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Journal of Membrane Science 374 (2011) 83–92

Contents lists available at ScienceDirect

Journal of Membrane Science

journa l homepage: www.e lsev ier .com/ locate /memsci

hin carbon/SAPO-34 microporous composite membranes for gas separation

ang Li, Jianhua Yang ∗, Jinqu Wang, Wei Xiao, Liang Zhou, Yan Zhang, Jinming Lu, Dehong Yinnstitute of Adsorption and Inorganic Membranes, State Key Laboratory of Fine Chemicals, Dalian University of Technology, Zhongshan Road 158, Dalian 116012, China

r t i c l e i n f o

rticle history:eceived 2 July 2010eceived in revised form 18 January 2011ccepted 9 March 2011vailable online 16 March 2011

eywords:arbon/SAPO-34 composite membraneicroporous

a b s t r a c t

Thin carbon/SAPO-34 microporous composite membranes, in which SAPO-34 particles were dispersedinto a carbon matrix, were successfully fabricated on porous a-Al2O3 tubes. The SAPO-34 molecularsieve content in the membrane precursor solution and coating cycles had a significant influence on theseparation performance of the composite membranes. The membrane (M1) with a SAPO-34 content of2 wt.% that was dip-coated twice demonstrated the best performance for gas separation. This membraneexhibited improved ideal selectivities for H2/CH4, CO2/CH4 and H2/SF6 of 97, 87 and 250, respectively,with a reduced H2 permeance of 10 × 10−8 mol m2 Pa−1 s−1. However, the H2/CH4 and CO2/CH4 of thepure carbon membrane were 64 and 40, respectively, at 298 K with a pressure drop of 0.1 MPa.

2/CH4 separationO2/CH4 separation

The CO2/CH4 and H2/CH4 selectivities of membrane M1 were as high as 258 and 112, respectively, forthe equimolar mixture of CO2/CH4 and H2/CH4, which was larger than the corresponding ideal selectivi-ties of 87 and 97 at 298 K, under a 0.1 MPa pressure drop. This effect was due to the blocked permeationof nonadsorbable gas by an adsorbed component or the competitive diffusion effect. It was found that theCO2/CH4 and H2/CH4 separation selectivities decreased with increasing temperature and pressure differ-ence. These results suggest that the SAPO-34 offers multiple roles in the improved selectivity by reducing

-34 m

pore size of carbon/SAPO

. Introduction

Membrane-based separation is the frontier of molecular-scaleeparation due to its high efficiency, low-energy requirementsnd simplicity [1–5]. Currently, organic polymeric membranes areidely applied to real-world applications such as H2 separation

nd purification, CO2 removal and air separation owing to theirelatively low cost and large-scale production. However, the mainrawback of these membranes is their poor stability at high tem-eratures and in the presence of strong solvents that limit theirse [6]. On the contrary, microporous inorganic membranes (withpore size less than 2 nm), such as zeolite-, carbon- and SiO2-basedembranes, possess a high thermal stability, excellent mechanical

trength against high pressure, and chemical stability. These mem-ranes have also been considered as a new generation of membraneandidates because they provide the potential for molecular recog-ition at the sub-nanometer level and the ability to accomplisheparation tasks in harsh conditions, such as at high tempera-

ures.

Among microporous inorganic membranes, carbon, crystallineluminosilicates (namely zeolite) and aluminophosphate mem-ranes are particularly intriguing [2,4,7–27]. Herein, microporous

∗ Corresponding author. Tel.: +86 411 84986147; fax: +86 411 84986147.E-mail addresses: yjianhua@dlut.edu.cn, youko16@hotmail.com (J. Yang).

376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2011.03.022

embrane and by its preferential adsorption and diffusion characteristics.© 2011 Elsevier B.V. All rights reserved.

crystalline aluminosilicate or aluminophosphate solid membranesare referred to as “crystalline molecular sieve solid” (denoted“CMS”) membranes. The CMS membranes have uniform andmolecular-sized pores, and they separate molecules based onsize selectivity and competitive adsorption. The strong interac-tions of CMS membranes with permeating components accountfor an ideal selectivity of 10,000 for the dehydration of organ-ics [19,21], even the reverse selectivity for the separation oforganics from water by pervaporation [21,28,29] and high CO2selectivity over CH4 or N2 [20,22,24]. However, a CMC membraneis a polycrystalline aluminosilicate or aluminophosphate film,and thus, the grain boundaries within polycrystalline molecularsieves exist inevitably. These boundaries provide a nonmolecu-lar sieve path for the permeation of separating molecules and arebelieved to degrade the separation selectivity [30–33]. They aretherefore a major challenge in the separation of small molecularmixtures that have weak or no interaction with a CMS mem-brane, e.g., a H2/CH4 mixture, even though some strategies suchas rapid thermal processing [33] were developed to minimizethe negative effect of the boundaries on separation performance.Carbon membranes are generally fabricated by the carbonization

of polymeric film [11]. The resultant ultramicroporous struc-ture featuring nominal pore diameters of 0.35–0.55 nm forms apure carbon sieve membrane ideally suited for small gas sep-aration for mixtures such as O2/N2, CO2/N2, CO2/CH4 [2,7–17],but the permeance of a pure carbon membrane is generally

8 brane

lt

mectoddTpbaoftbimSpcaawpcoswpttzpapt

stctrpcmtcoafonsu(tmacoiad

4 G. Li et al. / Journal of Mem

ow, below 10−9 mol m−2 s−1 Pa−1, which is far from satisfac-ory.

