Fabrication of graphene oxide composite membranes and their application for pervaporation dehydration of butanol☆

Xianfu Chen,Gongping Liu,Hanyu Zhang,Yiqun Fan*

Laboratory of Materials-Oriented Chemical Engineering,College of Chemistry and Chemical Engineering,Nanjing Tech University,Nanjing 210009,China

Keywords:Graphene oxide Membrane Pervaporation Dehydration Butanol

ABSTRACT As a newkind of 2D nanomaterials,graphene oxide(GO)with 2–4 layers was fabricated via a modified Hummers method and used for the preparation of pervaporation(PV)membranes.Such GO membranes were prepared via a facile vacuum-assisted method on anodic aluminium oxide disks and applied for the dehydration of butanol.To obtain GO membranes with high performance,effects of pre-treatments,including high-speed centrifugal treatment of GO dispersion and thermal treatment of GO membranes,were investigated.In addition,effects of operation conditions on the performance of GO membranes in the PV process and the stability of GO membranes were also studied.It is of benefit to improve the selectivity of GO membrane by pre-treatment that centrifuges the GO dispersion with 10000 r·min-1 for 40 min,which could purify the GO dispersion by removing the large size GO sheets.As prepared GO membrane showed high separation performance for the butanol/water system.The separation factor was 230,and the permeability was as high as 3.1 kg·m-2·h-1 when the PV temperature was 50°C and the water content in feed was 10%(by mass).Meanwhile,the membrane still showed good stability for the dehydration of butanol after running for 1800 min in the PV process.GO membranes are suitable candidates for butanol dehydration via PV process.

1.Introduction

A lot of works have been carried out to obtain high performance membranes by combining GO and common polymeric materials[8–12].Besides,membranes made with pure GO were also reported,which exhibited a lot of special properties for separation due to their 2D channels[5,13–15],hydrophilic surface[10,16,17]as well as the defects on GO sheets[18–20].Among the special properties of GO membranes,the priority permeability for water attracted a lot of interests.It was found that GO sheets bearhydroxyl,epoxy,and other hydrophilic functional groups on their openings and edges.Combined with the layered structure formed by GO laminates,these hydrophilic functional groups made GO membranes even completely impermeable to liquids,vapors,and gases,including helium,but allow unimpeded permeation of water[16].Similar GO membranes could be fabricated for the dehydration of organic solvents due to the high selectivity of water[21–25].

In the preparation process of such GO membranes,a series of facile membrane formation methods were employed including spin-coating method[13],dip-coating method[25],self-assemble method[6,26]and different types of filtration methods[21,24].Meanwhile,various preparation conditions,such as salt concentration,pH value[27]and pressure[24],were investigated.However,GO materials made from different raw graphite via different chemical oxidation routes are different in C/O ratio,particle size and shape[28].Even in one pot,synthesized GO sheets could be much different,especially in particle size.Hence,pre-treatment and purification of GO dispersion are very necessary.In addition,the functional groups and d-spacing of GO sheets are sensitive to water and temperature[16].Thus,GO membranes should be thermal treated under suitable conditions before application.

As an important kind of biofuels,butanol exhibits a huge potential to be the alternative of fossil energy.Membrane process is a high efficiency and low cost technique for the purification of butanol[29].To date,both organic and inorganic materials have been used to fabricate membranes for the dehydration of butanol in pervaporation(PV)processes.Organic membranes were easy to be fabricated and showed high selectivity to water[30,31],but their permeabilities were generally below 2 kg·m-2·h-1.Inorganic membranes performed higher permeabilities[32–34],which are usually in the range of 2–10 kg·m-2·h-1.An ideal membrane should combine the high performance of inorganic membrane and easy fabrication of organic membrane.However,to access such a membrane is still a big challenge we are facing[35].On this sight,GO membrane might be a promising one for butanol dehydration.However,seldom literature about such GO membranes for the dehydration of butanol was reported.

Here,we designed and fabricated GO membranes supported by ceramic substrates also via a simple vacuum-assisted filtration method,and applied these membranes for butanol dehydration in PV process.Effects of pre-treatments,including high-speed centrifugal treatment of GO dispersion and thermal treatment of GO membranes,were investigated.Besides,effects of operation conditions on the performance of GO membranes in the PV process and the stability of GO membranes were also studied.

2.Experimental

2.1.Materials

Fine grade synthetic graphite flakes of about 30 μm were supplied by Jinrilai Graphite Inc.(Qingdao,China)and used as received.Sulfuric acid[H2SO4,97%(by mass)],hydrochloric acid[HCl,10%(by mass)in water],hydrogen peroxide[H2O2,30%(by mass)in water]and Potassium permanganate(KMn O4)from Sinopharm Co.(Shanghai,China)were also used as received.Anodic aluminium oxide(AAO)macroporous membranes with an average pore size of 100 nm and a thickness of 50 μm from Merck Millipore Co.(USA)were used as substrates for GO membranes.

