surface modification of polyimide membranes by diamines for h2 and co2 separation

Surface Modification of Polyimide Membranes by

Diamines for H2 and CO2 Separation

Tai-Shung Chung,*1 Lu Shao,*2 Pei Shi Tin1

1Department of Chemical and Biomolecular Engineering, National University of Singapore, 119260, SingaporeFax: 65-67791936; E-mail: [email protected]

2Department of Applied Chemistry, Faculty of Science, Harbin Institute of Technology, Harbin, 150001, P. R. ChinaE-mail: [email protected]

Received: March 6, 2006; Revised: May 12, 2006; Accepted: May 15, 2006; DOI: 10.1002/marc.200600147

Keywords: crosslinking; gas permeation; membranes; modification; polyimides

Introduction

The separation of H2 and CO2 by polymeric membranes is

fraught with difficulties, as these membranes usually

exhibit both unfavorable diffusivity selectivity and solubi-

lity selectivity for H2/CO2. However, as global energy has

progressively become a major concern as a result of

resource depletion and newhighs in oil prices, hydrogen has

emerged as a strategically important fuel source in the

foreseeable future.[1,2] Not only is hydrogen an important

feedstock for the chemical industry, but also for fuel cells to

generate power/electricity.[3] A hydrogen-based energy

system also has the advantages of low emission, being

environmentally benign, clean, and efficient in achieving

sustainability goals.[2,4]

Hydrogen is a common element in the universe, but it is

not naturally available in the pure form. In other words,

hydrogen must be produced from other energy sources.

Generally, hydrogen production relies on conventional

technologies such as steam reforming of methane (natural

gas) or hydrocarbons, and partial oxidation and auto-

thermal reforming of hydrocarbons.[5] Currently, the steam-

methane reforming is the most favored route for large-scale

hydrogen production because of the availability of natural

gas. Through this process, synthesis gas which consists of

H2, CO, and unwanted CO2 is ultimately produced from the

reactions between methane and steam. As a consequence,

the removal of CO2 from H2 or the sequestration of CO2 is

an immediate as well as long-term goal for this sustainable

energy system.

Summary: The separation of H2/CO2 is technologicallyimportant to produce the next generation fuel source, hydrogen,from synthesis gas. However, the separation efficiency achievedby polymeric membranes is usually very low because of bothunfavourable diffusivity selectivity and solubility selectivitybetween H2 and CO2. A series of novel diamino-modifiedpolyimides has been discovered to enhance the separationcapability of polyimide membranes especially for H2 and CO2

separation. Both pure gas and mixed gas tests have beenconducted. The ideal H2/CO2 selectivity in pure gas tests is 101,which is far superior to other polymeric membranes and is wellabove the Robeson’s upper-bound curve. Mixed gas tests showan ideal selectivity of 42 for the propane-1,3-diamine-modifiedpolyimide. The lower selectivity is a result of the sorptioncompetition between H2 and the highly condensable CO2

molecules.However, both puregas andmixedgas data are betterthan other polymeric membranes and above the Robeson’supper-bound curve. It is evident that the proposed modificationmethods can alter the physicochemical structure of polyimidemembranes with superior separation performance for H2 andCO2 separation

Both pure gas andmixed gas separation properties of H2/CO2

for membranes derived from 6FDA-durene with respect tothe upper-bound curve.

Macromol. Rapid Commun. 2006, 27, 998–1003 � 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

998 DOI: 10.1002/marc.200600147 Communication

At present, the separation of CO2 from the steam-

methane reforming stream is mainly through absorption

technology (such as amine or hot potassium carbonate

aqueous solution) and pressure swing adsorption.[6] Hydro-

gen recovery using a polymeric membrane is highly

beneficial because of its inherent advantages, such as

simplicity in operation, lower capital cost and high energy

efficiency, as compared to conventional separation tech-

nologies.[7] So far, no suitable polymeric membranes have

been discovered for this application becausemost polymers

cannot discern H2 and CO2 molecules and thus show poor

H2/CO2 selectivity (about 0.5 to 5) arising from the

propinquity in H2 (2.89 A) and CO2 (3.30 A) kinetic

diameters. The flexible characteristics of thermally

motioned polymer chains with large interstitial spaces and

CO2-induced plasticization makes size-based separation

difficult.[8] Therefore, a polymeric membrane with anti-

plasticization characteristics and a higher H2/CO2 perm-

selectivity is essential for effectual hydrogen fuel produc-

tion. The chemical modification of polymers has been

found to effectively improve the plasticization resistance

and the separation characteristics of membranes for gas

separation.[9–13] However, most of them show very limited

enhancement of H2/CO2 separation.