Recently, incorporating inorganic particles into a carbonembrane, particularly crystalline molecule sieve particles, has

merged as an efficient strategy for developing novel membraneandidates for the separation of small gas mixtures. The resul-ant carbon/CMS composite membranes combine the advantagesf a carbon sieve membrane, such as narrow and small pore sizeistribution, with those of a CMS membrane, such as a reducediffusion path and strong interaction with permeation molecules.herefore, the composite membranes should improve separationerformance compared to pure carbon sieve membranes. Car-on/Y zeolite [34], carbon/silica [35], carbon/ZSM-5 zeolite [36]nd carbon/T zeolite [37] composite membranes were devel-ped and showed an improved permeability and/or selectivityor small gas mixtures such as CO2/CH4 and O2/N2 compared tohe carbon membranes. However, these carbon composite mem-ranes are self-supported, suffering from extreme fragility, which

s an important problem for industrial use. Meanwhile, their per-eability needs further improvement for practical applications.

upported carbon composite membranes on a porous substraterovide a solution to these problems. In our previous work, aarbon/silicalite-1 composite membrane was prepared on a porouslumina tube for O2/N2 separation [38]. The O2 permeance remark-bly improved from 7.6 × 10−10 to 1.6 × 10−8 mol m−2 s−1 Pa−1,ith the ideal selectivity of O2/N2 increasing from 3 to 5 in com-arison with the pure carbon membrane. Carbon/NaA and carbon/Lomposite membranes on a porous alumina support were devel-ped for CO2/N2 separation [39,40]. The carbon/NaA membranehowed the high CO2 permeance of 3.4 × 10−7 mol m−2 s−1 Pa−1

ith an ideal selectivity of 6. For the carbon/L membrane, theermeance of CO2 was about 2.0 × 10−7 mol m−2 s−1 Pa−1 whilehe separation factor was 44 for the equimolar CO2/CH4 mix-ure at room temperature. Zeng et al. [41] fabricated a nanosizedeolite NaA-filled carbon membrane on the inner surface of aorous alumina tube. The resulting membrane exhibited O2/N2nd CO2/N2 ideal selectivities of 15 and 159 with H2 and CO2ermeances of 3.6 and 13.0 × 10−10 mol m−2 s−1 Pa−1, respec-ively.

Obviously, the topological of the crystalline molecular sieveolids is crucial for the separation performance of the resul-ant carbon/zeolite composite membranes. In this study, thinarbon/SAPO-34 composite membranes were fabricated on aubular support by dip-coating and pyrolysis of phenolicesin/molecular sieve composites. The motivations for the incor-oration of the SAPO-34 molecular sieve particles into thearbon molecular sieve matrix are below: (1) the SAPO-34olecular sieves are highly thermal stable up to 1100 ◦C; (2)

hese sieves possess small crystalline pores of 0.38 nm, whichan provide molecular sieving selectivity for the separationf H2 or CO2 from CH4; (3) SAPO-34 molecular sieves werelso reported to adsorb more CO2 than CH4 [42], which isavorable for achievement of high selectivity of CO2 over CH4n the basis of a competitive adsorption separation mecha-ism. In terms of separation selectivity, the SAPO-34 molecularieves are considered to be advantageous over zeolite molec-lar sieves, such as Y (0.7 nm), ZSM-5 or silicalite-1 zeolite0.51–0.56 nm), NaA (0.41 nm) and L (0.71 nm,). It is expectedhat the carbon/SAPO-34 composite membrane has a high per-

eance and selectivity for small gas mixtures such as H2/CH4nd CO2/CH4. The influence of the SAPO-34 content in the pre-

ursor mixture of the composite membrane and coating cyclesn the separation performance for the small gases are exam-ned. In addition to their gas separation properties for H2/CH4nd CO2/CH4, single and binary systems were investigated inetail.

Science 374 (2011) 83–92

2. Experiments

2.1. Materials

The �-Al2O3 tubes (O.D. 13 mm, I.D. 9 mm and length 80 mmwith a membrane area of 32.66 cm2, Foshan Ceramics ResearchInstitute (FCRI)), with an average pore size of 0.2 �m and poros-ity of 30–40%, were used as supports. Ludox AS-40 colloidal silica(40%, Aldrich), aluminum isopropoxide (AIP, Tian Chemical Agent,China >99.5%) and H3PO4 (85 wt.% in aqueous, Tian ChemicalAgent) were used as silica, alumina, phosphoric oxide sources,respectively. Tetraethyl ammonium hydroxide (TEAOH 35 wt.%,Hangzhou Yanshan Chemical Agent) was used as a structure direct-ing agent. Phenolic resin (Tamanol 758, from Arakawa Chemical(USA)) was used as polymer precursor for the carbon membraneand carbon/SAPO-34 composite membranes.

