k0 k2 kn L L0 L1 LE Li Ln LN LS LS1 P0 P1 P2 Pi Pn Qi R R0 

R0 R1 R2 Ri RS Rt Tg V ε

Pressure or partial pressure difference across the membrane Effective area of the membrane Areas of the porous regions of the substrate’s outer surface Areas of the non-porous regions of the substrate’s outer surface Current through a resistor Gas permeance Ratio of gas permeability (k0 = P0 /P1 ) Ratio of gas permeability (k2 = P2 /P1 ) Ratio of gas permeability (kn = Pn /P1 ) Membrane thickness Effective depth of intrusion Thickness of the top layer Equivalent thickness Imaginary thickness Thickness of the nth top layer Nominal thickness Thickness of the substrate’s substructure Equivalent thickness with respect to P1 Gas permeability of the ubstrate materials Gas permeability of the first top layer Gas permeability of the second top layer Permeability of the membrane material to gas i Gas permeability of the nth layer material Flux of a gas i Resistance of the resistor Resistance of the intruded material with an effective depth of L0 Resistance of the substrate’s non-porous region with an effective thickness of L0 Resistance of the top layer with a thickness of L1 Resistance of the second top layer with a thickness of L2 Resistance to a gas permeate through the membrane Resistance of the rest substructure or the bulk substrate Total resistance of the composite membrane Glass transition temperature Voltage Surface porosity of the substrate

1. Introduction Separation and purification of gas and vapor mixtures are indispensable industrial processes in order to meet specific product requirements and purities [1–3]. Generally, conventional separation processes such as distillation, evaporation, absorption and drying, account for 10–15% of the world’s energy consumption [3]. Even though some of them (e.g. cryogenic distillation and adsorption/absorption) can separate gas and vapor mixtures, they

are not only energy-intensive but also produce pollutions [4,5]. Membrane-based separation technologies for gas and vapor mixtures usually do not involve in phase change, sorbents and their regeneration [6]. Therefore, membrane technologies are attractive because of their advantages such as energy efficiency, operational continuity and simplicity, small footprint, easy scale up, module compactness and modularity, cost competitiveness and environmental friendliness [4,5,7–11]. Comparing to the traditional distillation processes, the membrane-based processes may consume about 90% less energy [3]. The phenomenon of gas separation by means of membranes was discovered in the early nineteenth century [12]. However, only until the late 1970s, the first-generation industrial membrane for gas separation was realized due to the invention of asymmetric polymeric membranes [13,14] and composite membranes [15–18]. It has a composite configuration consisting of a thin defect-free selective layer and a porous and strong substrate to separate hydrogen from nitrogen during the ammonia production. The defect-free layer was obtained by coating the asymmetric substrate with a highly permeable material (e.g. silicone rubber) [15–17]. Since then, various gas-separation membranes have been designed for a wide range of applications, such as air separation, natural gas sweetening, hydrogen production and purification, biogas upgrading, olefin/paraffin separation, petrochemical industry application, volatile organic vapor recovery and dehumidification as well as CO2 capture from fossil-fuel-burning power plants [7,10,19–32]. Membrane-based processes are energy-efficient in gas separation and purification, in turn, the purified or upgraded gases (e.g. syngas, natural gas and biogas) can improve the efficiency of energy production of power plants, where the syngas, biogas and/or natural gas are used as fuels [19,33–37]. Therefore, the nexus between the membrane and energy production is strong. Besides, membranes can be used to produce oxygen-enriched air, which is able to increase the fuel-heat efficiency of a furnace/engine. Subsequently, more energy/electricity can be generated by power plants and/or combustion engines [4,14,38]. The current gas separation membranes must be further advanced in terms of permeance and selectivity in order to expand their market share and compete with the conventional separation technologies. Since gas permeance is inversely proportional to the dense-layer thickness, one must choose either highly permeable materials or reduce the thickness of the dense-selective layer in order to prepare more productive membranes. However, a highly permeable material usually has a low selectivity because of the trade-off relationship between permeability and selectivity for polymeric materials [39,40]. Therefore, the construction of composite membranes consisting of one or multi-thin layers on top of porous substrates (referred to as thin film composite (TFC) membranes thereafter) becomes the potential solution to maximize the selectivity, permeance and mechanical properties [14,16–18,27,41,42]. Comparing with conventional integrally skinned asymmetric membranes, the TFC membranes have the following advantages: (1) only small amounts of the selective-layer materials of approximately less than 2 g/m2 are needed to be coated on top of porous substrates as the selective layer in comparison with about 50 g/m2 needed for the fabrication of integrally skinned

C.Z. Liang et al. / Progress in Polymer Science 97 (2019) 101141

asymmetric membranes. Consequently, there is a significant saving on material costs; (2) it allows to optimize the membrane material and morphology in each layer independently according to the product requirements; and (3) there are fewer limitations on mechanical properties and processability of membrane materials as long as they can be formed or deposited as a thin layer on top of the substrates. However, there are some challenges to fabricate high-performance defect-free TFC membranes [14,22,27,42,43]: (1) effects of substrate resistance; (2) defective selective layers because of ultrathin thickness; (3) material intrusion into the substrates; and (4) the accelerating aging and plasticization of the ultrathin selective layer. These deleterious issues could compromise the performance of composite membranes. Bearing the benefits, motivations and challenges in mind, we aim to provide a critical review on composite membranes for gas and vapor separations so that this work may serve as a fundamental and useful guideline for producing high-performance TFC membranes. In this review, we focus on the fundamentals of gas transport theories across the composite membranes, their fabrication methods, and performance stabilities. We start by revisiting and analyzing various gas transport theories and models of composite membranes, then propose and derive a new general permeance equation, which is simple and suitable for explaining and predicting gas transport behavior through composite membranes. Secondly, the principles and methods of fabricating high-performance composite membranes are elucidated and explored. Thirdly, the gas transport phenomena across the composite membranes are analyzed. Fourthly, the aging and plasticization issues related to the thin film composite are discussed, and the strategies for addressing those problems are provided and discussed. Finally, we would highlight the directions, challenges and key areas for future research and development on gas/vaporseparation composite membranes.

