Kang Huang,Jianwei Yuan,Guoshun Shen,Gongping Liu,Wanqin Jin*

State Key Laboratory of Materials-Oriented Chemical Engineering,College of Chemical Engineering,Jiangsu National Synergetic Innovation Center for Advanced Materials,Nanjing Tech University,Nanjing 210009,China

1.Introduction

Gas separation membranes are increasingly becoming an important and convenient technology for the recovery of hydrogen from gas mixtures(H2/N2,H2/CO2,H2/hydrocarbons and so on),oxygen-nitrogen separation,natural gas separation,CO2capture,vapour-vapour separation and air dehydration[1].The attractive and significant reason is that the membrane technology can effectively separate gas mixtures under low pressure,obviously reduce required industry area and minimize necessary energy consumption with relatively low contamination,compared with traditional separation technologies[2,3].Up to now,numerous creative works about gas separation membranes are focused on achieving high flux and surprising selectivity[4-7].In order to obtain this target,three main routes are employed:1)designing and synthesizing new materials with special and excellent properties;2)improving current gas separation membrane materials by modification;3)developing novel and high-effective membrane processes based on current materials.The intrinsic excellent properties of the membrane materials are the precondition to achieve high separation performance.Beyond their outstanding properties,the membrane materials should also satisfy other necessary practical application conditions,such as low production cost,simple preparation process and easy scale-up.

Recently,considerable interest has been aroused by the emerging two-dimensional structural materials,such as MoS2,[8,9]phosphorene,[10]ZIF-7[11]and graphene,[12]due to their ultra-thin thickness and unique physicochemical property.Utilizing two-dimensional intriguing materials to fabricate thin membranes has been considered as a useful and effective way to overcome the current permeability/selectivity trade-off,[13,14]which often occurred in traditional polymer membranes[14,15].Among them,there is no doubt that graphene,a two-dimensional monolayer of sp2hybridized carbon atoms arrayed in a honeycomb pattern,exhibits the most imaginative and outstanding prospect,because ofa series of unique properties,such asgood chemical stability,excellent thermal conductance and strong mechanical strength[12,16-19].For example,Nairet al.found that graphene oxide(GO,the oxygen-containing analogue of graphene)membrane allowed unimpeded permeation of water while other molecules were blocked,because of the low-friction flow of water molecules through 2D capillaries between graphene sheets[20].Subsequently,based on this interesting discovery,significant amount of researches about GO membranes for water treatment,[21-26]liquid organic separation[27-30]and ion sieve[31,32]were investigated and showed great attractive performance.

GO's precise transport channels and atomic-scale pores also make it a potential candidate in gas separation.Up to now,some exciting and encouraging works have been achieved[33,34].However,because of the complicated membrane preparation process,[34,35]it is very hard to transfer their membrane to the practical application.On the other hand,the attractive ultra thin flat membrane structure brings ano the rcriticalissue:how to bear complex and harsh long-termoperation environments in the real industry(such as high pressure and unstable gas flow).Usually,extremely careful manipulations are needed for these few-layers GO membranes[33,34].Therefore,it is still necessary to design GO membranes which can satisfy the practical requirements and explore their gas separation.

In order to make up above shortages,we are apt to prepare porous ceramic supported GO membranes,because porous ceramic substrates not only can decrease thickness of membranes to realize high flux obviously,but also can offer a good mechanical strength for composite membranes[36,37].In this study,the porous ceramic hollow fibre was selected as the substrates due to its characteristic con figuration(low mass transfer resistance and high-packing density)and good chemical and thermal stability[38].Previously,we have proposed a convenient and rapid vacuum suction method to prepare GO membranes on the ceramic hollow fibre,which exhibited excellent pervaporation separation performance[27].The specialcon figuration makes GO membranes very easy to be scale-up.In addition,GO nanosheets stacked to form a cylinder shell around the ceramic hollow fibre,keeping it more stable than flat GO membranes.Herein,we will deeply study the gas separation performance of the ceramic hollow fibre supported GO membranes by:(1)optimizing microstructures of GO membranes;(2)exploring gas separation ability systemically,including single gas and binary mixture;(3)investigating stability of GO membranes in the gas system.Small gas molecules(H2,CO2,N2,O2and CH4)will be employed to investigate the potential separation in the whole study.