2.2. Preparation of SAPO-34 zeolite particles

Uniformly sized SAPO-34 particles of 200 nm used for incor-poration was prepared by the hydrothermal technique withthe assistance of a prehydrolysis strategy [43]. First, colloidalsilica was added to a 35-wt.% TEAOH aqueous solution in abeaker. The mixture was then heated to 353 K and maintainedat that temperature under stirring for a 16-h hydrolysis reac-tion. Second, after vigorous stirring for 5 h, the gel containingaluminum isopropoxide, phosphoric acid, and deionized waterwas mixed into the prehydrolyzed solution of colloidal silica andstirred for 24 h. The molar composition of the resultant syn-thesis solution was Al2O3:P2O5:0.3SiO2:1.2TEAOH:60H2O. Afterstirring for 24 h, the mixture was sealed in a Teflon-lined, stainlesssteel autoclave and placed in an oven with preheated tem-perature of 473 K under autogenous pressure for 24 h. Finally,the solid product was recovered by repeated centrifugationand washing with deionized water, and finally dried at 393 Kovernight.

2.3. Membrane preparation

A specified amount of the prepared SAPO-34 particles was dis-persed into the phenolic resin dissolved in ethanol with a PFweight percentage of 40% under ultrasonic vibration to form themembrane precursor mixture. The investigated weight contentsof SAPO-34 in the membrane precursor mixture were 0, 2, and4 wt.%. The resultant membrane precursor mixture was coatedby a simple and effective dip-coating of the outer surface of an�-Al2O3 support, in which each end was sealed with a Teflonrod and tape. The detailed procedures were as follows: the pre-treated �-Al2O3 support was dipped into the SAPO-34 suspensionusing a 90-s contact time and withdrawn vertically at a velocityof about 0.5 cm/s. Then, the support was dried at room temper-ament for 6 h. The drying and coating procedures were repeatedfor the specified runs. Finally, the coated supports were placed ina furnace, subjected to carbonization under N2 atmosphere witha heating rate of 1 K/min until 873 K was reached. The tempera-ture was then maintained at 873 K for 2 h and then cooled with arate of 1 K/min to room temperature. The carbon and carbon/SAPO-34 microporous composite membranes were obtained using thismethod.

2.4. Characterization

The as-prepared membranes were characterized by scanningelectron microscopy (SEM) using a KYKY2800B (KYKY technol-ogy development LTD., China) SEM at an acceleration voltage of15 kV and a working distance of 10 mm after coating with gold.

G. Li et al. / Journal of Membrane Science 374 (2011) 83–92 85

F 2) filteg eter a

TmarmuwtoutNpbocC

2

mgfobtcs

fltsdd

P

wtt

ig. 1. Schematic diagram of the gas permeation apparatus. (1) Feed gas cylinder, (auge, (7) back pressure regulator, (8) permeation cell, (9) soap bubble film flow m

he surface and cross-section morphologies of the as-preparedembranes were observed using X-ray diffraction (XRD) withPhilips Analytical (Amsterdam) X-ray diffractometer. Cu K�

adiation was used to analyze the crystalline microstructure of theembranes. Thermogravimetric analysis (TGA) was performed

sing an SDT 2960 Simultaneous DTA–TGA analyzer (TA, America)ith a heating rate of 10 ◦C/min in the range of room temperature

o 973 K under an oxygen atmosphere to determine the contentf the SAPO-34 zeolite in the composite membrane. The samplesed for the TGA, N2 and CO2 adsorption data was scrubbed fromhe obtained tubular carbon/SAPO-34 composite membrane. The2 adsorption isotherms of the carbon materials using the samereparation conditions for the carbon membrane were measuredy a BELSORP-18 (BEL Japan, Inc.). The CO2 adsorption isothermsf SAPO-34 powders, pure carbon materials and carbon/SAPO-34omposite materials were estimated from Cahn D-200 (THERMOAHN, America, Inc.).

.5. Gas permeation and separation

Fig. 1 shows the experiment apparatus. A membrane wasounted in a stainless steel module and sealed at each end with a

raphite gasket. The membrane module was placed in an electricurnace equipped with a thermocouple to control the temperaturef permeation. The pressure on the retentate side was controlledy a back pressure regulator, and the permeate pressure was main-ained at atmospheric pressure. The composition of feed gas wasontrolled by mass flow controllers. Gas permeate fluxes were mea-ured using soap film bubble flowmeters.