2. Theory of gas transport through membranes The gas permeation through a symmetric dense membrane made of gum elastic (i.e., natural rubber) was discovered in 1830 by Mitchell in the Philadelphia Medical Institute [12]. Graham postulated the gas transport across the dense polymeric membranes under the solution-diffusion mechanism in 1866 [44]. The fundamental theories about gas transport through membranes under pore-flow and solution-diffusion mechanisms were extensively studied by Barrer around 1950s [45–50]. To honor Barrer’s contributions, the gas permeability of membranes is mostly expressed in Barrer that is not an International System of Units (SI) unit. 1 Barrer = 1 × 10−10 cm3 (STP) cm/(cm2 s cmHg), STP stands for standard temperature and pressure [14,51]. The gas permeance or the pressure-normalized flux is normally expressed in terms of gas permeation units (GPU) and 1 GPU = 1 × 10−6 cm3 (STP)/(cm2 s cm Hg). The conversions between GPU and other SI gas permeance units can be found elsewhere [22,52]. In 1981, Henis and Tripodi [15–17] developed a resistance model (RM) to explain the gas transport behavior of composite membranes which consisted of a porous asymmetric substrate such as polysulfone (PSF) or polyacrylonitrile (PAN) and a silicone rubber coating layer. In their model, the gas permeation across composite membranes was analogous to the electric current flowing through an array of resistors in series-parallel. The use of RM helped predict the gas permeation behavior such as permeance and selectivity from the intrinsic permeability and physical properties of the porous substrates and coating materials. In fact, the Henis and Tripodi model (hereafter called HT model) was a specific case because it was based on the following assumptions: (1) the penetrant diffusion was one-dimensional across the membrane; (2) the

3

intrusion depth of the coating material was equal to the thickness of the dense-selective layer; (3) the gas permeability was independent of pressure. Since then, however, other models with some deviations from the HT prediction have been proposed [53–57]. In addition, various experimental and theoretical refinements have been made to improve the HT model for gas permeation in composite membranes by taking into account of various factors such as the intrusion depth, incomplete pore plugging, substrate properties and multi-layers of composite membranes [18,27,41,42,57–70]. Nevertheless, due to the beauty of its simplicity, the RM proposed by Henis and Tripodi is still the most popular and acceptable model for gas transport through composite membranes consisting of ultrathin dense-selective layers and porous substrates. Therefore, the objective of this section is to derive a more general and simple equation for gas transport through TFC membranes. In this section, the following discussion and development of permeance equations are based on the one-dimensional model (the direction of the penetrant diffusion is perpendicular to the membrane surface) with the consideration of other possible scenarios such as the material intrusion and the number of layers of TFC membranes. 2.1. Gas permeation and electrical circuit analogy The flux (Qi ) of a gas i through a defect-free symmetric dense membrane, which is made of only one material, can be expressed by the following equation: Qi =

Pi Ap p = L L (P A)

(1)

i

where Pi is the permeability of the membrane material to gas i, A is the effective surface area of the membrane, L is the membrane thickness, and p is the pressure or partial pressure difference across the membrane. Eq. (1) is mathematically equivalent to the Ohm’s law, which describes the current (I) flowing through a resistor [15–17]: I=

V R

(2)

where V is the voltage or electric potential across the resistor, R is the resistance of the resistor. Comparing Eq. (1) with Eq. (2),Qi is analogous to I, p is analogous to V , and L/(Pi A) is analogous to R. Therefore, the resistance (Ri ) to a gas permeate or flow through the membrane can be defined as: Ri =

L Pi A

(3)

By combining Eqs. (1) and (3), we have: Qi =

p Ri

(4)

2.2. Composite membranes comprising a single dense-selective layer As indicated in Table 1, composite membranes may consist of a single or multiple dense-selective layers on top of porous substrates. For composite membranes comprising a single denseselective layer, as illustrated in Fig. 1a, the first layer material which is on top of the porous substrate, usually intrudes into the substrate’s pores. The governing equation for the gas transport across the composite membrane (refer to Fig. 1) can be described as follows with the definitions and assumptions below: (1) The top layer (in the solid red color) is defect-free and dense, while the substrate layer (in the gray color) is porous. A is

4

C.Z. Liang et al. / Progress in Polymer Science 97 (2019) 101141

Table 1 Representative configurations and structures of TFC membranes.a Materials of composite membranes

Configuration

Singlelayer

Multilayer

Reference

Substrate

First layer

Second layer

Third layer

PAN PSF PSF PSF PES PVDF PEI PEI PSF PSF PSF PAN PAN PEI PAN PES

Silicon rubber or PDMS Silicon rubber or PDMS PEBA CA PDMS PDMS PDMS BEBA PAS PVP CN PDMS PDMS PDMS PTMSP PEG-hybrid

/ / / / / / / / PPO Silicone rubber Silicone rubber PEBA PIM PEBA Matrimid® PEG-hybrid

/ / / / / / / / / / / / / PDMS PDMS PEG-hybrid

[16,27,74] [18,65,69] [67,118] [59] [60] [75] [67,123] [131] [54] [41] [77] [122] [124] [70] [66] [21]

a Polyacrylonitrile (PAN), polydimethylsiloxane (PDMS), polysulfone (PSF), cellulose acetate (CA), polyether sulfones (PES), polyvinylidene fluoride (PVDF), polyetherimide (PEI), polyether block amide (PEBA), polyaminosiloxane (PAS), polydimethylpheny1ene oxide (PPO), polyvinylpyridine (PVP), cellulose nitrate (CN), polymer of intrinsic microporosity (PIM), polytrimethylsilylpropyne (PTMSP), polyethylene glycol (PEG).

Fig. 1. The schematic diagram of (a) the cross-section of composite membranes comprising a single dense-selective layer with intrusion and (b) the analog of an electrical circuit.

the total surface area of the composite membrane, A = A1 + A2 where A1 and A2 are the areas of the porous and non-porous regions of the substrate’s outer surface, respectively. (2) L1 is the thickness of the top layer, while L0 is the effective depth of intrusion where the pores are filled with the same material as the top layer. The effective thickness of the substrate’s nonporous region is equal to L0 , while LS is the thickness of the substrate’s substructure. (3) R1 represents the resistance of the top layer with a thickness of L1 ; R0 represents the resistance of the intruded material with  an effective depth of L0 , R0 represents the resistance of the substrate’s non-porous region with an effective thickness of L0 ; RS represents the resistance of the rest substructure or the bulk substrate. (4) P1 and P0 are the gas permeability of the top layer and substrate materials, respectively. The surface porosity of the substrate: ε = A1 /A; the ratio of gas permeability: k0 = P0 /P1 .

