Journal ofMaterials Chemistry A

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ZIF-8@PBI-BuI c

aPolymer Science and Engineering Division,

HomiBhabha Road, Pune-411008, India.

020-25902618; Tel: +91 020-25902180bPhysical Chemistry Division, CSIR-Nationa

Road, Pune-411008, India. E-mail: r.baner

Tel: +91 020-25902535

† Electronic supplementary informationmaterials, IR and NMR spectra of polmechanical properties of composite mem

Cite this: J. Mater. Chem. A, 2014, 2,12962

Received 4th February 2014Accepted 30th May 2014

DOI: 10.1039/c4ta00611a

www.rsc.org/MaterialsA

12962 | J. Mater. Chem. A, 2014, 2, 129

omposite membranes: eleganteffects of PBI structural variations on gaspermeation performance†

Anand Bhaskar,a Rahul Banerjee*b and Ulhas Kharul*a

The composites of metal organic frameworks (MOFs) and polymers look promising as membrane materials

for gas separation, provided benefits of both the components can be shown successfully. This work shows

that the structural architecture of polybenzimidazole (PBI) is highly advantageous in offering attractive gas

permeation properties of its composites with MOFs. PBI-BuI and its N-substituted (methyl and 4-tert-

butylbenzyl) derivatives were blended with ZIF-8. In general, homogeneous blend formation of ZIF-8

was achieved with all three polymers, as supported by SEM. Wide angle X-ray diffraction, mechanical

property analysis and density measurements of the composite membranes were performed in order to

understand the effects of physical blending of MOFs and polymers. Gas permeability analysis of the

composite membranes revealed that the properties of MOFs as well as those of polymers arising from

their structural architecture are responsible for governing the permeability and selectivity of the resulting

composites.

Introduction

Membrane based gas separation processes are gainingincreasing importance due to their inherent process advantagessuch as smaller foot prints, ease of operation, low operationalcost, etc.Most of the commercial gas separation membranes arepolymeric in nature due to their economical fabrication intohigh surface area congurations (hollow bres and spiralmodules). Moreover, they have good mechanical stability.1

Increasing demands from industry drive multidimensionalefforts towards performance elevation of gas separation poly-meric membranes that will meet the criteria of operationalsustainability under stringent conditions in an economicalmanner. These include design of new polymers,2 post-treatmentof membranes,3 composite membranes with porous or non-porous llers,1,4 etc. The methodology of incorporating porousllers such as zeolites,5 CNTs,6 CMS,7 MOFs,8 etc. offers anincrease in permeation properties while simultaneouslyreducing the effects of plasticization and physical aging of thepolymer matrix. The traditional mixed matrix llers (zeolites,silica or carbon based materials) face compatibility issues with

CSIR-National Chemical Laboratory, Dr

E-mail: uk.kharul@ncl.res.in; Fax: +91

l Chemical Laboratory, Dr HomiBhabha

jee@ncl.res.in; Fax: +91 020-25902636;

(ESI) available: Synthesis procedure ofymers, TGA, elemental mapping andbranes. See DOI: 10.1039/c4ta00611a

62–12967

polymer matrices due to their inorganic nature,9 limitingadvantages of composite membranes that could be anticipated.

MOFs, a new class of porousmaterials, have a diverse range ofpore dimensions, topologies and chemical functionalities.Additionally, they also showhigher porosity10 andgas adsorptionthan other porous materials11 (e.g. zeolites and carbons). Whenblended with a polymer, the organic nature of their ligands isanticipated to offer better MOF compatibility with the hostpolymer matrix. Owing to these advantages, MOFs are beingprojected as promising ller candidates for making compositepolymeric membranes that would offer improved gas perme-ation properties. Although MOFs have been grown successfullyon inorganic supports,12 their scale up for practical applicationsseems to be a remote possibility. Thus, polymers such as poly-sulfone,13 Matrimid,14 polyphenylene oxide,15 Ultem,16 and PBI17

have been employed tomake compositemembranes withMOFs.Some of these studies showed enhanced gas permeability;although,MOF aggregation or poor interfaces betweenMOF andpolymer matrix have been observed.14 Focused efforts arerequired while considering characteristics of the host polymerand MOF, so that better benets of both these individualcomponents can be extracted to offer MOF@polymer compositemembranes with improved gas permeation properties.

