co2 from flue gas

Journal of Membrane Science 279 (2006) 1–49

Review

Polymeric CO2/N2 gas separation membranes for the capture ofcarbon dioxide from power plant flue gases

Clem E. Powell, Greg G. Qiao ∗Cooperative Research Centre for Greenhouse Gas Technologies, Department of Chemical and Biomolecular Engineering, University of Melbourne, Australia

Received 15 July 2005; received in revised form 23 November 2005; accepted 23 December 2005Available online 17 February 2006

Abstract

Global warming has been identified as one of the world’s major environmental issues. While it is impossible to completely stop the effects ofanthropological global warming, it is possible to mitigate these effects via a variety of options. One such option is the reduction of greenhouse gasemissions by the capture of carbon dioxide from flue gases followed by underground sequestration. For this technology to become widespread,new methods of capturing carbon dioxide must be devised. While capture of carbon dioxide with amine solvents is the most mature technology,another possible contender is gas separation membranes. This review will focus on novel materials for gas separation. In particular, polymeric gass©

KP

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0d

eparation membranes are examined. Possible design strategies, synthesis, fabrication and role of novel materials are discussed.2006 Elsevier B.V. All rights reserved.

eywords: Geosequestration; Gas separation; Membranes; Polymeric; Carbon dioxide; Nitrogen; Polysulfones; Polyimides; Polycarbonates; Polyarylates;olypyrrolones

ontents

1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.1. Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.2. Carbon capture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2. Gas separation membranes for CO2/N2 separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.1. Synthetic strategies for polymeric gas separation membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2. Polymer structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2.1. Polyarylates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.2.2. Polycarbonates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.2.3. Polyimides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.2.4. Polypyrrolones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.2.5. Polysulfones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.2.6. Copolymers and polymer blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.3. Pressure/temperature effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.4. Cross-linking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.5. Mixed-matrix membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202.6. Asymmetric membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Appendix A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24A.1. Polyacetylenes (Figs. A1–A3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24A.2. Poly(arylene ether) (Fig. A4) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25A.3. Polyarylates (Figs. A5–A7) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

∗ Corresponding author. Tel.: +61 3 8344 8665; fax: +61 3 8344 4153.E-mail address: [email protected] (G.G. Qiao).

376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2005.12.062

2 C.E. Powell, G.G. Qiao / Journal of Membrane Science 279 (2006) 1–49

A.4. Polycarbonates (Figs. A8–A10) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28A.5. Poly(ethylene oxide) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29A.6. Polyimides (Figs. A11–A15) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30A.7. 6FDA-based polyimides (Fig. A16). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34A.8. Poly(phenylene oxide)s (Fig. A17) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36A.9. Poly(pyrrolone) (Fig. A18) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36A.10. Polysulfones (Figs. A19–A22) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37A.11. Others (Figs. A23–A27) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40A.12. Copolymers and polymer blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42A.13. Cross-linking polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

1. Introduction

Anthropogenic climate change is rapidly becoming one of themajor issues in environmental science. Global temperatures areprojected to rise between 1.4 and 5.8 ◦C by 2100 in the absenceof climate change policies [1]. This increase in global tempera-tures is likely to cause a number of negative effects; includingrising sea levels, changes in ecosystems, loss of biodiversity andreduction in crop yields [2]. These effects can be partially allevi-ated by reductions in emissions of greenhouse gases. Reductionof greenhouse gas emissions can occur in a number of ways; suchas improvements in energy efficiency, increased use of non-fossilfuel power sources, improved soil management and the geologi-cal sequestration of carbon dioxide from significant greenhousegas producing point sources [3].

Membranes have been investigated for over 150 years [4,5],and since 1980 gas separation membranes have been used com-mercially [6]. Gas separation membranes are used in a number ofindustrial processes; such as the production of oxygen enrichedair, separation of CO2 and H2O from natural gas, purificationof H2, and recovery of vapours from vent gases. A number ofreviews examining gas separation membranes have been pub-lished [6–13]. Different strategies towards the construction ofmore efficient membranes have suggested by Koros and Maha-jan [14].

Recent work has investigated the practicality of using gassptnpttbhbomatrasm

and flat asymmetric membranes will be discussed, but in lessdetail.

A variant on polymeric membranes is a carbon membrane.These membranes are constructed by heating a polymeric mem-brane above its decomposition temperature. These membranescan give excellent performance, but tend to have significant prob-lems such as brittleness and cost. Because the mechanism of gastransport is very different in carbon membranes compared withpolymeric membranes, carbon membranes will not be consid-ered in this review. A number of useful references on carbonmembranes have been published [16–21].

1.1. Theory

What follows is a brief introduction to some terms requiredin the experimental characterization of gas transport propertiesof membranes; a more complete discussion can be found else-where [8,10]. Generally, gas molecules are transported through apolymeric membrane by a solution–diffusion mechanism (othermechanisms included a molecular sieve effect and Knudsen dif-fusion). The relationship between permeability, diffusivity andsolubility can be described by the following equation:

P = DS

where P is the permeability coefficient (cm3 (STP)c −2 −1 −1

ttfimo

bumo1

s

wo

eparation membranes for the capture of carbon dioxide fromower station flue gases. While cost effective carbon seques-ration is a complex multi-faceted operation with many sig-ificant hurdles to overcome (such as the cooling and com-ression of the flue gases prior to separation), there is a needo summarise the capabilities of membranes for carbon cap-ure. There are a number of roles where gas separation mem-ranes can be used to reduce emissions of greenhouse gases;owever, this review will concentrate on the capture of car-on dioxide from flue gases and therefore focus on the usef membranes for the separation of carbon dioxide/nitrogenixtures. A recent paper by Carapellucci and Milazzo looks

t the possible use of membranes for carbon dioxide separa-ion from flue gases from an engineer’s perspective [15]. Thiseview will adopt a chemist’s view, examining the gas perme-tion properties of membranes in the light of their chemicaltructure. For this reason, attention will focus on dense filmembranes. Other membrane structures, such as hollow fibre

m s cmHg ; a measure of the flux of the membrane), Dhe diffusivity coefficient (cm2 s−1; a measure of the mobility ofhe molecules within the membrane) and S is the solubility coef-cient (cm3 (STP) cmHg−1; a measure of the solubility of gasolecules within the membrane). The common measurement

f P is the barrer (10−10 cm3 (STP) cm−2 s−1 cmHg−1).While P is a measure of a polymer’s permeability, a mem-

ranes permeance must also be measured. This is quantifiedsing a term known as a gas permeation unit (GPU), com-only used to describe the gas transport of a membrane, as

pposed to a membrane material. The GPU has a unit of0−6 cm3 (STP) cm−2 s−1 cmHg−1.

Experimentally, P is determined via the following relation-hip:

P

l= Q

A�p

here l is the effective thickness of the membrane, Q the measuref the gas permeation rate through the membrane, A the surface

C.E. Powell, G.G. Qiao / Journal of Membrane Science 279 (2006) 1–49 3

area of the membrane and �p is the pressure difference acrossthe membrane.

The ideal selectivity of one gas A over another gas B, α, isdefined as

α = PA

PB

The dual mode sorption model provides another means ofdescribing the sorption of gas molecules into a glassy mem-brane. The gas molecules are assumed to fit into two cate-gories; molecules absorbed directly into the polymer matrixand molecules absorbed into micro-cavities within the polymermatrix. The concentration of molecules absorbed in the poly-mer matrix, CD, and the concentration of molecules absorbedinto micro-cavities, CH, can be described by the following equa-tions:

CD = kDp, CH = C′Hbp

1 + bp

where kD is the Henry’s Law coefficient, C′H the hole saturation

constant, p the pressure and b is the hole affinity constant. Hence,the total concentration of absorbed molecules can be describedas

C = kDp + C′Hbp

1 + bp

Ab

ptp[tpuboomi

the molecular sieves. Mixed-matrix membranes are discussedfurther in Section 2.5.

The upper bound can be described by the following equation:

αA/B = βA/B

PλA/BA

where βA/B and λA/B are constants for each gas pair. Freemanhas suggested a theoretical model for this relationship [24]. Fromthis model, βA/B and λA/B can be calculated from the followingequations:

λA/B =(

dB

dA

)2

− 1

and

βA/B = SA

SBSλA/B

A exp

{−λA/B

[b − f

(1 − a

RT

)]}

where d is the kinetic diameters of gases A and B, S the solubilitycoefficient of gases A and B, a the constant equal to 0.64, andb and f are the constants depending on the polymer. The con-stant b = 9.2 and 11.5 cm2 s−1 for rubbery and glassy polymersrespectively. The value of f can vary by a very large degree, from0 to 14,000 cal mol−1.

The fractional free volume (FFV) is a measure of the theo-retical volume of the polymer divided by the actual volume oft

F

wvPt

(

wskbl

P

w3a

osttc“gs

n experimental investigation of the dual-sorption model haseen recently published by Chung and co-workers [22].

There appears to be a trade-off between selectivity andermeability, for example a highly selective membrane tendso have a low permeability. Robeson has suggested that theermeability–selectivity trade-off possesses an upper bound23]. Fig. 1 provides an example of this upper bound. Subsequento the publication of Robeson’s paper, only a few examples ofolymeric membranes have been published which exceed thepper bound. Koros and Mahajan have suggested that it maye possible to exceed the upper bound significantly by the usef “mixed-matrix membranes” [14]. These membranes consistf a polymeric membrane with a large volume of sub-micro-olecular sieves. This approach could combine the processabil-

ty of membranes with the high performance characteristics of

Fig. 1. Oxygen–nitrogen selectivity vs. oxygen permeability (from Ref. [23]).

he polymer. The fractional free volume is usually defined as

FV = V − V0

V

here V is the volume of the polymer per unit mass and V0 theolume per unit mass occupied by the polymer chains. Park andaul have suggested a definition of FFV, one which assumes that

he FFV is not constant for all gas types [25]:

FFV)n = V − (V0)nV

, (V ) =K∑

k=1

γnk(VW)k

here γnk is a set of empirical factors which depend on gas n andubstituent k. (VW)k is the van der Waals volume for substituent. Using this revised FFV, the authors can get a good correlationetween experimental permeabilities and permeabilities calcu-ated with the following relationship:

= A exp

(− B

FFV

)

here A and B are the constants for the gas. For carbon dioxide at5 ◦C and 10 atm pressure, A = 1750 and B = 0.860. For nitrogent 35 ◦C and 2 atm pressure, A = 112 and B = 0.967 [25].

Some attempts have been made to model the passagef carbon dioxide through a polymeric membrane via aolution–diffusion mechanism [26–30]. Gases, in these models,end to move through gaps in the polymeric structure—calledhe free volume. Because of movement of the polymer chains, ahannel between gaps can be formed allowing gas molecules tojump” from one gap to another. Through this jumping motion,as molecules can effectively diffuse through the membranetructure. Selective transport of gases can be achieved by use of a


Fig. 2. Movement of CO2 through a 6FDA–PDA membrane (from Ref. [27]).

polymer which forms channels of a certain size. Large channelswill allow faster diffusion of gases through a membrane at thecost of less selectivity between different gases; smaller chan-nels will allow a much greater selectivity at the cost of lowerpermeation rates. A more complete discussion of gas transportthrough polymeric membranes has been published by Maier [8](Figs. 2 and 3).

Pavel and Shanks have performed a molecular dynamicsstudy of the diffusion of carbon dioxide and oxygen througha variety of polyesters [31]. A relationship between calculateddensity and calculated diffusion was not observed. There wasa relationship between the calculated diffusion coefficient andthe calculated Voronoi free volume. The gas molecules wereobserved to travel through transient cavities through the polymerstructure. The distribution of these holes had a significant depen-dence on the conformation of the phenyl rings. The calculateddiffusion coefficients followed a para > ortho > meta series.

Another method to determine permeabilities theoretically isa group contribution approach [32,33]. The authors have used anumber of literature polymers to calculate volume and perme-ability contributions for a large number of structural moietiescommonly present in polymers used in the construction of gasseparation membranes. The permeabilities can then be calcu-lated using the following expression:

ln∑

wa

F(

Table 1Examples of group contributions to permeabilities

Group Volume(A3)

CO2

permeability(barrer)

N2

permeability(barrer)

39.0 3.34 0.327

40.75 0.021 0.00139

65.56 309.0 15.5

74.5 10.66 0.508

68.0 34.18 0.877

54.94 16.0 0.678

773.19 278.7 27.5

Data from Refs. [32,33].

One advantage of a group contribution approach is the abilityto quickly examine the effects of structural alterations on per-meabilities. Some examples of the volume and permeabilities ofvarious groups are tabulated in Table 1.

1.2. Carbon capture

Geological sequestration of carbon dioxide requires two sep-arate steps: the first is the capture of carbon dioxide, and thesecond is the transport and sequestration of the captured carbondioxide. This review will only concentrate on the first process;however, for an example of the sequestration aspect, a reviewof the Weyburn pilot project, located in Saskatchewan Canada,is informative [34]. A number of different techniques have beenproposed for the capture of carbon dioxide. These include chem-ical absorption, physical absorption, pressure swing absorption,

n P =i=1

φi ln Pi

here φi is the volume fraction of group i, and Pi is the perme-bility contribution of group i.

ig. 3. Movement of CO2 through the cavities of a 6FDA–ODA membranefrom Ref. [27]).


temperature swing absorption, cryogenic distillation and sepa-ration via membranes.