2.Experimental

2.1.Materials

GO powder prepared by modified hummer's method[39]was purchased from Nanjing JCNANO Tech Co.,Ltd.,China.The ceramic α-Al2O3hollow fibre support was prepared as our previous method[7].99.999%H2,H2,CO2,N2,O2and CH4were used as the gas sources,which were brought from Nanjing Special Gas Co.,LTD,China.Deionized water was also employed in the whole experiment.

2.2.Preparation of GO membrane

A typical process to prepare GO membranes is described as follow:Firstly,preparation of GO aqueous solution.GO powder was dissolved into deionized water,and at the same time the mixture solution was treated by ultrasound equipment for 1 h to form a high concentration GO aqueous solution.In this step,GO powder was exfoliated to nanosheets.Then,above GO solution was centrifuged at 3000 r·min-1for 10 min in order to remove agglomerated powder and impurity.After this,the as-prepared solution was diluted 1000 times to form a very low concentration solution(about 0.001 mg·ml-1).Secondly,fabrication of GO membranes.GO membranes were prepared by our previous reported method(i.e.,Vacuum Suction method)[27].The detailed steps are listed as below.One side of the ceramic hollow fibre was sealed and the other side was connected to a vacuum pump.Then,the whole hollow fibre was immersed in the GO aqueous solution.With the pressure driving,GO flakes were stacked on the surface in order.Through changing the operation time,different thickness GO membranes were fabricated.Finally,the as-prepared GO membrane was dried in a vacuum oven at 45°C over 48 h.The quality of as prepared GO membranes was examined by testing the H2and CH4single gas permeation.

2.3.Characterization

The morphologies of the GO membranes and the ceramic hollow fibre were characterized by field emission scanning electron microscope(FESEM S4800,Hitachi,Japan).The working parameters were a voltage(HV)of 5 kV and a work distance(WD)of 8 mm.Fourier transform infrared spectroscopy were recorded by using a FTIR spectrophotometer(AVATAR-FT-IR-360,Thermo Nicolet,USA)over the range of 4000-500 cm-1.The X-ray photoelectron spectroscopy(XPS)was carried out through an X-ray photoelectron spectrometer(Thermo ESCALAB 250,USA)with monochromatized AlKαradiation.Atomic force microscopy(AFM,XE-100,Park Systems,Korea)was used to detect the size of GO flakes and the surface morphologies of the GO membrane.

2.4.Gas permeation test

Gas permeation experiments were performed by small gas molecules(H2,CO2,N2,O2and CH4)on the permeation setup.Fig.1 shows the schematic of the gas separation setup.All the measurements were performed using the Wicke-Kallenbach technique with an on-line gas chromatography(Agilent Technologies 7820A)at room temperature.Before test,the membrane was activated at 45°C.And,all the results were tested three times,making sure that the results were reliable.

For the single gas measurement,the feed flow rate was set to 30 ml·min-1.When studying the influence of humidity,the dried gas flow would go through a water bottle.For the binary mixture,the feed side was fed at a total volumetric flow rate of 60 ml·min-1with each gas of 30 ml·min-1.When investigating the influence of H2fraction in the feed,the total flow was kept at 60 ml·min-1.In all measurements,helium was used as sweep gas at a flow rate of 30 ml·min-1.Atmosphere pressure was applied to both sides of the permeation cell.The temperature was controlled by a circulation oven.

Fig.1.Schematic diagram of the gas separation setup.“MFC”and “GC”are mass flow controller and gas chromatography(Agilent Technologies 7820A),respectively.“F”and“P”are the flow rate and pressure,respectively.

The membrane permeance(Fi)is defined as:

whereNiis the permeate rate of componenti(mol·s-1),ΔPiis the trans membrane pressure difference ofi(Pa),andAis the effective membrane area(m2).

The idealselectivity is calculated by the ratio of single gas permeances.

The separation factor was calculated as:

wherexandyare the molar fraction of the one component in the feed and permeate,respectively.