The polybenzimidazole (PBI), as a family of polymers, couldbe a promising candidate for the preparation of compositemembranes with MOFs. It provides good compatibility withMOFs, as evident from ZIF@PBI composite studies.17 PBI has arigid molecular structure and excellent thermo-chemicalstability and it retains mechanical properties at high tempera-ture.18 When blended with MOFs, these properties could be

This journal is © The Royal Society of Chemistry 2014

Fig. 1 Scheme of composite membrane preparation.

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highly benecial in view of practical signicance demandingstringent material criteria for sustainable membrane operation.The primary PBI is generally known to possess low gas perme-ability.19 In order to utilize the potential of MOFs; it would beprudent to make their composites with structurally modiedPBIs possessing high gas permeability. It was shown that thegas permeability of PBI can be signicantly improved by intro-ducing bulky groups in its molecular structure20 without muchcompromise on their thermal properties. These molecularvariants of PBI when used for blending with ZIF-8, formedcomposite membranes that are anticipated to retain theinherent properties of PBI.

In the present work, PBI-BuI and its two N-substitutedderivatives are used as the host materials to make ZIF-8@polymer composite membranes. PBI-BuI, one of themembers of the PBI family, has an excellent combination ofhigh thermal stability and gas permeability (10–14 times higherpermeability for different gases than that of base PBI-I).21 TheN-substitution of PBI-BuI further elevates its gas permeabilitywithout serious threats to its thermal stability.20 In the presentwork, ZIF-8 is used as a ller to form composite membranes.ZIF-8 is a member of the sub-family of MOFs, called zeoliticimidazolate frameworks (ZIFs) having tunable pore sizes andchemical functionality coupled with exceptional thermalstability.22 ZIF-8 is made by linking of Zn(II) and 2-methyl-imidazolate, giving a sodalite (SOD) topology with a theoreticalpore aperture of 3.4 A.22 It has a high surface area of 1630 m2

g�1.22 The obtained composite membranes were evaluated forrequisite characteristics (thermal, spectral and gas permeation)in order to evaluate the approach of blending high permeabilityPBIs with ZIF-8 to obtain composite membranes.

Experimental

ZIF-8 was synthesized by a facile ambient temperature synthesismethod,23 while PBI-BuI was synthesized by solution poly-condensation (Scheme S1†). Structural variants of PBI-BuIpossessing chosen alkyl groups (methyl and 4-tert-butylbenzyl)were synthesized by the N-substitution of its imidazole moiety(Scheme S2†).

Preparation of composite membranes

The solution casting method was employed to make compositemembranes while varying the MOF content in them (Fig. 1). ThePBI solution in DMAc (3% w/v) was mixed with a stocksuspension of the MOFand stirred for 24 h (bath sonicatedintermittently 4 times) in order to obtain a homogeneoussuspension. It was then poured on a at glass surface main-tained at 80 �C and the solvent was evaporated under a dryatmosphere for 16 h. The membrane was peeled off from theglass surface and was kept in a vacuum oven at 100 �C for a weekin order to remove traces of the solvent.

The thus-obtained membranes were named Zn@P, where ‘Z’refers to ZIF-8, ‘n’ represents the percentage of ZIF-8 loading(w/w) into the composite membrane, while ‘P’ represents thename of the polymer (PBI-BuI, DMPBI-BuI or DBzPBI-BuI).

This journal is © The Royal Society of Chemistry 2014

Physical property characterization

The X-ray diffraction (XRD) analysis of the compositemembraneswas carried out on a Phillips PAnalytical diffractometer in reec-tionmodeusingCuKa radiation (l¼ 1.54 A). The2q range from5�

to 40� was scanned at a scan rate of 2.5� min�1. Thermogravi-metric analysis (TGA) of the composite membranes was carriedout in air (temperature range: 50–900 �C) using a TGA-STA6000 ata heating rate of 10 �C min�1. Scanning Electron Microscopy(SEM) was performed on a FEI Quanta 200 3D ESEM (dual beam)instrument with a eld emitter as an electron source. The SEMimages of the membrane cross-section were taken aer freezefracture of the membranes in LN2. Samples for the SEM analysiswere gold sputtered before scanning. The composite membraneswere also characterized for their density using a specic gravitybottle. For this purpose, an organic solvent (decalin), whichshowed negligible sorption (#0.5 wt%) for the present polymers,was chosen. Mechanical properties of the composite membraneswere studied using a Linkam TST-350 microtensile testinginstrument, where the membrane samples were analyzed at astrain rate of 5 mm s�1. The density and mechanical propertyanalysis for eachcompositemembranewere repeatedwithat leastve samples prepared under identical conditions.

