Polymeric Membranes for Natural Gas Processing: Polymer ... … · Polymeric Membranes for Natural Gas Processing: Polymer Synthesis and Membrane Gas Transport Properties Jimoh K

Polymeric Membranes for Natural GasProcessing: Polymer Synthesisand Membrane Gas Transport Properties

Jimoh K. Adewole and Abdullah S. Sultan

Contents1 Natural Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Natural Gas Processing Using Membrane Separation Technology . . . . . . . . . . . . . . . . . . . . . . . . . 43 Functional Polymeric Membranes for Natural Gas Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 Classification of Membrane Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 Theoretical Background of Gas Separation in Polymeric Membranes . . . . . . . . . . . . . . . . . . . . . . 116 Synthesis, Preparation, and Transport Properties of Functional Polymeric Membrane for Natural

Gas Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156.1 Thermal Rearrangement and Cross-Linking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156.2 Grafting of Polymer Backbone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266.3 Template Polymerization Technique and Use of Porogens . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276.4 Sulfonation Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296.5 Technique for Preparing Polymers of Intrinsic Microporosity (PIM) . . . . . . . . . . . . . . . . . 296.6 Membrane Preparation Using 3D Printing Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

7 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

AbstractPolymers are macromolecules made up of repetition of some simpler units calledmonomers. To the general public, polymers are used as consumer products in theforms of plastics, fibers, rubber, adhesive, paints, and coatings. Today, new areasof applications of polymers have emerged due to the continuous growth of new

J. K. AdewoleCenter for Integrative Petroleum Research, College of Petroleum Engineering & Geosciences, KingFahd University of Petroleum & Minerals, Dhahran, Saudi Arabiae-mail: [email protected]

A. S. Sultan (*)Department of Petroleum Engineering, College of Petroleum Engineering & Geosciences, KingFahd University of Petroleum & Minerals, Dhahran, Saudi Arabiae-mail: [email protected]

# Springer International Publishing AG, part of Springer Nature 2019M. A. Jafar Mazumder et al. (eds.), Functional Polymers, Polymers and PolymericComposites: A Reference Series, https://doi.org/10.1007/978-3-319-92067-2_26-1

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classes of polymers. Varieties of functional polymers have been developed forvarious applications including organic catalysis, separation, biotechnology, med-icines, optoelectronics, photographic, building, and fuel. Natural gas separationusing functional polymers is a new area of application where not much informa-tion is available. This work seeks to present an overview of the synthesis,preparation, and separation performance of functional polymers in natural gasseparation.

List of Abbreviations6FDA 2,20-bis(3,4-Dicarboxyphenyl) hexafluoropropane dianhydrideAPTMDS bis(3-Aminopropyl)-tetramethyldisiloxane)BAS bis(Aminopropyl) polydimethylsiloxaneCD CyclodextrinCH4 MethaneCMS Carbon molecular sieveCO2 Carbon dioxideDADE Diamino diphenyl etherEDA Ethylene diamineGPU Gas permeation unitHPAAc Hydroxyl poly (amic acid)HPI Hydroxyl ployimideMDI 4,4-Metylenediphenyl diisocyanateNG Natural gasPBO PolybenzoxazolesPBT Polybenzothiazoles (PBT)PDMC Propane-diol monoesterified cross-linkable polyimidePEG Polyethylene glycolPEO Polyethylene oxidePHA PolyhydroxyamidePIM Polymers of intrinsic microporosityPIOFG Ortho-positioned functional groupsPPG Poly(propylene glycol)PTMS Poly[1-(trimethylsilyl)-1-propyne]S-PEEK Poly(ether ether ketone)TCDA 2,3,5-Tricarboxy cyclopentyl acetic dianhydrideTHF TetrahydrofuranTR Thermal rearrangement/thermally rearrangedTR-PBI Thermally rearranged microporous polybenzimidazoleXLPEGDA Cross-linked poly(ethylene glycol diacrylate)

List of SymbolsDD Gas diffusion coefficient by Henry modes of sorptionED Activation energy of diffusion (kJ/mol)EP Activation energy of permeation

2 J. K. Adewole and A. S. Sultan

DH Gas diffusion coefficient by Langmuir modes of sorptionΔHS Enthalpy of the gaskD Henry’s law constantb Langmuir affinity constant

C0H

Langmuir capacity constant

L Membrane thickness (dense-selective layer thickness)P PermeabilityαAB Permselectivity or selectivity or separation factor of component A from

BDA0 Pre-exponential factorSA0 Pre-exponential factorPA0 Pre-exponential factorA or B Gas components (or permeated and rejected component, respectively)D Diffusivity coefficientR Universal gas constant, 8.31 J/molS Solubility coefficientT Temperature, K

1 Natural Gas

Currently, the world cleanest, safest, and most efficient energy source is natural gas[1–3]. The carbon dioxide emission factor of commercial natural gas is 26% lowerthan that of oil and 41% lower than the emission from coal during combustion[4]. Natural gas is often found in crude oil or gas reservoirs, condensate wells, andcoal beds [2] with methane (CH4) as its primary constituent [5]. In addition, naturalgas contains some heavier hydrocarbons, water (in the form of vapor), mercury,nitrogen, helium, and acid gases [6]. Table 1 contains a detailed composition of atypical raw natural gas. It is important to note that natural gas composition variesfrom place to place [7]. The composition shown in Table 1 is the range obtained forEleme (Nigeria), Alberta (Canada), Western Colorado (USA), Miskar (Tunisia),Southwest Kansas (USA), Rio Arriba County (New Mexico), and Bach Ho (Viet-nam). In the present global economy, natural gas is indeed one of the fastest growingprimary sources of energy [2, 8–10]. In 2012, it constitutes about 24% of the globalenergy mix and it is expected to rise to 28 by 2040 (Fig. 1). The natural gas worldmarket is estimated as US$22billion per annum [11]. Clearly, for natural gas tomaintain its position as one of the world’s most sought after energy sources,alternative cheaper and energy efficient means of processing technique (such asthe membrane) is needed.

Polymeric Membranes for Natural Gas Processing: Polymer Synthesis and. . . 3

2 Natural Gas Processing Using Membrane SeparationTechnology

Today, membrane gas separation technology is a commercially available process forgas separation and purification. This include separation of nitrogen from air (oxygenenrichment), recovery of hydrogen from process streams, dehydration of gasstreams, recovery of vapors from gases, and natural-gas processing [16–18]. Thecomposition of crude natural gas displayed in Table 1 clearly reveals that the rawnatural gas needs further treatment in order to meet a specified quality standard. Thestandard that is specified by pipeline transmission and distribution companies isshown in Table 2. Membrane separation is one of the major separation techniquesthat are used in natural gas purification [1, 9, 19]. In membrane natural gas pro-cessing, research attention has been focused more on the removal of CO2 dueto its abundance in the raw gas than H2S. The removal of CO2 will enhance thecalorific value (energy content) of the natural gas, significantly reduce the volumeof gas to be transported through pipeline and cylinders, prevent atmospheric pollu-tion, and reduce the susceptibility of the pipeline to corrosion [11, 20]. Presently,

Table 1 Natural gas compositions [8, 12–14]

Component Composition range (mol%)

Helium (He) 0.0–1.8

Nitrogen (N2) 0.21–26.10

Carbon dioxide (CO2) 0.06–50.00

Hydrogen sulfide (H2S) 0.0–3.3

Methane (CH4) 29.98–90.12

Ethane (C2H6) 0.55–14.22

Propane (C3H8) 0.23–12.54

Butanes (C4H10) 0.14–8.12

Pentanes and heavier (C5H12 +) 0.037–3.0

4%7% 2%

33%

30%

24%

27%

5%

7%7%

26%

28%

1

2

3

4

5

6

Oil

Natural Gas

Coal

Hydro

Nuclear

Other

a b

Fig. 1 Forecast of changes in the global energy mix. (a) 2012. (b) 2040. (Copied fromKontorovich et al. [15] with permission from Elsevier)


commercially available CO2 separation techniques include adsorption, cryogenicdistillation, and membrane [21].