3.Results and Discussion

3.1.Basic characterization of GO materials and hollow fibre support

As we know,the properties of membrane materials have great effect on the eventual separation performance.Before experiments,we first characterized the basic properties of GO materials,including AFM,FTIR,XPS and Raman spectrum.Fig.2(a)shows the AFM image of GO flakes deposited on the mica substrate.The size of GO flake is about 1μm size.The depth pro file indicates that the GO sheetis approximately 1 nmin thickness.The FTIR spectrum(Fig.2(b))proves the presences of O--H stretching vibrations(3415 cm-1),C═O stretching vibrations from carbonyl and carboxylic groups(1733 cm-1),unoxidized sp2C═C bonds in the carbon lattice(1624 cm-1),and C--O stretching vibrations from epoxy groups(1051 cm-1).These functional groups were further confirmed by the XPS measurement.As shown in Fig.2(c),the XPS C1s spectrum of GO clearly indicates four kinds of C atoms in different functional groups:C--C(～284.8 eV),C--O(～286.8 eV),C═O(～287.8 eV),and C(O)O(～289.0 eV).The C--O groups(representing hydroxyl and epoxide groups)comprise approximately 45.3%of the total C1s peak area,whereas C═O and C(O)O are 11.07%and 5.4%,respectively.The results show that the ratio of O/C in GO is approximately 0.6,which is relatively high as compared to the reported values[40].XPS and FTIR results are well in agreement with the Lerf-Klinowski Model of the GO sheet[41,42].When the Hummer method produced amount of oxygen-containing groups,some intrinsic defects were also created at the same time.The present of defects can be supported by the Raman spectrum.As shown in Fig.2(d),theID/IGratio of the GO powder is about 1.05,which will be assigned to higher defects/disorders in the GO flake[33].The diverse carbon functional groups and intrinsic defects on the GO structure will be beneficial for the gas separation through the molecular interaction and sieving.

Fig.2.(a)AFM image of GO flakes deposited on the mica substrate;(b)FTIR,(c)XPS and(d)Raman spectrum characterization of GO;(e)Optical picture of the ceramic tube and hollow fibre;(f)FESEM images of the ceramic hollow fibre(insert i:cross-section;insert ii:surface).

Fig.3.The thickness of the ceramic hollow fibre supported GO membrane as a function of the membrane preparation time.

The structure of the ceramic hollow fibre was also investigated.Fig.2(e)gives an optical picture of the hollow fibre and the traditional ceramic tube support.Compared with tube,the hollow fibre owns a slender shape with smaller diameter(about1.5 mm),implying a higher packing density.Fig.2(f)shows the detail features of the hollow fibre by FESEM.Its asymmetric structure(i.e.,a thin separation dense layer integrated with finger-like porous layers on both sides in Fig.2(f-i)reduces the mass transfer resistance of supports.The relatively smooth surface makes(Fig.2(f-ii))GO sheets easy to stack and reduces the formation of big holes.

3.2.Optimizing structures of GO membranes

In order to obtain high flux and selectivity,the micros tructures of the hollow fibre supported GO membranes were optimized systematically.By adjusting the preparation time,GO membranes with different thickness were achieved.Fig.3 shows that the thickness of the GO membranes increases with the operation time increasing.However,there is a nonlinearity between the membrane thickness and operation time.The reason is that the resistance was reinforced with increasing the membrane thickness,which inhibited more GO sheets to stack on the surface.The insert in Fig.3 presents two typical GO membranes prepared at 10 and 120 min,respectively.Obviously,the membrane colour becomes darker when the thickness increases.Additional,both of them show a continuous and uniformly layer,indicating that the vacuumsuction method is very effective to fabricate tubular GO membranes.

Fig.4.FESEM images of the GO membrane prepared under different operation times:(a-c)the surface images;(e-f)the cross-section images.

Fig.4 shows the microstructures of three typical GO membrane which were prepared at 5,30 and 120 min,respectively(the corresponding membranes are marked as T5,T30 and T120,respectively).When the time is too short,there are some pin holes on the surface of GO membrane(Fig.4(a)).The insert of Fig.4(a)gives a clearer and enlarged image.From Fig.4(b)and(c),continuous and complete membranes can be observed.And,the surface becomes smoother with the growth of thickness.Fig.4(d),(e)and(f)exhibit the corresponding cross-section images of T5,T30 and T120 membranes,respectively.All of them attach well with the ceramic hollow fibre,which may be attributed to the hydrogen bond between the oxygen containing functional groups of the GO membrane and the hydroxy group on the surface of the ceramic hollow fibre.