Membranes are a low cost means of separating gases, whenhigh purity gas streams are not vital. There are a number of issuesassociated with the capture of carbon dioxide from flue gaseswhich limit the use of membranes. The concentration of carbondioxide in flue gases is low, which means that large quantities ofgases will need to be processed. The high temperatures of fluegases will rapidly destroy a membrane, so the gases need to becooled to below 100 ◦C, prior to membrane separation. Likewise,the membranes will need to be chemically resistant to the harshchemicals contained within flue gases, or these chemicals willneed to be removed prior to the membrane separation step. Addi-tionally, creating a pressure difference across the membrane willrequire significant amounts of power, which will in turn lowerthe thermal efficiency of the power plant.

White et al. have recently published a review on the geolog-ical sequestration of carbon dioxide, comprehensively coveringboth the capture and storage of carbon dioxide [35]. This reviewincludes a brief discussion on the possibility of membrane sep-aration of carbon dioxide.

Also recently published is a US Department of Energy reportexamining membrane separation of carbon dioxide for the pur-poses of geological sequestration [18]. In addition to polymericmembranes, the report examines inorganic membranes in detail.The authors are unconvinced about the usefulness of polymericmp[oiomaariphai

cftdwdc

igaTebg

Table 2Typical non-nitrogen components of untreated flue gases from Eastern LowSulfur Coal

Species Concentration

H2O 5–7%O2 3–4%CO2 15–16%Hg complexes 1 ppbCO 20 ppmVarious hydrocarbons 10 ppmHCl 100 ppmSO2 800 ppmSO3 10 ppmNOx 500 ppm

Data from Ref. [37].

tionally, untreated flue gas streams also contain considerableparticulate matter. Some industrial processes, such as the pro-duction of steel, produce flue gases with a considerably higherCO2 concentration.

For a membrane to be useful for the capture of carbon dioxide,it should posses a number of properties, namely:

• high carbon dioxide permeability.• high carbon dioxide/nitrogen selectivity,• thermally and chemically robust,• resistant to plasticisation,• resistant to aging,• cost effective, and• able to be cheaply manufactured into different membrane

modules.

Sakellaropoulos and co-workers have investigated the use ofpolysulfone and polyimide membranes for separation of CO2/N2from flue gases [38]. They conditioned the polymers with expo-sure to high pressure CO2, which caused a significant increasein the performance of the polyimide membrane; however, thiseffect tended to decay with time.

Other potential uses for membranes in the sequestration ofcarbon dioxide, is the incorporation of membranes into a pre-combustion separation step. This technique works by reaction ofhgfBdoit

2

2m

m

embranes for the capture of carbon dioxide, citing their lowroductivities. Similar concerns are expressed by White et al.35]. Baker has pointed out that the compression and coolingf flue gases prior to a membrane separation require signif-cant amounts of energy, which in turn lowers the efficiencyf the power plant [6]. Conversely, a modelling study of poly-eric membranes in gas turbine power plants by Carapellucci

nd Milazzo is more encouraging [15]. The authors find thatmulti-step polymeric membrane system can effectively sepa-

ate out carbon dioxide from gas flues. A potential problem isdentified; oxygen in waste gases can significantly reduce theurity of the carbon dioxide gas stream because of the relativelyigh permeability of oxygen through membranes. This can bevoided in external combustion plants, where the excess oxygens kept to a minimum.

Kazama et al. have recently investigated the possible role ofardo polyimide membranes for the capture of carbon dioxiderom flue gases [36]. It was found that polymeric gas separa-ion membranes were less cost effective at capturing carbonioxide than amine solvents from power plants; however thisas reversed when steel works were examined. The significantifference between the power plants and steel works was theoncentration of carbon dioxide in the waste gas stream.

While the composition of flue gases varies greatly depend-ng on the fuel source, power plant and prior treatment, someeneral statements can be made. Flue gases tend to be oxidisingnd generally consist of N2, O2, H2O, CO2, SO2, NOx and HCl.able 2 lists typical components of untreated flue gases fromastern low-sulfur bituminous coal [37]. Conversely, CO2 cane separated from fuel gases prior to combustion. Fuel gases areenerally reducing with CO, H2, H2O, CO2 and H2S [35]. Addi-

ot steam and methane to form a carbon dioxide and hydrogenas mixture. At this stage, the carbon dioxide can be separatedrom the hydrogen gas which is then used to produce energy.aker has suggested using N2/O2 separation membranes to pro-uce oxygen enriched air which can then be used for combustionf fossil fuels [6]. Both of these techniques are attempts toncrease the concentration of CO2 and overcome one of the fac-ors which disadvantage gas separation membranes.

. Gas separation membranes for CO2/N2 separation

.1. Synthetic strategies for polymeric gas separationembranes

Polymers used for CO2/N2 separation membranes have toeet certain criteria. One is their ability to permeate the gas


Fig. 4. Synthetic strategy for polymer structure used as CO2 separation membranes.

through the membrane; so a reasonable gas flux is achieved dur-ing separation. The second criterion is the separation of carbondioxide from other gases. The third one is that the polymericmembrane needs to provide good thermal and mechanical prop-erties; hence the separation can be conducted effectively andsometimes at elevated temperatures.

The above criteria are commonly met by synthesis of a blockpolymer system as shown in Fig. 4. The copolymer usuallyprocesses a hard block and a soft block. The hard block canbe synthesised by polymer with well-packed and more rigidstructures; therefore it forms a glassy segment of the polymerchain.

On the other hand, the soft block can be synthesised froma polymer with more flexible chains and that can form rub-bery segments on the polymer chain. When a polymeric mem-brane formed by use of these copolymers, glassy polymersegments will form a structural frame and provide mechani-cal support. If the hard block is formed by high temperaturepolymers such as polyimides, it can also provide better ther-mal resistance. On the other hand, the rubbery segments usuallyform continuous microdomains and the nature of the flexiblechain structure allows the transportation of gas; hence pro-vide a good permeability. Usually the balance of the hard andsoft block ratio provide the good separation without loss ofpermeability.

The glass segments usually gave lower free volume while thervcb

2

apstapippd

gas separation membranes, for this reason, some polymers arecovered in more detail at the expense of others.

Fig. 6 displays the literature values carbon dioxide permeabil-ity versus carbon dioxide/nitrogen selectivity of a large numberof polymers. The displayed polymers have been selected fromAppendix A and all have been measured at 35 ◦C and the car-bon dioxide permeability at 10 atm. A clear upper bound isobserved. While the poly(imide ethylene oxide) and cross-linkedpoly(ethylene oxide)s can appear to give both very high perme-abilities and selectivities, this may be due to the lower pressureat which they were measured, relative to many other polymers.For this reason, these polymers are not displayed in Fig. 6. Alsomissing are some of the extremely highly fractional free vol-ume polymers. These have been neglected in the interests ofclarity. Both the classes of polymers are discussed later in thetext. Additionally, the results presented here are from pure gasmeasurements. Mixed gas tests would result in different val-ues. While mixed gas data would give results more applicableto industrial applications, there is considerably more data onpure gas measurements. The permeability of carbon dioxide fre-quently shows a strong dependence on the carbon dioxide partialpressure, whereas the nitrogen permeability dependence is con-siderably less. The permeability tends to drop with increasingpressure. If the membrane is vulnerable to plasticisation, thepermeability will reach a minimum, and then rise steeply withincreased pressure.

smTrmddfaa[fgoaoa

ubbery segments provide higher free volumes. The high freeolume means the better gas permeation. The high free volumean also by achieved by intruding bulky structures into the softlocks.

.2. Polymer structure

This section aims to detail different polymers and their CO2nd N2 gas transport properties. Attention has been focusedrimarily on glassy section or block of the polymers in thecientific literature recently. Examples of polymers used inhe construction of gas separation membranes include poly-cetylenes [9], polyaniline [39–42], poly(arylene ether)s [43],olyarylates [44–47], polycarbonates [48–52], polyetherim-des [53,54], poly(ethylene oxide) [55,56], polyimides [57–74],oly(phenylene oxide)s [75–77], poly(pyrrolone)s [78–82] andolysulfones [48,50,83–94]. Examples of these polymers areisplayed in Fig. 5. There is an extensive body of literature on

It should be noted that permeabilities can be altered con-iderably by conditioning, such as thermal annealing of theembrane; this is particularly pronounced in glassy polymers.his introduces difficulties in comparing results which in turn

educes the utility of gas permeation tests as a tool for deter-ining structure/activity relationships. For example, the carbon

ioxide permeability of the polyimide 6FDA–durene has beenetermined by three different laboratories, with values rangingrom 400 to 456 barrer [65,71,95]. Upon thermally annealing6FDA–durene membrane at 100 ◦C for 7 h then 180 ◦C for

nother 7 h, it led to a drop in permeability from 400 to 230 barrer95]. Another important variable is the casting solvent used toorm the membrane. Mohr and Paul, for example, have investi-ated the effect of varying the casting solvent on the permeabilityf poly(4-methyl-1-pentene) [96]. The carbon dioxide perme-bility was found to vary from 15.0 to 93.8 barrer dependingn the casting solvent (see Table 3 for more details). Chungnd co-workers have performed a study on the effect of cast-


Fig. 5. Example of polymeric structures.

ing solvents on the permeabilities of a copolyimide membrane[97].

Another important issue concerning gas separation mem-branes is the effect of impurities on the membranes. Pereiraand Admassu have investigated the effect of impurities such aspump compressor oils on the performance of a number of gasseparation membranes [98,99].

Glassy polymers are vulnerable to aging effects. This isparticularly pronounced in thin films. Huang and Paul have pub-lished a method of monitoring the aging effects on gas transportproperties of glassy polymers [100]. Fig. 6. Selectivity vs. permeability of polymeric membranes.


Table 3Effect of casting solvent on the gas transport properties of poly(4-methyl-1-pentene)

Casting solvent Crystallinity P(CO2) P(N2) α(CO2/N2)

Chloroform 57 15.0 1.1 13.6p-Xylene 62 42.6 2.7 15.8cis-Decalin 65 27.8 1.6 17.4Cyclopentane 41 93.8 7.4 12.7Cyclohexane 51 83.0 6.5 12.8Carbon tetrachloride 56 77.7 6.0 13.0

Permeabilities in barrer, pressure = 2 atm, temperature = 35 ◦C. Data from Ref.[96].

Gas transport properties are affected by the method by whichthe membrane is constructed. Yilmaz and co-workers have inves-tigated the effect on gas transport properties by altering thepreparation of a polycarbonate membrane [101]. By altering theconcentration of polymer in the casting solvent, slight changesto the permeability are observed. Changes of a similar magni-tude are also observed when dichloromethane is used instead ofchloroform as the casting solvent. Other preparation variablesinvestigated include drying time, and annealing time.

Another study by Tsujita and co-workers has examined theeffect of thermal and pressure conditioning on the gas transportproperties of polycarbonates [102]. The authors found that heat-ing the polycarbonate above Tg, and then quenching the filmwould increase the CO2 sorption. As the heating was increased,the CO2 sorption capacity of the membranes likewise increased.This increased sorption was attributed to an increase in C′

H.Koros and co-workers have also investigated the carbon dioxidepermeability of various polycarbonates at high pressures [103].For these reasons, it is best to compare membranes that havebeen prepared with identical methodology.

Materials for effective separation of gases can follow one oftwo overall strategies; increasing the rate of diffusion of carbondioxide through the polymeric structure and increasing the sol-ubility of carbon dioxide in the membrane. Unfortunately, thereis frequently a trade-off between selectivity and permeability.The introduction of mixed-matrix membranes may allow supe-rt

dtittFta

•

• Para substituted phenyl rings tend to give, relative to meta,significantly higher permeabilities, usually at the cost of selec-tivity.

• Closely packed polymer chains tend to give better selectivity,at the cost of permeability.

• Modification of the regions of the polymer which form a gapbetween cavities, can in principle, be used to alter selectivityfor different gas pairs. This may be achieved by the synthesisof polymers which consist of alternating bulky and flat groups.A more complete discussion of this can be found elsewhere[8].

Improving the solubility ratio of a membrane is, conceptually,much simpler than improving the diffusion selectivity. Carbondioxide is considerably more polarisable than nitrogen, allowingthe addition of functional groups to interact with them and thusincrease the solubility of carbon dioxide within the membrane.Increasing the solubility ratio of a polymer for carbon diox-ide over nitrogen may allow the upper bound to be exceeded.Poly(ethylene oxide) is an example of a polymer with high car-bon dioxide solubility. One disadvantage of a high solubility isthat this tends to lead to increased rates of plasticisation. Aninvestigation into the permeabilities and solubilities of carbondioxide, nitrogen and various hydrocarbons in a variety of com-mercial rubbery polymer membranes has been conducted byFreeman and co-workers [104].

mcoistnpttis6p(ozmaoib

iapapsa

ior performance; however, there is considerably less scope forhe rational design of these membranes.

