Separation and Purification Technology 122 (2014) 431–439

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Separation and Purification Technology

journal homepage: www.elsevier .com/ locate /seppur

Perfluorooctanol-based liquid membranes for H2/O2 separation

1383-5866/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.seppur.2013.11.019

⇑ Corresponding author at: Department of Chemical System Engineering, TheUniversity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. Tel./fax: +8103 5841 7300.

E-mail address: oyama@vt.edu (S.T. Oyama).

Pornravee Leelachaikul a, Bernardo Castro-Dominguez a, Atsushi Takagaki a, Takashi Sugawara a,Ryuji Kikuchi a, S. Ted Oyama a,b,⇑a Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japanb Environmental Catalysis and Nanomaterials Laboratory, Department of Chemical Engineering, Virginia Polytechnic Institute & State University, Blacksburg, VA 24061-0211,United States

a r t i c l e i n f o a b s t r a c t

Article history:Received 10 September 2013Received in revised form 15 October 2013Accepted 18 November 2013Available online 23 November 2013

Keywords:PerfluorocarbonPerfluorooctanolSupported liquid membraneH2/O2 separationTime-lag measurement

Hydrogen is an important substance in the chemical industry which can be generated by different meth-ods, including photocatalytic water splitting. Photocatalysis is an environmentally friendly technique togenerate H2, but produces it in a mixture with O2. This paper reports the use of a perfluorooctanol-basedliquid membrane to separate the gases. The membrane shows an excellent separation performance atroom temperature with an average H2/O2 selectivity of 100 with a H2 flux of 2.9 � 10�10 mol m�2 s�1 Pa�1

(1030 barrers) for a 50/50 mixed gas feed. The separation was tested at various operating conditions, andit was found that higher temperatures and flow rates did not affect considerably the performance of themembrane, but decreased its stability. The diffusivity and solubility of H2 and O2 in the perfluorooctanolmembrane were measured by a time-lag technique, resulting in diffusivities of 1.4 � 10�4 and3.2 � 10�5 cm2 s�1 and solubilities of 4.4 � 10�4 and 5.4 � 10�4 mol m�3 Pa�1 for H2 and O2, respectively.The membrane was tolerant of water, but increase in hydrogen partial pressure decreased permeance.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Membrane technology is emerging as a non-energy intensivemethod capable of competing with conventional gas separationtechnologies. The cost for gas separation through membranes isfavorable when compared to cryogenic distillation, absorptionand pressure swing adsorption [1] and could even replace theseconventional processes for some separations. Membrane processesdo not require a phase transition of the gas resulting in lower en-ergy consumption [2].

The permeability of the membrane is a feature that requiresoptimization because it is inversely proportional to the operatingcosts. The optimization can be achieved by developing new mate-rials and structures, and improving the thickness of the membrane,membrane configuration and system design [2]. Consequently, dif-ferent types of membranes have been developed such as zeolitic[3], polymeric [4], metallic [5] and others [6]. Among the differenttypes of membranes, supported liquid membranes (SLMs) haveemerged as excellent candidates to achieve the separation of gases.

Supported liquid membranes (SLMs) are thin liquid barriersimpregnated into porous supports (Fig. 1) which favor the selectivepassage of specific molecules. These SLMs have achieved higher

fluxes than solid membranes since gases have higher solubilityand diffusivity in liquids than in solid materials [7,8]. The separa-tion occurs based on the difference in solubility and diffusivity ofthe solute gas in the liquid and its transport mechanism has beenproposed as follows [9]:

(1) Absorption of the gas into the liquid at the gas and liquidinterface.

(2) Diffusion of the gas through the liquid using the concentra-tion gradient created by the feed and permeate as the driv-ing force.

(3) Desorption of the gas to the permeate stream at the liquid–gas boundary layer.

The major problem in SLMs is the loss of the liquid from thepores which may be caused by evaporation, dissolution and flow(pressure difference can push the liquid out of the porous support)[10]. Ionic liquids (ILs) which are liquid salts below 100 �C [11]have been used as a liquid medium since they show uniquephysical properties such as negligible vapor pressure and goodthermal stability [11,12]. The application of ILs as SLMs has beendirected mainly for CO2 purification since this gas presents highersolubility than most other gases [13]. Several authors havereported the use of ILs. For example, Baltus et al. [14] utilized1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide[C4mim][Tf2N] supported on a porous anodic alumina membraneachieving a CO2/N2 ideal selectivity of 130 with a CO2 permeance

Fig. 1. Supported liquid membrane (SLM).