Although the substrate’s substructure is usually designed to widely open (highly porous) and RS is very small for most cases, it could be large for other cases when the gas/vapor permeance is very high. For the sake of mathematical simplicity, we drop the bulk resistance of the substrate (RS ) first, and include and discuss it later (in Section 2.5). Therefore, Eq. (5) becomes:

Based on the above concept and the schematics in Fig. 1, the total resistance (Rt ) of the composite membrane with a single defect-free (i.e., dense-selective) layer can be expressed as:

Q P = = J= L pA



Rt = R1 +

Rt = R1 +



R0 + R0

+ RS

(5)



R0 + R0

= R1 +

1

(6)



(1/R0 ) + (1/R0 )

Because the gas permeability of the top and substrate layers are P1 and P0 , respectively, one can substitute Eq. (3) into Eq. (6) and obtain: Rt =

L1 1 + P1 A (P1 A1 /L0 ) + (P0 A2 /L0 )

(7)

According to the definition of gas permeance (J = P/L), gas permeance can be expressed as follows:



p/Rt pA



=

1 = Rt A



1 L1 + P1 (P1 /L0 )(A1 /A) + (P0 /L0 )(A2 /A)

−1 (8)

Substituting A = A1 + A2 and ε = A1 /A into Eq. (8) yields:



R0 R0

R0 R0

J=

L

1

P1

+

L0 P1 ε + P0 (1 − ε)

−1

(9)

C.Z. Liang et al. / Progress in Polymer Science 97 (2019) 101141

Let k0 =

J=

L

1

P1

+

P0 , then substituting P0 P1

L0 P1 [ε + k0 (1 − ε)]

Assuming J=

L0 ε+k0 (1−ε)

 L + L −1 1 i P1

=

= k0 P1 into Eq. (9), one obtains:

⎛

−1

=⎝

L1 +



L0 ε+k0 (1−ε)

 ⎞−1 ⎠

P1

(10)

= Li and substituting Li into Eq. (10), yield:

P1 P1 = L1 + Li LN

(11)

where Li is called the imaginary thickness because L0 varies from place to place and its true value is small and cannot be measured easily; the unit of Li is the same as L1 (i.e., ␮m) since ε and k0 are dimensionless; LN = L1 + Li , LN is called the nominal thickness, it consists of a real part (L1 , can be measured) and an imaginary part (Li ). 2.3. Composite membranes with two defect-free layers

(1) Both the 1st and 2nd top layers are defect-free and dense with thicknesses of L1 and L2 , respectively. (2) R1 and R2 represent the resistance of the 1st and 2nd top layers, respectively. (3) P1 and P2 are the gas permeability of the 1st and 2nd top layer materials, respectively. As displayed in Fig. 2, the total resistance (Rt ) of a two-layer defect-free composite membrane with intrusion to the substrate can be expressed as: 

Rt = R2 + R1 +

R0 R0

+ RS



R0 + R0

(12)

Similar to the aforementioned, RS can be dropped first, and considered later (in Section 2.5). Eq. (12) becomes: Rt = R2 + R1 +

1

(13)



(1/R0 ) + (1/R0 )

Eq. (13) can be rewritten using Eq. (3) as follows: Rt =

L2 L1 1 + + P2 A P1 A (P1 A1 /L0 ) + (P0 A2 /L0 )

(14)

According to the definition of the gas permeance, we have:



P = L

J=

p/Rt pA



=

1 = Rt A



L2 L1 1 + + P2 P1 (P1 /L0 )(A1 /A) + (P0 /L0 )(A2 /A)

−1 (15)

Substituting ε = A1 /A and A = A1 + A2 into Eq. (15), we obtain: J=

L

2

P2

+

L1 L0 + P1 P1 ε + P0 (1 − ε) P

−1

(16)

P

Let k0 = P0 and k2 = P2 , then substituting P0 = k0 P1 and P2 = 1 1 k2 P1 into Eq. (16), yields:

 J=

L2 L1 L0 + + P1 k2 P1 P1 ε + k0 P1 (1 − ε)

Let J=

L

L2 k2

E

P1

+ L1 = LE and

+

Li P1

−1

=

−1 L2

L0 ε+k0 (1−ε)

P1 P1 = LE + Li LN

=

k2

+ L1 P1

+

L0 ε+k0 (1−ε)

−1

P1

where LE is called the equivalent thickness with respect to the gas permeability of the reference material (i.e., P1 ), L2 and L1 can be measured. Similar to the previous cases, Li is called the imaginary thickness because L0 varies from place to place, its value is small and cannot be measured easily. LN = L1 + Li , LN is called the nominal thickness, it consists of a real equivalent part (LE ) and an imaginary part (Li ). The units of LE , Li and LN are the same as L1 (i.e., ␮m) since ε and k2 are dimensionless. 2.4. Composite membranes with n defect-free layers We can perform similar operations like the above for a composite membrane consisting of n-defect-free layers, where n ≥ 3 as shown in Fig. 3. The permeance equation for the n-layer composite membrane can be expressed as:

Ln

J=

kn

+ ... +

(17)

= Li , Eq. (17) becomes: (18)

L2 k2

+ L1

P1

where LE =

Similar to the previous case (Section 2.2), the governing equation for the gas transport through a composite membrane comprising two defect-free layers (refer to Fig. 2) can be described as follows with the additional definitions and assumptions:

5

Ln kn

+

Li P1

, kn =

−1  = Pn , P1

Li LE + P1 P1

−1 =

P1 P1 = LE + Li LN

and n ≥ 3. Pn is the gas permeability

of the nth layer material. For example, if n = 3, LE = L1 + where k1 =

P1 P1

(19)

L2 k2

+

L3 , k3

= 1.

2.5. Resistance of the substrate The resistance of gas transport through the membrane substrate (RS ) could be assumed as a sum of various transport modes such as Knudsen flow, viscous or Poiseuille flow and surface flow [5,53,54,67]. These transport modes are a function of temperature, pressure, geometry and size of pores, and bulk structures. A completely precise calculation of these modes is a formidable task [53,71]. Here, we use a black-box method to treat RS to avoid the complexity of various transport modes by assuming that the substrate has a permeance for a gas i, called Ji , which can be obtained by doing simple permeation measurements as follows: Ji =

P1 LS1

(20)

where LS1 is an equivalent thickness with respect to P1 , which is the gas permeability of the reference material (e.g. the 1st layer material of the composite membrane, as shown in Figs. 1–3). Since the resistance of the substrate is in series with the resistances of other coating layer(s) such as R1 , R2 and Rn , so once LS1 = P1 /Ji is determined, LS1 can be directly substituted into the permeance equations of Eqs. (11), (18) and (19), in which LS1 has the same power/order with L1 because they share the sample permeability P1 (by definition). For example, in Eq. (19), with the consideration of the substrate’s resistance, the equivalent thickness can be expressed as: LE = LS1 +

Ln

kn

(21)

2.6. The general gas permeance equation for TFC membranes Based on the definition of permeance and the above derivations, the general gas permeance equation for TFC membranes with defect-free top layers can be written as follows: J=

P P = LN LE + Li

(22)

where P is the gas permeability of the reference material (e.g., the 1st layer serves as the reference material and has a gas permeability of P1 ). LN is the nominal thickness, LN = LE + Li . It consists thickness (LE ) and an imaginary thickness (Li ). of a real/equivalent Ln LE = LS1 + , (the integer n ≥ 1), n is the number of layer(s) that k n

above the substrate; Ln is the thickness of the nth layer; kn = Pn /P is

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Fig. 2. The schematic diagram of (a) the cross-section of a two-layer defect-free composite membrane and (b) the analog of an electrical circuit.