The rate at which gases diffuse through a polymeric matrixepends on the amount and distribution of the free volume withinhe polymer. As the gas molecules jump from one transient cav-ty to another, manipulation of the polymer structure can alterhe barrier between cavities allowing carbon dioxide to diffusehrough the polymer structure significantly faster than nitrogen.urther, to the synthetic strategy described earlier, changes in

he polymer packing can significantly alter the permeabilities innumber of ways:

Incorporation of bulky side groups into the polymer’s back-bone tends to lead to an increase in the permeabilities, witha corresponding loss in selectivity. However, in many cases,the loss of selectivity is slight compared with the increase incarbon dioxide permeability.

The insertion of bulky substituents into the peripheral poly-er backbones should disrupt efficient packing of the polymeric

hains, leading to an increase in the permeabilities and a lossf selectivities. A number of different polymers have beennvestigated for this effect. The effect of incorporating bulkyubstituents into bisphenol A polycarbonate has been inves-igated by Muruganandam and Paul [52]. Relative to bisphe-ol A polycarbonate, tetramethyl- and tetrachloro-bisphenol Aolycarbonate show greater permeabilities, with reduced selec-ivities. Tetrabromobisphenol A polycarbonate exhibits bet-er selectivities, at the cost of a slightly reduced permeabil-ty. Tanaka et al. have investigated the effect of methyl sub-tituents on the gas permeability and solubility coefficients ofFDA-based polyimides [71]. A series of methyl-containingara- and meta-phenyl diamines have been reacted with 4,4′-hexafluroisopropylidene)diphthalic anhydride to give a rangef polyimides. As the number of methyl groups increases (fromero to four) a significant increase in the carbon dioxide per-eability is observed (from 9.20 to 440 barrer). As expected,decrease in the carbon dioxide/nitrogen selectivity is also

bserved upon the addition of extra methyl groups. This increasen permeabilities has been subscribed to an increase in the num-er of large free spaces.

The synthetic polymer with the largest measured permeabil-ties is poly(1-trimethylsilylpropyne). This polymer possessescarbon dioxide permeability of 28,000 barrer and a nitrogen

ermeability of 4970 barrer. These very large permeabilitiesre associated with a very large fractional free volume. Theseermeabilities tend to decrease with time due to slow crystalli-ation of the polymer. This effect can be counteracted by theddition of certain additives. Other polyacetylenes give lower,


Fig. 7. Examples of high free fractional volume polymers.

but still substantial permeabilities. Another polymer with anextremely large fractional free volume, and hence permeabilitiesis poly(dimethylsiloxane) which possesses a carbon dioxide andnitrogen permeabilities of 4550 and 351 barrer, respectively. Arecent communication by Budd et al. expands the field of highfractional free volume gas separation membranes [105]. Twoaromatic rigid yet twisted polymers (denoted as polymers ofintrinsic microporosity) have been synthesised and their per-meabilities measured. The carbon dioxide permeabilities rangefrom 1100 to 2300 barrer, while the nitrogen permeabilitiesrange from 92 to 42 barrer. Unlike other high fractional freevolume polymers, these polymers also incorporated reasonablesolubility selectivities. The structure of these polymers is dis-played in Fig. 7.

Polyaniline has been extensively studied in the literature[39–42]. Polyaniline which had been doped with HCl, dedopedwith ammonia and finally redoped with HCl have been reportedto give very high selectivities [39,40]. For example, an idealselectivity for CO2/N2 of 1560 has been reported [39]. Subse-quent studies, have found considerably lower selectivities, withHellgardt reporting a CO2/N2 ideal selectivity of only 17 [42].Because of the low CO2 permeability of polyaniline membranes(Hellgardt, for example, reports a CO2 permeability of approxi-mately 0.1 barrer [42]), it is unlikely that they will be useful forthe capture of carbon dioxide.

Yoshino et al. have examined a number of high molec-u[bmecpatp

ity literature, a direct comparison of gas transport properties isdifficult.

A number of polymers types have shown good gas transportproperties and considerable scope for structural variations. Thefollowing describes the main polymer types which have beenstudied as CO2 separation membranes.

2.2.1. PolyarylatesA large number of polyarylates have been synthesised and

their gas transport properties studied. The most common syn-thetic route involves reaction of a diol, such as bisphenol A, withan acid chloride to afford the desired polyarylate. This reactioncan be performed under a variety of conditions such as interfa-cial or a stepwise polycondensation. The structure of a commonpolyarylate is displayed in Fig. 8. The oxygen/nitrogen separa-tion properties of polyarylates have been reviewed by Paul andPixton [106].

The carbon dioxide permeabilities and carbon diox-ide/nitrogen selectivities for a number of polyarylates is graphedin Fig. 9. Most polyarylates possess carbon dioxide perme-ability less than 25 barrer; however, a small number possesspermeabilities as high as 85. The sterically bulky polyarylate,TBHFBPA/tBIA, exhibits the highest carbon dioxide and car-bon dioxide/nitrogen selectivity of 19, the structure of which isdisplayed in Fig. 10.

2

c

lar weight branched poly(ethylene oxide)-based polymers56]. These polymers were synthesised by using a com-ination of various monomers; ethylene oxide (EO), 2-(2-ethoxyethoxy)ethyl glycidyl ether (EM) and allylglycidyl

ther (AGE). These polymers give very good combinations ofarbon dioxide permeabilities and selectivities, with one exam-le possessing a carbon dioxide permeability of 773 barrer andcarbon dioxide/nitrogen selectivity of 46. Because these gas

ransport properties of these materials were tested at differentressures and temperatures to the majority of the gas permeabil-

.2.2. PolycarbonatesA number of polycarbonates have been synthesised and their

arbon dioxide/nitrogen gas transport properties studied. Poly-

Fig. 8. PA.


Fig. 9. Carbon dioxide permeability vs. selectivity for polyarylates.

carbonates are generally synthesised by reaction between a dioland phosgene under a variety of conditions. Like many poly-meric materials used in the synthesis of gas separation membranematerials, the ready availability of structural variants of bisphe-nol A has led to a large number of different polymers. Mostpolycarbonates tend to have a carbon dioxide permeability ofunder 40 barrers, and selectivities range from 15 to over 25.One notable exception to this is the polycarbonate TMHFPC,which possess a carbon dioxide permeability of 111 and a car-bon dioxide/nitrogen selectivity of 15.0. The structure of thispolymer is displayed in Fig. 11. The carbon dioxide permeabil-ity and carbon dioxide/nitrogen selectivities for a number ofpolycarbonates are graphed in Fig. 12.

2.2.3. PolyimidesPolyimides combine excellent thermal and chemical stability

with a very wide range of carbon dioxide permeabilities. Somepolyimides, particularly those incorporating the group 6FDApossess very high carbon dioxide permeability. An extensivereview of the gas separation properties of polyimides was pub-lished in 1996 [13].

Polyimides are generally synthesised by the reaction of adiamine with a diahydride in an aprotic solvent to form apolyamic acid. This polymer then undergoes a polyconden-sation reaction (either thermally or chemically) to form thedesired polyimide. An example of the synthesis of the polyimide6FDA–pPDA is displayed in Scheme 1.

A number of different diahydrides have been used in the syn-thesis of polyimides for gas separations; these include PMDA,6FDA and BTDA. The chemical structures of these dianhydridesis displayed in Fig. 13.

While most polyimides are synthesised using either PMDA,BPDA or 6FDA as the dianhydride, a number of authors havesynthesised novel dianhydrides for the use in the formation ofnovel polyimides [66,107]. The structures of these starting mate-rials are displayed in Fig. 14. These dianhydrides were thenreacted with a range of diamines to give a number of poly-imides. The polyimide, PI–TMMPA, has been observed to giveextremely high permeabilities. This result is unsurprising giventhe large steric bulky nature of the polymer (Fig. 15).

Polyimides incorporating the group 6FDA have been theobject of much research, as they tend to combine both highselectivities and high permeabilities. Three reasons are com-monly given for this behaviour: (a) the CF3 groups considerablyincrease the stiffness of the chain, allowing the membrane tomore effectively separate molecules on the basis of steric bulk;(b) effective chain packing is reduced by the large CF3 groupswot

F

Fig. 10. TBHFBPA/tBIA.

Fig. 11. TMHFPC.

hich leads to an increase in the permeability; (c) the formationf charge-transfer complexes is reduced, which in turn reduceshe likelihood of effective chain packing [9,108].

ig. 12. Carbon dioxide permeability vs. selectivity for polycarbonates.


Scheme 1. Synthesis of 6FDA–pPDA.

Another target of considerable research is the commercialpolyimide, Matrimid 5218, which is shown in Fig. 16. Matrimidhas been functionalised by brominations. Bromination tendedto increase both the carbon dioxide and nitrogen permeabilities,leading to only a small decrease in the carbon dioxide/nitrogenselectivity [109]. A study on the effect of cross-linking Matrimid5218 is discussed in Section 2.4. Fig. 17 displays the carbondioxide permeability and carbon dioxide/nitrogen selectivitiesof a variety of polyimides.

2.2.4. PolypyrrolonesPolypyrrolones are structurally similar to polyimides; how-

ever, they are considerably more rigid. Polypyrrolones are ofinterest because of their high thermal and chemical resistance,which can be greater than that of similar polyimides. However,despite this, the number of different polypyrrolones which havebeen studied is quite limited. The structures of these polymersare displayed in Fig. 18. Because of their high rigidity, it hasbeen suggested that polypyrrolones can act as a molecular sieve.

For this reason, a significant volume of the literature coverscopolypyrrolones and copoly(imides pyrrolone), as varying theproportion of the mers can vary the rigidity.

Polypyrrolones are generally synthesised by reaction of dian-hydride and a tetraamine functionalised monomer. The synthesisof the polypyrrolone 6FDA–TAB is displayed in Scheme 2. Thepresence of the tetraamine groups required for reaction signifi-cantly limits the possible structural variations.

Zimmerman and Koros have published a series of papersinvestigating copolymers of 6FDA/PMDA–TAD [79,80]. Theresults of these are discussed more fully in the copolymersection. Walker and Koros have investigated a polypyrrolone6FDA–TADPO and compared it to a number of polymers [82].

Zimmerman and Koros have performed an investigationinto a copolypyrrolone, 6FDA/PMDA–TAB, the structure ofwhich is displayed in Fig. 19 [79,80]. As the proportion ofPMDA increases, the carbon dioxide permeability decreases andthe selectivity increases. The temperature dependence of thesecopolypyrrolones has also been studied [80].


Fig. 13. Common dianhydrides used in the synthesis of polyimides.

Fig. 14. Catechol-based dianhydrides.

Fig. 15. PI–TMMPA.

Fig. 16. Matrimid 5218.

Fig. 17. Carbon dioxide permeability vs. selectivity for polyimides.

Fig. 18. Polypyrrolone structures.

Scheme 2. Synthesis of 6FDA–TAB.


Fig. 19. 6FDA/PMDA–TAB.

2.2.5. PolysulfonesConsiderably amounts of research have been performed on

the gas transport properties of polysulfones, because of theirreasonable performance and low cost. Polysulfones are alreadyused in industrial gas separation processes [6]. Polysulfonesare generally synthesised by a condensation reaction between abisphenol and a dihalogenated diphenylsulfone. The most exten-sively studied polysulfone is PSF, formed using bisphenol A.Two routes for the synthesis of PSF are displayed in Scheme 3.Most other polysulfones are structurally related to this polymer.An extensive study of the gas transport properties of PSF hasbeen performed by Paul and co-workers [54].

By substitution of bisphenol A with a different diol, largenumbers of PSF derivatives have been synthesised. These dis-play a wide range of carbon dioxide permeabilities and selectiv-ities. More exotic polysulfones have also been examined. Pixtonand Paul have investigated the transport of different adamantane-based polysulfone membranes [88]. Aitken and Paul have like-wise examined naphthalene-based polysulfone membranes [91].The structures of these polymers are displayed in Figs. 20 and 21.

Some polysulfones have been functionalised by a bromina-tion [110]. This increases the reactivity of the polymer, leadingto the potential for more structural variation. Guiver and co-workers have abstracted the bromine atom with butyl lithiumand subsequently performed a reaction with iodotrimethylsi-lane [85]. Bonfanti et al. have further reacted the polymers witha series of acetylenes utilising a PdII/CuI catalyst (Scheme 4)[87]. A number of different polymers were successfully syn-thesised with different degrees of bromination and alkylation.Both the bromination and alkylation steps were not carried outto completion, ensuring that the final products were composedof a mixture of unreacted monomer, brominated monomer andalkylated monomer.

A number of polysulfones have been modified by reactionwith butyl-lithium followed by addition of a pendent group[83–86]. The reaction site on the polymer can be altered byusing a brominated polysulfone as a precursor. Using this tech-nique a number of novel polysulfones structurally related to PSF(Fig. 22), HFPSF (Fig. 23), PPSF (Fig. 24) and TMPSF (Fig. 25)

Fig. 20. Adamantane-based polysulfone membranes.