For many reasons, membrane separation has witnessed a much more rate ofgrowth than other separation techniques [17]. This is particularly due to some ofthe special features of membrane separation technique which distinguishes it fromothers. Advantages of membrane gas separation include higher separation efficiency,faster separation, simplicity, and high space economy [7, 23–26]. The use of anefficient gas processing technology for producing a cleaner source of energy willconstitute an excellent approach in addressing environmental issues that are associ-ated with greenhouse gas emission at relatively lower cost.

3 Functional Polymeric Membranes for Natural GasProcessing

Review of available literature on membrane gas separation techniques revealed thatcurrent research efforts in improving the competitiveness of membrane gas separa-tion with other gas separation processes can be classified into two: namely [27],engineering, which deals with the design and optimization of membrane modules,and materials, which involves the design and development of novel membranematerials.

In materials development, research efforts are twofolds:

(i) Synthesis of new novel polymers for preparation of new membranes.(ii) Modification of existing commercial polymers to improve their gas separation

properties.

In both cases, the synthesis of functional polymers and functionalization ofexisting polymers have dominated research endeavors. Functional polymers aremacromolecules which contain specified chemical groups or polymer that has certainspecific physical, chemical, and biological properties. The selection of such poly-mers for certain application depends on the presence of the functional groups itcontains. Usually, the properties of such polymers are determined by the presence of

Table 2 Natural gas specifications for various applications [8, 13, 14, 22]

Major components Mol% range

Methane �75

Ethane �10.0

Propane �5.0

Butanes �2.0

Pentanes and heavier �0.5

Nitrogen and other inerts �3.0

Carbon dioxide 2.0–5.0

Total diluents gases 4.0–5.0


functional groups which are different from those of the backbone chains [28]. Thepolymers can be based on simple linear backbones, three-dimensional (such asstars, hyperbranched polymers) or dendrimers (tree-like structures polymers). Allthese polymers have been used in the preparation of membrane materials for gasseparation. For example, CO2 has an affinity to amine, polar ether oxygen linkages,and other functional groups. Thus, polymers which bear any of these functionalgroups are often preferred for preparing membrane materials for CO2 removal fromnatural gas. Table 3 showed commercial membranes that are presently being used inindustrial gas plants. Also, the performance of other commercially available mem-branes in CO2 removal from CH4 is shown in Fig. 2. These membranes are madefrom polymers that contain one or more functional groups.

The presence of these functional groups in a polymer will enhance the solubilityand solubility selectivity of such polymers with respect to CO2. In addition, thepresence of the functional groups could also enhance the diffusivity of the naturalgas component in membrane especially when they are positioned as branches withinthe polymer matrixes. The presence contributes seriously to intrasegmental rota-tional mobility and intersegmental chain packing of polymers [32]. For instance,Table 4 shows two membrane samples, one with ether functional group and the otherwithout. It can be observed that the one with ether has higher solubility as comparedto the other one. Also, the second without ether has higher diffusivity which is aresult of the gap created by the presence of H2 branching that is not available in thefirst one.

The unit of P is Barrer; S is 108 cm2

s

� �, D is cm3 STPð Þ

cm3 polymerð Þ-atm� �

.

The effect of substitution of the bulkier functional groups (such as tri-fluoromethyl) in place of the methyl groups has also been investigated for iso-propylidene moiety of the diamine. The results shown in Table 5 revealed onceagain that the polymer with ether oxygen linkage has the highest CO2 solubility thanother polymers. This is majorly due to the interaction between the ether linkage andCO2. Moreover, polymers with bulkier functional groups (placed as branches) havehigher diffusivity. For instance, the diffusivity of 6FDA-MDA with H branch isabout 64% lower than 6FDA-6FpDAwhich contains bulkier functional group unitsas branches. This is because the substitution of the bulkier central moiety for lessbulky ones leads to simultaneous disruption in intermolecular packing and suppres-sion of intrarotational flexibility in the diamine segment of polyimides [32].

Permeability unit (Barrer); solubility unit 108 cm2

s

� �, cm3 STPð Þ

cm3 polymerð Þ-atm� �

.

The main aim of this work is to provide an overview of the method of synthesisand transport properties of polymers that are presently being used for natural gas

Table 3 Commercial membranes used for CO2 removal from natural [7, 16, 29–31]

Membrane Selectivity CO2/CH4

Cellulose acetate 10.0–28.0

Polysulfone 22.0

Polyimide 32.5


processing. Particularly, the writing will be limited to functional polymers forseparation of major components of natural gas which include helium, nitrogen,carbon dioxide, hydrogen sulfide, methane, ethane, propane, Butanes, pentanes,and heavier hydrocarbon. In addition, functional polymeric membrane gas transportproperties in terms of permeability, solubility, diffusivity, and selectivity will beconsidered. Polymeric membrane materials are often subdivided into two classes,namely, rubbery and glassy polymers. Rubbery polymers are those that are above theglass-transition temperature while glassy polymers are those below the glass-transition temperature [1]. The glassy polymers often possess high selectivity buttheir permeability are low [33]. Moreover, the permeability may deteriorate overtime as a result of polymer aging. In the contrary, the rubbery polymers are known tobe highly permeable but their selectivity and plasticization resistance properties arelow. Herein, attention has been given to both classes of polymers.

4 Classification of Membrane Materials

Polymeric membranes have been applied in a verity of areas such as filtration(microfiltration, nanofiltration, and ultrafiltration), reverse osmosis, pervaporation,desalination, membrane distillation, membrane crystallization, dialysis, gas andvapor separation, and so on. The classification of membrane materials is carriedout based on many factors. Membrane materials can be either synthetic or natural.Natural membrane materials are those polymers that are often found in animals and

Fig. 2 Gas separation (CO2/CH4) performance of some commercial polymeric membranes. (Cop-ied from Adewole et al. [4] with permission from Elsevier)

Table 4 Comparison of effect of functional groups on gas transport and separation properties ofpolymers [3]

Name Structure P S D Permselectivity

4,4-Oxydianiline(4,4-ODA)

23 4.89 3.58 60.5

Methylenedianiline(MDA)

19.3 3.96 3.70 44.9


Table

5Com

parisonof

effectof

bulkierfunctio

nalgrou

pson

gastransportandseparatio

nprop

ertiesof

polymersat10

atm

and35

� C[32]

Nam

eStructure

Solubility

Diffusivity

6FDA-O

DA

4.89

3.58

6FDA-M

DA

3.96

3.70

6FDA-6FpD

A4.72

10.3


plants. Examples include polysaccharides and rubbers [34]. Synthetic membranescan be further classified into organic (polymers), inorganic (such as ceramic, metals,zeolites, and carbon [35]), or mixed-matrix (containing both organic and inorganicmaterials). Figure 3 depicts a pictorial representation of membrane material classi-fication. Moreover, both the synthetic and natural polymers can be used in gasseparation in the form of solid or liquid membranes.

In terms of morphology, membrane materials can be classified into porousand nonporous membranes. The scanning electron microscope (SEM) surfaceimages of porous and nonporous membranes are shown in Fig. 4. According tothe cross-sectional structure, membranes can be isotropic (symmetric membranes)or anisotropic (asymmetric membrane with a thin porous or nonporous selective layerthat is supported by a much thicker porous substructure) [34]. Asymmetric mem-branes are commonly prepared in such a way that they are integrally anisotropic.Dual-layer (or composite) membrane is an example of asymmetric membrane(Fig. 5). The dual-layer technique provides the opportunity to separately optimize

Glassy Rubbery Ceramic Glass

MembraneMaterials

Natural

Biomaterials e.g.Cellulose

Organic(Polymer)

Metals Zeolite

Inorganic

Synthetic

Fig. 3 Membrane material classification for gas separation

Fig. 4 Surface images of porous and nonporous membranes. (a) Nonporous membrane. (b) Porousmembrane


the performance of the selective layer and the porous support. The top selective layercould be a barrier with pores in the nanometer range or a nonporous polymer. Theporous sublayer provides the mechanical support that is needed during operation.