Herein, a new approach to enhance the gas selectivity of

polyimidemembranes, aimed to achieve superior separation

efficiency for H2/CO2, is reported. A novel physicochemical

surface modification has been carried out by the incorpo-

ration of various linear diamines at room temperature into

polyimide membranes, with the following objectives: 1) to

form three-dimensional cross-linked/network structures in

the polyimide membranes, 2) to optimize the interstitial

space suitable for H2 and CO2 separation, 3) to introduce

hydrogen bonds in a cross-linked/network structure, and 4)

to alter the chemical environments of the polyimide

membranes by partially transferring the imide group into

an amide group.

Experimental Part

Dense membrane films were prepared from 6FDA-durenepolyimide (synthesized in our lab)[14] vacuum dried at 250 8Cfor 48 h to give a thickness of about 50� 5 mm. A series oflinear aliphatic cross-linking reagents, ethylenediamine(EDA), propane-1,3-diamine (PDA), and butane-1,4-diamine(BuDA), were purchased from Aldrich and used to modify thepolyimide films. The concentration of the cross-linkingsolutions was 1.65 mol �L�1 in methanol. The chemicalmodification was performed by immersing the films in thediamine/methanol solution for a stipulated period of time (1, 5,or 10 min). The modified films were washed with freshmethanol immediately after removal from the reagent solutionto wash away any residual solution, followed by dryingnaturally at room temperature for approx. 1 d.

Pure gas permeabilities were determined by a constantvolume method with a precalibrated permeation cell, as

reported elsewhere.[15] The permeabilities were obtained at35 8C and 3.5 atm. The gas permeability was determined fromthe rate of pressure increase obtainedwhenpermeation reacheda steady state. The ideal selectivity of a membrane for gas A togas B was evaluated through the ratio of their permeability.

For binary gas permeation measurements, a permeation celland a gas chromatograph were combined in order to allowstraightforward determination of gas permeability. A binarygas mixture containing 50% of H2 in CO2 was used as the feedgas with argon as a carrier gas at the permeate side. Themeasurements were conducted at 35 8Cwith a total pressure of7 atm. Detailed experimental design, procedures, and theequations involved have been described elsewhere.[16]

Results and Discussion

The cross-linking reaction between the 6FDA-durene and

the diamines is confirmed by the gel content examination,

where the polyimide films cannot be fully dissolved in the

original solvent (dichloromethane) after treatment with

linear diamines. In particular, the gel content of a 1 min

EDA cross-linked film can simply achieve approx. 77%,

which indicates that the cross-linking reaction occurs very

rapidly. The measurement of gel content has been reported

elsewhere.[14,16] FTIR-ATR is applied to further validate

the cross-linking reaction, and to explore the reaction

mechanism andmonitor the reaction development. Figure 1

shows the typical infrared bands for the original and PDA

cross-linked polyimides. The FTIR-ATR spectrum of

6FDA-durene (imide group) is characterized by bands

around 1 786, 1 716, and 1 351 cm�1. The intensities of

these imide characteristic peaks gradually decrease with an

increase in cross-linking period. The appearance of an

amide characteristic peak at around 1 516 cm�1 indicates

the thermodynamic occurrence of a cross-linking reaction

between the diamine and polyimide even at room

temperature. The C–F peak at around 1 241 cm�1 is set as

the reference peak to quantitatively determine the progress

of reaction, since the C–F group is inert to the diamine

cross-linking. From the relative intensity of the imide C O

asymmetric stretch to C–F stretch, where it declines from

1.099 (original), to 0.672 (1 min PDA cross-link), to 0.045

(5 min PDA cross-link) and finally to 0.034 (10 min PDA

cross-link), the kinetic property of the cross-linking

reaction can be obtained.