Gas permeability measurements

The gas permeability (P) using pure gases, viz., He, H2, N2, CH4

and CO2 was obtained at 35 �C using a variable volume method(Fig. S7†).21 An upstream pressure of 20 atm was used whilemaintaining the permeate side at the ambient pressure. Threesamples of the membrane prepared under identical conditionswere analysed. The averaged data are given with variation inpermeability of the order of �10–15% for different gases. Theideal selectivity (a) was calculated from the ratio of the indi-vidual gas permeability.

Results and discussionCharacterization of MOFs and polymers

The WAXD patterns of as-synthesized ZIF-8 matched well withthe simulated data (Fig. S1†), conrming successful formation

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Fig. 3 SEM images of (a) Z30@PBI-BuI, (b) Z30@DMPBI-BuI and (c)Z20@DBzPBI-BuI.

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of ZIF-8. ZIF-8 nanocrystals of 60–80 nm of rhombic dodeca-hedral shape could be observed from Fig. S2.† The IR and1H-NMR spectra of PBI-BuI and its N-substituted derivatives(DMPBI-BuI and DBzPBI-BuI) as given in Fig. S3 and S4,†respectively, matched well with those reported previously.24

Detailed description of the correlation of these spectra with thestructure of the respective polymer is given in the ESI.†

Characterization of composite membranes

The ZIF-8 basedmembranes with PBI-BuI and DMPBI-BuI couldbe successfully prepared up to 30% of ZIF-8 loading. They couldbe analyzed easily for the physical properties and gas perme-ability investigation. Unfortunately, composite membranesbased on DBzPBI-BuI and ZIF-8 could be prepared only up to20% of ZIF-8 loading. Beyond this concentration of ZIF-8, theobtained composite membranes were brittle, could not be usedfor permeation analysis and thus were not characterizedfurther.

The WAXD spectra of PBI-BuI, DMPBI-BuI and DBzPBI-BuIshowed an amorphous hollow structure with an average inter-segmental d-spacing (dsp) of 4.56 A, 4.64 A and 5.30 A; respec-tively. Higher dsp values for N-substituted polymers than that ofPBI-BuI indicated their more open polymer matrix.

In the WAXD pattern of composite membranes, sharpmultiple peaks belonging to ZIF-8 in addition to the charac-teristic amorphous hollow structure of host polymers werepresent (Fig. 2). The presence of all the sharp peaks belonging toZIF-8 in the spectra of composite membranes conrmed thatthe structure of ZIF-8 remained intact.

SEM images of the cross-section of composite membranes(Fig. 3) showed an almost homogeneous distribution of ZIF-8nanoparticles without any agglomeration. This is further sup-ported by the cross-sectional elemental mapping (Fig. S5†). Thehomogeneous distribution of ZIF-8 without causing any particleagglomeration could be the result of the adopted membrane

Fig. 2 WAXD patterns with photograph of ZIF-8 composites.

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preparation methodology. The ZIF-8 nanoparticles were addedto a polymer solution in the form of their stock suspension. Thisavoided ZIF-8 drying and thus particle agglomeration. In allthree types of composites, SEM images of the membrane cross-section did not show any voids at the polymer-MOF interface.This indicated good compatibility between the polymer and ZIF-8. The voids present at Cu-BPY-HFS-Matrimid14a and Matrimid/[(Cu)3(BTC)2]14b interfaces reported earlier were absent in the

This journal is © The Royal Society of Chemistry 2014

Fig. 4 Decrease in density of composite membranes with ZIF-8loading.