During operation, all the membrane materials are often packed within a certainhousing called modules. There are two major types of membrane modules: thehollow fiber module (containing hollow fiber membranes) and the spiral woundmodule (housing flat sheet membranes). The two types of module are shown inFigs. 6 and 7, respectively. Majority of the large-scale membrane gas separationplants are operated based on these two modules. A photograph of membrane CO2

separation plant is shown in Fig. 8.

Feed Gas

Permeate Gas

Gas A + B

Gas A

Hollow-Fiber Membrane

Gas B

Reject Gas

Fig. 6 Hollow fiber membrane module. (Reprinted from [37])

Fig. 5 Cross-sectional images of dual-layer polymeric membranes. (a) Flat sheet dual-layer. (b)Hollow fiber dual-layer. (Reprinted from [36] by permission from Elsevier)


5 Theoretical Background of Gas Separation in PolymericMembranes

Theoretically, the permeation of gas through nonporous dense polymeric membranesis commonly described using the Solution-diffusion mechanism. Based on thismechanism, gas molecules are first sorbed by the membranes at the upstream

Fig. 8 Photograph of a CO2 separation plant. (Reprinted from [7] with permission from Elsevier)

Anti-telescopingDevice

Perforated Central Tube

Permeate

Concentrate

Feed ChannelSpacer

Permeate CollectionMaterial

Membrane

Feed Channel Spacer

Outer Wrap

Membrane

Feed Solution

Fig. 7 Schematic representation of spiral wound membrane module. (Copied from [38])


(high-pressure) interface, the sorbed gas then diffuses through the membrane, and itis later desorbed at the downstream (low pressure) chamber [39–41]. A schematicillustration of a gas permeation setup is shown in Fig. 9. Gas molecules movementthrough polymeric membranes is controlled by diffusivity coefficient (D) and solu-bility coefficient (S). The solubility coefficient is thermodynamic in nature and isdetermined by the following factors:

(i) The inherent condensability of the penetrant(ii) Polymer–penetrant interactions(iii) The amount of excess volume existing in the glassy polymer

Diffusivity coefficient, D, is a kinetic parameter which determines the mobility ofindividual molecules through the polymer chains [42]. It is determined by packingand motion of the polymer segments and by the size of the penetrant molecule.Increasing the diffusivity or the solubility or both for a particular gas by structuralmodification of the polymer allows one to raise the gas permeability. Moreover, suchmodification could also lead to improvement of the permselectivity of a desired gasover an undesired one [4].

P

P

P

Pressure Switch

Polymer sample holder

Heating Jacket

Pt-100Gas

res

ervo

ir

ISCO Pump Upstream chamber Downstream chamber

DataAcquisition

Tubing

Wiring

Fig. 9 Schematic diagram of a permeation setup for measuring gas permeability. (Copied fromAdewole et al. [43] with permission from Springer)


The permeability (P), which provides information about the ability of the mole-cules to pass through a membrane, is therefore defined as the product of diffusivitycoefficient and solubility coefficient:

P ¼ DS (1)

The commonly used unit of permeability in membrane gas separation is thebarrer. Conversion from barrer to other unit can be done using the given expression:1 barrer = 1 � 10�10cm3(STP)cm/(cm2scmHg) = 7.5005 � 10�18m2s�1Pa�1 [44].

The ability of a membrane to separate a gas A from a mixture of gases (A and B)referred to as the selectivity (αAB).

αAB ¼ PA

PB(2)

The Eq. (2) above is used for calculating selectivity involving single (pure) gaspermeation through a membrane. For the permeation of mixed gases, the selectivity(which is referred to as mixed gas selectivity or the separation factor) is calculatedusing the following equation [20, 45, 46]:

Separation factor ¼Composition of A Downstream

Composition of B Downstream

� �

Composition of A Upstream

Composition of B Upstream

� � ¼yAyB

� �

xAxB

� � (3)

where A is the gaseous component which permeated through the membrane. B is thegaseous component which was retained or rejected from permeating through themembrane. The x is the mole fraction of the gaseous component which is present inthe feed while y is the mole fraction of the gaseous component in the permeate. It isimportant to note that the type of the permeation experiment (either pure or mixedgas permeation) needs to be mentioned while reporting the selectivity. By combiningEqs. (1) and (2), it is possible to express the selectivity as:

αAB ¼ DA

DB� SASB

(4)

The DADB

is often referred to as the mobility selectivity or diffusivity selectivity.

Also, the SASB

is referred to as the sorption selectivity or solubility selectivity [40].

A parameter that is used for expressing membrane performance depends on thetype of membrane module. For hollow fiber membranes, the separation performanceis commonly expressed in term of permeance. The permeance of a hollow fibermembrane to gas A is the permeability of the membrane (PA) divided by themembrane thickness (L ). Mathematically,


Permeance ¼ PA

L(5)

For dual-layer hollow fiber, L is the dense-selective layer thickness and can bedetermined as follows [47]:

L ¼ PA dens _e_filmð ÞP

L= ÞA dual-layer_hollow_fiber_membraneð Þ� (6)

Accordingly, the selectivity is expressed as:

αAB ¼P

L= ÞAð = PL= ÞBð (7)

The permeance is commonly expressed using the gas permeation unit(GPU) (1GPU is equivalent to 1 � 10�6cm3(STP)/(cm2scmHg) or7.5005 � 10�12ms�1Pa�1) [47, 48]. It is a common practice to use Eq. 7 forasymmetric membranes especially when the thickness of the active layer is notknown.

The relationship between the operating temperature and the gas diffusion coeffi-cient in polymeric membrane can be represented by the Arrhenius equation [40]:

DA ¼ DA0exp�ED

RT

� �(8)

where DA represents the diffusion coefficient (cm2/s) of gas A. DA0, ED, T, and R are

the pre-exponential factor, the activation energy of diffusion (in kJ/mol), the tem-perature (in K), and the universal gas constant, respectively. Here, the value of theR is 8.31 J/mol.

Similarly, the solubility as a function of temperature can be expressed by the van’tHoff equation:

SA ¼ SA0exp�ΔHS

RT

� �(9)

SA0 and ΔHS are the pre-exponential factor and the enthalpy of solution of thegas A, respectively.

The gas permeability can therefore be expressed as a function of temperature bycombining Eqs. 8 and 9 substituting them into Eq. 1 as follows:

PA ¼ PA0exp�EP

RT

� �(10)

EP ¼ ED þ ΔHS (11)


PA0and EP are the pre-exponential factor, and the activation energy of permeation,respectively.

Gas transport through polymeric membrane is also affected by the upstream(feed) pressure as well as downstream (permeate) pressure of membrane module.The effect of pressure on gas transport properties of the two major classes ofpolymeric membrane is very much distinguishable. For example, in rubbery poly-mers, the well-known solution-diffusion model is used to describe gas transportbehavior as a function of feed pressure. On the other hand, gas transport behavior inglassy polymers is more complicated and other equations are needed. Changes in gaspermeability as a function of pressure is often explained using the dual-mode modeland partial/total immobilization models [40]. In this model, solubility is expressedas:

S ¼ kD þ FC0Hb

1þ bp(12)

where kD,C0H, and b are constants referred to as Henry’s law constant, the Langmuir

capacity constant, and the Langmuir affinity constant, respectively. In the case fortotal immobilization model, F is set to 1.