2.2.Preparation of graphene oxide

Modified Hummers method was used to prepare GO from graphite[36].50 ml sulfuric acid was added to 2 g graphite powder and the temperature of the dispersion was maintained at about 7°C and stirred for 30 min.Then the dispersion was heated to 20°C and stirred for 60 min.Subsequently,7 g KMnO4was added in 4 times every 30 min.At this time,the dispersion changed in color from black to dark green.Then the dispersion was heated to 35°C and stirred for 120 min.After 100 ml deionized water was added,the dispersion was heated to 98°C and stirred for 5 min.The oxidation reaction was terminated by adding 200 ml deionized water.The dispersion turned brown in color.To remove the excess oxidant,5 ml 30%(by mass)hydrogen peroxide dispersion was added,and the dispersion was stirred for 30 min.At this time,a dispersion with light yellow was obtained.Synthesized GO was separated and washed with 100 ml 10 wt.%hydrochloric acid for five times in a Buchner funnel.Filtered GO cakes were redispersed in pure water and filtered and washed several times until no sulfate ions could be detected by the precipitation method with 0.02 mol·l-1barium chloride solution.Filtered GO cakes were redispersed in pure water again with the assistance of ultrasonic wave for 60 min.The sonication power and frequency were controlled at 450 W and 20 kHz,respectively.Then,the dispersed GO dispersion was centrifuged at 3000 r·min-1for 5 min to remove the graphite oxide particles.The purified GO dispersion was dried by a freeze drying method.

2.3.Fabrication of GO membranes

50 mg GO was dispersed in 200 ml water by an ultrasonic assisted method to form a homogeneous GOdispersion.To purify the GO dispersion,we designed a high-speed centrifugal pre-treatment to remove large size GO sheets.GO dispersion containing 250×10-6GO was centrifugal pre-treated at a high rotate speed of 10,000 r·min-1for several times with 10 min each time.After each time of centrifugal treatment,the precipitation was removed from GO dispersion carefully.Then,the purified GO dispersion was diluted to 50×10-6.

She was in his thoughts, and the knife trembled in the hand of the little mermaid: then she flung it far away from her into the waves;37 the water turned red where it fell, and the drops that spurted up looked like blood

AAO micro filtration membranes were used as substrates for GO membranes.To improve the strength of AAO membranes,they were supported by macroporous ceramic disks(? 40 mm × 2.5 mm)and sealed by a solvent resistant binder.GO membranes were prepared on the enhanced AAO substrates by a vacuum assisted filter method.The thickness of membrane was controlled by adjusting the volume of added GO dispersion.Until no liquid could be observed on the surface of fresh GO membrane,the vacuum conditions were kept for two more hours to remove the excess water in GO membrane.As prepared fresh GO membranes were dried at given temperatures for 12 h to remove the residual water.

2.4.Characterization techniques

Fourier transform infrared(FTIR)spectra of GO were measured using an Nicolet 8700 infrared microspectrometer(Thermo,USA)in the range of 4000–1000 cm-1.Raman spectra(LabRAM HR 800,Horiba,France)were recorded using an excitation wavelength of 514 nm(2.41 eV).An X-ray diffractometer(XRD)(Mini flex 600,Rigaku,Japan)was used to measure the interlayer with focused monochromatized Cu-Kα radiation at a wavelength of 0.15418 nm,operating in the 2θ range of 5°–50°at a scan rate of 20(°)·min-1.The average d-spacing value was calculated using Bragg's law from the X-ray scattering data.Thermogravimetry(TG)analyses were performed in a STA 449F3 instrument(NETZSCH,Germany),heating the sample to 500 °C at a rate of 10 °C·min-1under a 100 ml·min-1air flow.The particle size distribution of the GO was characterized by dynamic light scattering(DLS,Microtrac,Zetatrac,USA).FESEM(S-4800,Hitachi,Japan),TEM(JEOL 1011,JEOL,Japan)and AFM(XE-100,Park,Korea)were employed to characterize the microtopographies of samples.Contactangle between water and the surface of membrane was measured by a contact angle apparatus(A-100p,DropMeter,China).