432 P. Leelachaikul et al. / Separation and Purification Technology 122 (2014) 431–439

of 4.0 � 10�10 mol m�2 s�1 Pa�1. Scovazzo et al. [15] reported theuse of imidazolium ionic liquids immobilized in a hydrophilicpolyethersulfone (PES) support to separate CO2/CH4 and CO2/N2

at 30 �C. The CO2/CH4 and CO2/N2 selectivities were obtained inthe range of 10–27 and 12–21, respectively with a CO2 permeancerange of 7.9 � 10�10 to 1.9 � 10�9 mol m�2 s�1 Pa�1. Close et al.[16] tested the CO2/N2 separation using several ILs impregnatedon a nanoporous alumina support at 23 �C displaying CO2/N2 selec-tivities between 12 and 21 with CO2 permeances between7.7 � 10�9 and 1.8 � 10�8 mol m�2 s�1 Pa�1. The separation ofH2/O2 has been rarely reported in the literature. Neves et al. [17]utilized 1-decyl-3-methylimidazolium tetrafluoroborate [C10MIM][BF4] supported on a hydrophobic polyvinylidene fluoride (PVDF)to separate H2, O2, N2, CH4 and CO2 at 30 �C achieving CO2/N2,CO2/CH4 and H2/O2 selectivities of �22, 69 and 1.3, respectivelywith CO2 and H2 permeabilities of 720 and 110 barrers(1.9 � 10�9 and 2.9 � 10�10 mol m�2 s�1 Pa�1 with an estimatedthickness of 125 lm), respectively.

This work focuses on the application of SLMs to separate H2 andO2 generated from photocatalytic water splitting. Photocatalyticwater splitting is a clean method that produces H2 by decomposingwater into H2 and O2 by means of solar energy in the presence of aphotocatalyst. For this application, membranes should maintaintheir performance in the presence of water. However, the proper-ties of ILs such as solubility and viscosity change significantly inhumid conditions [18]. Therefore, this paper reports the use of aperfluorocarbon (PFC) based SLM using 1H, 1H, 2H, 2H perfluoro-1-octanol (PFO) as the liquid barrier impregnated in a porousalumina support. Previously, the use of a perfluorocarbon (perflu-orotributylamine) was reported for the separation of O2/N2,H2/N2 [19] and H2/O2 [24] showing good separation properties.

Perfluorocarbons are hydrocarbon in which hydrogen has beenpartially or totally replaced by fluorine atoms. PFCs were chosen tofabricate SLMs since they have unique properties including hydro-phobicity, low vapor pressure, high solubility for gases andexcellent chemical and thermal stability [20,21]. The permeanceand selectivity of the PFO membrane were studied at different con-ditions such as temperature and feed flow rate for single andmixed gases, partial pressure and different levels of humidity in or-der to measure its capability. The diffusivity and solubility of H2

and O2 in the PFO membrane were analyzed by means of a time-lag technique to gain insight in the gas transport mechanism ofthe membrane.

Table 1Physical properties of 1H, 1H, 2H, 2H perfluoro-1-octanol (PFO) at 25 �C.

Structure Molar mass (g mol�1) Vapor pressur

364 51

a Viscosity of PFO at 28 �C.

2. Experimental

2.1. Materials

Porous alumina supports with ID of 7 mm, OD of 10 mm andpore diameters of 60 nm were obtained from the Noritake Corpo-ration. The PFC, 1H, 1H, 2H, 2H perfluoro-1-octanol (PFO) wasbrought from Wako Pure Chemical Industries, Ltd. with a purityof >96%. The physical properties of PFO are shown in Table 1. PureH2, O2 and Ar gases (purity > 99.9%) were obtained from ToatsuYamazaki Co., Ltd.

2.2. Support characterization

The support was characterized by imaging the structure withscanning electron microscopy (SEM) and determining the porosity.The porosity of the support was analyzed by two methods: (1) SEMimage analysis and (2) a water saturation technique. The determi-nation of the porosity by the SEM photos consisted of a grayscaleanalysis. The SEM images are grayscale photos that show particlesand pores at different shade intensities. Particles, which reflect theelectron beam of the SEM, tend to be lighter than the pores. There-fore, by determining which gray intensity corresponds to either apore or a particle, it is possible to estimate the porosity. To estab-lish this pore–particle classification the SEM picture was trans-formed into a black and white image, where black representedthe pores and white denoted the particles. The ratio of the areacovered by the black color to the total area is the porosity of thesupport. The classification of the brightest point considered aspores required the determination of a segregation point (SP). TheSP was adjusted until the black and white image showed clear par-ticle and pore regions. A MATLAB program was used to calculatethe number of pixels that composed each gray tone and to obtaina histogram of the pixel distribution at each gray level. The SPwas used to specify the fraction of pixels in the histogram that cor-responded to pores and particles, allowing the calculation of theporosity of the support. The program listing is in the supplemen-tary information file.

The water saturation method consisted of measuring the degreeof water uptake within the pores of the support. In this techniquethe support was dried at 120 �C overnight to remove any water inthe pores. After measuring the dry weight of the support, it wasimmersed in water for 4 h, allowing the water to saturate thepores. The wetted support was weighed and the difference inweight before and after wetting was calculated to obtain the wateruptake of the support. The volume of water that the support heldwas proportional to the pore volume.

2.3. Membrane synthesis

The porous alumina support was cut to 30 mm pieces andjoined to dense alumina tubes at both ends with glass glaze byheating at a rate of 5 �C min�1 to 1000 �C and held for 10 min.The connection was done in order to facilitate the manipulation

e (Pa) Density (g cm�3) Surface tension (dyne cm�1) Viscosity (mPa s)a

1.65 16.4 14.3

P. Leelachaikul et al. / Separation and Purification Technology 122 (2014) 431–439 433

of the membrane. The support was dried overnight at 200 �C beforethe impregnation of the liquid. The PFO was deposited on the sup-port immediately after drying in order to avoid water condensationinside the pores. The PFO was impregnated into the outer surfaceof the alumina support with a pipette. Several layers were applieduntil a visible liquid film formed. Afterwards, the support wasweighed to determine the amount of liquid deposited in thesupport.