Using a modified form of Eq. 12 allows the permeability of gases to be expressedusing the dual-sorption model as follows [16]:

P ¼ kDDD þ C0HbDH

1þ bp2(13)

where DD is the diffusion coefficient for gas molecules absorbed by Henry sorptionmode andDH the coefficient for gas molecules absorbed Langmuir mode of sorption.

Equation (1) provides a better measure of the gas transport properties andseparation performance of polymer because all the parameters included in theseequations are experimentally measurable. Therefore, all these parameters will beused as indices of the degree of natural gas purification capability of the functionalpolymers in this discussion.

6 Synthesis, Preparation, and Transport Propertiesof Functional Polymeric Membrane for Natural GasProcessing

6.1 Thermal Rearrangement and Cross-Linking

High thermal property and chemical resistance are some of the properties that areneeded for polymers to be used for separation NG components. Aromatic polymerssuch as polybenzoxazoles (PBO) and polybenzothiazoles (PBT) are two excellenttypes of polymers with these properties. However, the high chemical resistance(to common solvents) makes it difficult for them to be used in membrane


preparation. This challenge was overcome by using thermal rearrangement methodwhich involves a postfabrication polymer – modifying reaction. The process iscarried out by thermally rearranging the aromatic polyimides containing functionalgroups (PIOFG) such as –OH and –SH in order to obtain a dense PBO and PBTmembrane [49]. The whole procedure is a two-stage procedure:

(i) Synthesis of hydroxyl-containing polyimides: Polyimides containing ortho-positions functional groups (PIOFGs) were prepared via a two-step process.First, formation of hydroxyl-containing poly(amic acids). Then thermalimidization.

(ii) Synthesis of thermally rearranged membrane: Thermal treatment was done atvarying temperature (300 to 450 �C) and time 1 to 3 hours under argon gas in aquartz tube furnace.

A schematic of the proposed scheme for thermal rearrangement is shown inFig. 10.

The transport properties of TR membranes were evaluated with respect to per-meability, solubility, and diffusivity. Results of these properties showed that gastransport properties depend on the final heat treatment temperature and the method ofimidization used for preparing the polyimide. Previous reports [50] showed thatthermal rearrangement of polyimide was triggered at around 350 �C and at 450 �C,full conversion of polyimide to thermally rearranged polymer occurred. At thetemperature between 350 �C and 450 �C, TR membranes were believed to containboth unchanged segments and thermally rearranged segments. Usually, polyimideshows higher H2 permeability than CO2 permeability. This is in consistent with theorder or increasing kinetic diameters of penetrant molecules. This means that the

Fig. 10 Proposed thermal rearrangement mechanism from hydroxyl-containing polyimide topolybenzoxazole. (Taken from Park et al. [49] with permission from Elsevier)


diffusion selectivity plays a major role. On the other hand, TR polymer treated at450 �C has permeability in the order: CO2 > H2 > He>O2 > N2 > CH4 with highgas permeability values. By increasing the treatment temperature, the gap betweenthe permeabilities of CO2 and H2 reduces and at 450 �C the order of the permeabilityvalues for these two gases are completely reversed. The change in behavior suggeststhat CO2 solubility is higher in TR polymer than H2 solubility and this contribute tothe higher value of CO2 permeability as compared to that of H2. Table 6 shows thecomparison of gas transport properties of polyimides made from two imidizationmethods. The gas selectivity decreases with increase in treatment temperature;however, TR polymer still looks promising because it has higher CO2 permeabilitywith corresponding high CO2/CH4 selectivity than other conventional polymers suchas cellulose acetate and polyimides.

The huge success recorded on the use of thermal rearrangement method for thesynthesis of TR-PBO membranes encouraged researchers to further dig into inves-tigating the thermally rearranged polymers. Importantly, a detailed fundamentalunderstanding of the behavior of the TR polymers is needed in order to develop abetter procedure for the design and synthesis of these polymers.

Wang and Chang [52] carried out a systematic investigation on the evaluation ofthe physicochemical and gas transport properties of a representative thermallyrearranged polymer (polyhydroxyamide (PHA)) with respect to temperature. Thestudy was aimed at providing solid evidence that will help to elucidate the funda-mentals of thermal cyclization and provide molecular illumination of thetemperature-induced chain rearrangement. Therefore, the effect of thermal cycliza-tion temperature on the thermal rearrangement process was investigated. The effectof the temperature on the chemical structure transformation, thermal stability, phys-ical properties, gas separation performance, and gas sorption properties of theresulting membranes were evaluated. Results revealed that the thermal cyclizationtemperature is a factor that affects the degree of thermal conversion, influences thethermal history of polymers, and causes thermal cross-linking.

Table 7 shows the effect the temperature has on gas separation performance of theresulting membrane. Membranes that are cyclized at higher temperature exhibitbetter performance. According to the authors, the superior performance recordedfor these membranes is due to the fact that a higher thermal cyclization temperaturecaused the suppression of polymer chain segmental rotation, resulted in lower chainpacking density, and enhanced the formation of microvoids.

The work carried out by Wang and Chang [52] on TR polymers involved the useof PHA as processors owing to its high rate of conversion at relatively lowertemperature. Wang et al. [53] used another similar polymers but with a highernumber of functional groups. The authors employed poly(hydroxyl amic acid)

Table 6 Gas transport properties of polyimide obtained by various imidization methods [49, 51]

Method of imidization CO2 permeability (Barrer) CO2/CH4 selectivity

Chemical imidization 17 62

Thermal imidization 6 43


(PHAA) as a precursor to synthesize PBO. PHAA comprises of hydroxyl andcarboxyl functional group giving room for the exploration of different stages ofthermal cyclization of PHAA in a stepwise manner. The first step involved thetransformation of PHAA to poly(imide benzoxazole) (PIBO) at temperature up to300 �C. During the second step, PIBO was finally transformed to polybenzoxazole(PBO) at 400 �C. This was followed by a systematic investigation of the resultingevolution of the structure and gas separation performance of the ensuing membrane.The results of the physicochemical and gas transport properties are shown in Table 8.It was concluded that gas separation efficiency of polymer membrane can be tailoredby the use of appropriate thermally induced structure transformation and polymerchain rearrangement.

It is worth mentioning that the authors observed a bimodal distribution fortemperatures 350 �C and 400 �C. A detailed explanation of these results can befound in the original article [53].

Various research articles have been published on alternative approaches tomolecularly design advanced membrane materials using thermal rearrangement.The conversion of hydroxyl-containing imides to PBO was reported by Tulloset al. [54]. This method was used by many researchers to synthesize membranesfor gas separation purposes [49, 55]. Another approach for producing highlypermeable polymeric membrane is by controlling the free volume cavities viathe synthesis of poly(benzoxazole-co-imide) [56] and the incorporation of hetero-cyclic compound (such as pyrolone or ether) during copolymerization[57]. Thus thermally rearranged poly(benzoxazole-co-pyrrolone) was reported by

Table 8 Gas transport properties of poly(benzoxazole) membranes prepared from poly(hydroxyamide amic acid) via stepwise thermal cyclization

Permeability Ideal selectivity

Membrane Temperature d-Spacing CO2 CH4 N2 CO2/N2 CO2/CH4

PHAA 130 5.28 2.12 0.05 0.10 21.1 40.2

PIBO 200 5.39 5.06 0.10 0.24 21.1 49.4

PIBO 220 5.48 9.65 0.20 0.46 21.0 48.5

PIBO 300 5.63 30.2 0.69 1.53 19.8 43.6

PBO 350 5.77 113 3.43 5.73 19.7 33.0

PBO 400 6.16 456 17.4 23.8 19.2 26.2

Table 7 Effects of cyclization temperature on gas transport properties

MembraneCyclizationtemperature FFY

Permeability Ideal selectivity

CO2 CH4 N2

CO2/CH4

CO2/N2

PHA – 0.134 5.76 0.13 0.24 43.0 24.2

PBO 1 300 0.195 44.7 1.56 2.28 28.6 19.6

PBO 2 350 0.198 84.2 3.48 4.42 24.0 18.9

PBO 3 425 0.202 183 8.53 10.10 21.4 18.1

PBO 4 450 0.220 532 28.9 30.30 18.4 17.6


Choi et al. [57] while thermally rearranged poly(ether-benzoxazole) membrane wasreported by Calle et al. [58].