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present case. Polymers chosen for the present work belong tothe PBI family possessing two imidazole groups per repeat unit.The absence of any voids in the present composite membranesindicates that PBI and its structural variants have goodcompatibility with ZIF-8. This could be correlated with theelectron donor ‘N’ present in the imidazole moiety of PBI, which

Fig. 5 Variation in tensile strength of ZIF-8 based compositemembranes.

Fig. 6 Variation in H2 permeability ( ) and its selectivity over CH4 ( ) and N(c) ZIF-8@DBzPBI-BuI based composite membranes.

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could have positive interactions with the metal ion at the MOFsurface.17a

TGA showed that ZIF-8 is stable up to 450 �C in air (Fig. S6†).This value is in agreement with the reported one.23 PBI-BuI,DMPBI-BuI and DBzPBI-BuI are thermally stable up to 500 �C,450 �C and 350 �C, respectively, in air (Fig. S6†). DBzPBI-BuIshowed comparatively lower thermal stability than other twopolymers due to the presence of the tert-butylbenzyl group in it.The degradation of composite membranes in air yielded onlymetal oxides as a char. From these data, accurate estimation ofthe ZIF-8 content in the membrane was made. It matched wellwith the ZIF-8 quantity taken during the preparation ofcomposite membranes (Table S1†).

When the experimental density of the composite membranewas compared with that of calculated ones (based on thecomposition), it was found that composite membranes showedlower density than the corresponding theoretical values(Table S1†). The percent change in the density [Dr (%)] wascalculated by the following equation:

Dr ð%Þ ¼ rT � rE

rT� 100

where rT is the theoretical density and rE is the experimentaldensity. As could be seen from Fig. 4, Dr (%) increased with ZIF-8 loading in the composite membranes based on all the threePBIs. This could be attributed to the disruption of chainpacking caused by ller nanoparticles, which creates additionalfree volume. Song et al. also have observed similar extra freevolume creation in Matrimid-ZIF-8 composites.25 PBI-BuI andits N-substituted variants have a rigid backbone, which could besignicantly perturbed by ZIF-8 nanoparticles, leading to thecreation of the extra free space.

Analysis of mechanical properties of composite membranesshowed decreasing tensile strength (Fig. 5) and modulus withan increase in ZIF-8 loading (Table S1†). A decrease in modulusthan that of the respective pristine polymer shows the lowertoughness of the composite membranes. In general, compositemembranes with 10 and 20% loading showed no signicantlowering in the strength. Only at 30% loading, a signicantdecrease in the tensile strength was observed. This indicatesthat only aer 30% ZIF-8 loading the continuity of the polymerphase is decreased.

2 ( ) with MOF loading in (a) ZIF-8@PBI-BuI, (b) ZIF-8@DMPBI-BuI and

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Fig. 7 Variation in CO2 permeability ( ) and its selectivity over CH4 ( ) and N2 ( ) with MOF loading in (a) ZIF-8@PBI-BuI, (b) ZIF-8@DMPBI-BuIand (c) ZIF-8@DBzPBI-BuI based composite membranes.

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It is worthy to note that all the present membranes couldwithstand 20 atm upstream pressure, when mounted in the gaspermeation cell, indicating their strong mechanical robustness.To the best of our knowledge, this is the highest ever pressuretested for the MOF–polymer composite membranes and thusshowed the success of choosing PBI as a family of polymers tomake composite membranes with ZIF-8.

Gas permeability analysis

In all the cases of ZIF-8 based composites with three PBIs, thegas permeability increased with an increase in ZIF-8 loading(Table S2†). As could be seen from Fig. 6a and 7a, the H2 andCO2 permeability in the case of PBI-BuI based compositesincreased almost linearly. A three times increase in H2 perme-ability from 6.2 Barrer (for pristine PBI-BuI) to 22.1 Barrer (for30% composite) coupled with increased H2/CH4 selectivity from155 to 184 and H2/N2 selectivity variation from 73 to a smalldecrease up to 67 was an appreciable outcome of the PBI-BuIbased composites. Although CO2 based selectivities (Fig. 7a) areslightly decreased, the CO2/CH4 selectivity remained appre-ciable even with 30% of ZIF-8 loading as 43.6. This is higherthan the selectivity for commonly used gas separationmembrane materials such as Matrimid,26 polysulfone27 andpolycarbonate28 [a(CO2/CH4) ¼ 36, 22 and 19, respectively].