2.4. Permeance measurement

The permeance measurements were carried out in a cylindricalapparatus surrounded by an electrical furnace (Fig. 2). The temper-ature of the system was measured by a thermocouple located onthe outside of the cylindrical device, while the membrane was lo-cated at the center. The gases (O2, H2 and Ar) were introducedthrough the center of the membrane, whereas permeated gaseswere swept with Ar on the outer side of the tubular membrane.The composition of the gases was analyzed by a gas chromato-graph (Shimadzu, GC-8APT) fitted with a thermal conductivitydetector (TCD).

To determine the permeance (Pi of gas i across the membraneEq. (1) was used:

Pi ¼Fi

ADPið1Þ

Here, Fi is the molar flow rate of gas i (mol s�1), A the area of themembrane (m2) and DPi the partial pressure difference of gas i be-tween the feed and the permeate (Pa). The selectivity (aij) of gas iwith respect to j was obtained from the following equation:

aij ¼Pi

Pj

ð2Þ

Note that Eqs. (1) and (2) neglect any axial change in composi-tion of the feed gas. This is a reasonable assumption since the feedflow rate was set high enough to maintain the ratio of feed flow tothe permeance flow (stage cut) to be less than 1%.

Single and mixed gas permeance measurements wereperformed. The single gas permeation test consisted of introducingH2 or O2 diluted with Ar at a molar ratio of 1.0 through the centerof the tubular membrane as the feed gas. The dilution was made tokeep the partial pressure of H2 and O2 the same as for the mixedgas experiments. The mixed gas permeation test consisted of

Fig. 2. Schematic of the permeance test unit.

premixing and introducing H2 and O2 to the membrane at a molarratio of 1.0. The Ar flow rate on the permeate side was always keptat the same flow as the feed flow rate for all conditions and wassufficient to effectively sweep all permeated gases. The flow ratesutilized were chosen to avoid external mass transfer limitationsin the measurements. The outlet stream of the feed gas was dilutedwith Ar before venting to prevent the handling of an explosive gasmixture.

The base condition was operated at 40 �C with a total flow rateof 50 cm3 min�1. The partial pressure of species i in the feed was50.7 kPa while at the permeate side was negligible by use of sweepgas. Note that the total pressure used in all experiments was atmo-spheric pressure. This pressure was chosen because it is dangerousto pressurize H2 and O2 mixtures. The experiment was repeated atdifferent temperatures (30, 40 and 50 �C) and gas flow rates (25, 50and 100 cm3 min�1) to study the effect of these operating condi-tions. The temperature of 40 �C was chosen to be the base temper-ature since photocatalytic water splitting requires the separationof H2/O2 to be at close to room temperature. In addition, due tochanges in weather conditions throughout the year, for better con-trol, the measurement temperature was chosen to be slightlyabove room temperature. A partial pressure experiment on themixed gas was performed by diluting the H2–O2 mix with Ar.The partial pressure was examined at 25%, 50% and 75% Ar dilutionwhich corresponds to a H2–O2 mixture partial pressure of 76.0,50.7 and 25.3 kPa, respectively.

For the analysis at humid conditions, water vapor was gener-ated in a bubbler filled with distilled water. Pure H2 was used asthe vapor carrier (Fig. 2). The water content of the vapor carrierwas estimated by the water vapor pressure in the bubbler at a gi-ven temperature. A heater was used to control the temperature inthe bubbler and in the lines. The gases O2 and H2 along with waterwere premixed before being introduced to the membrane. The per-meate stream was sent to a water trap to condense any vapor be-fore entering the GC. The experiment was done at a humidity of5.0%, 8.4% and 14.1% with a total feed flow rate of 50 cm3 min�1

and 40 �C. A summary of operation condition used in the experi-ments is shown in Table 2.

External mass transfer resistances at the boundaries of themembrane can affect its performance. Therefore, the different flowrates used in the experiments were analyzed for mass transferresistances. At steady state, the rates of mass transfer (Ni) throughall resistances are equal and therefore can be expressed as:

Ni ¼DPi

Hkrþ 1

Pmþ H

kp

ð3Þ

where kf and kp are the mass transfer resistances at the feed and thepermeate, respectively, ðPmÞ the permeance of the membrane and HHenry’s law constant.