Similarly, the change in the intrinsic properties of TR-PBO as a result of theincorporation of a cardo moiety was investigated by Yeong et al. [59]. The workresulted into the synthesis of sets of new copolyimide with different composition ofcardo-copolybenzaxole membranes with enhanced gas transport properties. Sanderset al. [60] employed polycondensation method to prepare thermally rearrangedpolyimide membranes. The authors used HAB and 6FDA as precursors. A newsynthesis route which involves alkaline hydrolysis of carbonyl-group of poly-pyrrolone and thermal treatment was investigated by Han et al. [61]. Highly perme-able thermally rearranged microporous polybenzimidazole (TR-PBI) membrane wasproduced using this method. A new copolyimide was prepared by Lu et al. [62]. Thispolymer was subsequently thermally treated in air and N2 atmosphere to attainthermal oxidative and thermally rearranged membranes, respectively. Table 9 con-tains the summary of the gas transport properties of these polymers.

The thermal rearrangement method has not been limited to flat sheet membrane.Some articles have been published on thermally rearranged hollow fiber membraneas well. The first TR hollow fiber membrane was reported by Kim et al. [63]. Themembrane was fabricated by spinning hydroxyl poly(amid acid) (a soluble precursorpolymer) via a phase separation process. A hollow fiber membrane (with a highpermeability) was produced by thermal imidization and subsequent thermalrearrangement of hydroxyl polyimide. The effect of thermal treatment parameter(time from 10 to 60 mins and temperature from 450 �C to 500 �C) on gas separationperformance of the membrane was also investigated. An optimum treatment param-eter of 500 �C and 10 mins was found to produce a membrane with the highestselectivity of CO2/CH4 of 22. The membrane has a permeance of 2326 and 105 GPUfor CO2 and CH4, respectively. Various other reports and description of thermalrearrangement procedure for polymers have also been published on the use ofthermal rearrangement [64–68].

A cross-link is a formation of intermolecular connections through chemicalbonds. It is used to develop materials with superior resistance to swelling andplasticization. The possibility of swelling and plasticization phenomenon is veryhigh when using a polymeric membrane to separate natural gas with a high concen-tration of CO2 or separation at high feed pressure. Cross-linking methods that havebeen reported in the literature for developing plasticization resistant membranesinclude chemical, thermal, and photochemical cross-linking [3, 69, 70].

Table 9 Gas transport properties of polyimides prepared by thermal oxidative and thermalrearrangement procedure

Permeability Selectivity

CO2 CH4 N2 CO2/N2 CO2/CH4

TO 31.22–653.93 3.83–13.42 3.04–15.15 43.16–94.74 48.73–75.20

TR 24.54–5087.49 16.62–245.45 20.44–212.78 15.37–34.65 20.73–42.62

TO thermal treatment in Air, TR thermal treatment in nitrogen


The mechanism of cross-linking shown in Fig. 11 has been employed in a varietyof ways to produce membranes with better gas separation performance. For example,esterification, cross-linking, mono-esterification, trans-esterification reaction, imidering opening reactions, thermal cross-linking by decarboxylation, and chemicalcross-linking using diamino compounds are some of the methods that have beenused successfully prepare membranes with better plasticization resistance properties[71–73].

Cross-linking often results in a reduction in chain mobility and increase in Tg. Ithas been successfully employed for producing membranes with superior perfor-mance at high pressure. When a polymeric membrane is cross-linked, its plasticiza-tion pressure increases, consequently, the cross-linking processes helps in stabilizingthe membrane at a higher partial pressure of CO2 [74]. Membranes with higherpacking density, smaller free volume, better resistance to CO2-induced plasticizationphenomenon, and higher Tg has been produced using thermal annealing.

Staudt-Bickel and Koros [75] carried out cross-linking in order to determine itseffect on the reduction of swelling and plasticization. The work was focused onsimultaneous inhibition of chain mobility and intrasegmental packing by changingthe backbone structure of the polymer. Polyimides with strong polar associatingfunctional groups were synthesized. In addition, cross-linking polyimides andcopolyimide were also synthesized in the study. Gas permeability and perm-selectivity of CO2/CH4 were investigated. It was observed that the presence of -C(CF3)- linkage and polar carboxylic group enhanced the CO2/CH4 perm-selectivityof the polyimides. The effects of cross-linking degree were evaluated using chemicaland thermal cross-linking approach. Plasticization and swelling properties wereinvestigated up to 41 atm feed stream pressure using pure CO2 as feed gas. Theauthors concluded that cross-linking of polyimide can enhance the plasticizationresistance properties of the cross-linked membrane. However, such enhancement isoften accompanied with reduced gas permeability.

Copolymer membranes were synthesized and evaluated for natural gas pro-cessing by Wind et al. [5]. The authors used cross-linked polyimides copolymers(6FDA-DAM,DABA and 6FpDA,DABA). Transport properties of natural gas com-ponents (used as 50/50 CO2/CH4 mixtures and multicomponent mixtures) wereinvestigated at 35 �C and an upstream pressure up to 55 atm. The cross-linkingprocedure was performed at a range of temperature (220 �C to 295 �C) and1,4-butylene glycol and 1,4-cyclohexanedimethanol were used as cross-linkingagents. Posttreatment annealing was done at temperatures between 130 �C and295 �C. In order to ensure a clearer interpretation of the results, the effect of varioustreatments (such as thermal treatment and chemical cross-linking) were decoupledwith respect to separation performance of the membrane. The composition of themulticomponent gas mixtures is CO2 (30.01–30.02), CH4 (61.0–62.95), C2H6

(3.99–4.0), C3H8 (2.99–3.0), C4H10 (2.01), and toluene (0.0299) (in mol%) whilethe mixed gas is 50 mol% each of CO2 and CH4. The results obtained on theperformance analysis of the membrane were compared with membranes made ofcommercially available polymers (such as Matrimid and cellulose acetate). It wasobserved that the resulting membranes from these studies have better transport and


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penetrant-induced plasticization properties than the commercially available poly-mers. The summary of the results of transport and plasticization resistance propertiesare shown in Table 10. It can be observed from the table that by covalently cross-linking, its plasticization resistance property can be improved. The table alsorevealed that thermal annealing can serve as an effective tool for suppressing CO2-induced plasticization challenges.

In a similar study by Omole et al. [76], separation capability of an asymmetrichollow fiber membranes made from propane-diol monoesterified cross-linkablepolyimide (PDMC) was investigated. The ability of PDMC to effectively separateCO2 from gas mixture was studied under aggressive operating conditions. The cross-linked membranes showed a good separation performance for CO2 and CH4. Aselectivity value of about 49, CO2 permeability of 161 barrer, were obtained underthe operating condition of 65 psia, 35 �C, and 10% CO2. The membranes alsoshowed good resistance to CO2-induced plasticization at a CO2 partial pressure up to400 psia. Thus, plasticization was effectively suppressed. The improved plasticiza-tion resistance was however accompanied by a decline in membrane permeability.The accompanied deterioration of the permeability was as a result of densification ofthe selective top layer during the annealing process.

High temperature cross-linking is a common procedure for modifying polyimide.There is, therefore, a need to develop a low-temperature protocol in order to reducethe cost of membrane preparation. Liu et al. [77] developed a simplelow-temperature (room temperature) chemical cross-linking technology for mem-brane preparation. The authors tested this technology using 6FDA-durene cross-linked with a p-xylenediamine methanol solution. Gas (such as N2 and CO2)permeation properties of the cross-linked polyimides were investigated at 35 �Cand 10 atm. Low-temperature chemical cross-linking was also applied by Tin et al.[70] for the modification of a commercially available polyimide (Matrimid® 5218).