For the measurement of the mixed gases, the feed gas wasowed at a rate of 40 ml/min and the sweep gas of Ar was con-rolled at a rate of 20 ml/min. The composition of feed and permeatetreams was analyzed by a gas chromatograph with a thermal con-uctivity detector. The permeance Pi for the permeating gas i isefined as:

N

i = i

A �pi

here Ni is the permeating flux of component i (mol/s), �Pi is theransmembrane pressure difference of component i (Pa), and A ishe membrane area (m2). The separation selectivity or separation

r, (3) mass flow controller, (4) non-return valve, (5) shut-down valve, (6) pressurend (10) gas chromatograph.

factor (S.F.) ˛ij is defined as the ratio of permeance of permeatinggases i and j:

˛sepi/j

= Pi

Pj

3. Results and discussion

3.1. Characterization of the prepared SAPO-34 particles

Fig. 2 shows the XRD patterns and SEM image of as-preparedSAPO-34 particles. The XRD patterns in Fig. 2(a) represent thecharacteristic peaks of the SAPO-34 phase without the pres-ence of any other impure phase, confirming the formation of theSAPO-34 molecular sieves. The strong intensities indicate that theas-synthesized SAPO-34 particles are of high crystallinity. From theSEM images of the as-prepared SAPO-34 particles in Fig. 2(b), it canbe seen that the particles are quite uniformly sized at about 200 nm.

3.2. Characterization of the prepared carbon/SAPO-34microporous membranes

3.2.1. The effects of the SAPO-34 content in the membraneprecursor mixture and the number of coating times on theproperties of carbon/SAPO-34 composite membranes

The synthesis conditions of carbon/SAPO-34 composite mem-branes and their gas permeation results are listed in Table 1.The XRD patterns of the M0 pure carbon membrane and thecarbon/SAPO-34 composite membranes M1-2 are shown in Fig. 3.The XRD patterns of the M2 membrane (4 wt.%, SAPO-34 parti-cles) clearly represent the characteristic peaks of the SAPO-34phase, revealing that SAPO-34 crystals were successfully embed-ded into the carbon matrix. These patterns also suggest that nodamage was made to the SAPO-34 framework during the prepara-tion of a carbon/SAPO-34 composite membrane, especially during

the pyrolysis process. The intensities of the diffraction peaks of theM1 were much weaker than those of M2 peaks most likely due tothe decrease in the SAPO-34 content in the M1 membrane com-pared to the M2 membrane. Fig. 4 shows the SEM images of themembrane M0-2. At a SAPO-34 content of 2 wt.%, the SAPO-34 par-

86 G. Li et al. / Journal of Membrane Science 374 (2011) 83–92

ima

titSbtmmocaoH9otwbc

tbnMtF

tawCamdfd

TTa

Fig. 2. XRD patterns (a) and SEM

icles were observed to be relatively homogeneously distributedn the carbon matrix. The Si and C element mapping across thehickness of the membrane (data not shown here) revealed that theAPO-34 particles are fairly evenly distributed throughout the car-on matrix across the thickness of the membrane. However, whenhe SAPO-34 content was increased to 4 wt.%, the surface of the

embrane became very rough and serious cracks occurred in theembrane (M2). These effects may result from the high tendency

f SAPO-34 crystals to agglomerate at a high SAPO-34 content. Theross-section images reveal that the membranes of M0 and M1-2re very thin, about 2 �m. As shown in Table 1, the incorporationf SAPO-34 improves the selectivity of the carbon membrane. The2/N2 and H2/CH4 ideal selectivities of membrane M1 are 33 and7, respectively, which are higher than the corresponding valuesf the pure carbon membrane, 20 and 64, respectively. However,he H2 permeance was reduced. Membrane M2 exhibited a muchorse performance with the largely increased permeance of H2

ut much smaller ideal selectivities of H2/N2 and H2/CH4 becauseracks formed in the membrane M2, as was observed using SEM.

More coating cycles were applied in order to further improvehe performance of the composite membrane. The membranesecame thicker with increasing coating cycles. However, when theumber of coatings was larger than three, the membrane (M3,4) exhibited a much degraded performance due to the forma-

ion of cracks in the membranes, as observed in the SEM images ofig. 5.

From the above sections, it has been clearly observed thathe M1 membrane showed the best selectivity for H2, CH4, N2nd CO2 gases, and can be obtained by two coating cycles andith a 2-wt.% incorporated content of SAPO-34. The H2/CH4 andO2/CH4 ideal selectivities of the membrane were quite high: 97

nd 87, respectively. The reproducibility of the carbon/SAPO-34embranes was good; 80% of the products using the same proce-

ures were reproduced. The M1 membrane was therefore used forurther investigation of the permeance dependencies on the kineticiameter, pressure and temperature in a later section.

able 1he effect of SAPO-34 content in the membrane precursor solution and the number of coat 298 K and pressure drop of 0.1 MPa.

Sample SAPO-34 content (wt.%) Coating times Permeance (m

H2 C

M0 0 2 18 1M1 2 2 10M2 4 2 68 6M3 2 3 30.8 3M4 2 4 46 5

ge (b) of the SAPO-34 particles.