Figure 2 illustrates the diamine-polyimide reaction

mechanism of chemical cross-linking modification. X-ray

photoelectron spectroscopy (XPS, spectra not shown) is

subsequently carried out to corroborate the cross-linking

mechanism. Again, since the fluorine content is constant

during the modification, the nitrogen/fluorine ratio is

compared among the samples. From the increment of the

N/F ratio (0.31 for the original! 0.71 for the 1 min PDA

cross-linked! 0.95 for the 5 min PDA cross-linked), it is

confirmed that the nitrogen in PDA is chemically bonded to

6FDA-durene, as shown in Figure 2.

Surface Modification of Polyimide Membranes by Diamines for H2 and CO2 Separation 999

Macromol. Rapid Commun. 2006, 27, 998–1003 www.mrc-journal.de � 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

In addition, X-ray diffraction (XRD) patterns are

obtained for PDA cross-linked polyimide membranes.

The polyimide of 6FDA-durene possesses a d-space value

of 6.40 A and it is decreased after cross-linking with PDA

(i.e., 5.98, 5.90, and 5.86 A for 1, 5, and 10 min PDA cross-

linking, respectively). The shift of this peak indicates an

increase in the regularity of packing structure relative to

unmodified polyimide. Although the PDA reagents break

the rigid polyimide chains to form relatively flexible

polyimide-amide networks during chemical reaction, as

illustrated in Figure 2, the insertion of PDA agents between

polymer chains seems to have greater effects on d-space

compaction. The decrease in d-space after cross-linking

should be mainly attributed to the insertion of cross-linking

reagents, which occupy the inter-spaces between the chains

and rearrange the chain configuration, thus decrease the

interstitial space and alter the free volume distribution. This

explains why the packing regularity increases after cross-

linking modification. Furthermore, the strong hydrogen

bonding of inter-chains and intra-chains may also be the

possible reason for the reduction in d-space after cross-

linking. More information can be acquired through further

analysis of XRD data. It appears that a shoulder appears at

2y¼ 20–308 after cross-linking and it intensifies with

cross-linking time. In addition, the intensity of the main

peak is found to decrease with the cross-linking density.

These findings further illustrate a tighter infrastructure of

the cross-linking network in the membrane at a higher

cross-linking density.

Undoubtedly, from the above characterizations, the

optimization of the physicochemical structure of the

polyimidemembranes by linear diaminesmay considerably

affect the separation performance of the membranes.

Hence, pure gas permeation measurements were conducted

to investigate the effect of cross-linkingmodification on the

polyimide gas separation properties. The Robeson H2/CO2

trade-off line for 6FDA-durene polyimide and its cross-

linked films are shown in Figure 3.[17] As compared to

conventional polymeric membranes that have low H2/CO2

selectivity and are located far below the upper-bound

curve,[18–22] it is obvious that a high separation perform-

ance can be achieved through our proposed cross-linking

Figure 1. FTIR-ATR spectra of unmodified and PDA cross-linked 6FDA-durene.

C

C

O

O

N

CCF3 CF3

C

CO

N

O

CH3

CH3 CH3

CH3

m

H2NR

NH2

NHO

O HNO ONH

R

NH

(6FDA-durene)

(Diamine)

(Cross-linked networks)

Figure 2. Possible cross-linkingmechanism between the 6FDA-durene and the diamines (R¼ (CH2)n, n¼ 2–4).

1000 T. S. Chung, L. Shao, P. S. Tin


method. The ideal selectivity of the cross-linked mem-

branes increases tremendously after modification and

surpasses the upper-bound limit of polymeric membranes.

Accordingly, the diamine cross-linking technology is a

promising strategy to produce high performance mem-

branes for H2/CO2 separation, as it is competent in altering

and optimizing the chemical component and physical

structures in polyimide membranes. In particular, the best

performance for H2/CO2 separation is observed for the

PDA cross-linked polyimide. The permeation results

obtained are consistent with the FTIR-ATR data. It appears

that under the same cross-linking conditions, PDA is the

most effective cross-linking reagent in modifying 6FDA-

durene.