Fig. 21. Naphthalene-based polysulfone membranes.
Fig. 22. Novel polysulfones-based off PSF.


Scheme 3. Synthesis of PSF.

have been synthesised and their gas transport properties exam-ined. An NMR study of some polysulfones modified by additionof a trimethylsilyl group found a decrease in chain mobility whenthe Me3Si group was ortho to the sulfone group through the etherlinkage [111].

Fig. 26 displays the carbon dioxide permeability and carbondioxide/nitrogen selectivity for a variety of polysulfones.

2.2.6. Copolymers and polymer blendsCopolymers offer the potential to fine tune permeabilities

and reduce the costs of polymer synthesis. A copolymer willtend to have permeabilities which are intermediate comparedwith the homopolymers which make it up. Likewise an expen-sive and effective polymer can be significantly reduced price

by the formation of a copolymer which incorporates cheapermonomers.

A series of copolyimides formed from reaction of NTDAwith a mixture of sulfonated and non-sulfonated diamines wassynthesised by Espuche and co-workers [63]. The structure ofthese copolyimides is displayed in Fig. 27. As the proportionof sulfonated groups increases, the carbon dioxide permeabilitydrops dramatically.

The CO2/N2 gas separation properties of a largeseries of poly(ethylene oxide) segmented copolymers withpolyurethanes, polyamides and polyimides have been investi-gated by Okamoto and co-workers [112]. These copolymersgenerally gave both high selectivities and a high CO2 permeabil-ity, likely due to the high solubility of carbon dioxide into the


Scheme 4. Synthesis of alkynylated PSF.

poly(ethylene oxide) segments. Of particular note is the poly-mer PMDA-pDDS/PEO4(80) (Fig. 28) which exhibits a CO2permeability of 238 barrer and a CO2/N2 selectivity of 49. Thiscombination of both high selectivities and permeabilities maybe related to the low pressures used during the determination ofthe gas transport properties.

Chung and co-workers have synthesised and determined thegas transport properties of a series of copolyimides incorporat-ing the 6FDA–durene and 6FDA–pPDA subunits [65]. As theproportion of 6FDA–pPDA increases, a decrease in the perme-abilities is observed. The increase in permeabilities for certaincopolyimides is greater than expected from the addition rule ofthe semi-logarithmic equation. This unexpected increase is dueto a greater increase in the solubility coefficients than antcip-itated. Similarly 6FDA copolyimides have been synthesised[61,113].

Dense films of 6FDA–durene and 6FDA–durene/mPDA(50:50) have also been reacted with N,N-dimethylamino-ethyleneamine in hexane to perform an amidation on the groupring [58]. The reaction is displayed in Scheme 5. By increasingthe reaction time, the degree of amidation was also increased. Adecrease in the permeabilities was observed with an increase inthe amount of amidation.

Similarly, a series of three component polyimides have beensynthesised and their gas permeabilities tested [68]. 6FDA has

been reacted with various ratios of two diamines, FDA andHFBAPP, to give a number of copolyimides. Pure 6FDA–FDAhas greater carbon dioxide and nitrogen permeabilities than pure6FDA–HFBAPP. As the weight fraction of FDA increases, thepermeabilities increase.

Copolysulfones incorporating both bisphenol A and 1,1-bi-2-naphthol have had their gas transport properties studied [114].As the naphthol concentration increases, both the carbon dioxideand nitrogen permeabilities decrease.

A copoly(imide pyrrolone), 6FDA–TAB/DAM has beenexamined by Burns and Koros [115]. The structure of thiscopolymer is displayed in Fig. 29. Increasing the proportionof DAM leads to an increase in the permeability and a decreasein the selectivity.

The oxygen/nitrogen separation properties of 6FDA–TABand the copolymer 6FDA/PMDA–TAB has been examined byZimmerman and Koros [78]. While, the authors do not investi-gate carbon dioxide transport, the paper contains an interestingdiscussion on the relationship between entropy and permeabil-ity. The polypyrrolones studied in the paper, are extremely rigid,and function more like molecular sieves than polymeric mem-branes.

Sakellaropoulos and co-workers have investigated the gastransport properties of a series of polysulfone–polyimide poly-mer blends [116,117]. The authors found that the effect of


Fig. 23. Novel polysulfones-based off HFPSF.

increasing the proportion of polyimide on the transport proper-ties of gases which do not interact significantly with the polymermatrix can be described by a simple relationship. The permeabil-ity of carbon dioxide, however, is lower for the polymer blendsthan what would be expected if it follows a similar relation-ship. The authors suggest that this could be due to interactionsbetween the carbon dioxide and polyimide or densification andconsolidation of the membrane induced by the presence of highpressure carbon dioxide.

Okamoto and co-workers have investigated a series of starpolymers incorporating a 6FDA-based polyimide core andpoly(ethylene oxide) arms [118]. Tris(4-aminophenyl)aminewas reacted with 6FDA to give a mixture of hyper-branchedpolyimide cores. By varying the ratio of 6FDA to tris(4-aminophenyl)amine the composition of the polyimide cores

Fig. 24. PPSF and novel polysulfones-based off PPSF.

could be altered. These cores were then reacted with a vari-ety of poly(ethylene oxide) chains. These poly(ethylene oxide)molecules varied in both molecular weight (2000, 2840 or 5000)and terminal groups (amino or hydroxyl). The membranes couldthen be doped with a variety of ions.

A series of poly(imide siloxane) copolymers have been syn-thesised by Smaihi et al. and their gas transport propertiesdetermined [119]. The synthesis of these network membranesis displayed in Scheme 6. The amic acid precursors were castand then heat to form both the imide and siloxane bonds. Thenitrogen and carbon dioxide gas transport properties of thehybrid membranes was tested at both 90 and 190 ◦C. Marandand co-workers have likewise investigated the gas permenta-tion properties of thermally annealed polyimide-organosilicatehybrid membranes [59].

Silica-polyimide membranes have likewise been prepared byMorooka and co-workers [120]. The membranes were synthe-sised by mixing a polyamic acid with a mixture of tetraethylorthosilicate, 3-glycidyloxypropyltrimethoxysiland and a cou-pling agent. The solution was then cast onto alumina supporttubes which were then heated at 350 ◦C to form the networkedmembrane. The presence of the silica increased the carbon diox-ide permeability at the cost of selectivity. Nanocomposite mem-branes incorporating polyacetylenes have also been synthesisedby Peinemann and co-workers [121].


Fig. 25. Novel polysulfones-based off TMPSF.

Fig. 26. Carbon dioxide permeability vs. selectivity for polysulfones.

Fig. 27. NTDA–BDSA/ODA.

Fig. 28. PMDA–pDD

Fig. 29. 6FDA–T

2.3. Pressure/temperature effects

The relationship between temperature and gas transport prop-erties can be expressed with the following equations:

P = P0 exp

(− EP

RT

), D = D0 exp

(−ED

RT

),

S = S0 exp

(− Hs

RT

)

where P0, D0 and S0 are the initial conditions, EP and ED theactivation energies for permeation and diffusion, respectively

S/PEO4(80).

AB/DAM.


Scheme 5. Reaction of N,N-dimethylaminoethyleneamine and 6FDA–durene.

and Hs is the heat of sorption. This implies that increasing thetemperature should lead to an increase in the permeability anda decrease in the selectivity of a membrane.

Lin and Freeman have investigated the gas transport prop-erties of poly(ethylene oxide) for a large number of gases atdifferent temperatures [55]. The permeabilities of both carbondioxide and nitrogen were measured at 25, 35 and 45 ◦C. The per-meabilities of both gases increased dramatically upon heating;however, the CO2/N2 selectivity suffered a significant loss. Theeffect of changing the upstream pressure was also investigated.The permeability of carbon dioxide increases with pressure,

indicating plasticisation of the membrane, whereas nitrogen per-meability decreases slightly with increased pressure, as expectedfrom the dual-mode sorption model. The authors applied the fol-lowing empirical model to these results:

PA = PA(1 + m�p) = PA,0(1 + mp2)

where PA,0 is the permeability at an infinitely small pressuredifference, m an adjustable constant, p2 the upstream pres-sure and �p is the pressure difference across the membrane.From this they determine that for CO2 PA,0 = 12 ± 1.0 barrerand m = 34 ± 10 atm−1, whereas NO2 PA,0 = 0.25 ± 0.02 barrerand m = −10 ± 7 atm−1.

Stern and co-workers have investigated the gas transportproperties of a series of 6FDA–polyimide membranes [72]. Ofparticular interest, an empirical relationship between pressureand permeability was developed:

log P = n + m�p

where n and m are the constants. By fitting experimental data tothis equation, n and m have been determined (Table 4).

Villaluenga and Tabe-Mohammadi have examined the effectof pressure variations on both glassy and rubbery polymers[122]. The effect of altering the pressure was much more sig-nificant when the rubbery polymer was examined, whereas theglassy polymer was unaffected by pressure changes. However,

TP

P

6666

D
Scheme 6. Synthesis of poly(imide siloxane) copolymers.
able 4ressure dependence of permeabilities

olymer CO2 N2

n m (×105) n m (×105)

FDA–mPDA 0.986 −70.71 −0.427 −10.19FDA–mMPDA 1.512 −55.13 0.101 15.15FDA–DATr 1.700 −71.42 0.344 −22.95FDA–DBTF 1.412 −76.97 0.098 −32.61

ata from Ref. [72].


as the authors did not use a highly condensable gas, such ascarbon dioxide, the glassy polymers are expected to be lesssusceptible to pressures changes. Koros and co-workers haveinvestigated the effect of carbon dioxide plasticisation in a smallnumber of polyimide membranes [123]. They investigated theeffects of increasing the temperature on the gas transport prop-erties of two polyimides: 6FDA–DAF and 6FDA–IPDA [73].Costello and Koros have similarly examined 6FDA–6FpDA and6FDA–6FmDA over a large temperature range (35–325 ◦C) andhave suggested that 6FDA–6FpDA would be a candidate forincorporation into a high temperature application.

Koros and co-workers have performed an in-depth studyof the temperature effects on the gas transport properties ofthe polypyrrolone 6FDA–TADPO [124]. The 6FDA–TADPOmembranes were heated from 35 to 200 ◦C. Both the pre-exponential factor P0 and the activation energy EP have beendetermined. 6FDA–TADPO possesses a P0 of 5.0 × 101 and2.0 × 102 barrers for carbon dioxide and nitrogen, respectively.Above 150 ◦C, the P0 is better expressed as 9.3 × 102 and5.0 × 103 barrers. The activation energies, EP are listed as 0.28and 3.1 kcal mol−1 for carbon dioxide and nitrogen, respectively.Above 150 ◦C, the activation energies are 2.7 and 5.8 kcal mol−1.

2.4. Cross-linking

chbs

hboid

Table 5Effect on increasing proportion of K-bismuththiol on gas transport properties ofpoly(ethylene oxide-co-epichlorohydrin)

Cross-linking agent CO2

permeabilityN2

permeabilityCO2/N2

selectivity

1.1 g/100 g polymer 15.0 2.3 6.52 g/100 g polymer 14.9 1.0 155 g/100 g polymer 16.1 0.5 32

Permeabilities measured at 25 ◦C. Data from Ref. [125].

was kept constant at 5%). The selectivity increases with theethylene oxide concentration, peaking at 93 mol%. At this con-centration the selectivity is 63.0. Increasing ethylene oxide con-centration causes a dramatic decrease in the selectivity.

Hirayama et al. have investigated the effects of cross-linkingpoly(ethylene oxide) on the carbon dioxide and nitrogen gastransport properties [126]. Methacrylate terminated monomersincorporating poly(ethylene oxide) of various lengths (Fig. 30)were photo polymerised. All of the polymers gave good CO2/N2selectivities, and most exhibited large CO2 permeabilities at tem-peratures up to 100 ◦C. A patent published by Kita et al. hasdemonstrated excellent CO2/N2 properties for cross-linked var-ious poly(ethylene oxide) polymers [127]. These polymers wereprepared by the polymerization of various oxirane compounds.Cross-linking was achieved via a variety of methods, includ-ing reaction with peroxides, azo containing compounds andhydrosilylation reactions. Recently a large volume of research oncross-linked poly(ethylene oxide) has been published by Free-man and co-workers [128,129]. The authors have also publisheda useful paper on material selection for solubility selective poly-mers which possess good carbon dioxide permeabilities whilestill possessing excellent carbon dioxide/light gas selectivity[130].

An unsuccessful attempt at improving gas transport prop-erties by cross-linking a polyarylate has been performed byWright and Paul [131]. The authors synthesised a copolyary-

d by

Cross-linking offers the potential to improve the mechani-al and thermal properties of a membrane. Koros and Mahajanave suggested that cross-linking can be used to increase mem-rane stability in the presence of aggressive feed gases and toimultaneously reduce plasticisation of the membrane [14].