Table 10 Gas separation and plasticization resistance properties of cross-linked copolyimidesmembrane [4]

Polymermembrane Method

T(�C)

P (CO2)

(Barrer)

αCO2/CH4

Plasticizationpressure(atm)

6FDA-DAM:DABA 2:1

Annealing 130 135 28 20–25

220 115 33 >22.5

295 95 30 35

Cross-linked with butylenesglycol monoester

220 40 26 >55

295 150 30 >30

Cross-linked withbenzenedimethanolmonoester

295 43 37 33

6FDA-6FpDA:DABA 1:2

Cross-linked with ethyleneglycol monoester

220 24 47 27

6FDA-DAM:DABA 2:1

Annealing 220 47 23–28 >40

50–55 23–27 >10


Pure and mixed gas permeation properties of the membranes were performed usingCH4 and CO2 gases. Moreover, the authors also studied the effect of cross-linking onplasticization resistance properties of the membrane. The outcome of the studiesshowed that plasticization resistance increased significantly. The plasticization pres-sure was found to increase from 15 atm to 32 atm following a 7-day polymer cross-linking. This is more than 100% increment. Unfortunately, the increase in plastici-zation resistance property was accompanied by 45% decrease in permeability. Therewas no significant change in the ideal selectivity.

A critical observation on the side effect of cross-linking revealed that cross-linking is often accompanied by a reduction in permeability. To solve this problem,rapid quenching was introduced into membrane cross-linking procedure. The use ofrapid quenching was used for solving the challenge of permeability reduction withno compromise to the plasticization resistance properties of the membranes. In somecases, improved plasticization resistance property was recorded. For instance,decarboxylation-induced cross-linking was utilized by [74] to improve the plastici-zation resistance of 6FDA-DAM:DABA (2:1) copolyimides. Two sets of membranesamples were employed for the studies. One set of membrane samples was annealedwhile the other set was rapidly quenched from above its Tg. Gas permeability andsolvent dissolution tests were performed on both samples. The sample that wassubjected to rapid quenching was observed to exhibit better plasticization resistanceproperty. The plasticization pressure was observed to increase from 28 atm to morethan 49 atm. The results of these plasticization resistance properties are shown inTable 11. In addition, the normalized CO2 permeability was found to be stable up to49 atm. Also, the samples did not dissolve in solvents such as cyclohexanone, THF,and NMP which were well-known solvents for dissolving polyimides. In fact, thequenched membranes did not dissolve after boiling in NMP for 18 hr. It wasconcluded that both responses that were observed are due to cross-linking whichoccurred in the polyimide.

Table 11 Plasticization properties of polyimide membrane prepared using thermal treatment andchemical cross-linking [4]

Treatmenttemperature

Cross-linkingagents/method Polymer

P (CO2)(barrer)

Plasticizationpressure (atm)

Noncross-linked 6FDA-mPD 9 5–7

35 �C Ethylene glycol inDMAc

6FDA-DABA 7.5 24

Noncross-linked 6FDA-mPD /DABA (9:1)

7 15

35 �C Ethylene glycol inDMAc

6FDA-mPD /DABA (9:1)

11 36

220 �C, 23 h Decarboxylation 6FDA-DAM:DABA (2:1)

0.65a 25-32

220 �C,23 h + Quenching

Rapid quenchingfrom above Tg

6FDA-DAM:DABA (2:1)

0.64a >49

aThe normalized permeability which was calculated as a ratio of the measured permeability to theinitial permeability that was measured at 2 atm


The outcomes of many investigations on the use of chemical cross-linkingindicate that the technology is promising in developing plasticization-resistantmembrane. However, the use of chemical cross-linking has its own challenges.Some of these challenges include the formation of ester linkages when diol is usedas cross-linking agents. It is highly possible for the linkages so formed to behydrolyzed when a membrane is used in the presence of aggressive acid gas feedstreams. The hydrolysis is known to be capable of reversing the effects of the cross-linking, reduce the membrane separation efficiency, and make it (the membrane) tobe more susceptible to penetrant-induced plasticization [74]. For this reason, a newmethod of cross-linking which does not cause formation ester linkages was devel-oped. The new cross-linking approach involves the use of a high-temperaturedecarboxylation process. The decarboxylation of the acid pendant (at high temper-ature) will create free radical sites that are capable of initiating the cross-linkingprocess in the absence of a diol cross-linking agent.

Clearly, the use of decarboxylation approach has a variety of advantages anddisadvantages. One of the major concerns of the high-temperature decarboxylationas a cross-linking technique is the possibility of causing the collapse of the transitionlayers as well as the substructure of asymmetric hollow fiber membrane that wasdeveloped using decarboxylated materials. In order to address this challenge, anattempt was made to investigate the possibility of carrying out thermal cross-linkingat a temperature below the glass transition temperature. Consequently, a new thermalcross-linking method was reported by Qiu et al. [72]. Using this method, a glassypolymer can be easily cross-linked at a temperature that is lower than the glasstransition temperature of such a polymer. The sub-Tg method of cross-linking is apromising method that could facilitate a large-scale method of fabricating asymmet-ric membranes. The sub-Tg method was used to develop a polyimide (6FDA-DAM:DABA (3:2)) membrane for application in CO2 separation. The properties of theresulting membranes were evaluated using solubility, thermal, and permeation tests.The gas transport properties and gas separation performance tests were performedusing aggressive gas mixtures as feed streams at high feed (upstream) pressure. Thefeed streams consist of both the pure and mixed gases of CO2 and CH4. The resultsof the permeation tests revealed that there was significant improvement in theplasticization resistance properties of the membranes. Tested membrane samplesshowed no sign of plasticization at feed pressure as high as 48 bar and 69 bar for pureand mixed gas permeation tests, respectively. The results of the solubility test wereused to corroborate the increment in the plasticization resistance properties. It wasobserved that some membranes did not dissolve in hot solvents such as NMP at100 �C when soaked in the hot NMP for 1 week. The authors also carried out furtherstudies to discover the range of temperatures at which cross-linking occurred. It wasobserved that there was no cross-linking at a temperature below 300 �C [72].

Diamino-organosilicone (bis(3-aminopropyl)-tetramethyldisiloxane)),APTMDS) is another cross-linking agent that was investigated by Chen et al.[78]. The authors employed two different routes to accomplish the cross-linkingand the preparation of two different cross-linked polyimide membranes. In addition,both routes were implemented at ambient temperature. The results of gas separation


performance of the resulting membranes are shown in Tables 12 and 13. From thesetables, it can be observed that both methods produced membranes with enhanced gasseparation properties.

Research focus on CO2 separation has targeted polymeric materials with rela-tively higher CO2 selectivity. Interestingly, the rubbery polymers constitute the mostpromising materials in this respect [16]. Outstanding rubbery polymers such as thosecontaining polyethylene oxide (PEO) segments have better solubility selectivitiesdue to their molecular interaction with the CO2 quadrupole. On the other hand, PEOis crystalline. Therefore, to achieve useful gas separation performance from poly-mers in which PEO has been incorporated, the PEO crystallization must besuppressed. This will give room to achieving high fluxes and retard swelling.Some of the means by which this could be accomplished include cross-linking,polymer blending, or block-copolymerization [16]. A comprehensive description ofpolyethylene glycol can be found in Adewole et al. [4]. Within the polyether family,PEO is as an outstanding membrane forming material particularly suitable forprocessing and purification of gas mixtures containing polar gases (such as CO2

and H2S) and nonpolar ones (such as H2, N2, and CH4) [79, 80]. This is due to itspolarity and CO2 affinity [81]. PEO contains an ether oxygen functional group whichfavorably interacts with CO2 at the molecular level which then result in increase inselectivity toward the gas [82].