Since the apparent permeance ðPappÞ obtained experimentally isgiven by:

Ni ¼ PappDPi ð4Þ

The permeance of the membrane which comes from the secondresistance term of Eq. (3) can be expressed as:

Pm ¼1

Papp

� Hkr� H

kp

" #�1

ð5Þ

The mass transfer resistances are estimated from the Sherwoodnumber defined as follows:

Sh ¼ dHkDab¼ 1:86Re0:33Sc0:33 dH

L

� �0:33

ð6Þ

where Re is the Reynolds number given by:

Table 2Summary of operating conditions.a

Nomenclature (cm3 min�1) Temp (�C) Feed to inside (cm3 min�1) Feed to outside (cm3 min�1)

H2 or O2 Ar Ar

Single gas measurement25 40 12.5 12.5 2550 30 25 25 5050 40 25 25 5050 50 25 25 50100 40 50 50 100

Feed to inside (cm3 min�1) Feed to outside (cm3 min�1)

H2 O2 Ar

Mixed gas measurement25 40 12.5 12.5 2550 30 25 25 5050 40 25 25 5050 50 25 25 50100 40 50 50 100

Dilution (%) Feed to inside (cm3 min�1) Feed to outside (cm3 min�1)

H2 O2 Ar Ar

Partial pressure analysis at 40 �C25 18.8 18.8 12.5 5050 12.5 12.5 25.0 5075 6.25 6.25 37.5 50

Humidity (%) Feed to inside (cm3 min�1) Feed to outside (cm3 min�1)

H2 O2 H2O Ar

Humid conditions at 40 �C5 23.8 23.8 2.5 508.4 22.9 22.9 4.2 5014.1 21.5 21.5 7.1 50

a Flow rates were measured at room temperature.

434 P. Leelachaikul et al. / Separation and Purification Technology 122 (2014) 431–439

Re ¼ dHvql ð7Þ

where q is the density and l the viscosity of the fluid, m thesuperficial velocity L the length of the membrane and dH thehydraulic diameter given by the diameter of the pipe in the feedand by dH = Do � Di given the formation of an annulus. Do and Di

are the outer (16 mm) and inner diameters (10 mm) of the annulusrespectively. The properties of the fluid were calculated from puregas properties shown in Table 3.

The Schmidt’s number used to estimate the Sherwood numberwas defined by:

Sc ¼ lqDab

ð8Þ

where the diffusion coefficient Dab was estimated as follows:

Dab ¼ 0:0018583

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiT3 1

Maþ 1

Mb

� �s1

pr2uabð9Þ

where T is the temperature, Ma and Mb the molecular weight of mol-ecules a and b, p the pressure (1 atm), r is the average value be-tween H2 or O2 and Ar and uab are Lennard Jones parametersbetween Ar and H2 or O2.

Table 3Fluid properties used for the estimation of the mass transfer resistances.

Density at 20 �C (kg m�3) Viscosity at 20 �C (Pa s)

H2 0.086 8.7 � 10�6

O2 1.309 2.0 � 10�5

Ar 1.635 2.3 � 10�5

2.5. Diffusivity and solubility measurement

A time-lag technique was used to estimate the diffusivity (D) andsolubility (S) of the gases in the PFO membrane. The method was firstdescribed by Daynes who calculated the diffusivity of several gasesthrough a rubber membrane [23]. The time-lag method models thetransient mass transfer of permeated species across a material. Gasspecies start permeating through the membrane and continuouslyincrease until reaching steady-state. At steady-state the sum of thespecies permeated through a hollow cylinder (Q) as a function oftime (t) can be described by the following equation [24]:

Q ¼ 2peLC0D

ln rcr0

� � � t �r2

0 � r2C þ ðr2

0 þ r2CÞln rc

r0

� �4Dln rc

r0

� �0@

1A ð10Þ

Here, e is the porosity of the support, L the length of the membrane(m), C0 the concentration of the gas in a liquid at the liquid interfaceof the feed side (mol m�3), rc the diffusion path (m), r0 the inner ra-dius (m). The diffusion path (rc) is calculated from Eq. (11), where r1

is the outer radius and s tortuosity of the support:

rc ¼ s � r1 ð11Þ

The time-lag (tlag) represents the time required for the system to re-spond. The diffusivity can be computed from this value (Eq. (12))

Molecular weight (g mol�1) r (Å) [35] uab [22]

2.01 2.915 0.551831.99 3.433 0.502739.95 3.432

P. Leelachaikul et al. / Separation and Purification Technology 122 (2014) 431–439 435

while the steady-state slope (m1) can be used to find the gas con-centration in the liquid (Eq. (13)). The solubility can be estimatedfrom Eq. (14).

tlag ¼r2

0 � r2C þ ðr2

0 þ r2CÞln rc

r0

� �4Dln rc

r0

� � ð12Þ

m1 ¼2peLC0D

ln rcr0

� � ð13Þ

C0 ¼ SP ð14Þ

Here, P is the partial pressure of the gas (Pa).The execution of the time-lag technique requires the purge of

all gases from the membrane. Hence, Ar was introduced on the in-side and the outside of the membrane before H2 or O2 was fed intothe membrane. The permeate stream was connected directly to athermal conductivity detector for measuring the change in compo-sition in real time. Time delays due to the experimental set upwere cautiously calibrated. The liquid loss during the experimentalprocedure was also considered. The estimation of the liquid lossconsisted of interpolating the liquid weight in the support betweenthe time of injection and the end of the measurement.

3. Results and discussion

3.1. Support characterization

A SEM image of a cross-section of the membrane (Fig. 3) showsthat the support is composed of two layers: a coarse section withlarge pores and a fine section with small pores. The coarse sectionwas composed of a-Al2O3 particles of �2 lm in diameter andcomprised the main part of the support. The fine section was com-posed of a-Al2O3 particles of�200 nm and covered the coarse layerwith a thickness of �30 lm on the inner side of the tubular sup-port. A SEM grayscale analysis showed a support porosity of 0.32(A.1 Supplementary Information).