Copolymers of poly(ethylene oxide-co-epichlorohydrin)ave been cross-linked with various concentrations of K-ismuththiol [125]. The effect of increasing the proportionf cross-linking agent is tabulated in Table 5. The effect ofncreasing the proportion of ethylene oxide on the carbonioxide–nitrogen selectivity was also examined (K-bismuththiol

Fig. 30. Monomers use
Hirayama et al. [126].


Fig. 31. FBP/XTA-C110/tBIA.

late, FBP/XTA-C110/tBIA, and exposed it to 350 ◦C to inducecross-linking. The copolyarylate is displayed in Fig. 31. Uponheating to 350 ◦C, the cyclobutene group can cause cross-linkingby linear addition and cycloaddition. Thermal decomposition ofthe polymer was observed during the cross-linking stage. Uponheating, a small increase in the permeability of the copolyarylatewas observed (CO2 permeability increases from 33 to 37 barrer,N2 permeability increases from 2.17 to 2.35 barrer). The authorssuggest that changes in the gas transport properties are due todegradation reactions.

Rezac et al. have blended 6FDA–IPDA with a diacetylenecontaining oligomer 6FDA–DIA [132]. Cross-linking can thenbe achieved by heating the membrane to 340 ◦C under vac-uum. Cross-linking decreased the solubility of the membranes indichloromethane and N-methylpyrrolidone dramatically. Cross-linking the polymers did not significantly alter the gas perme-abilities; however, increasing the proportion of 6FDA–DIA didlower both the carbon dioxide and nitrogen permeabilities. Nak-agawa and co-workers have incorporated a photosensitiser intoa polyimide membrane and irradiated with UV light to inducecross-linking [133].

The use of diamines to cross-link polyimides was patentedin 1991 [134]. The use of diamines to cross-link polyimides hasbeen a topic of considerable interest in the literature since 2001.Chung and co-workers have cross-linked 6FDA–durene andMatrimid 5218 dense membranes by soaking the membranes in ascwsqodeHtsttx5t

bmdi

possibly due to reduced diffusion of the cross-linking agent intothe membrane. Both the carbon dioxide and nitrogen permeabili-ties decreased significantly with increased cross-linking. Similarwork has been performed on 6FDA–durene and a PAMAM den-drimer [136]. Increased cross-linking generally leads to increasein the carbon dioxide/nitrogen selectivities at the cost of carbondioxide permeability.

Koros and co-workers have synthesised carboxylic contain-ing polyimide membranes which can then be cross-linked byeither a transesterification reaction or reaction with a metal ion[123,137–139]. These membranes have been investigated fortheir usage in CO2/natural gas separations; however, it is likelythat they will give similar results for CO2/N2 separations.

A patent by Hayes has investigated photochemical cross-linking of copolyimides which include a benzophenone func-tional group [140]. Improvements in the selectivities for somegas pairs were noted upon irradiation induced cross-linking. Thecross-linking is thought to occur between the benzophenonemoiety and an alkyl substitute on the diamine section of thepolyimide.

Won et al. have investigated the effects of surface modifi-cation of polyimide (Matrimid 5218) and polysulfone (PSF)membranes by an ion beam [141]. In both cases, there was asignificant drop in the CO2 permeability, with a correspondinglarge increase in the CO2/N2 selectivity.

2

t

Fe

olution of p-xylenediamine in methanol [57,64]. The methanolauses the membrane to swell, while the p-xylenediamine reactsith two carbonyl groups cross-linking the polyimide. Expo-

ure of 6FDA–durene to the p-xylenediamine/methanol solutionuickly causes a reduction in the permeabilities (the magnitudef the reduction depends on the exposure time). The carbonioxide/nitrogen selectivity is also observed to decrease uponxposure to the solution. When other gas systems (such ase/N2 and O2/N2) were investigated, an increase in the selec-

ivities was observed. Matrimid 5218 gives similar, althoughlower, results for carbon dioxide/nitrogen separations, however,he effects of cross-linking on gas permeabilities did not begino become significant until after a week of exposure to the p-ylenediamine/methanol solution. The change in the Matrimid218 membranes carbon dioxide permeability over exposureime is graphed in Fig. 32.

Membranes formed from the polyimide 6FDA–durene cane cross-linked by reaction with a diaminobutane dendrimer inethanol solution [135]. Longer reaction times led to a larger

egree of cross-linking. As the dendrimer generation numberncreased, the speed at which cross-linking occurred decreased,

.5. Mixed-matrix membranes

Mixed-matrix membranes refer the incorporation of solid par-icles within a membrane. Generally the presence of particles can

ig. 32. Carbon dioxide permeability vs. p-xylenediamine/methanol solutionxposure time.


have three effects on the permeabilities; they can act as molecularsieves altering the permeabilities, they can disrupt the polymericstructure increasing permeabilities and they can act as barrierreducing permeabilities. Generally the literature has focused onthe incorporation of molecular sieves into a polymer matrix.Often these molecular sieves possess superior gas transport prop-erties, but have significant problems with their processability. Anumber of articles providing an overview of mixed-matrix mem-branes have been previously published [142,143].

The Maxwell equation can be used to model the gas transportproperties of mixed-matrix membranes [142]. The behaviour ofgases within a mixed-matrix will vary depending on the spatialrelationship of the particles within the matrix. If a gas moleculecan make a continuous path through the particles without signif-icant transport through the polymeric material, it would exhibitdifferent behaviour to a gas molecule which is forced to take adiscontinuous path through both the polymeric material and thesolid particles. If the mixed-matrix material acts as in a contin-uous fashion then the permeability can be modelled as

Peff = P1Φ1 + P2Φ2

where Pn is the permeability and Φn is the volume fraction ofcomponent n. Whereas, if the material acts in a discontinuousfashion, then the permeability can be described as

P = P1P2

F

P

wamctit

a5(ali[

CtortMc

m

Table 6Theoretical and experimental CO2 permeabilities of mixed-matrix membranes

Carbon dioxide permeability (barrer)

Bruggemanmodel

Maxwellmodel

ModifiedMaxwellmodel

Experimental

Matrimid 521817% CMS 13.1 13.0 11.0 10.319% CMS 13.5 13.3 11.2 10.633% CMS 17.0 16.5 12.1 11.536% CMS 17.8 17.2 12.3 12.6

Ultem® 100016% CMS 2.33 2.21 2.5120% CMS 2.59 2.40 2.9035% CMS 4.23 3.44 4.48

Data from Ref. [144].

tion of particles, as well as the formation of spaces between thesolid particle and the polymeric material. This can allow thegases to flow non-selectively around the solid particles. Koroshas suggested experimental procedures to minimise these issues;sonication and decantation of smaller solid particles can increasethe viscosity of the casting solution reducing both aggregation,sedimentation, and priming the particles with small amounts ofthe polymer can increase polymer–particle interactions, as canreducing the rate of evaporation of the casting solution [145].Thompson et al. have tried to avoid these problems by synthesis-ing zeolite containing polymeric hybrid materials using covalentbonding of the zeolite to the polymer matrix [146].

Chung et al. have formed mixed-matrix membranes fromMatrimid 5218 and a benzylamine-modified fullerene [147].Fig. 33 displays the carbon dioxide and nitrogen permeabili-ties versus increased fullerene content. Addition of the fullereneappears to lower the permeabilities of all of the examined gases.The authors suggest that the fullerene is causing rigidificationof the polymer matrix, leading to a decrease in the diffusioncoefficient.

Koros and co-workers have investigated a number of differentpolymeric membranes (polycarbonate, polysulfones and 6FDApolyimides) formed on silicon dioxide impregnated ceramic

FM

effΦ1P2 + Φ2P1

or a dilute suspension of spheres in a matrix:

eff = Pc

[Pd + 2Pc − 2Φd(Pc − Pd)

Pd + 2Pc + Φd(Pc − Pd)

]

here Pd and Pc refer to the permeabilities of the dispersednd continuous phase, respectively. In principle, a mixed-matrixembrane will exhibit discontinuous behaviour when the con-

entration of solid particles is low, and will tend towards con-inuous behaviour when the concentration of the solid particless high. Effective medium theory (EMT) has likewise been usedo model mixed-matrix membranes [142].

Koros and co-workers have compared theoretical perme-bilities with experimental derived permeabilities of Matrimid218 and Ultem® 1000 mixed with carbon molecular sievesCMS) [144]. The authors investigated two models, Maxwellnd Bruggeman (Table 6). The Bruggeman model uses the fol-owing equation when examining a random dispersion of spher-cal particles:

(Peff/Pc) − (Pd/Pc)

1 − (Pd/Pc)

] (Peff

Pc

)−1/3

= 1 − Φd

onsiderable differences between the calculated and experimen-al permeabilities were observed. This was particular in the casef Matrimid 5218. The authors suggested that this was due toigidification of the polymer where it was in close proximityo a CMS particle. The authors used a modified version of the

axwell equation to account for this and obtained a much closeorrelation with the experimental results (Table 6).

Three potential problems with the formation of effectiveixed-matrix membranes are the aggregation and sedimenta-

ig. 33. Effect of increased proportion of benzylamine modified fullerene inatrimid 5218 mixed-matrix membranes gas transport properties.


substrates [148]. The authors observed an increase in the selec-tivities compared with dense membranes of the polymers, aswell an increase in the permeabilities.

A series of mixed-matrix membranes incorporating pow-dered polypyrrole and polycarbonate have been synthesised andtheir gas transport properties determined by Yilmaz and co-workers [149]. Both the nitrogen and carbon dioxide permeabil-ities increase as the concentration of polypyrrole is increased.Other mixed-matrix membranes using polymeric particles havebeen examined [60]. Interestingly, when PMDA–ODA wasmixed with poly(styrene-co-4-vinylprydine) nanoparticles incertain proportions an increase in both the carbon dioxide per-meability and carbon dioxide/nitrogen selectivity was observed[60].

Spontak and co-workers have formed mixed-matrix mem-branes from cross-linked poly(ethylene oxide) and variousnanoparticles [150,151]. While these membranes were exam-ined primarily for their CO2/H2 separation performance, someCO2/N2 data was collected.

2.6. Asymmetric membranes

For membranes to be used in practical applications, it is nec-essary for them to be constructed into a useful morphology. Anextensive review of membrane structures is beyond the scopeofi

pbaettmattdaas[

Fig. 34. Structure of a typical asymmetric membrane (from Ref. [155]).

Conceptually, the simplest type of asymmetric membrane isone formed via a Loeb-Sourirajan phase separation process. Thisgives a membrane with two components: a defect free selectionlayer (similar to that found on a dense film membrane) and aporous support layer (Fig. 34). These membranes can be man-ufactured as either flat sheets or hollow fibres (Fig. 35). Foran asymmetric membrane to have similar selectivities to theintrinsic selectivity of the material, it is important that the gasflux of the substructure be considerably greater than the fluxthrough the selective layer [153]. Flat sheet asymmetric mem-branes have been constructed from a number of different poly-meric materials, including polysulfones [154,155], polyetherim-ide [156,157] and polyimides [158]. Hollow fibre membraneshave been constructed using a number of different polymericmaterials, including polysulfones [159–162], polyethersulfones[163], polyphenylene oxide [164], polyetherimide [165], poly-imides [166–168], polycarbonates, polyimide–poly(ethyleneoxide) copolymers[169] and polymer blends [170]. Recentpatents include useful details about the synthesis of variousasymmetric membranes [171,172]. Lin and Chung have inves-tigated the effect of aging on the performance of hollow fibremembranes constructed with 6FDA–durene [173]. Khulbe etal. have carried out atomic force microscopy and contact anglegoniometery studies on a polyethersulfone–polyimide blendhollow fibre membrane [174].

w fib

f this review. What follows is a brief introduction into thiseld.

Most studies on the gas transport properties of polymers areerformed on dense homogeneous membranes. Dense mem-ranes are generally synthesised by casting a solution of solventnd polymer onto a flat surface, and then allowing the solvent tovaporate. As such, their permeability has a close relationshipo the intrinsic permeability of the polymer. These studies allowhe efficiency of a material to be evaluated, as far as experimental

argins of error permit. However, the construction of a gas sep-ration module requires a more complex membrane structureo be used. Dense membranes tend to be significantly thickerhan the selective layer of asymmetric membranes. This leads toense membranes possessing considerably lower gas fluxes thanlternative membrane structures. Ultra thin membranes havelso be constructed by more exotic techniques, such as colli-ion of hyperthermal protons with absorbed organic precursors152].

Fig. 35. SEM pictures of a hollo
re membrane (from Ref. [175]).


A number of recent studies have been carried out on the effectof preparation variables and aging of hollow fibre membranes[175–178].

Chung and co-workers have constructed dual-layer hollowfibre membranes using polyimide and a poly(ether sulfone) toconstruct each layer [179]. The membrane was then cross-linkedby soaking the hollow fibres in a solution of p-xylenediamine inmethanol.

The carbon dioxide/nitrogen selectivity of a hollow fibremembrane system consisting of cardo polyimides has been dra-matically increased (from 35 to 81) by the incorporation of aninner layer consisting of a zero-generation polyamidoamine den-drimer. The carbon dioxide gas flux, does however, drop byalmost 50% [36].