3.3. Gas permeation properties of the membrane M1

3.3.1. Permeation properties of single gasesIt is generally accepted that the pore dimensions of carbon

molecular sieve membranes range from 0.3 to 0.7 nm, with amaximum between 0.35 and 0.55 nm, and they depend largely onthe fabrication procedures [2]. On the other hand, studying thegas permeance of various molecules with different kinetic diam-eters of permeating molecules can provide a clue for the roughestimation of the pore size of a membrane based on the derivationof the observed ideal selectivities from those of Knudsen diffusionmechanism [44,45]. Fig. 6 shows the observed permeance as afunction of kinetic diameter at 298 K (H2: 0.289 nm, CO2: 0.330 nm,N2: 0.365, CH4: 0.380 nm, SF6: 0.550 nm [46]) for the membraneM1. The gas permeance decreased sharply with increasing kineticdiameter, which suggested that the membrane M1 possessesa relatively narrow pore size distribution and absence of largedefects. The ideal selectivities of H2/N2, H2/CH4, H2/SF6 werehigh: 33, 97 and 250, respectively, and are much higher than thecorresponding Knudsen diffusion values of 3.74, 2.83 and 8.37.The H2 and CO2 permeances were relatively high: 10.0 × 10−8 and8.7 × 10−8 mol m2 Pa−1 s−1, respectively. Judging from the largedeviation of the ideal selectivities of H2/CH4 and H2/SF6 from theKnudsen diffusion value, the average pore size of the membraneM1 was estimated around 0.38 nm, with fewer large pores near0.55 nm through which SF6 can permeate.

The permeance of H2 and CH4 as a function of temperaturewas further investigated, as shown in Fig. 7. The permeance ofeach gas increased with increasing temperature, indicating thatthe permeation of these gases is governed by the activation dif-fusion. However, the CH4 permeance increased more than that

of H2 due to the larger size of the CH4 molecule of [8], result-ing in the decrease of the H2/CH4 ideal selectivity from 97 to69 in the temperature range from 298 K to 973 K. This result isconsistent with the high ideal selectivity of H2/CH4 and furthersupports the affirmation that the pore size of the membrane M1

ting times on the permeation properties of carbon/SAPO-34 composite membranes

ol m−2 Pa−1 s−1 × 10−8) Ideal selectivity

O2 N2 CH4 H2/CH4 CO2/CH4 H2/N2

4 0.9 0.28 64 50 208.7 0.3 0.10 97 87 333 12.6 7.15 9.5 8.8 5.48.4 5.5 3.2 9.6 12 5.61.0 12.4 10.2 4.5 5.0 3.7

G. Li et al. / Journal of Membrane Science 374 (2011) 83–92 87

with

i0

3

fvi

mottiHi2tpe

tpi7itd(tmiaastflsp5

Fig. 3. XRD patterns of the carbon/SAPO-34 membrane prepared

s close to the diameter of the CH4 molecule, which is around.38 nm.

.3.2. Permeation properties of binary gas mixturesGenerally, the permeation performance of the membrane as a

unction of temperature and pressure difference for the gas mixturearies from the performance for the single gas system due to thenfluence of the second components.

The permeances of H2 and CH4 for the equimolar H2/CH4 (1:1)ixture as a function of temperature under a pressure difference

f 0.1 M are illustrated in Fig. 8. The permeance and selectivity inhe mixture show a similar dependency on temperature to that ofhe single system. Both H2 and CH4 permeances increased withncreasing temperature, but CH4 permeance increased more than

2, behaving activation diffusion properties. The H2/CH4 selectiv-ty decreased from 112 to 78 with increasing temperature from98 K to 973 K. However, a lower permeance and higher selec-ivity were observed for the mixture system compared to theure gas H2/CH4 system, owing to the competitive permeationffect.

The permeance and selectivity of the membrane M1 at 298 K forhe equimolar H2/CH4 and CO2/CH4 mixtures as a function of theressure difference between two sides of membrane are presented

n Figs. 9 and 10, respectively. The H2 permeance decreased from.2 × 10−8 mol m−2 Pa−1 s−1 to 5.9 × 10−8 mol m−2 Pa−1 s−1 with an

ncreasing pressure difference from 0.1 MPa to 0.25 MPa, whereashe CH4 permeance slightly increased with the increasing pressureifference. Although both H2 and CH4 showed activated diffusionFigs. 7 and 8), this pressure dependency of permeance appearso be based on the surface diffusion mechanism since the per-

eance of less adsorbable gas (H2) decreased with an increasen the partial pressure of the more adsorbable gas (CH4). Thedsorbed concentration gradient of H2 did not increase linearlys the feed pressure increased, and the feed side approachedaturation. Thus, the permeance decreased. In contrast, because

he adsorption capacity of CH4 was further from saturation, theux increased sharply (more largely than linearly) with pres-ure, and thus the permeance slightly increased upon a drop inressure. The resultant H2/CH4 selectivity decreased from 112 to5 as the pressure difference increased from 0.1 M to 0.25 M. In

different SAPO-34 contents in the membrane precursor solution.

the case of the CO2/CH4 mixture separation, a similar pressuredependence of permeance with the H2/CH4 mixture was observed;the CO2 permeance decreased upon an increasing pressure dif-ference, while CH4 slightly increased with increasing pressure.As a result, the CO2/CH4 selectivity decreased from 258 to 156when the pressure difference increased from 0.1 MPa to 0.25 MPa.It is clear that the CO2/CH4 selectivity is greater than that ofthe H2/CH4, which is due to the diffusion of non-adsorbable gasCH4 being significantly hindered by the adsorbable component(CO2).