A water saturation method showed that the support had awater uptake of 0.52 cm3. Since the total volume of the supportis 1.38 cm3, the porosity of the membrane was estimated to be0.38. The average porosity of the support was hence �0.35.

Fig. 3. Cross-sectional SEM image of the support.

3.2. Permeance tests

The gas flow rates used in the inner and outer sides of the mem-branes resulted in Reynolds numbers ranging between 4 and 70,suggesting a laminar flow pattern along the membrane. The diffu-sion coefficients for H2 and O2 with argon were calculated to be1.21 � 10�4 and 3.72 � 10�5 m2 s�1, respectively. The resistanceswere found to be small in both sides of the membrane but increasewith flow (Table 4).

The apparent or measured permeance of the membrane wascompared with the real permeance of the membrane. The realpermeance accounts for external mass transfer resistances. How-ever, the difference between the apparent and real permeancewas found to be so small that the error introduced in the calcula-tion was in the order of 10�9–10�11%. Table 5 shows the errors esti-mated when comparing the two permeances with an error %defined as:

% error ¼ Pm � Papp

Papp

!� 100 ð15Þ

From these results, it is possible to conclude that the experi-mental measurements do not have external mass transfer limita-tions and consequently the apparent permeance corresponds tothe real permeance of the membrane.

The permeances of H2 and O2 were measured as single gases(Fig. 4). In the course of the measurements the PFC evaporatedand this was registered as a weight loss. The weight loss of thefluorocarbon was observed in all experimental conditions. The per-meance of the gases is displayed as a function of the fractionalweight loss of the liquid in the support which is related to thethickness of the liquid. At the beginning of the test, the supportwas oversaturated with liquid, causing the gas flux to be diminu-tive, since the liquid layers acted as a resistance to the gas flux. Pro-gressively, the liquid in the support was lost and the supportedliquid layer became thinner resulting in higher gas fluxes throughthe membrane. Further studies, such as SEM images of the PFO inthe membrane, are needed to determine the distribution of theliquid in the pores of the alumina support.

The existence of liquid excess was confirmed by wiping off theexcess liquid from the surface of the support at the beginning of aseparate experiment. Membranes without excess liquid showedhigher permeances than those with excess liquid at initial stages.The estimated maximum liquid amount in the support with aporosity of 0.35 was 0.71 g. In comparison, the amount of liquidloaded on the support in all experiments was determined byweighing to be 0.76 ± 0.04 g. Considering the excess liquid to beuniformly distributed on the surface of the support, the excess li-quid layer was estimated to have a thickness of 0.02 mm.

At a fractional weight loss of 0.15 (Fig. 4) the H2 permeance in-creased dramatically from the 10�13 level to 10�10 mol m�2 s�1 Pa�1

while the O2 permeance increased only gradually. This suggests thatthe transport mechanism of H2 is limited by solubility when themembrane is oversaturated with excess liquid while O2 is limitedby diffusion, so is not so dependent on the thickness of the liquidlayer. At a fractional weight loss of liquid higher than 0.2, no excessliquid was present, and the H2 permeance reached a plateau region.In this region the gas separation displayed an average H2/O2 selectiv-ity of 50 with a H2 permeance of 2.2 � 10�10 mol m�2 s�1 Pa�1 (700barrers) and an O2 permance of 4.4 � 10�12 mol m�2 s�1 Pa�1. Theresults show that the performance of the PFO membrane is excel-lent. For instance, the Robeson’s upper limit [25] for this separationin polymeric membranes has permeabilities of 7400 to 1 barrerswith selectivities of 2 to 100, respectively. In the field of ionic liquidmembranes, Neves et al. [17] reported a H2/O2 selectivity of 1.3 witha H2 permeability of 110 barrers. The superior performance of

Table 4Resistances present at the feed and the permeate sides of the membrane at different flow rates.

Flow rate (cm3 min�1) 25 50 100 25 50 100

Mass transfer coefficient at the feed, kf (m s�1) Mass transfer coefficient at permeate, kp (m s�1)

H2 0.017 0.021 0.027 0.025 0.032 0.039O2 0.008 0.009 0.012 0.011 0.014 0.018

Table 5Error between the apparent permeance and the permeance of the membrane.

Flow rates (cm3 min�1) 25 50 100

1.5 � 10�9% 1.2 � 10�9% 9.9 � 10�10%3.4 � 10�11% 2.7 � 10�11% 2.1 � 10�11%

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.010-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

Perm

eanc

e / m

ol m

-2s-

1 Pa-

1

Fractional weight loss of liquid

H2 O2

25 cm3 min-1

50 cm3 min-1

100 cm3 min-1

(a)

(b)0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

Perm

eanc

e / m

ol m

-2s-1

Pa-1

Fractional weight loss of liquid

H2 O2

50 OC 40 OC 30 OC

Fig. 4. H2/O2 permeance in single gas measurement at (a) different flow rates at40 �C (b) different temperatures at 50 cm3 min�1.