The effect of filling agents on the structural, mechanicaland gas transport properties of silicon coated polysulfone hol-low fibre membranes has been investigated by Shilton and co-workers [180]. Carbon black (CB), vapour grown carbon fibres(VGCF) and TiO2 were all trialled as fillers. The concentrationof VGCF and TiO2 was 5%, whereas CB was trialled at 2, 5 and10% (w/w). The introduction of VGCFs caused a reduction in theCO2 permeability and an increase in the N2 permeability, lead-ing to a change in selectivity from 42 to 14. TiO2 also induced anincrease in the N2 permeability; however, the CO2 permeabil-ity was essentially unchanged. The new selectivity was 28. Theintroduction of 2% CB led to only minor changes for the per-mia3g(ifica

mpssslmhbcisaav

3

u

can play an important role in the development of this technology,one such role is the development of novel polymeric materialsfor the separation of carbon dioxide from nitrogen in power plantgas flues.

While this review concentrates on flue gas CO2/N2 separa-tion, there are a number of different locations where membranescan play a significant role. In addition to the power generationindustry, there are a number of different point sources whichproduce large CO2 emissions (steel foundries, etc). N2/O2 sep-aration membranes can be used to create oxygen enriched air,which can be used in combustion, to get concentrated CO2 fluegases. CO2 can also be separated from fuel gases leading tolower CO2 emissions.

The carbon dioxide and nitrogen gas transport proper-ties of a number of polymeric membranes have been dis-cussed. A number of different classes of polymers havebeen surveyed for their utility as materials for effective gasseparation membranes. Some types of polymers, such aspolycarbonates, polysulfones and polyimides, combine goodperformance while possessing considerable scope for struc-tural variation. One possible strategy towards the synthesisof high performance gas separation membranes is the con-struction of polymers consisting of both hard and soft sec-tions.

While many polymers have been described here, the classof polymer with the largest volume of research are polyimides,pttii

wsp2o

umep

phmto

tppueilo

eability of both gases, whereas 5% CB led to a small increasen the N2 permeability and a small decrease in the CO2 perme-bility. 10% CB reversed this trend and led to a selectivity of3. With all fillers, the gas transport properties for the CO2/N2as pairs were worse than polysulfone. The addition of the fillerswith the exception of VGCF) did lead to an increase in the burst-ng pressures of the membranes. The authors postulated that thellers increased the fractional free volume; however, the carbon-ontaining fillers interacted with the carbon dioxide leading toreduction in its permeability.

Another type of asymmetric membrane is the compositeembrane. Composite membranes utilise two or more different

olymeric materials in their construction. Often an expensiveelective layer is combined with a cheaper layer which providestructural support. Frequently a three layer system is synthe-ised by the addition of a protective layer above the selectiveayer. Composite membranes can take a number of different

orphologies, such as hollow fibre. Pinnau and co-workersave published an interesting discussion of composite mem-ranes [181]. A number of composite membranes have beenonstructed from a number of different polymeric materials;ncluding poly(ethylene oxide) containing polyimides [169],ulfonated poly(phenylene oxide) [75], polyimides [182,183]nd polysulfones. Composite membranes can be constructed byvariety of techniques including dip coating [184] and chemicalapour deposition [185].

. Conclusions

Geosquestration of carbon dioxide is an emerging technologysed to reduce the impact of fossil fuel combustion. Chemists

articularly polyimides which incorporate 6FDA. This is due toheir gas transport properties, good physical properties, poten-ial structural variations and ease of membrane formation. Its expected that this trend towards increased research on poly-mides will continue in the near future.

The high solubility of carbon dioxide in certain polymersill ensure that polymers with high carbon dioxide/nitrogen

olubility selectivity will increase in interest. For example, theolyimide/polyethylene oxide polymers described in Section.2.6 have excellent gas separation properties and are furtherf future research.

Also of interest are polymers with a high fractional free vol-me. While most of polymers offer very high permeabilities andodest selectivities, some recent advances which also have mod-

rate solubility selectivities have lead to excellent gas transportroperties.

Likewise, carbon membranes can offer excellent gas transportroperties; however their fragility, difficulty of manufacture andigh cost impose significant limitations on their utility. Mixed-atrix membranes offer a realistic technique to combine the gas

ransport properties of carbon membranes with the practicabilityf polymeric membranes.

Additionally, while gas transport properties are impor-ant, it is equally important to ensure that membranes arehysically durable and resistant to both chemical attack andlasticisation, while still being flexible enough for man-facture into various membrane types. For reason, it isxpected that future research will concentrate on improv-ng these properties through techniques such as cross-inking after manufacture into appropriate membrane morphol-gy.


Appendix A

What follows is a listing of polymers with their structures and their carbon dioxide/nitrogen gas separation properties.

A.1. Polyacetylenes (Figs. A1–A3)

Fig. A1. Chemical structures of some polyacetylenes used in the formation of gas separation membranes.




Carbon dioxide and nitrogen gas permeability data for poly(acetylene) dense films.

Name Temperature (◦C) P(CO2) (barrer) P(N2) (barrer) α(CO2/N2) Reference

Poly(trimethyl-prop-1-ynyl-silane) 25 19000 1800 10.6 [9]Poly(3,3-dimethyl-but-1-yne) 25 560 43 13.0 [9]Poly(1-(dimethyl-trimethylsilanylmethyl-silanyl)-propyne) 25 310 21 14.8 [9]Poly(1-[dimethyl-(2-trimethylsilanyl-ethyl)-silanyl]-propyne) 25 150 14 10.7 [9]Poly(trimethyl-(2-prop-1-ynyl-phenyl)-silane) 25 290 24 12.1 [9]Poly(1-prop-1-ynyl-2-trifluoromethyl-benzene) 25 130 7.3 17.8 [9]Poly(dec-2-yne) 25 130 14 9.3 [9]Poly(1-chloro-dec-1-yne) 25 170 16 10.6 [9]Poly(1-chloro-oct-1-yne) 25 130 11 11.8 [9]Poly(1-chloro-hex-1-yne) 25 180 10 18.0 [9]Poly(hexyl-dimethyl-prop-1-ynyl-silane) 25 71 4.3 16.5 [9]Poly(trimethyl-(1-pentyl-prop-2-ynyl)-silane) 25 120 8.7 13.8 [9]Poly(hexyl-dimethyl-(1-propyl-prop-2-ynyl)-silane) 25 70 6.3 11.1 [9]Poly(prop-1-ynyl-benzene) 25 25 2.2 11.4 [9]Poly(but-1-ynyl-benzene) 25 40 4.5 8.9 [9]Poly(oct-1-ynyl-benzene) 25 48 5.5 8.7 [9]Poly(chloroethynyl-benzene) 25 23 1.0 23.0 [9]Poly(1-ethynyl-2-methyl-benzene) 25 15 3.0 5.0 [9]Poly(dimethyl-phenyl-(1-propyl-prop-2-ynyl)-silane) 25 54 2.5 21.6 [9]

A.2. Poly(arylene ether) (Fig. A4)

Carbon dioxide and nitrogen gas permeability data for poly(arylene ether) dense films.

Name Feed pressure (atm) Temperature (◦C) P(CO2) (barrer) P(N2) (barrer) α(CO2/N2) Reference

6666

FPT–6FBPA 1.0 35 25.29 2.18 11.6 [43]FPT–BPA 1.0 35 18.53 1.37 13.5 [43]FPPy–6FBPA 1.0 35 29.46 2.39 12.32 [43]FPPy–BPA 1.0 35 21.44 1.70 12.6 [43]


Fig. A4. Fluoro-containing novel poly(arylene ether).

A.3. Polyarylates (Figs. A5–A7)

Carbon dioxide and nitrogen gas permeability data for polyarylate dense films.


BPA/IA 10/2 35 5.4 0.24 22.5 [45]BPA/tBIA 10/2 35 24.2 1.20 20.2 [45]HFBPA/IA 10/2 35 19.1 1.11 17.2 [45]HFBPA/tBIA 10/2 35 56.9 3.88 14.7 [45]PhTh/IA 10/2 35 6.74 0.28 24.1 [45]PhTh/tBIA 10/2 35 23.8 1.09 21.8 [45]FBP/IA 10/2 35 12.4 0.57 12.4 [45]FBP/tBIA 10/2 35 36.8 1.93 19.1 [45]TBBPA/IA 10/2 35 4.93 0.18 27.4 [46]TBBPA/tBIA 10/2 35 21.5 0.90 23.9 [46]TBHFBPA/IA 10/2 35 25.6 1.07 23.9 [46]TBHFBPA/tBIA 10/2 35 85.1 4.47 19.0 [46]TBPhTh/IA 10/2 35 8.34 0.29 28.8 [46]TBPhTh/tBIA 10/2 35 30.6 1.28 23.9 [46]TBFBP/IA 10/2 35 20.4 0.70 29.1 [46]TBFBP/tBIA 10/2 35 69.5 2.94 23.6 [46]DMBPA/IA 10/2 35 1.24 0.063 19.7 [44]DMBPA/tBIA 10/2 35 8.0 0.39 20.5 [44]TMBPA/IA 10/2 35 12.0 0.58 20.7 [44]TMBPA/tBIA 10/2 35 44.6 2.52 17.7 [44]DiisoBPA/IA 10/2 35 5.16 0.27 19.1 [44]DiisoBPA/tBIA 10/2 35 16.1 1.08 14.9 [44]DBDMBPA/IA 10/2 35 5.45 0.22 24.8 [44]PhAnth/IA 10/2 35 9.0 0.36 25 [47]PFF

hAnth/tBIA 10/2 35 25.9 1.35 19.2 [47]BP/IA 10/2 35 12.4 0.57 21.8 [47]BP/tBIA 10/2 35 36.8 1.93 19.1 [47]


Fig. A5. Structures of symmetric polyarylates used by Pixton and Paul [44–46].

Fig. A6. Structures of asymmetric polyarylates used by Pixton and Paul [44].


Fig. A7. Structures of anthrone-based polyarylates studied by Pixton and Paul [47].

A.4. Polycarbonates (Figs. A8–A10)

Carbon dioxide and nitrogen gas permeability data for dense films incorporating polycarbonates.


PC 10/5 35 6.5 0.33 19.7 [50]PC 1 35 6.0 0.289 21 [52]PC 10 35 6.8 0.32 21 [51]TMPC 1 35 17.58 1.09 16.1 [52]TMPC 10 35 18.6 1.0 18.6 [51]TCPC 1 35 6.66 0.36 18.5 [52]TBPC 1 35 4.23 0.182 23.2 [52]HFPC 10 35 24 1.6 15.0 [51]TMHFPC 10 35 111 7.4 15.0 [51]NBPC 10/2 35 9.1 0.47 19.4 [49]PCZ 10/2 35 2.2 0.105 21.0 [49]PC–AP 2 35 9.48 0.361 26.3 [48]FBPC 2 35 15.1 0.592 25.5 [48]


Fig. A8. Structures of some polycarbonates.

A.5. Poly(ethylene oxide)

Carbon dioxide and nitrogen gas permeability data for poly(ethylene oxide) dense films.


PEO 7.8 25 8.1 0.07 140 [55]PEO 4.4 35 13 0.24 55 [55]PEO 7.8 35 15 0.24 63 [55]PEO 14.6 35 17 0.22 79 [55]PEO 4.4 45 40 0.99 40 [55]PEO 7.8 45 46 1.0 48 [55]PEO 14.6 45 52 1.0 52 [55]EO/EM/AGE (80/20/2) Not reported 35 773 16.8 46 [56]EO/EM/AGE (77/23/2.3) Not reported 35 680 15.5 44 [56]EO/EM/AGE (96/4/2.5) Not reported 35 580 12.1 48 [56]


Fig. A9. Structures of some aromatic halogenated polycarbonates.

Fig. A10. Structures of some polycarbonates.

A.6. Polyimides (Figs. A11–A15)

Carbon dioxide and nitrogen gas permeability data for dense films incorporating polyimides.