3.3.3. Comparison of permeation properties with carbon/zeolitecomposite membranes

The permeation properties of the carbon/SAPO-34 membranewere compared with those of various carbon/zeolite compositemembranes reported in our previous work and in the literature,as shown in Table 2, in order to discuss the effects of the molecu-lar sieve structure on the performance of the resultant compositemembrane. It is clear that the pore size of the molecular sieves hassignificant influence on the permeation performance of the com-posite membranes. For the small pore size of NaA (0.41 nm) andSAPO-34 (0.38 nm) molecular sieves, the incorporation of molec-ular sieves reduced the permeance of single gas H2, CO2, and N2but improved the ideal selectivities of H2/N2, H2/CH4, and H2/N2.The incorporation of zeolite with medium pore size or large poresize (ZSM-5, silicalite-1, L zeolite) largely improved the perme-ance of carbon/zeolite membranes with the selectivity almost keptthe same as that of the pure carbon membrane (except carbon/KYcomposite membrane) [34]. The reason for this opposite effect ofincorporating molecular sieves into the carbon matrix is discussedbelow.

4. Roles of SAPO-34 molecular sieves in the performance ofcarbon/SAPO-34 composite membranes

As discussed above, the average pore size of the M1 membraneis close to the CH4 molecule diameter of 0.38 nm, based on thedependence of H2 and CH4 permeance on the temperature for bothsingle and binary H2 and CH4 systems together with dependenceof the gas permeance on the kinetic diameter of the permeating

88 G. Li et al. / Journal of Membrane Science 374 (2011) 83–92

Fig. 4. SEM images of carbon/SAPO-34 mixed matrix membranes with different SAPO-34 contents in the membrane precursor solutions: M0 (a and b); M1 (c and d); M3 (eand f).

Fig. 5. SEM images of the carbon/SAPO-34 membrane M4 prepared by four coating cycles: left: surface view; right: cross section.

G. Li et al. / Journal of Membrane Science 374 (2011) 83–92 89

Table 2Performance of pure carbon membranes, carbon/SAPO-34 and carbon/zeolite composite membrane synthesized derived from various polymer and crystalline molecularsievea.

Sample Polymer/loadingb (wt.%) Permeance (mol m−2 Pa−1 s−1 × 10−8) Ideal selectivity

H2 CO2 O2 N2 CH4 H2/CH4 CO2/CH4 H2/N2 O2/N2 Ref.

PC PF 18 14 – 0.9 0.28 64 50 20 – This workC/SAPO-34 PF/2 10 8.7 – 0.3 0.10 97 87 33 – WorkPC PF 58 170 48 11 – – – 5.3 4.4C/NaA PF/2 3.6 13 1.2 0.8 – – – 43.9 14.6 [41]PC-600 PFA 0.994 0.495 – 0.029 0.015 68.54 34.17 – –PC-600/L PFA/3 10.02 5.72 – 0.28 0.16 62.63 35.75 – – [40]PIZ0-700c PAA 253 158 12 0.81 – – – 312 14.8PIZ1-/ZSM-5c PAA/4.76 3348 1548 275 25 – – – 135 12.6 [36]PC PF – 7.65 2.5 – – – – – 3.06C/silicalite-1 PF/8 – 158 30.8 – – – – – 5.13 [38]PC Matrimid 611 – – 10 61 – –CM-KY Matrimid 266 – – 2.15 124 – – [34]

PC: pure carbon matrix membrane.a Measured at 298 K and 0.1 MPa trans-membrane drop.b Loading is referred to the content of crystalline molecular sieve in the polymer precursor mixture.c Permeability [bar]: 1 bar = (1 × 10−10 cm3(STP)cm/cm2 s cmHg).

0.25 0.30 0.35 0.40 0.45 0.50 0.551E-10

1E-9

1E-8

1E-7

1E-6

SF6

CH4

N2

CO2

H2

Per

mea

nce

(mol

m2 P

a-1 s

)

Fd

mlar

Fo

200 300 400 500 600 700 800 900 10001E-10

1E-9

1E-8

1E-7

1E-6

20

40

60

80

10 0

12 0

14 0

160

Per

mea

nce

(m

ol m

-2 s

-1 P

a )-1

H2

CH4

S.F.