Table 12 Gas separation performance of cross-linked polyimide membranes [78]

Polymer Cross-linking methodP (CO2)(barrer)

Idealselectivity

Separationfactor

6FDA-ODA Original sample 19 53 49

Immersion in APTMDS/methanol

12–17 66–90 39–62

Direct mixing of APTMDS inpolyimide solution

12–14 71–82 55–64

6FDA-ODATemMPD(50/50%)

Original sample 57 15 14

Immersion in APTMDS/methanol

18–54 15–41 14–40

Direct mixing of APTMDS inthe polyimide solution

24–33 19–24 15–24

Table 13 Gas transport and plasticization resistance properties of cross-linked polyimide mem-branes [78]

PolymerCO2 permeability(barrer)

Plasticizationpressure (atm)

6FDA-ODA 15 20–24

Cross-linked 6FDA-ODA 8 >41

6FDA-ODA(50%)-TemMPD (50%) 48 14

Cross-linked 6FDA-ODA (50%) -TemMPD(50%) (direct mixing)

12 >41


One of the strategies that can be used to improve the gas separation capability ofPEO-based membrane is to destroy the crystalline nature of PEO and make it tobecome more amorphous. A family of cross-linked poly(ethylene glycol diacrylate)(XLPEGDA) was prepared by Lin and Freeman [80]. The effect of cross-linking onthe crystallinity and gas permeability of XLPEGDA was investigated. The resultsrevealed that the cross-linking have successfully disrupted the crystallinity. Nosignificant effect was observed with respect to the permeability and diffusivity.Expectedly, it was observed that the solubility selectivity for the CO2/CH4 gas pairincreased due to the interaction of PEO functional group with CO2. Lin and Freeman[83] also measured the permeabilities of CO2 and CH4 through XLPEGDA mem-brane as a function of temperature and feed fugacity. The authors observed thatpermeability of CH4 is independent of feed fugacity while that of CO2 increased withincreasing fugacity. Therefore, it can be inferred that the cross-linking have nosignificant effect on the plasticization resistance behavior of the polymer. Duringthe permeation test, the significant quantity of CO2 sorbed into the polymer causedan increase in polymer chain flexibility as well as an increase in fractional freevolume.

Li et al. [84] prepare a series of poly(urethane-urea)s polymers using4,4-metylene diphenyl diisocyanate (MDI), various types of polyether diols, andethylene diamine (EDA). Membrane produced using the prepared copolymers weresubjected to pure gas permeation tests using CO2 and CH4. The data obtained fromthe permeability measurements revealed that the CO2 permeability increased instan-taneously with increase in feed pressure for all the samples. Such type of transportphenomenon is commonly associated with rubbery polymers. Usually, rubberypolymers always exhibit a high tendency for CO2 sorption. Mixed as well as puregas permeation tests results of highly permeable poly(ethylene) oxide [85], cross-linked poly(ethylene) oxide [86], and blend of polyimide-b-ethylene oxide)/poly-ethylene glycol [87] membranes did not show any promising performance withrespect to plasticization resistance. Therefore, it can be concluded that polyether-based polymers are not suitable for high-pressure removal of CO2 gas from CH4.Studies on how the plasticization pressure of these class of highly permeable poly-mers can be improved are recommended for further research. Other new membranesand thermal treatment methods have been reported. Details of these can be found inthe literature [54, 88–95].

6.2 Grafting of Polymer Backbone

The performance properties of polymeric membranes can be improved by theincorporation or the substitution of small functional groups onto the rotating unitof the polymers. This simple method has been applied by some researchers toenhance the permeability and selectivity of membranes. Pixton and Paul [96] studiedthe gas transport properties of 2,2-(4-hydroxy-phenyl) adamantane polysulfone and1,3-(4-hydroxyphenyl)adamantane polysulfone membranes and compared theresults with that of bisphenol A-based polysulfone. The results of the pressure-


dependent permeability tests that was carried out on the membranes (with a thicknessin the range of 50–75 μm) revealed that both polymers can resist CO2-inducedplasticization up to a pressure of 20 atm. It is worth mentioning that 20 atm is themaximum pressure of the experiment. Therefore, there is every possibility for theplasticization pressure to be more than 20 atm. The recorded improvement in theplasticization suppression properties is, however, dependent on the thickness of themembrane. This became evident from the studies that were later carried on ultra-thinfilm polysulfone membranes where the plasticization pressure was found to be lower[19]. To date, varieties of new and more advanced grafting techniques have beenreported for modifying polymers that are used for applications in gas separation.Some of these techniques can be found in the open literature [97–99].

6.3 Template Polymerization Technique and Use of Porogens

Template polymerization is a concept that was derived from natural polymerizationprocesses. The process mimics the polymerization techniques that are found innature. Examples include DNA replication and biosynthesis of proteins [100]. Tem-plate polymerization the technique provides scientists with the opportunity to controland tune polymer properties during polymerization [101]. It is a versatile processthat can be employed in synthesizing ultrathin polymeric membranes. It can be madeto proceed in various ways such as template polycondensation, ring opening poly-merization, polyaddition, and ionic or radical polymerization [100, 101]. The pro-cess is a convenient way of producing poly complexes, poly complex composites,and polymer blends.

The term “porogen” is an acronym for “pore generator” [102]. The mechanismsof porogen-induced pore formation have been investigated by a number of authors.Traditionally, this method is used for the preparation of porous materials for appli-cations in dielectric industries [103, 104]. The porogenation process can beperformed via a sacrificial thermal decomposition of the porogens in the range of350 �C and 400 �C [103–105]. Marti et al. [106] provided a detailed description ofanother new approach for using porogens for membrane preparation. Recently, thisapproach has been found to be very useful in developing membranes with betterresistance to plasticization phenomenon. The techniques of template polymerizationand use of porogens can be traced back to the preparation of carbon molecular sieve(CMS). A comprehensive explanation of CMS and its preparation are presented inMohamed [107], Kita [108], and Williams and Koros [109].

Aromatic polyimides have been used as polymer precursors for the preparation ofCMS membranes due to their excellent balance of productivity and separation factoror ideal selectivity [110]. CMS membranes are fabricated by heating up the poly-imides to a temperature range of 700–900 �C. The high-temperature heating processoften results in polyimides degradation [110]. Relatively low-temperature pyrolysiswas therefore proposed. Islam et al. [111] and Weiliang et al. [112] employedlow-temperature pyrolysis for preparing flexible pyrolytic membranes. Sulfonatedpolyimides were pyrolyzed at a temperature range of 370 �C and 450 �C to produce


dense flat sheet membranes. The membrane contains a large number of microvoidswhich were caused by “template-like effect” of SO3H groups. Gas (CO2) perme-ability of the membranes was found to have increased by 378 barrer in comparisonwith the polymer precursor.

Similar approach has been used by other authors such as Xiao and Chung [113](using beta-cyclodextrin thermal labile molecule with large dimensions) and Chuaet al. [114] (using thermal labile saccharide units with different molecular weightsand structures: glucose (180 g/mol), sucrose (342 g/mol), and raffinose (504 g/mol)).Based on the permeability of CO2, membrane prepared using cyclodextrin, porogenswas found to be the best membrane (Table 14). A further investigation into theplasticization behaviour of polyimide containing cyclodextrin porogens wasperformed by Askari et al. [69, 115]. A summary of the plasticization properties ofthe membrane developed using the copolyimide is displayed in Table 15.