Since the EDA, PDA, and BuDA have the similar

molecular width (around 3 A simulated by Cerius2

software), the effectiveness of cross-linking reagents in

the polyimide modification is most probably influenced by

their molecular lengths. Among all the diamine reagents

used, BuDA has the longest molecular length (around 8 A),

which may restrict its diffusion into polyimide membranes

and thus reduce the reaction rate. On the other hand,

although EDA has the shortest molecular length (around

5.5 A), the shorter distancewill induce themutual influence

between the two functional groups (–NH2) and thermody-

namically restrain the cross-linking reaction. As a result,

the PDAwith the medium molecular length (around 6.7 A)

achieves the balance between these two impediments and

possesses the higher capability in cross-linking the poly-

imide membranes. For instance, the ideal selectivity of

6FDA-durene membranes increases dramatically from 1 to

38.5 and 101 after cross-linkingwith PDA for 5 and 10min,

respectively. These values are far higher than other high

performance polymers. The unforeseen ideal selectivity of

H2/CO2 (101) for the 10 min PDA cross-linked membrane

definitely surpasses the performance of up-to-date poly-

meric materials (i.e., the left bottom of Figure 4) and is

even comparable to the performance (H2/CO2 separation

factor is around 98) of inorganic (silica) membranes.[23] It

is anticipated that the H2/CO2 selectivity can be further

improved by increasing the cross-linking time with a

reduction in permeability. In addition, although the diamino

cross-linking starts on the membrane surface, it is

reasonable to expect that the reaction can penetrate deeper

to the bulk of the membrane. This is because the diamine

has amolecularwidth of 3 A,which is far smaller than the d-

space (more than 5.86 A) of the polymer, especially

in the membranes swollen by methanol. The swelling

of membranes provides a great opportunity for and

facilitates the diffusion of such small reagents across the

membranes.

Figure 4 shows a comparison between mixed and pure

gas data. The Robeson’s upper-bound curve is absent from

the comparison because it is derived mainly from pure gas

permeation properties. Because of competition on sorption,

the permeation properties are affected by the presence of

10001001010.10.01

1

10

100

Traditional Polymeric Membranes

PDA-10min

PDA-5minBuDA-5min

EDA-5min

H2/C

O2 Id

eal S

elec

tivity

H2 Permeability (Barrers)

Trade-off Line

Original 6FDA-durene

PDA-1min

Figure 3. Pure gas separation properties of H2/CO2 for membranes derived from 6FDA-durene withrespect to the upper-bound curve.



other penetrants in a gas mixture.[24,25] Hence, H2/CO2

separation properties for modified membranes have been

investigated by using an equal molar binary system to

characterize the true separation performance of the mem-

branes. The CO2 permeability is comparable to the pure gas

result. In spite of this, the presence of CO2 molecules not

only lowers the permeability of H2 through the membranes

but also results in a smaller separation factor than the ideal

selectivity ofH2/CO2 (i.e., 42 (mixed gas) vs. 101 (pure gas)

for the 10 min PDA cross-link sample). This is because the

slower molecules (CO2) often prevent the faster molecules

(H2) from permeating through the membranes, especially

for highly soluble CO2 molecules. Nevertheless, our

permeation data from the binary system is excellent and

encouraging because both the permeability and selectivity

of the membranes are still above the Robeson’s upper-

bound curve (for pure gas).

Conclusion

A new modification approach has been developed for H2/

CO2 separation by linear diamine cross-linking on poly-

imidemembranes. This study demonstrates for the first time

that diamine cross-linked membranes possess high separa-

tion performance and provide impressive separation

efficiency for H2/CO2 separation. This advancedmembrane

shows a remarkably high potential in hydrogen recovery,

and no doubt will go a long way to solve the world-wide

dwindling-energy problem.

Acknowledgements: The authors thank the NUS for fundingthis research with the grant number of R-279-000-165-112 and R-279-000-184-112. Professor D. R. Paul at University of Texas isespecially appreciated for his valuable suggestions on mixed-gastests.

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10001001010.1

1

10

100

PDA-5min-Binary

PDA-10min-Binary

PDA-10min

PDA-5minBuDA-5min

EDA-5min

H2/C

O2 Id

eal S

elec

tivity

H2 Permeability (Barrers)

Original 6FDA-durene

PDA-1min

Figure 4. Comparison between pure gas and mixed gas separation properties of H2/CO2 formembranes derived from 6FDA-durene.

1002 T. S. Chung, L. Shao, P. S. Tin


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surface modification of polyimide membranes by diamines for h2 and co2 separation

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