436 P. Leelachaikul et al. / Separation and Purification Technology 122 (2014) 431–439

perfluorocarbons has recently been reported for perfluorotributyl-amine with a H2/O2 selectivity of 40 and a H2 permeance of3.0 � 10�10 mol m�2 s�1 Pa�1 and an O2 permeance of 7.5 � 10�12 -mol m�2 s�1 Pa�1 at the same conditions (Single gases, 40 �C and50 cm3 min�1) [26].

Several conditions were investigated for the single gas perme-ance measurements. Increases in flow rate slightly increased thepermeance of the gases, and this could be the result of a small de-gree external mass transfer limitation. Temperature increased thepermeance of both gases, likely as a result of an increase in diffu-sivity, since solubility decreases with increasing temperature. Kre-ulen et al. [27] reported that the diffusivities of gases throughliquids increase as the liquid viscosities decrease. Therefore, thediffusivity of gases through the PFO membrane is expected to in-crease at higher temperatures. Iiconich et al. [13] reported that

high temperatures increased the permeability of CO2 and He in1-n-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, and explained that this was due to diffusional pathwaysbecoming less restricted. The permeance through a liquid mem-brane is related to the gas solubility, diffusivity and thickness ofthe membrane (l):

P ¼ DSl

ð16Þ

The behavior of O2 (Fig. 4b) revealed the existence of two plateauregions, which can be explained by considering the structure ofthe support. At the beginning the liquid is contained in the coarsea-Al2O3 and fine a-Al2O3 layers where the first steady flux appears.The appearance of a second plateau can be the result of liquiddepletion from the larger pore coarse a-Al2O3 layer. The liquidevaporates from the coarse a-Al2O3 first since the capillary forceholding the liquid is lower than in the fine a-Al2O3.

The 50/50 H2/O2 mixed gas measurement exhibits an averageH2/O2 selectivity of 100 with a H2 permeance of 2.9 � 10�10 -mol m�2 s�1 Pa (1030 barrers) at 50 cm3 min�1 and 40 �C. The H2/O2 separation improved when both H2 and O2 were mixed and intro-duced into the membrane. The average H2 permeance increasedfrom 2.2 � 10�10 mol m�2 s�1 Pa (700 barrers) in the single gas testto 2.9 � 10�10 mol m�2 s�1 Pa (1,030 barrers) for mixed gas whilethe O2 permeance decreased from 4.8 � 10�12 mol m�2 s�1 Pa (15barrers) for single gas to 3.0 � 10�12 mol m�2 s�1 Pa (11 barrers)in mixed gas. This phenomenon could be explained by consideringthe possible interactions between H2 and O2. The structure of the li-quid can be considered to have pockets or solubility sites that holdthe permeating species. In mixed gas permeation the more mobileH2 species preferentially occupy the sites and block the diffusionof O2 through the membrane. In this way H2 seems to inhibit thepermeation of O2 leading to higher H2 selectivities.

The results of the mixed gas test showed that higher flow ratesand temperatures produced higher permeabilities and H2 selectiv-ities (Table 6). The reason is that high temperatures promote fasterdiffusion through the liquid layer resulting in higher permeabili-ties. At the same time, high temperatures decrease solubility.Consequently, temperature promotes diffusivity but reduces solu-bility, and O2 appears to be negatively affected by this solubility–diffusivity tradeoff. The results in both, single and mixed gasmeasurements show that an increase in flow rate or temperatureenhanced the gas flux. The lifetime of the membrane, which isthe period where the membrane can operate, was drastically influ-enced by temperature and flow rate (Table 6).

A partial pressure analysis was performed to understand theinfluence of solubility on the transport mechanism. In the mea-surements the partial pressure of the feed gas containing a 50/50H2/O2 mix was diluted with Ar to 25.3, 50.7 and 76.0 kPa whilekeeping the composition constant. There was a slight decrease inpermeance of both gases with increasing partial pressure, exceptat the highest pressure (Fig. 5). A maximum H2/O2 selectivity of330 with a H2 permeance of 2.6 � 10�9 mol m�2 s�1 Pa�1 (12,000barrers) was achieved when operating at a total H2/O2 partial pres-sure of 50.7 kPa. The decrease in permeance at higher partial pres-sures can be the result of high occupation of solubility sites whichlimit the hopping rate of molecules. This suggests that there is anoptimum partial pressure in the PFO liquid membrane operation.

Table 6H2 permeance, average H2/O2 selectivity and membrane lifetime at different conditions for mixed gas measurement.

Flow rate (cm3 min�1) Temperature (�C) H2 permeance (mol m�2 s�1 Pa�1) (barrera) Average H2/O2 selectivity Membrane life time (hr)

25 40 2.0 � 10�10 (900) 65 7.850 40 2.9 � 10�10 (1030) 100 8

100 40 3.9 � 10�10 (1100) 130 3.850 30 2.2 � 10�10 (880) 70 1050 50 3.7 � 10�10 (1250) 120 6

a The membrane thickness used to estimate permeability depended on the amount of liquid in the support.

0 20 40 60 80 10010-13

10-12

10-11

10-10

10-9

10-8

10-7

H2

O2

selectivity

Total H2-O2 partial pressure / kPa

Perm

eanc

e / m

ol m

-2s-

1Pa

-1

1

10

100

1000

Max

imum

H2/O

2 se

lect

ivity

Fig. 5. H2 and O2 permeance and maximum H2/O2 selectivity at different partialpressures.