PMDA–BAPHF 6.8 35 11.8 0.66 17.8 [74]PMDA–3BAPHF 6.8 35 6.12 0.29 21.1 [74]PMDA–4,4′-ODA 6.8 35 1.14 0.049 23.3 [74]PMDA–4,4′-ODA 10 35 2.71 0.10 27.1 [186]PMDA–3,3′-ODA 6.8 35 0.50 0.018 27.8 [74]PMDA–4,4′-ODA 10/2 35 1.18 0.0454 26.0 [70]PMDA–3,3′-ODA 10/2 35 3.55 0.145 24.5 [70]PMDA–MDA 10 35 4.03 0.20 20.2 [186]



PMDA–IPDA 10 35 29.7 1.50 19.8 [186]PMDA–BAPHF 10/2 35 17.6 0.943 18.7 [70]PMDA–BATPHF 10/2 35 24.6 1.50 16.4 [70]BPDA–BAPHF 10/2 35 4.95 0.245 20.2 [70]BPDA–BATPHF 10/2 35 9.15 0.563 16.3 [70]BPDA–BAHF 10/2 35 27.7 1.39 19.9 [70]BPDA–BAFL 1 25 23 0.61 37.7 [187]BPDA–mTrMPD 10/2 35 137 8.42 16.3 [71]BTDA–4,4′-ODA 10/2 35 0.625 0.0236 26.5 [70]BTDA–BAPHF 10/2 35 4.37 0.195 22.4 [70]BTDA–BATPHF 10/2 35 6.94 0.370 18.8 [70]BTDA–BAHF 10/2 35 10.1 0.45 22.4 [70]BTDA–mTrMPD 10/2 35 30.9 1.55 19.9 [71]BTDA–BAFL 1 25 15 0.39 38.5 [187]PI 10 35 2.00 0.063 31.7 [60]oMeCat–durene 1 30 27 0.83 33 [66]mMeCat–durene 1 30 20 0.59 34 [66]DMeCat–durene 1 30 63 2.05 31 [66]mtBuCat–durene 1 30 71 2.55 28 [66]oMeptBuCat–durene 1 30 67 2.5 27 [66]TMeCat–durene 1 30 200 8.1 25 [66]mMetCat–MDA 1 30 22 0.65 34 [66]mtBuCat–MDA 1 30 63 2.2 29 [66]TMeCat–MDA 1 30 110 3.8 30 [66]TMeCat–TMB 1 30 39 1.2 33 [66]DBuCat–TMB 1 30 95 4.9 19 [66]mtBuCat–DMOB 1 30 6.7 0.21 32 [66]TMeCat–6FiPDA 1 30 54 1.9 28 [66]6F 3 Not reported 114 5.8 19.6 [107]TMMPD 3 Not reported 600 35.1 17.1 [107]IMDDM 3 Not reported 196 10.8 18.1 [107]ODA 3 Not reported 25 0.97 25.8 [107]Matrimid 5218 10 35 6.5 0.25 25.6 [57]

Fig. A11. Common polyimide moieties.


Fig. A12. Some diamine repeating units.


Fig. A13. Some diamine repeating units.

Fig. A14. Catechol-based polyimides synthesised by Kricheldorf and co-workers [66].


Fig. A15. Structures of some bulky novel polyimides.

A.7. 6FDA-based polyimides (Fig. A16)

Carbon dioxide and nitrogen gas permeability data for dense films incorporating the polyimide 6FDA.


6FDA–pPDA 10 35 15.3 0.80 19.12 [71]6FDA–pDiMPDA 10 35 42.7 2.67 16.0 [71]6FDA–durene 10 35 440 35.60 12.4 [71]6FDA–durene 10 35 456 35.50 12.85 [65]6FDA–mPDA 10 35 9.20 0.447 20.6 [72]6FDA–mPDA 6.8 35 8.23 0.36 22.7 [72]6FDA–mMPDA 10 35 40.1 2.24 17.9 [72]6FDA–mMPDA 6.8 35 42.52 2.12 20.1 [72]6FDA–mTrMPDA 10 35 431 31.6 13.6 [72]6FDA–DATr 6.8 35 28.63 1.31 21.9 [72]6FDA–DBTF 6.8 35 21.64 1.17 18.5 [72]6FDA–PHDoeP 6.8 35 8.59 4.50 1.91 [69]6FDA–PPDoeP 6.8 35 15.05 1.12 13.4 [69]6FDA–PMDoeP 6.8 35 4.78 0.181 26.4 [69]6FDA–PEPE 6.8 35 6.88 0.255 27.0 [69]6FDA–PBEPE 6.8 35 2.50 0.099 25.3 [69]6FDA–PTEPE 6.8 35 1.94 0.068 22.6 [69]6FDA–PMeaP 6.8 35 2.41 0.086 28.0 [69]6FDA–3,4′ODA 10 35 6.11 0.259 23.6 [70]6FDA–APAP 10 35 10.7 0.473 22.6 [70]6FDA–pp′ODA 10 35 16.7 0.733 22.8 [70]6FDA–BAPHF 10 35 19.1 0.981 19.5 [70]6FDA–BATPHF 10 35 22.8 1.30 17.5 [70]6FDA–BAHF 10 35 51.2 3.11 16.5 [70]6
FDA–1,5-NDA 10 35 23 1.1 21 [62]



6FDA–durene 24 h amidation 10 Not Reported 11.6 1.33 8.75 [58]6FDA–durene/mPDA (50/50) 10 Not Reported 84.6 5.18 16.4 [58]6FDA–durene/mPDA (50/50) 4 h amidation 10 Not Reported 54.9 3.38 16.2 [58]6FDA–durene/mPDA (50/50) 6 h amidation 10 Not Reported 49.1 3.27 15.0 [58]6FDA–durene/mPDA (50/50) 12 h amidation 10 Not Reported 46.0 2.94 15.6 [58]6FDA–durene/mPDA (50/50) 24 h amidation 10 Not Reported 36.0 2.06 17.5 [58]6FDA–durene/mPDA (50/50) 48 h amidation 10 Not Reported 24.5 1.38 17.8 [58]6FDA–FDA/HFBAPP (1/1) 1.1 kg/cm2 30 465.0 19.9 23.4 [68]6FDA–ODA 10 35 23 0.83 27.7 [186]6FDA–4,4-ODA 6.8 35 22.0 0.94 23.4 [74]6FDA–MDA 10 35 19 0.81 23.5 [186]6FDA–4BDAF 6.8 35 19 0.98 19.4 [74]6FDA–3,3′-ODA 6.8 35 2.1 0.10 21 [74]6FDA–3BDAF 6.8 35 6.3 0.24 26.3 [74]6FDA–IPDA 10 35 24.3 0.87 27.9 [73]6FDA–IPDA 10 45 25.6 1.09 23.5 [73]6FDA–IPDA 10 55 27.4 1.39 19.7 [73]6FDA–DAF 10 35 19.5 0.81 24.1 [73]6FDA–DAF 10 45 20.2 0.95 21.3 [73]6FDA–DAF 10 55 21.3 1.15 18.5 [73]PI-1 1 30 32 1.4 22.9 [67]PI-3 1 30 360 16.5 21.8 [67]PI-4 1 30 62 2.4 25.8 [67]PI-5 1 30 190 7.3 26.0 [67]6FDA–BAFL 1 25 98 3.3 29.7 [187]

Fig. A16. 6FDA–terphenylene polyimides synthesised by Fritsch and co-workers [66].


Fig. A17. Structures of some poly(phenylene oxide) derivates.

A.8. Poly(phenylene oxide)s (Fig. A17)

Carbon dioxide and nitrogen gas permeability data for poly(phenylene oxide) dense films.


PPS 1.5 35 1.60 0.046 34.8 [77]PDMPO 1.5 35 65.5 3.5 18.7 [77]PDPPO 1.5 35 39.9 1.5 26.6 [77]PDMPO 689.1 kPa 22 90.0 3.7 24.3 [76]PDMPO (20.0% brominated) 689.1 kPa 22 93.6 3.8 24.6 [76]PDMPO (37.4% brominated) 689.1 kPa 22 97.1 3.7 26.2 [76]PDMPO (60.0% brominated) 689.1 kPa 22 159.9 8.0 20.0 [76]

A.9. Poly(pyrrolone) (Fig. A18)

Carbon dioxide and nitrogen gas permeability data for polypyrrole dense films.


6FDA–TAB 10/3 35 54.0 2.6 20.8 [79]6FDA–TADPO 10/3 35 27.6 1.2 23.0 [82]BBL 10/3 35 0.12 0.0026 46.3 [81]


Fig. A18. Structures of some poly(pyrrolone)s.

A.10. Polysulfones (Figs. A19–A22)

Carbon dioxide and nitrogen gas permeability data for dense films incorporating polysulfones.


PSF 10/5 35 5.6 0.25 22.4 [93]PSF 10/>5 35 4.9 0.2 24.5 [50]TMPSF 10/5 35 21 1.06 19.8 [93]HFPSF 10/5 35 12 0.67 17.9 [94]TMHFPSF 10/5 35 72 4.0 18 [92]PSF-F 10/5 35 4.5 0.20 22.5 [94]PSF-O 10/5 35 4.3 0.20 21.5 [94]PSF-P 10/1 35 6.8 0.32 21.3 [90]TMPSF-F 10/5 35 5.5 0.61 9.0 [92]TMPSF-P 10/1 35 13.2 0.57 23.2 [90]BIPSF 10/2 35 5.6 0.24 23.3 [89]TMBIPSF 10/2 35 31.8 1.21 26.3 [89]1,5-NPSF 10/2 35 1.6 0.057 28.1 [91]2,6-NPSF 10/2 35 1.5 0.051 29.4 [91]2,7-NPSF 10/2 35 1.8 0.074 24.3 [91]DMPSF 10/5 35 2.1 0.091 23.1 [93]HMBIPSF 10/2 35 25.5 1.2 23.3 [89]DMPSF-Z 10/5 35 1.4 0.057 24.6 [93]PSF-AP 2 35 8.12 0.278 29.2 [48]FBPSF 2 35 13.8 0.484 28.5 [48]PSF-M 1 35 2.8 0.11 25.5 [90]TMPSF-M 10/1 35 7.0 0.28 25.0 [90]PSF-BPFL 1 25 10 0.25 40 [187]3,4′-PSF 1 35 1.5 0.066 22.7 [90]12P

,3-ADM PSF 10/2 35 7.2 0.33 21.8 [88],2-ADM PSF 10/2 35 9.5 0.46 20.6 [88]SF (6% Br, 92% C CSiMe3) 1 35 36.5 2.1 17.4 [87]



PSF (3% Br, 47% C CSiMe3) 1 35 18.5 1.24 14.9 [87]PSF (21% Br, 77% C CCMe3) 1 35 28.2 1.7 16.6 [87]PSF (5% Br, 45% C CCMe3) 1 35 16.4 0.9 18.2 [87]PSF 1 35 5.6 0.25 22.4 [84]PSF-s-HBTMS 1 35 21 0.96 22.2 [84]PSF-o-HBTMS 1 35 70 3.29 21.3 [84]PSF-CH2-TMS 1 35 18 0.95 18.9 [84]EM3 1 35 29 1.3 22 [86]EM2 1 35 6.2 0.24 26 [86]EM1 1 35 4.8 0.16 30 [86]SM3 (degree of substitution = 2.0) 1 35 18 0.77 23 [86]SM3 (degree of substitution = 1.0) 1 35 10 0.38 26 [86]SM1 1 35 5.1 0.17 30 [86]PPSF 1 35 3.2 0.10 32 [86]RM3 1 35 27 1.9 14 [86]RM2 1 35 6.7 0.60 11 [86]RM1 1 35 6.9 0.61 11 [86]HFPSF 1 35 12.0 0.67 17.9 [84]HFPSF-o-HBTMS 1 35 105 5.63 18.6 [84]HFPSF-s-TMS 1 35 41 2.0 20 [83]HFPSF-o-TMS 1 35 84 4.7 18 [83]HFPSF-TMS 1 35 110 6.3 18 [83]TM6FPSF 1 35 72 4.0 18 [83]TM6FPSF-s-TMS 1 35 96 5.2 19 [83]TMPSF-TMS 1 35 32 1.51 21.3 [85]TMPSF-s-TMS 1 35 66.3 3.07 21.6 [85]TMPSF-HBTMS 1 35 72 3.36 21.4 [85]

Fig. A19. Structures of PSF and some related methylated polymers.


Fig. A20. Some polysulfone structures.

Fig. A21. Some polysulfone structures incorporating bisphenol Z.


Fig. A22. Polysulfone structures incorporating aromatic substituted bisphenols.

A.11. Others (Figs. A23–A27)

Carbon dioxide and nitrogen gas permeability data for other polymeric dense films.


HQDPA–PDA 7 30 0.598 0.016 37.4 [53]HQDPA–PDA 7 100 1.70 0.111 15.3 [53]HQDPA–DBA 7 30 0.683 0.015 45.5 [53]HQDPA–DBA 7 100 2.10 0.125 16.8 [53]HQDPA–MDBA 7 30 1.18 0.034 34.7 [53]HQDPA–MDBA 7 100 2.37 0.160 14.8 [53]HQDPA–EDBA 7 30 2.26 0.077 29.4 [53]HQDPA–EDBA 7 100 4.18 0.292 14.3 [53]12H 5 35 4.6 0.21 21.9 [189]6H6F 5 35 8.6 0.44 19.5 [189]6F6H 5 35 8.9 0.42 21.2 [189]12F 5 35 12.9 0.76 17.0 [189]PBK 10/2 35 3.3 0.13 25.4 [188]PBK-S 10/2 35 3.27 0.11 29.7 [188]PBSF 10/2 35 10.8 0.47 23.0 [188]PPES Not reported 25 0.92 0.027 34 [190]PPESK Not reported 25 0.75 0.042 18 [190]

Fig. A23. PEI structure.


Fig. A24. PEI studied by Li and co-workers.

Fig. A25. Some poly(ether ketone) structures.


Fig. A26. Poly(arylether bissulfone)s and poly(arylether bisketone) studied by Paul and co-workers [188].