Gas separation measurements of single H2and CH4were utilized to examine the membranes' quality.As shown in Fig.5,the gas permeances of H2and CH4decline together,when the membrane thickness increases.But the H2/CH4selectivity of T30 GO membrane gets a peak and meanwhile the membrane still has a good H2permeance.Obviously,with increasing the membrane thickness,the H2permeance declines quickly.This is why Nairet al.found that the thick GO membrane was impermeable to gases because of the higher membrane thickness[20].Considering permeance and selectivity,we selected the 30 min operation time as the most optimized condition to prepare the ceramic hollow fibre supported GO membranes.Fig.6 shows the FESEM image of T30 GO membrane after rotating 45°and the corresponding AFM image,indicating that the membrane is very intact with lots of ripples and the membrane thickness is about 300 nm.The corresponding XRD result(the inset in Fig.6(a))shows the d-spacing size of the GO membranes is～0.81 nm.

Fig.5.The H2 and CH4 permeance and corresponding H2/CH4 selectivity of GO membrane as a function of the membrane preparation time.

3.3.Single gas separation performance

Single gas permeations of T30 GO membrane,including H2,CO2,O2,N2and CH4,were tested in detail.From Fig.7,the permeances of these small gas molecules decreases in the order H2＞CH4＞N2＞O2＞CO2,with increasing the molecular weight.The corresponding idealselectivity(Fig.8)of H2/CO2,H2/O2,H2/N2and H2/CH4are 15.0,7.5,7.2 and 6.4,respectively.In contrast to other gases,CO2shows a sharp down in the permeance and the highest hydrogen selectivity,which can be attributed to the chemicalnature of GO material.As we know,there are numerous carboxylic acid groups distributed at the edge of GO flakes.Strong interplay between these polar groups and C--O bonds in the nonpolar CO2molecules would happen.For CO2transfer,CO2as a Lewis acid or a Lewis base participates in hydrogen bonding,which inhibits it from transferring within the stacked GO structure[34].The similar phenomenon was also found in the porous metal-organic framework(MOF)ZIF-78 membrane[43].The two polar functional groups--NO2in ZIF-78 structure made it exhibit the highest affinity for CO2,which blocked the diffusion of CO2molecules through the ZIF-78 channels.

Then,we investigated the influence of humidity for the gas transfer.As shown in Fig.7,most gas permeances(except CO2)decrease when the humid steam is added in the feed,because the water molecules in the GO channels limit the gas transfer.However,a slight increase of CO2is observed.Kimet al.[34]found the same trend when GO membrane was used to separate humid gas.This result further confirms that CO2molecules have special interaction with the carboxylic acid groups in GO.Because of the growth ofCO2permeance,the corresponding H2/CO2ideal selectivity declines obviously under the wet condition(Fig.8),indicating that wet gas has disadvantages for hydrogen recovery from H2/CO2mixture.

Fig.7.Single gas permeance(H2,CO2,O2,N2 and CH4)with dry feed or hydrated feed as a function of the molecular kinetic diameter.

Fig.6.(a)FESEM image and(b)AFM image of T30 GO membrane.The inset of(a)is the XRD result.

Fig.8.Selectivity of H2/CH4,H2/N2,H2/O2 and H2/CO2 with dry feed or hydrated feed.

3.4.Binary gas separation performance

Single gas measurements only can give the ideal selectivity,because the gas molecular transfer in this process is relatively independent.However,interplays between different gases are generally not negligible,which may result in a prominent deviation of the mixture separation factor from the ideal selectivity.Table 1 lists all the single gases and binary mixtures separation performances.Like single gas test,the ceramic hollow fibre supported GO membrane shows a same regular for mixtures.But the corresponding permeance and separation factor have a bit of decrease because of the competitive adsorption and diffusion between different gas molecules.This phenomenon was often observed in zeolite membranes.For example,a CVD modified ZSM-5 membrane exhibited a higher H2/CO2ideal selectivity(17.5)than the mixture separation factor(10.8)[44].Another AlPO4membrane also showed a lower separation factor(9.7)in H2/CO2binary system than the ideal selectivity of 23.9[45].Generally speaking,although the H2/CO2separation factor drops,it still reveals a useful separation ability(10.2)for practical hydrogen recovery application.The Robeson plot for H2-CO2selectivities versus CO2permeabilities ofpolymer membranes has been widely used to compare the performance of membranes[46].As shown in Fig.9,our GO hollow fibre membranes exhibit superior properties.