Fig. 3. The schematic diagram of (a) the cross-section of an n-layer defect-free composite membrane, and (b) the analog of an electrical circuit.

the ratio of gas permeability of the nth layer to the reference layer or the reference material, Pn is the gas permeability of the nth layer material (i.e., k1 = P1 /P1 = 1). The imaginary thickness is defined L0 as Li = ε+k (1−ε) , 0 < ε < 1. 0

Thus far, a simple and general permeance equation for gasseparation TFC membranes has been developed. The derivatives and equation can be further refined to achieve the ultimate goal of a simple and universal permeance equation. Challenges that remain in this area include (1) the process of how to account for defective layer(s) that are attached to the substrate; (2) the development of a simple equation with the consideration of two-dimensional gas diffusion; (3) methodologies to determine the real depth of the material intrusion. 3. Fabrication of composite membranes The conventional defect-free integrally skinned asymmetric membranes can be regarded as a special type of composite membranes, in which the defect-free skin layer is the first layer and the microporous substructure is the substrate of the composites. The fabrication of TFC membranes consisting of a porous substrate

is complex because it involves in many variables such as material intrusion, surface porosity of substrates, compatibility and processability between the top and substrate materials. In this section, we will first examine the preparation of membrane substrates and then discuss the methods of fabricating composite membranes. 3.1. Preparation of porous membrane substrates TFC membranes with porous substrates have been widely studied not only for water treatments [72–74], but also for gas separation because the porous substrates function as mechanical supports that have resistance for mass transport. The substrate resistance for gas separation can be significant depending on the physicochemical properties of the material used in each layer and gaseous penetrants [16,37,67,74–82]. Therefore, the optimization of substrate’s properties and morphology in composite membranes is sometimes as important as the formation of defect-free thin selective layer(s) [21,27,67,76,81,83]. Beside the requirements of strong mechanical strength and good thermal and chemical stabilities, the desired substrate should have (1) a highly porous bulk structure; (2) a porous and thin skin

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layer; (3) a smooth outer surface; and (4) a high surface porosity but with small surface pores and a narrow pore size distribution. Such characteristics not only minimize the gas transport resistance across the membrane but also minimize the material intrusion into the substrate [17,18,74,77–80,82–84]. The substrates of composite membranes are either in the form of tube/hollow fiber or flat sheet/disc. Generally, they can be made from inorganic and organic materials. The inorganic membrane substrates can be made from materials such as alumina, titanium, zinc oxide, ceramic, glass frit, metal and carbon, etc. They are thermally and chemically stable, but they are expensive and fragile [5,7,8,10,85–87]. The organic membrane substrates are usually made of polymers such as polyacrylonitrile (PAN), poly(ether sulfones) (PES), polysulfones (PSf), poly(vinylidene fluoride) (PVDF), polytetrafluoroethylene (PTFE), polyetherimide (PEI), polyimide (PI), polyamide (PA); cellulose acetate (CA) and polyethylene (PE); and they are can be fabricated by various methods, which include the phase inversion, the melt extrusion, the controlled stretching, the electro-spinning and the track etching [10,15–18,21,22,26,27,37,41,53–57,60–63,69,74,75,79,80,82,83, 88–91]. Currently, phase-inversion processes, including non-solvent induced phase separation (NIPS), thermally induced phase separation (TIPS), vapor induced phase separation (VIPS) and solvent evaporation, are commonly used for the fabrication of porous substrates [5,7,11,89,91–95]. Among various phase-inversion processes, NIPS is mostly studied and widely applied due to its versatility, convenience, scalability and low cost [6,11,89,93]. It is a de-mixing process, where a polymer dope solution is de-mixed into a polymer-rich phase that forms the membrane after the precipitation, and a polymer-poor phase that creates membrane pores after the subsequent removal of the solvents. The NIPS method has provided a wide range of opportunities to prepare membranes with a plethora of morphologies [6,11,89,92,93,95]. However, to some extent, NIPS is an empirical and heuristic method partly due to the complexity of membrane formation [11,89,91–93]. A rule of thumb is as follows: (1) substrate porosity increases while

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mechanical strength decreases with a decrease in polymer concentration of polymer dopes and (2) substrate porosity can be increased by adding pore-forming agents into dopes such as polyethylene glycol (PEG), poly(vinyl pyrrolidone) (PVP), silica, supercritical carbon dioxide and copolymer additives [11,89,93]. Though polymeric hollow fibers are the dominant membranes for gas separation industries [4,7,10], the fabrication of hollow fiber membranes is much more complex than that of flat-sheet membranes. This is because the former depends on many factors such as chemistry of dopes, inner and external coagulants, and process parameters such as spinneret dimension, air gap, temperature, humidity and take-up speed [11,96–99]. Experiences suggest that apart from adding pore forming agents, temperature increases in the coagulant bath and the suitable choice of the polymer dope and spinneret can favorably influence the porosity and gas permeance of hollow fiber membrane substrates [97,97,98,99]. Second, hollow fiber membrane substrates with a desirable substructure and a thin skin layer full of small pores may be produced by tuning the dope concentration, bore fluid chemistry and spinning conditions [21,27,83,96–101]. Fig. 4 shows the scanning electron microscopy (SEM) images and pore size distribution of such a porous hollow fiber substrate made of PAN [83,100]. Flat-sheet polymeric membrane substrates with uniform surface pores in the range of 10–100 nm have been recently produced by using block copolymers with self-assembling properties. The block copolymers could firstly form a segregated block-copolymer template consisting of dense microphases. Subsequently, one of the blocks was selectively dissolved or etched, thus forming uniform pores [102–106]. Fig. 5 displays the representative SEM images of an asymmetric flat-sheet membrane substrate made of polystyrene-b-poly (4-vinylpyridine) diblock copolymer [102]. 3.2. Fabrication of TFC membranes with porous substrates 3.2.1. Composite membranes with slightly defective substrates Most commercially available gas separation membranes consisting of integrally skinned asymmetric membranes belong to this

Fig. 4. Morphologies and properties of the hollow fiber membrane substrate made of PAN. (A) The enlarged outer surface; (B) the outer-inner interface; (C) the enlarged inner surface; (D) the cross-section; (E) the outer skin layer; (F) the physical properties of the substrate. Reproduced from references [83,100]. Copyright 2017. Reproduced with permission from Elsevier and John Wiley and Sons.