Fig. A27. Structure of poly(phthalazinone ether ketone) (PPEK) and poly(phthalazinone ether sulfone) (PPES).

A.12. Copolymers and polymer blends

Carbon dioxide and nitrogen gas permeability data for copolymers and polymer blend dense films.


6FDA–TAB 10/3 35 54.0 2.8 19.3 [79]6FDA/PMDA–TAB (50:50) 10/3 35 15.8 0.70 22.6 [79]6FDA/PMDA–TAB (25:75) 10/3 35 3.13 0.098 31.9 [79]6FDA/PMDA–TAB (10/90) 10/3 35 1.11 0.036 30.8 [79]6FDA–TAB/DAM (75/25) 3 35 73.7 3.1 23.8 [115]6FDA–TAB/DAM (50/50) 3 35 155 6.6 23.5 [115]6FDA–DAM 3 35 370 29.5 12.5 [115]6FDA/PMDA(1:6)–TMMDA (CH2Cl2 cast) 10 35 187 11.7 16.0 [97]6FDA/PMDA(1:6)–TMMDA (NMP cast) 10 35 144 8.76 16.4 [97]6FDA/PMDA(1:6)–TMMDA (DMF cast) 10 35 88.6 5.16 17.2 [97]MDI–BPA/PEG(75) 2 35 31 0.70 44 [112]



MDI–BPA/PEG(80) 2 35 48 1.0 47 [112]MDI–BPA/PEG(85) 2 35 59 1.20 49 [112]L/TDI(20)–BPA/PEG(90) 2 35 47 0.92 51 [112]L/TDI(40)–BPA/PEG(85) 2 35 35 0.73 48 [112]IPA–ODA/PEO3(80) 2 35 58 1.1 53 [112]BPDA–ODA/DABA/PEO1(75) 2 35 2.7 0.048 56 [112]BPDA–mDDS/PEO1(80) 2 35 3.8 0.066 58 [112]BPDA–ODA/DABA/PEO2(70) 2 35 14 0.25 57 [112]BPDA–ODA/DABA/PEO2(80) 2 35 36 0.64 56 [112]BPDA–ODA/PEO3(75) 2 35 75 1.4 52 [112]BPDA–mDDS/PEO3(75) 2 35 72 1.4 53 [112]BPDA–mPD/PEO4(80) 2 35 81 1.5 54 [112]BPDA–ODA/PEO4(80) 2 35 117 2.3 51 [112]PMDA–ODA/DABA/PEO1(80) 2 35 14 0.27 52 [112]PMDA–ODA/PEO2(75) 2 35 40 0.74 54 [112]PMDA–mPD/PEO3(80) 2 35 99 2.0 50 [112]PMDA–APPS/PEO3(80) 2 35 159 3.1 51 [112]PMDA–APPS/PEO4(70) 2 35 136 2.6 53 [112]PMDA–mPD/PEO4(80) 2 35 151 2.9 52 [112]PMDA–ODA/PEO4(80) 2 35 167 3.2 52 [112]PMDA–pDDS/PEO4(80) 2 35 238 4.9 49 [112]PMDA/BTDA–BAFL (50:50) 1 25 43 1.3 33 [187]PMDA/BTDA–BAFL (90:10) 1 25 130 3.8 34 [187]BPDA–BAFL/HMDA (50:50) 1 25 0.54 0.014 39 [187]PPES Not reported 25 0.92 0.027 34 [190]PPES/PPEK (3:1) Not reported 25 2.94 0.074 40 [190]PPES/PPEK (1:1) Not reported 25 4.12 0.089 46 [190]PPES/PPEK (1:3) Not reported 25 2.06 0.026 39 [190]PPHHHHPPPPPPPPPNNNN66666666666666666

PES/PPEK (1:4) Not reported 25 1.77 0.052 34 [190]PEK Not reported 25 0.75 0.042 18 [190]QDPA–DPA/MDPA 7 30 0.957 0.023 41.2 [53]QDPA–DPA/MDPA 7 100 2.34 0.147 15.9 [53]QDPA–DPA/EDPA 7 30 1.34 0.036 37.6 [53]QDPA–DPA/EDPA 7 100 3.25 0.207 15.7 [53]I 10 35 2.00 0.063 31.7 [60]I/10PS 10 35 2.33 0.085 27.4 [60]I/15PS 10 35 2.32 0.090 25.8 [60]I/20PS 10 35 2.90 0.91 3.19 [60]I/25PS 10 35 4.29 0.91 4.71 [60]I/10PSVP 10 35 3.58 0.13 28.4 [60]I/15PSVP 10 35 3.71 0.14 26.5 [60]I/20PSVP 10 35 5.65 0.15 38.4 [60]I/25PSVP 10 35 6.55 1.55 4.31 [60]TDA–BDSA(30)/CARDO/ODA 3 30 70 1.7 41 [63]TDA–BDSA(30)/CARDO 3 30 164 4.5 36 [63]TDA–BDSA(30)/BAPHF 3 30 23 0.64 36 [63]TDA–BDSA(30)/ODA 3 30 5.2 0.1 52 [63]FDA–FDA/HFBAPP (1/1) 1.1 kg/cm2 30 465.0 19.9 23.4 [68]FDA–durene/pPDA (80/20) 10 35 230 16.88 13.62 [65]FDA–durene/pPDA (50/50) 10 35 126 7.74 16.28 [65]FDA–durene/pPDA (20/80) 10 35 59.26 2.81 21.09 [65]FDA–durene/3,3′-DDS (75/25) 10 35 84.7 5.91 14.3 [61]FDA–durene/3,3′-DDS (50/50) 10 35 19.8 1.09 18.2 [61]FDA–durene/3,3′-DDS (25/75) 10 35 5.12 0.26 19.7 [61]FDA–3,3′-DDS 10 35 1.84 0.08 22.7 [61]FDA–6FpDA-DABA-12.5 4 35 34.0 2.01 16.9 [59]FDA–6FpDA–DABA-12.5 annealed 4 35 70.8 4.50 15.7 [59]FDA–6FpDA–DABA-12.5 (22.5% TMOS) 4 35 30.9 1.70 18.2 [59]FDA–6FpDA–DABA-12.5 (22.5% TMOS) annealed 4 35 47.6 3.16 15.1 [59]FDA–6FpDA–DABA-12.5 (15.0% MTMOS) 4 35 44.0 2.53 17.4 [59]FDA–6FpDA–DABA-12.5 (15.0% MTMOS) annealed 4 35 110 7.07 15.6 [59]FDA–6FpDA–DABA-12.5 (15.0% PTMOS) 4 35 32.3 1.80 17.9 [59]FDA–6FpDA–DABA-12.5 (15.0% PTMOS) annealed 4 35 91.8 5.59 16.4 [59]FDA–6FpDA–DABA-12.5 (22.5% PTMOS) 4 35 30.7 1.88 16.3 [59]



6FDA–6FpDA–DABA-12.5 (22.5% PTMOS) annealed 4 35 90.9 5.87 15.5 [59]6FDA–6FpDA–DABA-25 4 35 20.3 1.20 16.9 [59]6FDA–6FpDA–DABA-25 annealed 4 35 77.3 4.85 15.9 [59]6FDA–6FpDA–DABA-25 (22.5% TMOS) 4 35 15.7 1.06 14.8 [59]6FDA–6FpDA–DABA-25 (22.5% TMOS) annealed 4 35 79.8 4.87 16.4 [59]6FDA–6FpDA–DABA-25 (15.0% MTMOS) 4 35 16.6 1.07 15.5 [59]6FDA–6FpDA–DABA-25 (15.0% MTMOS) annealed 4 35 81.1 5.07 16.0 [59]6FDA–6FpDA–DABA-25 (22.5% MTMOS) 4 35 16.6 1.07 15.5 [59]6FDA–6FpDA–DABA-25 (22.5% MTMOS) annealed 4 35 60.1 3.83 15.7 [59]6FDA–6FpDA–DABA-25 (15.0% PTMOS) 4 35 18.4 0.94 19.6 [59]6FDA–6FpDA–DABA-25 (15.0% PTMOS) annealed 4 35 104 6.25 16.6 [59]6FDA–6FpDA–DABA-25 (22.5% PTMOS) 4 35 19.1 0.98 19.5 [59]6FDA–6FpDA–DABA-25 (22.5% PTMOS) annealed 4 35 104 6.25 16.6 [59]Poly(5:5 BPA/BN) 5 35 5.71 0.19 30.1 [114]Poly(7:3 BPA/BN) 5 35 4.62 0.16 28.9 [114]

A.13. Cross-linking polymers

Carbon dioxide and nitrogen gas permeability data for cross-linked membranes dense films.


Poly(ethylene oxide-co-epichlorohydrin) (1:1) 1.1% 3 × 105 Pa 25 15.0 2.3 6.52 [125]Poly(ethylene oxide-co-epichlorohydrin) (1:1) 2% 3 × 105 Pa 25 14.9 1.0 14.9 [125]Poly(ethylene oxide-co-epichlorohydrin) (1:1) 5% 3 × 105 Pa 25 16.1 0.5 32.2 [125]DM14/MM9 (100/0) 96.7 kPa 25 45 0.66 68 [126]DM14/MM9 (100/0) 96.7 kPa 50 107 2.8 38 [126]DM14/MM9 (90/10) 96.7 kPa 25 62 0.90 69 [126]DM14/MM9 (90/10) 96.7 kPa 50 133 3.4 39 [126]DM14/MM9 (70/30) 96.7 kPa 25 96 1.5 66 [126]DM14/MM9 (70/30) 96.7 kPa 50 195 5.4 36 [126]DM14/MM9 (50/50) 96.7 kPa 25 144 2.25 64 [126]DM14/MM9 (50/50) 96.7 kPa 50 260 7.2 36 [126]DM14/MM9 (30/70) 96.7 kPa 25 210 3.3 63 [126]DM14/MM9 (30/70) 96.7 kPa 50 350 10.6 33 [126]DB30/MM9 (100/0) 96.7 kPa 25 93 1.5 63 [126]DB30/MM9 (100/0) 96.7 kPa 50 200 5.7 35 [126]DB30/MM9 (90/10) 96.7 kPa 25 105 1.6 64 [126]DB30/MM9 (90/10) 96.7 kPa 50 210 5.8 36 [126]DB30/MM9 (70/30) 96.7 kPa 25 141 2.1 67 [126]DB30/MM9 (70/30) 96.7 kPa 50 270 7.7 35 [126]DB30/MM9 (50/50) 96.7 kPa 25 179 2.9 62 [126]DB30/MM9 (50/50) 96.7 kPa 50 330 9.7 34 [126]DB30/MM9 (30/70) 96.7 kPa 25 250 4.2 60 [126]DB30/MM9 (30/70) 96.7 kPa 50 410 12.4 33 [126]DM9/MM9 (90/10) 96.7 kPa 25 18.3 0.3 68 [126]DM9/MM9 (90/10) 96.7 kPa 50 51 1.3 38 [126]DM23/MM9 (90/10) 96.7 kPa 25 145 2.2 66 [126]DM23/MM9 (90/10) 96.7 kPa 50 290 7.6 38 [126]DB10/MM9 (90/10) 96.7 kPa 25 6.7 0.11 61 [126]DB10/MM9 (90/10) 96.7 kPa 50 27 0.79 34 [126]DB69/MM9 (90/10) (cooling) 96.7 kPa 25 240 4.3 56 [126]DB69/MM9 (90/10) (cooling) 96.7 kPa 50 510 14.2 36 [126]DB69/MM9 (90/10) (heating) 96.7 kPa 25 98 1.6 62 [126]DB69/MM9 (90/10) (heating) 96.7 kPa 50 400 11.4 35 [126]DM14/MM23 (30/70) (cooling) 96.7 kPa 25 240 3.9 62 [126]DM14/MM23 (30/70) (cooling) 96.7 kPa 50 420 12 35 [126]DM14/MM23 (30/70) (heating) 96.7 kPa 25 250 4.0 62 [126]Matrimid 5218 10 35 6.5 0.25 25.6 [57]



Matrimid 5218, 1 day cross-linking 10 35 7.4 0.29 25.6 [57]Matrimid 5218, 3 days cross-linking 10 35 6.0 0.24 25.2 [57]Matrimid 5218, 7 days cross-linking 10 35 5.1 0.21 24.6 [57]Matrimid 5218, 14 days cross-linking 10 35 4.7 0.19 24.1 [57]Matrimid 5218, 21 days cross-linking 10 35 3.4 0.15 22.2 [57]Matrimid 5218, 32 days cross-linking 10 35 1.9 0.13 15.0 [57]6FDA–durene, 5 min cross-linked 10 35 136 11.1 12.3 [64]6FDA–durene, 10 min cross-linked 10 35 91.8 6.53 14.1 [64]6FDA–durene, 15 min cross-linked 10 35 70.0 6.05 11.6 [64]6FDA–durene, 30 min cross-linked 10 35 30.3 2.87 10.6 [64]6FDA–durene, 60 min cross-linked 10 35 2.14 0.40 5.35 [64]

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co2 from flue gas

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