Table 1Single gases and binary mixtures separation performances under dry state

Fig.9.Comparison of H2/CO2 gas separation performances of GO hollow fibre membranes with Robeson upper bound[46].

3.5.Effect of the H2 concentration in the feed for H2/CO2 mixture

In general,the H2/CO2rate in the realmixture is very complicated and cannot be one to one.In order to assess the influence of the feed composition on the hydrogen recovery,the separation of H2/CO2binary mixture was explored under different H2concentrations in the feed.As shown in Fig.10,with increasing the H2concentration,the H2permeance has a slight growth because of the enhanced driving force.In contrast,an imperceptible downtrend exists in the CO2permeance line.As a result,the H2/CO2separation factor is almost unchanged and almost keeps a constant(about 10),which is independent of H2fraction in the feed.

Fig.10.Effect of the H2 concentration in the feed for H2/CO2 mixture separation performance.

Fig.11 presents the corresponding H2and CO2fraction in the permeate as a function of H2concentration in the feed.Obviously,a high H2concentration mixture over 90%in volume will be obtained,when the feed is equimolar.According to Fig.11,for 20%H2concentration of H2/CO2mixture,the final H2concentration mixture will be more than 95%only after twice purification in theory.

Fig.11.H2 and CO2 fraction in the permeate as a function of H2 concentration in the feed.

3.6.Effect of temperature on H2/CO2 separation

Fig.12 represents the variation of the H2and CO2permeances and the ircorres ponding separation factors from their equimolar binary mixture in the temperature of 50-200°C.Both the permeances of H2and CO2increase quickly with increasing operation temperature because of an activated diffusion process.The corresponding separation factor shows a down trend,indicating that some inevitable pores were formed in the GO laminates by heating.On the other hand,these pores would also contribute to improving the permeance.X-ray photoelectron spectroscopy(Fig.13)shows that most of the oxygen containing functional groups have disappeared after heating treatment,implying that this process is irreversible.Therefore,low temperature(room temperature)is very suitable for GO membrane used in gas separation industry field.

Fig.12.Effect of temperature on H2/CO2 separation.

3.7.Repeatability and stability of GO hollow fibre membranes

As shown in Table 2,the ceramic hollow fibre supported T30 GO membranes exhibit very good repeatability on the separation of equal molar H2/CO2binary mixture at room temperature.All the membranes were prepared under the same conditions.The single permeances of H2and CO2are around 1.3 × 10-7and 0.09 × 10-7mol·m-2·s-1·Pa-1,respectively,and the corresponding ideal selectivity is around 15.For binary mixture,the as-prepared GO membranes show similar results as listed in Table 2.This well reproducibility will benefit to the practical application.What's more,we also summarize the H2-CO2separation performance of one of the as-prepared GO hollow fibre membranes during the whole test process.As shown in Fig.14,the GO hollow fibre membrane showed a very good stability under different test conditions.

Fig.13.X-ray photoelectron spectroscopy result of the GO membrane after heating treatment.

Table 2Repeatability of GO membranes

Fig.14.The long term stability of GO hollow fibre membrane under different test conditions.

4.Conclusions

The ceramic hollow fibre supported GO membranes were studied systematically in the present work.The optimized GO membranes possess a good balance between H2permeance and selectivity.Considering the uncomplicated fabrication process and high packing density,the gas separation ability of ceramic hollow fibre supported GO membranes will create a great amount of brilliance in the gas separation field.The high specific surface area of GO material also makes GO membranes very attractive,due to the resource saving and cost-effective.In addition,because of their special oxygen containing functional groups,it may further handle the chemical nature of GO membranes by modifying surface or channels,the gas separation ability of GO membranes could become adjustable and various.

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