H2/C

H4 s

epar

atio

n se

lect

ivity

Kenetic Diameter (nm)

ig. 6. Permeances of single gases through the membrane M1 as a function of kineticiameter at 298 K and pressure drop of 0.1 Mpa.

olecules. The membrane M1 possesses a higher selectivity withower permeance than the carbon membrane for both the H2/CH4nd CO2/CH4 systems, suggesting the contribution of the incorpo-ation of the SAPO-34 particles improves the separation efficiency.

200 300 400 500 600 700 800 900 10001E-10

1E-9

1E-8

1E-7

1E-6

20

40

60

80

100

120

140

Per

mea

nce

(m

ol m

-2 s

-1 P

a )-1

Temperature (K)

H2

CH4

S.F.

H2/C

H4 Id

eal s

epar

atio

n se

lect

ivity

ig. 7. Permeances and separation selectivities of the membrane M1 as a functionf temperature for single gas system of H2 and CH4.

Temperature (K)

Fig. 8. Permeances and separation selectivities of the membrane M1 as a functionof temperature for equimolar H2/CH4 mixture.

To clarify the contribution of SAPO-34 particles on the performanceof M1, the TGA, N2 adsorption at 77 K and CO2/CH4 adsorptionmeasurements for the carbon/SAPO-34 composite material were

performed. The TGA curve in Fig. 11 revealed that the amountof SAPO-34 in the separating layer of the carbon/SAPO-34 com-posite membrane M1 was high, about 30 wt.%, despite the lowercontent of 2 wt.% in the precursor mixture of the membrane. The

0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.260

2

4

6

8

10

Pressure drop (MPa)

Per

mea

nce

108 (

mol

m2 P

a-1 s

-1)

CH4

10

H2

0

40

80

120

160

Sep

arat

ion

Sel

ectiv

ity (

H2/C

H4 )

Fig. 9. Permeances and separation selectivities of the membrane M1 as a functionof trans-membrane pressure drop for separation of equimolar H2/CH4 mixture.

90 G. Li et al. / Journal of Membrane Science 374 (2011) 83–92

0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.260

2

4

6

8

10

CH4

10

Pressure Drop (MPa)

Per

mea

nce

108 (

mol

m2 P

a-1 s

-1)

CO2

0

50

100

150

200

250

CO

2/C

H4 S

epar

atio

n se

lect

ivity

[-]

Fo

H30potoeontbcKractospts3t(

200 400 600 800 1000 120020

30

40

50

60

70

80

90

100

110

Wei

ght l

oss

(wt.%

)

ig. 10. Permeances and separation selectivities of the membrane M1 as a functionf trans-membrane pressure drop for equimolar CO2/CH4 mixture.

orvath–Kawazoe (HK) pore size distribution of the carbon/SAPO-4 composite material exhibited two peaks centered at about.48 nm and 0.54 nm, while that of the carbon material exhibitedeaks centered at about 0.57 nm (Fig. 12), suggesting the pore sizef the carbon/SAPO-34 membrane was reduced by the incorpora-ion of SAPO-34 into the carbon matrix. Note that the pore sizef the M1 determined from the gas permeation data was differ-nt from that by the N2 adsorption method, since overestimationf pore size of zeolites and microporous molecular sieves occursormally when using the H–K method [47]. Another reason forhe pore size deviation is probably because the pores determinedy N2 adsorption include some dead pores through which gasesannot permeate. Tin et al. also found that the incorporation ofY zeolite induced polymer chain rigidization and subsequentlyesulted in a denser structure of the carbon matrix near zeolitend, thus, a higher selectivity but a lower flux [34]. Therefore, itan be concluded that the improved gas separation selectivity andhe reduced permeance are due to the reduced effective pore sizef the carbon matrix. Another important reason for the improvedelectivity of the composite membrane might be due to the smallore window of SAPO-34, 0.38 nm, which could have improved

he selectivity due to both adsorption and diffusion effects ashown from the SAPO-34 amount in the resultant carbon/SAPO-4 composite membrane as high as 30 wt.%. It is worth notinghat for the zeolite with a medium pore size or large pore sizeZSM-5, silicalite-1, L zeolite), the incorporation of zeolite largely

Fig. 12. Adsorption isotherms (a) and pore size distribution (

Temperature (K)

Fig. 11. TGA curve of the carbon/SAPO-34 composite scrubbed from the tubularcarbon/SAPO-34 membrane (M1).

improved the permeance while the selectivity was kept the sameas that of the pure carbon membrane as mentioned above, indi-cating that zeolite plays a different role in the performance ofcarbon/zeolite membranes. In our previous work, the pore sizeof the carbon/L zeolite composites was found to be increased byincorporated L zeolite due to the larger pore size of the L zeo-lite, 0.7 nm [40], contributing the largely improved permeance ofCO2. Liu et al. also suggested that the largely improved perme-ances of H2, CO2 and O2 of the carbon/ZSM-5 composite membranewere due to the presence of the ZSM-5 zeolite, which served asa shorter diffusion path, rather than selectivity of the moleculesieves.