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Fig. 5. SEM images of the morphologies of the asymmetric flat-sheet membrane made of polystyrene-b-poly(4 vinylpyridine) diblock copolymer. (A) The cross-section; (B) the enlarged cross-section; (D) the enlarged cross-section and top surface; and (D) the enlarged top surface. The average top surface pore size is about 10 nm [102]. Copyright 2007. Reproduced with permission from Springer Nature.

class of composite membranes because the uncoated integrally skinned asymmetric membranes are slightly defective with a surface porosity of about or less than 10−5 [15–17]. Generally, there are two approaches to repair the defects. One is to plug the defects with a highly permeable material such as silicon rubber to recover the selectivity of the integrally skinned asymmetric membranes [11,15–18,59,107–111], while the other is to deposit a highly selective material on top of asymmetric membranes that not only seals the defects but also enhances the overall selectivity [112–114]. 3.2.2. Composite membranes with highly porous substrates When the substrates are highly porous and have a large surface porosity, techniques such as solution coating, interfacial polymerization, lamination, chemical vapor deposition, and plasma polymerization have been employed to fabricate the composite membranes for gas separation. The solution coating method includes processes such as dip coating [18,27,41,55,67,69,75,82,83,100,101,115–125], spin coating [126–128], continuous coating [21,62,129–131], spray coating [132], brush coating [132], pouring [75,123], and solution casting [123,133]. The coating process, particularly dip coating, is popular in both industry and academia because of its simplicity, ease of scale-up and cost-effectiveness. However, during the coating process, the intrusion of the first layer’s material into the substrate pores is frequently encountered or inevitable. Fig. 6 schematically describes the dip coating processes with and without controlling the intrusion. The intrusion of coating materials narrows the channels for gas transport and prolongs the transport distance, thus leads to a low gas permeance [42,57,62,75]. To minimize or reduce the intrusion, a number of techniques have been developed. For example: annealing [21,59,62], quenching [134], pre-wetting [27,62,122,130,135,136], guttering [66,122,124,137,138], post pore opening [139] and pore sieving [140]. In general, annealing reduces pore sizes so as to reduce the intrusion. Quenching increases solution viscosity to retard the intrusion.

Pre-wetting temporarily seals substrate’s pores, blocking the intrusion paths. The pre-wetting agents are preferably immiscible with the coating solutions but can be evaporated or leaching out easily in the subsequent heating or solvent exchange processes. Guttering is to introduce a non-porous intermediate layer between the substrate and the outer selective layer. The gutter layer is preferably made from highly permeable materials compatible with the adjacent layers. For example, PDMS, poly(1-(trimethylsilyl)1-propyne) (PTMSP) and metal-organic frameworks (MOF) are popular gutter layer materials because they have high gas permeability [22,66,122,124,137,138]. So far, PTMSP is the most permeable glassy polymer [141] and PDMS is the most permeable rubbery polymer [142]. Though MOFs are less flexible than polymers, they are much more permeable than both PTMSP and PDMS [138]. In the post pore opening method, the surface pores of substrates are created only after the top layer is formed [139]. Pore sieving is realized by carefully choosing appropriate solvents and coating polymers so that the polymer chains are fully swollen up by the solvents with a large dimension which can be easily sieved out by the tiny substrate pores [140]. Though the composite membranes made by the aforementioned methods have better performance, they come at a price. For example, they might be time-consuming, labor-intensive, expensive and poor in reproducibility. In addition, to deposit a very thin selective layer of < 1000 nm on top of porous substrates by solution coating, it is necessary to use a dilute solution of 0.1–1 wt% which often has a low viscosity and high tendency to intrude into the substrate pores. To solve the dilemma, one practical way is to use a direct coating method (see Fig. 6A) with a novel coating solution. In other words, if the coating solution consists of solutes with an extremely high molecular weight or network, then the large solute molecules may bridge over the substrate pores and prevent the intrusion. Peter and Peinemann [58], Li et al [27] and Liang et al [83,100] have employed the same strategies to minimize the solution intrusion.

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Fig. 6. Schematics of dip coating processes for fabricating TFC composite membranes. (A) Direct dip coating; (B) dipping coating with a pre-wetting treatment. The prewetting agent temporarily fills out the surface pores of substrates. Once the substrate is coated with the coating solution, the pre-wetting agent is removed by evaporation and/or solvent exchange.

Fig. 7. Schematic representation of the novel interfacial polymerization (IP) process for fabricating TFC membranes. The PDMS gutter layer is introduced in between the substrate and the IP layer. Reproduced from reference [150]. Copyright 2012. Reproduced with permission from RSC Publishing.

Interfacial polymerization (IP) has been widely employed to fabricate reverse osmosis and nanofiltration membranes for water reuse and seawater desalination [143,144]. It typically involves an aqueous solution of reactive precursors firstly deposited in the pores of a porous substrate, then the substrate is contacted with a water-immiscible solvent solution containing other reactants, the polymerization takes place at the interface between the two immiscible solutions to form a thin film membrane layer. TFC membranes prepared by IP usually show low gas permeances [145–148] because the less cross-linked and water-swollen hydrogel initially intruded inside the substrate pores becomes rigid with substantial resistance for gas transport [145,146]. As described in Fig. 7, such problems could be solved by introducing a gutter layer prior to the interfacial polymerization so that the intrusion can be mitigated [149,150]. The performance of composite membranes fabricated by IP can be further optimized by controlling parameters such as monomer concentration, duration and temperature of polymerization and curing [145–150].

Lamination simply involves the attachment of a pre-prepared thin film on top of membrane substrates coherently. In other words, an ultrathin membrane film is first prepared by either a phase inversion or film casting method. Then, it is transferred to laminate onto the substrate surface [54,67,134,151,152]. As a result, material intrusion is virtually eliminated or completely avoided. Similarly, chemical vapor deposition (CVD) is a thin-film-forming technique to deposit precursors via vapor phase onto the surface of porous substrates, then form the functional thin layer on top of substrate surface via chemical reactions [153–156]. Since the reactions mainly occur on the top surface, the intrusion into the substructure is minimized. Plasma polymerization is another thin-film-forming process, where plasma polymers are deposited on top of substrates [157]. Plasma polymerization is usually performed within a vacuum system, where helium, argon, or other inert gas is introduced to induce plasma formation and polymerization. It has the advantage to form an ultrathin defect-free film with a thickness ranging

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Fig. 8. Schematic representation of the co-extrusion phase-inversion spinning process. The bore fluid is shown in green, the polymer dope of the inner layer is in yellow, and the polymer dope of the outer layer is in blue. The three fluids are simultaneously coextruded through the spinneret into the coagulant (e.g. water). Typically, the air gap is needed and the die-swell phenomenon exists.