Table 14 Gas transport properties of partially pyrolyzed polyimide membranes using variousporogens [4]

Porogens Temperaturea (�C) P (CO2)αCO2/CH4

Sulfonic group 370–450 145–380 –

Carboxylic acid 425–475 39.1–94.8 60–47

Cyclodextrin 300–450 56–8000 31–17

Saccharides 200–425 533–1389 24.9–26.9aThis is the temperature of partial pyrolysis

Table 15 Penetrant-induced plasticization properties of 6FDA-Durene/DABA co-polyimides withα, β, and γ-cyclodextrin [69]

Polymer Treatment temperature (�C) Plasticization pressure (atm)

Original polyimide 200 5–10

300 10–15

425 15–20

Polyimide grafted with- α-CD 200 7–10

300 10–15

425 25

Polyimide grafted with β-CD 200 7–10

300 10

425 >30

Polyimide grafted with γ-CD 200 10

300 10

425 >30


6.4 Sulfonation Method

Sulfonation involves the replacement of a hydrogen atom of an organic compoundwith a sulfonic acid (-SO3H) functional group. It is often accomplished reaction ofthe organic compound (such as a polymer) with sulfuric acid at high temperatures.Sulfonation is one of the new methods that have been proposed for modification ofgas transport properties of polymers. Sulfonated polymeric membranes usuallyexhibit enhanced separation performance than the original ones. Studies onstructure–properties relationship of sulfonated polymers was conducted by Pirouxet al. [116] for a large series of sulfonated copolyimides. The copolyimides weresynthesized with a naphthalenic dianhydride, a sulfonated diamine, and variousnonsulfonated diamines. It was observed that the contribution of the dispersedsulfonated phase to the gas transport greatly depends on the structure of the non-sulfonated diamine.

Previously, polymer structures are commonly modified by postsulfonation.Unfortunately, the practice of postsulfonation lead to lower thermal and mechanicalstabilities of the polymers compared with the original polymer. In addition, it verydifficult to achieve a high degree of sulfonation using postsulfonation process. Toovercome this problem, sulfonation of monomers prior to polymerization wasproposed. Khan et al. [117] prepared a series of sulfonated poly(ether ether ketone)(S-PEEK) membranes by the sulfonation of its monomers. Commercial-scale appli-cation of the membranes (that were prepared from the S-PEEK) for CO2 separationfrom gas mixtures was investigated. The authors also investigated the effects ofvarious degree of sulfonation and the presence of different types of ions (H, Na, Li,Mg, Ba, and Al) on the gas separation performance of the membranes were inves-tigated. Results obtained for the plasticization pressure and CO2 permeability aredisplayed in Table 16.

6.5 Technique for Preparing Polymers of Intrinsic Microporosity(PIM)

Percentage crystallinity is one of the major morphological properties which deter-mine the transport properties of polymers that are used for making membrane. Lesscrystalline (amorphous) are commonly recommended candidate for membrane

Table 16 Transport and plasticization properties of S-PEEK with sodium ions [117]

PolymerDegree ofsulfonation

CO2 permeability(barrer)

Plasticization pressure(bar)

S-PEEK-Na-1 0.44 8 11

S-PEEK-Na-2 0.76 10.5 32

S-PEEK-Na-3 1.10 14.5 >40


applications. A successful synthesis of nanoporous polymers was reported by Buddet al. [118]. The new set of polymers were prepared from certain microporousnetworks of functional units including phthalocyanine, porphyrin, and hexa-azatrinaphthylene. Due to the microporous nature of the starting materials, theresulting polymer from this synthesis protocol is often called polymers of intrinsicmicroporosity (PIMs) [119].

PIMs are nanoporous (0.4–0.8 nm), soluble, solution proccessable, and thermallystable (initial decomposition at around 460 �C) polymers. They have very highsurface areas (of ~800 m2/g) and free volume [119, 120]. A detailed description ofthe concept of PIMs can be found in Budd et al. [118]. Following the pioneeringarticle that was published on PIMs synthesis, the polymers have been furtherinvestigated for a variety of applications [120]. In gas separation application, PIMshave high permeability as well as high selectivity that are above the Robeson’s upperbound for a number of gas pairs. Table 17 shows the separation performance resultsof PIMs in separating CO2 from CH4. Phthalazinone-based copolymers with intrin-sic microporosity (PHPIMs) was reported by [121]. A high temperature aromaticdouble nucleophilic subcondensation polymerization method was used to synthesizethe polymer. Gas separation performance of membrane prepared from the polymerswas investigated. The effect of the stiffness of the phthalazinone unit was investi-gated in order to explore the possibility of using this property to design a new high-performance membrane for gas separation. The results of the investigation showedthat the introduction of phthalazinone moiety resulted into amorphous glassy copol-ymers which exhibit excellent thermal stability and gas separation properties.

Table 17 Gas transport properties of PIMs [4]

Membrane

Experimental parameters Permeability (Barrer)

Feedpressure

Temperature(�C) CO2 CH4

PIM-1a 0.2 bar 30 23,000 125

PIM-7b 0.2 bar 30 1100 62

Polybenzodioxane- PIM-1 1 atm 25 4390 310

Thioamide-PIM 1 bar 25 1120 56

Carboxylated PIMs 50 psig 25 620–6715 –

Cross-linked PIMs 3.4 atm 30 1291–2345 52.6–192

PIMs (Trifluoromethyl andPhenylsulfone groups

50 psig 25 731–3616 –

aPrepared using 5,5,6,6-tetrahydroxy-3,3,3,3 tetramethyl-1,1-spirobisindane andtetrafluoroterephthalonitrilebPrepared using 5,5,6,6-tetrahydroxy-3,3,3,3 tetramethyl-1,1-spirobisindane and 7,7,8,8-tetra-chloro-phenazyl-3,3,3,3-tetramethyl-1,1-spirobisindane


6.6 Membrane Preparation Using 3D Printing Technologies

Three-dimensional (3D) printing is a new technology whose applications have beenextended into almost all aspects of research and development. The technology is alsoreferred to as additive manufacturing or rapid prototyping. It involves a layer-by-layer building of materials to create materials of various geometries from 3D imagedata [122, 123]. The 3D technology has been recently applied for membranematerials fabrication. It is a technology that is expected to revolutionize membranematerials and modules fabrication. In membrane technology, the applicable additivemanufacturing techniques can be grouped into four: namely, photopolymerization,powder, material extrusion, and lamination [124, 125]. The most promising amongthese is the photopolymerization. It is the curing of photoreactive polymers (photo-polymers) with a laser, UV, or light [124].

Presently, the majority of the polymeric membranes produced by 3D printingtechnology have been evaluated for liquid. Thus, very few publications can befound on gas separation. Polymers that have been investigated for membrane fabri-cation include polydimethylsiloxane membrane for use as gas-liquid contactor[126–128], m-phenylene diamine [129, 130], polyamide-12 using selective lasersintering [131] and photocurable formulation of diurethane dimethacrylate(DUDA), poly(ethylene glycol) diacrylate (PEGDA), dipentaerythritol penta-/hexa-acrylate, and 4-vinylbenzyl chloride [125].

7 Conclusion

Advances in the method of synthesis and gas transport properties of functionalpolymeric membrane materials were reviewed. The transport properties of bothrubbery and glassy polymeric membranes that were prepared by these methods ofsynthesis were described in terms of permeability, solubility, diffusivity, and selec-tivity. It was generally observed that the presence of functional groups such as polarether oxygen linkages, hydroxyl, and amine enhances the separation efficiency ofpolymeric membranes. The intermolecular interaction of these functional groupswith CO2 enhances its solubility within the membrane matrix and hence the solu-bility selectivity of the membrane is improved. The size of such functional groupscontributes in the enhancement of CO2 diffusivity and hence diffusivity-selectivityof the membranes. It was also revealed that porogens such as sulfonic, carboxylicacid, cyclodextrin, and saccharides could be excellent candidates for improving theseparation properties of polymeric membranes. Therefore, the introduction of any ofthese functional groups or similar ones into the polymer backbone is one of thepromising methods for producing membranes with superior natural gas separationperformance.


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