It is interesting that the CO2/CH4 selectivity of 258 for the mem-brane M1 of the binary equimolar CO2/CH4 mixture was muchhigher than that of 87 for pure gas system. However, its H2/CH4selectivity for the H2/CH4 mixture, the non-adsorbable system, isslightly bigger than for the pure gas system. Li et al. found thatthere was a slight decrease in the mobile H2 through the pureSAPO-34 membrane with a concomitant increase in the tardierCH4, N2, or CO, and the H2/CH4 selectivity of the pure SAPO-34 membrane in the binary mixture was observed to be smallerin the pure gas system [48]. This result means that the reduced

b) of carbon/SAPO-34 composite and carbon materials.

pore size of the carbon matrix through the incorporation of SAPO-34 contributes to the improved H2/CH4 selectivity of the M1.In the case of the CO2/CH4 system, the separation through thecomposite membrane was governed by more than the molecularsieving mechanism. Fig. 13 reveals that the adsorption capability

G. Li et al. / Journal of Membrane

Fo(

opCicptCi[tttbSc3c

5

wmScocstah2cC

tdttsmr

[

[

[

[

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[

ig. 13. Adsorption isotherms and adsorption ratio for single gas CO2, CH4 at 25 ◦Cn the SAPO-34 particles ( ), carbon/SAPO-34 composite ( ) carbon matrix

).

f carbon matrix material to CO2 was improved by the incor-oration of SAPO-34 particles while the adsorption capability toH4 was less affected. As a result, the ideal adsorption selectiv-

ty of CO2 over CH4 increased a little for the carbon/SAPO-34omposite compared to that of the pure carbon. Krishna et al.ointed out that the high CO2/CH4 selectivity in the mixturehrough the SAPO-34 membrane resulted from the preference ofO2 adsorption in the pore mouth of SAPO-34 and the decreas-

ng diffusivity of the CH4 by the adsorbed CO2 in the pore mouth49,50]. It can therefore be inferred that for the separation ofhe CO2/CH4 mixture, the SAPO-34 particles largely contributeso the improved selectivity for CO2/CH4 mixture not only throughhe reduction of the pore size of carbon matrix but also throughoth its preferential adsorption and diffusion characteristics. TheAPO-34 offers multiple roles in the improved selectivity of theomposite membrane by reducing the pore size of carbon/SAPO-4 zeolite and by its preferential adsorption and diffusionharacteristics.

. Conclusions

Thin (∼2 �m) carbon/SAPO-34 mixed matrix membranes inhich SAPO-34 particles of 200 nm were dispersed into the carbonatrix were successfully prepared on porous tubular supports. The

APO-34 content in the membrane precursor mixture and coatingycle showed a significant impact on the separation performancef a carbon/SAPO-34 composite membrane. The carbon/SAPO-34omposite membrane (M1) demonstrated the best selectivity foreparation of H2/CH4 and CO2/CH4 gas systems when SAPO-34 con-ent was optimized at 2 wt.% in the membrane precursor mixturend two coating cycles were used. The membrane M1 exhibitedigh ideal H2/CH4, CO2/CH4, and H2/SF6 selectivities of 97, 87 and50, respectively, with reduced permeance compared to the purearbon membrane, which shows ideal selectivities for H2/CH4 andO2/CH4 of 64 and 40, respectively.

The selectivity of membrane M1 for the equimolar CO2/CH4 mix-ure was as high as 258 at room temperature and at a pressureifference of 0.1 MPa, which was significantly larger than that for

he single gas system. The selectivity for the equimolar H2/CH4 mix-ure was 112, which was higher than that of 98 for the pure gasystem under same conditions. The H2 and CO2 permeances in theixture were relatively high, 7.2 and 7.0 × 10−8 mol m−2 Pa−1 s−1,

espectively, smaller than that in the single gas system. The large

[

[

Science 374 (2011) 83–92 91

enhancement in the CO2/CH4 selectivity was attributed to theblocked permeation of nonadsorbable gas by the adsorbed CO2component. The selectivity decreases from 112 to 78 upon increas-ing temperature from 298 K to 973 K because the increase of H2permeance is smaller than that of CH4 upon the temperatureincrease. For the H2/CH4 and CO2/CH4 mixtures, both the perme-ance of CO2 and H2 decreased with increasing pressure difference,resulting in decrease in the selectivity. It was suggested that theSAPO-34 provides multiple roles in the improved selectivity of thecomposite membrane by reducing the pore size of carbon/SAPO-34 membrane and by its preferential adsorption and diffusioncharacteristics. This work demonstrates a novel membrane can-didate for small gas separation, especially for H2 purification andseparation at high temperatures. The application of the preparedcomposite membrane for the dehydrogenation reaction of hydro-carbons, such as the conversion of methane into benzene undernonoxidative conditions in a membrane reactor, is currently underinvestigation.

Acknowledgements

We greatly appreciate financial support for this project fromthe National Key Technology R&D Program (2006BAE02B05), theNational Natural Science Foundation of China (21076029) andthe Fundamental Research Funds for the Central Universities(DUT10ZD207).

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