from several nano- to micrometers, which is difficult to achieve with traditional coating methods. Since the plasma polymerization only takes place on the top surface, it has no intrusion issue. The separation performance of membranes prepared by plasma polymerization can be further improved by controlling the plasma conditions [117,157–163]. 3.3. Other advanced techniques to form TFC membranes In addition to the aforementioned methods, novel TFC composite membranes have been developed by emerging fabrication techniques and materials for gas separation. Since conventional TFC membranes needs at least two steps during the fabrication; namely, preparing the substrate and then depositing the thin selective layer. Dual-layer co-extrusion process offers advantages of saving time and minimizing intrusion to form the TFC membranes in one-step [91,114,164–169]. Fig. 8 illustrates a typical co-extrusion process to fabricate TFC membranes via phase-inversion process [165–167]. Different from single-layer spinning, the dual-layer co-extrusion process has a triple-orifice spinneret, where the bore fluid, inner and outer dopes are simultaneously co-extruded. Even though the two dopes may intrude each other during spinning, once they are precipitated and solvent exchanged, the intruded portion may enhance the interface strength without increasing the transport resistance because they are fully porous. In summary, the distinct advantages of the co-extrusion process are: (1) the outer-layer thickness can be controlled within the micrometer range by tuning its flow rate; (2) the TFC hollow fiber membranes are produced in a single step; and (3) the process is cost saving because low cost and mechanically strong materials can be used as the substrates. Using metal organic framework (MOF), zeolitic imidazolate framework (ZIF) and their hybrids with polymers as mixed matrix membranes becomes increasingly important for gas separation [170–175], but they are rarely fabricated into TFC configurations. Some breakthroughs have been made recently. Fu et al. [137] developed a continuous assembly technology via atom transfer radical polymerization (ATRP) for defect-free TFC membranes with ultrathin dense layers and impressively high separation

performance (i.e., CO2 permeance = 1260 GPU; CO2 /N2 selectivity = 43). As depicted in Fig. 9A, an ultrathin top selective layer ˜ nm is made of PEG-based macro cross-linkers polymerized of 40 on top of a PDMS initiator and gutter layer upon a microporous PAN substrate [137]. Xie et al. [138] applied the same method to fabricate another defect-free TFC membrane with different and unique architecture. As shown in Fig. 9B, an ultrathin selective ˜ nm is polymerized on a rough micro-scale MOF gutter layer of 30 layer. The polymer-on-MOF architecture displays good gas sep˜ aration performance (i.e., CO2 permeance = 3000 GPU; CO2 /N2 selectivity = 34) because the MOF layer is about 400 times more permeable than the PDMS layer. Besides, Ma et al [176] developed a ZIF-8/␥-alumina/␣-alumina nanocomposite membrane by means of vapor-phase atomic layer deposition (ALD). As illustrated in Fig. 9C, a thin layer of precursors (i.e., ZnO) is firstly deposited on a porous ␥-alumina support, followed by a ligand-vapor of 2methylimidazole, which converts the precursors into ZIF crystals. The resultant membrane has superior performance in terms of propylene permeance and propylene/propane selectivity (i.e., pure ˜ C3 C6 permeance = 100 GPU; C3 C6 /C3 C8 selectivity >100). Although the TFC membrane in Fig. 9C is made of inorganic materials (e.g. alumina and ZIF) that are expensive, if the inorganic substrate could be replaced by a cheaper polymeric one, the cost of making such high-performance ZIF-selective TFC membranes will be reduced significantly. Interestingly, a 3D- printing method recently has been developed and applied to precisely control the thickness and roughness of the selective layer of TFC membranes by electrospraying monomers directly onto a substrate [177]. Subsequently, the thin (e.g. 15–20 nm) and smooth selective layer (polyamide) was formed in situ through interfacial polymerization. Although the “as-printed” TFC membrane was designed for desalination, such method and strategy might have high potential to be applied to fabricate ultra-thin TFC membranes for gas separations. 4. Gas transport through composite membranes Gas transport across TFC membranes may be different from that of thick dense membranes [14]. This may arise from various

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Fig. 9. Schematics of emerging methods to prepare TFC membranes for gas separation. (A) Fabrication of an ultrathin film composite membrane via (i) depositing a PDMS gutter layer by spin coating; (ii) seeding an initiator layer by spin coating; (iii) forming a UFC membrane by continuous assembly of polymers in the presence of cross-linkers [137]. Copyright 2015. Reproduced with permission from RSC Publishing. (B) Fabrication of a polymer/MOF architecture via (i) in situ formation of the MOF layer by a hydrothermal treatment of anodisc; (ii) initiator grafting of the MOF membrane; (iii) formation of an ultra-thin polymer film on the MOF membrane by self assembly of polymers and atom transfer radical polymerization (ATRP) [138]. Copyright 2017. Reproduced with permission from RSC Publishing. (C) Fabrication of ZIF nanocomposite membranes via an all-vapor-phase processing method: (i) blocking pores of the ␥-Al2 O3 support with ZnO using atomic layer deposition; (ii) converting ZnO deposits to ZIF via the ligand-vapor treatments [176]. Copyright 2018. Reproduced with permission from The American Association for the Advancement of Science.

mechanisms involved during gas transport across a TFC membrane. As elucidated in Fig. 10, the gas transport takes place in three layers; namely, boundary layer, selective layer and porous substrate layer. There is a concentration polarization in the boundary layer. When a mixed gas is separated by a selective membrane, the fast gas (i.e., the more permeable component) depletes fast at the boundary layer adjacent to the membrane surface, while the slow gas accumulates. As a result, the concentration gradient of the slow gas builds up while that of the fast gas diminishes near the boundary layer, as presented in Fig. 10. This leads to a decreased effective driving force for the fast gas but an increased driving force for the slow gas, thus deteriorating the overall separation efficiency [5–8,165]. The concentration polarization becomes significant when the membrane has a very high permeance and/or selectivity, it may also happen under specific conditions of high operation pressure and stage cut. This is particularly true if the membrane separation involves highly condensable gases and vapors such as C3 gases, water vapor and volatile organic compounds as well as small gas species (i.e., H2 ) [37,69,178–182]. Special module and turbulencepromoting designs have been developed to minimize the effects of concentration polarization in the feed side [180,183]. Similarly, gas sweeping and applying vacuum are useful strategies to reduce the influence of concentration polarization in the permeate side [37,67,101,181–184] Fig. 10 also shows various gas transport mechanisms across the selective layer, such as solution-diffusion, size-exclusive and

facilitated transport mechanisms. For polymeric membranes, the dominant is the solution-diffusion mechanism [6,185,186]. For inorganic membranes such as, zeolites, carbon molecular sieves, ZIF and MOF, the governing mechanism is molecular-sieving [5,7]. For facilitated-transport membranes, the prevailing mechanism is the facilitated-transport [4,5,187]. The gas transport through the porous substrate layer (or bulk substructure) of the composite membrane could be under Knudsen diffusion, Poiseuille flow and surface flow, depending on substrate porosity, gas properties and operating conditions such as pressure and temperature [5,53,54,67,89]. Often, gas permeance is found to be inversely proportional to the thickness of the defect-free membrane. However, that is not always true for composite membranes. For defect-free TFC membranes made by coating polymer solutions onto porous substrates, it has been observed that the layer thickness increases with an increase in the concentration of coating solutions, but the gas permeance of the resultant composite membranes initially declines rapidly and then levels off gradually [100,101,124]. This interesting phenomenon is ascribed to the effect of solution intrusion [100]. The coating solution with a lower polymer concentration leads to a thinner coating layer but results in more material intrusion due to its lower bulk viscosity. Although a higher concentration results in a thicker coating layer, it causes less material intrusion because of its high bulk viscosity. As a result, the effective thickness of the selective layer for both cases remains the same or similar. These arguments and observations can be theoretically explained with the newly derived