25 50 75 1002

4

6

8

10

12

14

1625 50 75 100

TemperatureFlow rate

Temperature / oC

Rat

e of

liqu

id lo

ss /

10-4 g

min

-1

Flow rate / cm3 min-1

Fig. 6. Rate of liquid loss at different conditions.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.010-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

Perm

eanc

e / m

ol m

-2s-

1Pa

-1

Fractional weight loss of the liquid

H2 O2 0% 5% 8% 14%

Fig. 7. 50/50 H2/O2 mixed gas at different humid conditions.

P. Leelachaikul et al. / Separation and Purification Technology 122 (2014) 431–439 437

Increasing flow rates and temperatures reduced the lifespan ofthe membrane (Fig. 6). Both higher flow rates and temperature in-creased the liquid loss rate from the support. These effects can bereduced by using a support with smaller pore size since smallpores reduce the vapor pressure of the liquid due to the formationof a more curved interphase as expressed by the Kelvin equation:

PV

P0V

¼ e�rM=qRTdP ð17Þ

where r is the surface tension (dyne cm�1), M the liquid molarmass, q the liquid density (g cm�3), R the ideal gas constant, T thetemperature (K) and dP the pore diameter (m). Although, the smal-ler the pores size the higher the stability, the reduction of the poresize is limited by physical constraints. It is estimated that PFO has amolecular diameter of 0.9 nm which is the smallest pore size thatthe support can have to fit a molecule of PFO. However, to maintainthe physical properties of the liquid, it is required to have a poresize of �5 to 10 times the molecular diameter.

Studies on the stability of the membrane at humid conditionswere performed using the 50/50 H2/O2 mixed gas (Fig. 7). The re-sults showed that the PFO-based liquid membrane was not signif-icantly influenced by the presence of water vapor since the PFOcompound is immiscible in water [28]. However, the average H2/O2 selectivity decreased from 100 to 80, and the lifetime of themembrane was reduced (Table 7). At humid conditions watercould interact with PFO and reduce its viscosity. A reduction in vis-cosity affects the adherence of the liquid to the support causing thelifetime of the membrane to shorten. Fortunato et al. [29] reportedthe reduction of viscosity in ionic liquids due to the presence ofwater.

The stability of liquid membranes is a problem which limitstheir application in industry. Teramoto et al. [30] suggested thatthe liquid thickness of the membrane can be maintained by recy-cling the liquid lost from the support. The liquid can be recoveredfrom the exit of the permeate and feed streams and can then be

added directly to the feed. This concept was demonstrated by Atch-ariyawut et al. [31] who used a gas–liquid membrane contactor toseparate CO2 from CH4 by continuously pumping a liquid in thelumen of a hollow fiber membrane. The membrane was able tooperate for 15 days without changes in the CO2 flux. Hu et al.[32] employed an emulsion liquid membrane with a hollow fibercontactor to extract copper. The system was reported to have nostability issues. Therefore, one way to overcome the instability ofthe PFO liquid membrane is to apply the concept of a gas–liquidmembrane contactor.

3.3. Diffusivity and solubility in the membrane

Time-lag experiments were performed to determine the solu-bility and diffusivity of H2 and O2 across the perfluorooctanolliquid. Measurements were made at different H2 and O2 flow rates

Table 7Lifetime of the membrane at different humid conditions.

% Humidity Lifetime (h)

0 >85.0 6.58.4 6.214.1 4.2

Table 8Diffusivity and solubility of H2 and O2 in 1H, 1H, 2H, 2H perfluoro-1-octanol.

H2 O2

Diffusivity (cm2 s�1) � 105 14.3 ± 1.1 3.2 ± 0.1Solubility (mol m�3 Pa�1) � 104 4.4 ± 0.4 5.4 ± 0.2Permeability (mol m�1 s�1 Pa�1) � 1012 6.2 ± 0.1 1.7 ± 0.1Permeance (mol m�2 s�1 Pa�1) � 109 4.7 ± 0.1 1.3 ± 0.1

438 P. Leelachaikul et al. / Separation and Purification Technology 122 (2014) 431–439

in order to establish that the response curves were repeatable andconsistent (A.3, Supplementary Information). Time-lag measure-ments were conducted by increasing the pressure by 100 kPa in or-der to obtain clear response curves.

As shown in Fig. 8 the time-lag curves for H2 show a slow re-sponse at low flow rates and tend toward an asymptote as the flowincreases. These results are likely due to the initial gas interchangebetween Ar and H2 with low flow rates resulting in intermixing ofthe gases rather than displacement. Therefore, the curves at thehighest flows (50 and 75 cm3 min�1) were utilized for computingthe solubility and diffusivity of the gases.