The effect of structural variations in PBI-BuI was promi-nently seen in DMPBI-BuI based composites. An order ofmagnitude elevation in H2 permeability compared to that of thepristine case was achieved with 30% loading (Fig. 6b). This wascoupled with �70% decrease in H2/N2 and H2/CH4 selectivitythan that of unloaded DMPBI-BuI. Thus lower selectivity(27 and 37, respectively) values are still comparable with thoseof commonly used gas separation membrane materials likepolysulfone, for which the H2 permeability is 14 Barrer,27 whichis far below than the H2 permeability (127.5 Barrer) (Table S2†)of Z30@DMPBI-BuI.

An increase in CO2 permeability from 3.8 Barrer (for thepristine DMPBI-BuI) to 58 Barrer for the 30% compositemembrane was substantial (Fig. 7b). One of the peculiaritiesseen in the case of DMPBI-BuI based composites was that anincrease in permeability up to 20% ZIF-8 loading was linear,while a considerable elevation in permeability was observed at

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30% loading (Table S2†). The elevation in H2 permeability was10 fold, while that of CO2 permeability was 14 fold. Thisbehavior could be attributable to the extra free volume createddue to the rigid polymer chain backbone, as indicated by thedecrease in density as discussed above (Fig. 4). Such a creationof extra free volume could be possibly due to the bulk of methylgroups present in DMPBI-BuI. No visible defects were detectedin the SEM image of this composite (Fig. 3b), as could be seen inthe case of ZIF-8/Matrimid composites.14c

In the case of DBzPBI-BuI composites, the ZIF-8 loadingcould be successfully achieved only up to 20%. When theelevation in permeability at this level is compared with that ofthe unsubstituted PBI-BuI case, the effect of the bulky substit-uent, DBz, is prominently evident. A rise in H2 and CO2

permeability in DBzPBI-BuI composites up to 3 and 3.5 times,respectively, compared to that in the pristine DBzPBI-BuI wasobserved (Fig. 6c and 7c). On the other hand, a similarenhancement in the PBI-BuI based composite was just 2.2 and1.7 times, for H2 and CO2, respectively (Table S2†). Moreover,the H2 and CO2 permeability values of Z20@DBzPBI-BuI as 180.3and 89.8 Barrer, respectively; coupled with H2/N2 and H2/CH4

selectivity of 28.7 and 23.2, respectively (Table S2†) are highlypromising for practical separation of these gases. Improvedpermeation properties of all the three PBI based compositesconclude that benets of ZIF-8 porosity can be better shownwith the polymers of low diffusion resistance (high perme-ability), provided that they have good compatibility with theMOF, as ZIF-8 contributed to a signicant increase in thepermeability in all three polymer cases. It can thus beconcluded that ZIF-8 elevates permeability, rather thanaffecting size based discrimination of penetrants.

Conclusions

With ZIF-8@PBI-BuI composite membranes, good compati-bility and homogeneous ller distribution were obtained byblending of the as-synthesized nano-sized ller with polymers.A good improvement in gas permeation properties of ZIF-8based composite membranes was observed. PBI-BuI showed analmost linear increase in the gas permeability (3.5 times in H2

permeability at 30% MOF loading). The good compatibility and

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subsequent improvement in the gas permeability prevail evenaer elimination of H-bonding while substituting two nitrogenatoms of the PBI-repeat unit by alkyl groups. The ZIF-8@DMPBI-BuI composites showed an exponential increase in thegas permeability due to unique structural features of individualcomponents combined together positively. The ZIF-8@DBzPBI-BuI composites also showed an improvement in the gaspermeability with increasing ZIF-8 loading. Thus, both, prop-erties of MOF as well as the polymer used for makingcomposites, are important in governing gas permeation prop-erties of resulting composites. By appropriate selection of thepolymer and MOF (e.g. DMPBI-BuI and ZIF), an excellentcombination of high permeability and selectivity can beachieved.

Acknowledgements

Authors acknowledge funding from the CSIR's XII Five YearPlan Project (Grant no.: CSC 0122). AB acknowledges CSIR, NewDelhi, India, for a research fellowship.

Notes and references

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