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Fig. 10. Properties of gas/vapor transport through thin film composite membranes in gas/vapor separation processes. Top: the concentration polarization reduces the driving force. Bottom: various gas transport mechanisms across the selective layer.

general permeance equation, where the permeance is inversely proportional to the nominal thickness. The nominal thickness (Eq. (22)) may vary with many parameters such as coating thickness, surface porosity of the substrate and intrusion depth. 5. Aging and plasticization of TFC membranes for gas separation and energy production Even though some TFC membranes developed according to aforementioned methods have been commercialized for air separation, hydrogen recovery, CO2 capture, natural gas purification, biogas separation and air dehumidification [4,7,9,14,15,17,20,28,37,114,125,134,139,145,149,157], they still face challenging in harsh environments and highly condensable gases because polymeric membranes are often suffered from physical aging [4,186,188–199] and plasticization [4,37,142,200–209. Physical aging occurs because polymeric membranes made from glassy materials are fabricated in non-equilibrium states. The nonequilibrium polymeric chains tend to approach a thermodynamic equilibrium state by reducing their free volume or fractional free volume (FFV) via lattice contraction and free volume diffusion, as illustrated in Fig. 11 [210–212]. As a result, permeability decreases while selectivity increases. Usually, the thinner the membrane, the greater decrease in permeability [192,194,197,198]. For TFC

membranes comprise ultrathin dense-selective layer(s), significant drops in their permeance with time have also been observed [207,213–218]. Plasticization is caused by the sorption of highly condensable penetrants (e.g. CO2 , H2 S, C3 H6 and C3 H8 ) that enhances the mobility and free volume of polymer chains. As a result, permeability increases while selectivity diminishes [4,37,142,200–209]. Comparing to thick membranes, thin polymeric membranes display much faster plasticization phenomena [198,207,201–209]. Therefore, TFC membranes consisting of ultrathin dense-selective layer(s) also show severe plasticization phenomena [215–238]. A number of methods have been developed to suppress aging and plasticization for gas separation and energy production. Some representatives are: (1) cross-linking modifications [66,206,209,217–231]; (2) thermal treatment/annealing [232–235]; (3) plasma or UV treatment [236,216–238]; (4) polymer blends [239–242]; (5) addition of thermally labile units and then crosslinking [243–247], (6) addition of nano-particles [241,248–259]; (7) alcohol treatments [199,260–262]. Generally, cross-linked membranes have shown effectiveness to enhance anti-aging/plasticization resistance because the crosslinked polymer chains have a small fraction free volume and limited mobility. Thus, the cross-linked membranes often exhibit stronger aging resistance. However, one has to pay the price

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Fig. 11. Properties and phenomena of physical aging of polymeric membranes. (A) The schematic of polymer specific volume as a function of temperature. (B) Physical aging mechanisms in polymeric membranes: the lattice contraction (top) and the diffusion of free volume (bottom). (C) O2 relative permeability vs. aging time for thermally rearranged (RT) polyimide dense film membranes with different thicknesses [198]. (D) CO2 plasticization pressure curves for the TR thin and thick films. Open symbols are for thick films, while closed symbols are for thin films. The corresponding film thicknesses can be referred to figure C [198]. Copyright 2014. Reproduced with permission from Elsevier.

of a much slow permeance. Similar improvements have been observed for thermally annealed membranes with a more compact selective layer and a lower permeance. The plasma treatment can induce surface crosslinking and result in membranes with better plasticization resistance. However, long stability data are needed to show its effectiveness. Polymer blends provide a simple method but the resultant membranes may be applicable to less harsh environments because their threshold pressure for plasticization may be lower than the cross-linked ones. Membranes made from polymers consisting of thermally labile units, then decomposition and crosslinking show good separation performance. Similarly, membranes made from polymer/nanoparticle hybrids display impressive anti-aging and plasticization ability. However, they need to be tested in real harsh environments. The alcohol treatment usually involves in soaking membranes made of high free volumes (e.g. PTMSP and PIM membranes) in methanol for a certain time. It can revitalize membrane performance because the treatment increases the free volume of membranes. However, most aforementioned methods to enhance anti-aging and anti-plasticization properties are based on the studies from dense flat films or wholly integrally asymmetric hollow fibers coated with silicone rubber rather than from multi-layer TFC membranes. Since TFC membranes are made of different materials in each layer with distinctive physicochemical properties, the employment of the aforementioned approaches may improve anti-aging and anti-plasticization properties of the dense-selective layer but severely damage other layers. Therefore, one must choose proper materials for each layer in order to design TFC membranes

with anti-aging and anti-plasticization characteristics, particularly in a large-scale production. For example, Liu et al [220] produced a dual-layer hollow fiber membrane consisting of an ultra-thin polyimide outer selective layer and a PES inner supporting layer. Since the chemical crosslinking modifications could only take place in the polyimide selective layer, the PES supporting layer remained porous and strong because PES was immune to the cross-linker. Similarly, to minimize the effects of thermal treatment on the supporting layer of TFC membranes, dual-layer membranes comprising a low glass transition temperature (Tg ) ultra-thin selective layer embedded with nanoparticles and a high Tg porous support layer is preferred. By doing so, the annealing may only densify the selective layer, while the porous supporting layer with low transport resistance remains intact [263–265]. 6. Conclusion and outlook In this review, we have examined and summarized the fundamentals and advances of composite membranes for gas and vapor separation in the past four decades. The booming of membranebased gas separation technologies is mainly driven by economic benefits as well as the emerging challenges in the areas of energy and sustainability. The applications of gas separation membranes have expanded from traditional separations of permanent gases like hydrogen recovery, air separation and natural gas sweetening to condensable gas/vapor separations such the recovery of heavy gaseous hydrocarbons, separation of paraffin/olefin, and separation of volatile organic compounds. Another huge challenge

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and also an opportunity for composite gas separation membranes is the CO2 capture from flue gas and air. To tackle such situations, new-generation and high-performance polymeric composite membranes are in high demand. In addition to revisiting the basic principles of gas transport through TFC membranes, a simple universal equation for permeance has been derived for TFC membranes with multiple layers. The general gas permeance equation can be used for interpreting and predicting gas transport through membranes. It can also serve as a guideline for optimizing the fabrication of high-performance composite membranes. The methods for fabricating various types of polymer-based TFC composite membranes have been elucidated and discussed. In order to maximize the separation performance of TFC membranes, four main strategies have been and can be employed: (1) develop and employ materials with an intrinsically high permeability and selectivity as the dense-selective layer; (2) optimize the substrate’s chemistry, structure and porosity; (3) minimize material intrusion into the substrate and (4) reduce the dense-layer thickness. For polymeric composite membranes, the gas separation efficiency may deteriorate because of physical aging and/or plasticization. The accelerated aging and plasticization commonly happens in thin film polymeric membranes. Various strategies and methods to suppress and minimize the effects of aging and plasticization to achieve long-term stability have been discussed and elucidated. Although the progress and success of gas separation membranes have been witnessed in the past, there are huge demands for better TFC membranes in order to compete with the conventional processes such as cryogenic distillation and adsorption/absorption. Therefore, we outline the following key areas for future research and studies.