The results (Table 8) were reasonable, showing higher solubilityfor O2 than for H2. This was expected for perfluorocarbons, whichhave been used in artificial blood [33]. To our knowledge, the sol-ubility and diffusivity of gases in PFO have not been reported pre-viously, but the range seems to be consistent with other PFCsreported previously. Lowe [34] reported an O2 solubility of1.56 � 10�4 and 1.64 � 10�4 mol m�3 Pa at 37 �C in perfluorotribu-tylamine and perfluorodecalin, respectively. These values are sim-ilar with the experimental results for H2 and O2 solubilities of4.4 � 10�4 and 5.4 � 10�4 mol m�3 Pa, respectively. O’Brien et al.[35] reported that O2 had a diffusivity of 2.0 � 10�5 cm2 s�1 inperfluorotributylamine at 22 �C. The diffusion coefficients for H2

and O2 in PFO were 1.4 � 10�4 cm2 s�1 and 3.2 � 10�5 cm2 s�1,respectively. The higher values obtained for the PFO membraneare the result of possible defects between the alumina supportand PFO liquid. These defects would be due to incomplete wettingof the support surface.

The theoretical permeances calculated from the solubility and dif-fusivity are for the liquid and result in a H2/O2 selectivity of 3.6 with apermeance for H2 of 4.7 � 10�9 and for O2 of 1.3 � 10�9 mol m�2 s�1 -Pa�1. The permeances obtained from the steady-state experiments inSection 3.2 are for the membrane and showed a H2/O2 selectivity of 50with a permeance for H2 of 2.2 � 10�10 mol m�2 s�1 Pa�1 and for O2 of4.4� 10�12 mol m�2 s�1 Pa�1. To obtain quantities for the liquidthese values are divided by the porosity (e = 0.35), as only the exposedsurface of the liquid should be considered. This gives the same selec-tivity of 50 but a permeance of H2 of 6.3� 10�10 mol m�2 s�1 Pa�1

and for O2 of 1.4� 10�11 mol m�2 s�1 Pa�1. Thus, the permeances ofH2 and O2 obtained from the time-lag experiments are higher than

0 100 200 300 400 5000.0

1.0x10-5

2.0x10-5

3.0x10-5

4.0x10-5

5.0x10-5

Tota

l gas

per

mea

ted

/ mol

es

time / s

10 cm3 min-1

25 cm3 min-1

50 cm3 min-1

75 cm3 min-1

(a)

Fig. 8. Time-lag curves for (a)

from the permeance tests. These results can be explained by differ-ences in occupancy of solubility sites at transient and steady-stateconditions. In particular, time-lag measurements are performed withfreshly prepared membranes where all solubility sites are vacant. Incontrast, at steady-state, solubility sites in the liquid are partiallyoccupied. Since sites in the time-lag experiments are vacant there willbe a higher apparent solubility, and easier motion, so also higher dif-fusivity. The consequence is a much higher calculated permeancewith the time-lag experiments.

The discrepancy in H2/O2 selectivity can also be explained bythis phenomenon. The solubility of O2 is higher than that of H2,and at the steady-state site blockage will reduce the rate of diffu-sion for O2 more than H2 since diffusion involves jumps to unoccu-pied sites, giving rise to the observed higher H2/O2 selectivity atsteady-state conditions than in the time-lag experiments.

To summarize, a new type of membrane system has been de-scribed which has potential use in the separation of a difficultgas combination: mixtures of H2 and O2. The membranes consistsof liquid perfluorooctanol imbued in a porous alumina supportand work at close to room temperature near the conditions atwhich such mixtures are generated by photochemical methods.At a molecular level the liquid is considered to have solubility siteswhich accommodate the permeating species as they pass throughthe membrane.

4. Conclusions

The perfluorocarbon 1H, 1H, 2H, 2H perfluoro-1-octanol wasimpregnated onto a porous alumina support to produce a sup-ported liquid membrane capable of separating H2 and O2. Theexperimental conditions did not present external mass transferlimitations. The membrane had an average H2/O2 selectivity ofover 100 with a H2 permeance of 2.9 � 10�10 mol m�2 s�1 Pa�1

(1030 barrers) at 40 �C for a 50/50 mixed gas feed. Higher flowrates and temperatures did not affect the performance of the mem-brane, but lowered the lifetime of the membrane. A mixture of H2

and O2 increased the selectivity of the membrane as H2 inhibitedthe permeation of O2. A partial pressure analysis revealed thatthe permeance of H2 was slightly reduced as the driving forceincreased, but the separation performance of the membrane

0 100 200 300 400 5000.0

2.0x10-64.0x10-66.0x10-68.0x10-61.0x10-51.2x10-51.4x10-51.6x10-51.8x10-52.0x10-52.2x10-5

Tota

l gas

per

mea

ted

/ mol

es

time / s

10 cm3 min-1

25 cm3 min-1

50 cm3 min-1

75 cm3 min-1

(b)

Hydrogen and (b) Oxygen.

P. Leelachaikul et al. / Separation and Purification Technology 122 (2014) 431–439 439

remained unchanged. Humidity did not affect the performance ofthe membrane, but its lifetime was slightly reduced. Time-lag anal-ysis of the membrane showed a solubility of 4.4 � 10�4 and5.4 � 10�4 mol m�3 Pa�1 and a diffusivity of 1.4 � 10�4 cm2 s�1

and 3.2 � 10�5 cm2 s�1 for H2 and O2, respectively.

Acknowledgements

We acknowledge the fund from a Grant-in-Aid ScientificResearch (B) (No. 22360335) of the Ministry of Education, Culture,Sports, Science and Technology (MEXT), Japan. P.L. and B.C.D. arethankful for the scholarship from the Japanese Government(MEXT).

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.seppur.2013.11.019.

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