(1) Polymeric TFC membranes containing nanoparticles are promising candidates to circumvent the trade-offs between permeability vs. selectivity of polymeric materials and between aging/plasticization rates vs. dense-layer thickness. (2) Simplification and scale-up of current fabrication processes for TFC membranes, particularly for membranes containing nanoparticles, should be promoted in order to produce costeffective membrane products. (3) Novel strategies to conduct crosslinking or annealing modifications at the specific dense-selective layer without damaging the substrate structure are needed to facilitate the fabrication of TFC membranes with improved anti-aging and anti-plasticization characteristics. (4) TFC membranes comprising of polymeric layers(s) and inorganic layer(s) provide another opportunity and direction for future research and development. (5) Long-term stability studies and data in the range of 3–5 years are vital and imperative to expand TFC membranes for industrial applications. (6) Exploration of new applications is encouraged to take the full advantages of TFC membranes.

In summary, composite gas separation membranes are a promising technology for an energy and environmental sustainable future. The advances in membrane materials and membrane fabrication techniques in tandem with the changes in industrial applications and environments may accelerate the demand for next-generation gas separation membranes. This critical review may serve as a stepping stone in pursuing and making TFC membranes for next-generation gas and vapor separations.

Acknowledgements This research grant is supported by the Singapore National Research Foundation with the project entitled, “Membranes to remove air pollutants and excess humidity for better indoor air quality and energy saving” (grant number: R-279-000-453-279). The authors also would like to thank the research grant from National University of Singapore (NUS) under the project entitled “Membrane research for CO2 capture” (Grant No. R-279-000-404133). References [1] Maier G. Gas separation with polymer membranes. Angew Chem Int Ed 1998;37:2960–74. [2] Abetz V, Brinkmann T, Dijkstra M, Ebert K, Fritsch D, Ohlrogge K, et al. Developments in membrane research: from material via process design to industrial application. Adv Eng Mater 2006;8:328–58. [3] Sholl DS, Lively RP. Seven chemical separations to change the world. Nature 2016;532:435–7. [4] Baker RW. Future directions of membrane gas separation technology. Ind Eng Chem Res 2002;41:1393–411. [5] Mulder M. Basic principles of membrane technology. Dordrecht: Kluwer Academic Publishers; 1996. p. 575. [6] Koros WJ, Fleming GK. Membrane-based gas separation. J Membr Sci 1993;83:1–80. [7] Baker RW. Gas separation. In: Baker RW, editor. Membrane technology and applications. New York: John Wiley and Sons Ltd; 2012. p. 325–75. [8] Yampolskii Y, Freeman B, editors. Membrane gas separation. Chichester: John Wiley & Sons Ltd; 2010. p. 355. [9] Puri PS. Commercial applications of membranes in gas separations. In: Drioli E, Barbieri G, editors. Membrane engineering for the treatment of gases. London: The Royal Society of Chemistry; 2011. p. 215–44. [10] Ismail AF, Khulbe KC, Matsuura T. Gas separation membranes-polymeric & inorganic: polymeric and inorganic. New York: Springer International Publishing; 2015. p. 331. [11] Peng N, Widjojo N, Sukitpaneenit P, Teoh MM, Lipscomb GG, Chung TS, et al. Evolution of polymeric hollow fibers as sustainable technologies: past, present, and future. Prog Polym Sci 2012;37:1401–24. [12] Mitchell JK. On the penetrativeness of fluids. J Membr Sci 1995;100:11–6. [13] Loeb S, Sourirajan S. Sea water demineralization by means of an osmotic membrane. Adv Chem Ser 1963;38:117–32. [14] Baker RW, Low BT. Gas separation membrane materials: a perspective. Macromolecules 2014;47:6999–7013. [15] Henis JMS, Tripodi MK. A novel approach to gas separations using composite hollow fiber membranes. Sep Sci Technol 1980;15:1059–68. [16] Henis JMS, Tripodi MK. Composite hollow fiber membranes for gas separation: the resistance model approach. J Membr Sci 1981;8:233–46. [17] Henis JMS, Tripodi MK. The developing technology of gas separating membranes. Science 1983;220:11–7. [18] Pinnau I, Wijmans JG, Blume I, Kuroda T, Peinemann KV. Gas permeation through composite membranes. J Membr Sci 1988;31:81–8. [19] Scholes CA, Stevens GW, Kentish SE. Membrane gas separation applications in natural gas processing. Fuel 2012;96:15–28. [20] Bernardo P, Drioli PE, Golemme G. Membrane gas separation: a review/state of the art. Ind Eng Chem Res 2009;48:4638–63. [21] Chen HZ, Xiao YC, Chung TS. Multi-layer composite hollow fiber membranes derived from poly(ethylene glycol) (PEG) containing hybrid materials for CO2 /N2 separation. J Membr Sci 2011;381:211–20. [22] Dai Z, Ansaloni L, Deng L. Recent advances in multi-layer composite polymeric membranes for CO2 separation: a review. Green Energy Environ 2016;1:102–28. [23] Ramasubramanian K, Ho WW. Recent developments on membranes for post-combustion carbon capture. Curr Opin Chem Eng 2011;1:47–54. [24] Scholes CA, Smith KH, Kentish SE, Stevens GW. CO2 capture from pre-combustion processes-Strategies for membrane gas separation. Int J Greenhouse Gas Control 2010;4:739–55. [25] Zhang X, Singh B, He X, Gundersen T, Deng L, Zhang S. Post-combustion carbon capture technologies: energetic analysis and life cycle assessment. Int J Greenhouse Gas Control 2014;27:289–98. [26] Du JR Liu L, Chakma A, Feng X. Using poly(N,N-dimethylaminoethyl methacrylate)/polyacrylonitrile composite membranes for gas dehydration and humidification. Chem Eng Sci 2010;65:4672–81. [27] Li P, Chen HZ, Chung TS. The effects of substrate characteristics and pre-wetting agents on PAN-PDMS composite hollow fiber membranes for CO2 /N2 and O2 /N2 separation. J Membr Sci 2013;434:18–25. [28] Merkel TC, Lin H, Wei X, Baker R. Power plant post-combustion carbon dioxide capture: an opportunity for membranes. J Membr Sci 2010;359:126–39. [29] Zhu L, Swihart MT, Lin H. Unprecedented size-sieving ability in polybenzimidazole doped with polyprotic acids for membrane H2 /CO2 separation. Energy Environ Sci 2018;11:94–100.

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