Pervaporation process for the dehydration of butanol was carried out on the device showed in Fig.1.The device contained five major parts including stock tank,membrane module,temperature controller,cold trap and vacuum system.The pressure of permeate side was controlled below 100 Pa,and the feed temperature was controlled at a various values from 30 to 90°C with a deviation below 1°C.The flux(F)was determined by using the following equation:

where Q is the mass of permeate;A is the effective membrane area and t is the time interval of sample collection.Gas Chromatograph(GC,GC2014,Shimadzu,Japan)was used to analyze the contents of water in butanol.The separation factor of water and butanol was calculated using the standard equation:

where X and Y are the weight fractions of species in the feed and permeate,respectively.The pervaporation separation index(PSI)was calculated using the following equation[37]:

where F is the flux,and α is the separation factor.

Fig.1.PV set for the dehydration of butanol.

3.Results and Discussion

3.1.Characterizations of graphene oxide

GO was characterized by FTIR spectroscopy.As shown in Fig.2,a lot of hydrophilic functional groups were observed.Such functional groups have excellent water-absorbing capacity and play important roles in water transfer process in GO membranes.The peaks at around 3430 cm-1and 1635 cm-1refer to the–OH groups of absorbed water in GO.The peaks at 2930 cm-1and 2850 cm-1are attributed to –CH2– groups on GO sheets.The peak at 1720 cm-1corresponds to C=O groups from carbonyl or conjugated carbonyl groups.The peaks at 1380 cm-1,1264 cm-1and 1110 cm-1are due to the–OH groups from carboxyl,C–O–C groups from epoxy of ether,and C–O groups from alkoxy,respectively.These results agree well with the reports in literature[38,39].In contrast,the peaks of–OH and C–O–C groups almost disappeared for reduced GO.

To further study the structural properties of the GO powder,Raman spectroscopy was employed.As shown in Fig.3,G band at 1585 cm-1and D band at 1338 cm-1were observed clearly,which are assigned to the graphitized structure and local defects or disorders located at the edges of graphene,respectively[5,18].

Fig.2.FTIR spectrum of GO and reduced GO.

Fig.3.Raman spectrum of GO.

The GO gel was prepared from GO dispersion(2 mg·ml-1)via a filtration concentration process for TG analyses.The results are showed in Fig.4.It could be found that GO gel contained three types of water including free water,trapped water and bounded water[40,41].Free water behaves like normal water without any restraint,which could be removed mostly before 100°C corresponding to a mass loss of 45%;trapped water is confined in GO sheets and encounters arbitrary constrains while retaining translational mobility,which were almost removed by further increasing the temperature to 240°C.This process corresponded to a mass loss of 29%.Bound water is tightly bound to GO by different bonds allowing only local motions,which was also to be partly removed above 240°C corresponding to a small mass loss of 1%.Compared with the free water and the trapped water,the mass loss was much smaller for the decomposed functional groups on GO sheets.There are two main reasons for the small weight loss ratio at 240°C.First,the GO gel contains a lot of free water and trapped water due to the good hydrophilicity of GO sheets.Second,only the C–O(hydroxyls and epoxides)groups were decomposed at around 240°C,whereas the C=O and COOH components even did not change[42].

Fig.4.TG-DSC curve of GO gel.

Fig.5.XRD spectra of GO and graphite.

XRD spectra of GO and graphite are showed in Fig.5.It could be found that the 2 theta degree of the sharp peak for GO was much smaller than that of graphite.This was due to the oxidation process in which the introduction of functional groups enlarged the d-spacing of GO sheets.According to the Bragg equation[43],the d-spacing was calculated to be 0.343 nm and 0.885 nm for graphite and GO,respectively.The enlarged d-spacing made it possible for molecule transfer in GO membranes.

The morphologies of GO sheets are showed in Fig.6,in which Fig.6(a)and(b)shows TEM images of GO sheets.From Fig.6(a),we can clearly observe the corrugated surface of GO.Fig.6(b)shows the edges of GO sheets.It could be found that the GO sheets were of 2–4 layers.AFM image[Fig.6(c)]indicates that as-prepared GO sheets are about 3.6 nm thick and 1–3 μm in length and width.These results indicate that as-prepared GO sheets were a kind of proper 2D materials.Such 2D structure is benefit for the fabrication of ultrathin GO membranes and the improvement of permeability[18,35].

3.2.Effect of pre-treatment of GO dispersion on the properties of GO membranes

GO dispersion containing 250 mg·L-1GO was centrifugal pretreated at a high rotate speed of 10000 r·min-1for 10 min each time.After each time of centrifugal treatment,the precipitation was removed from GO dispersion carefully.The concentration ofGOwas measured by an UV–vis spectroscopy.From the UV–vis absorption spectrum of GO dispersions(Fig.7,right),it could be found that the curves were smooth and the absorbance values were relatively high at the wave length of 400 nm.Hence,the absorbance at400 nm was applied for the measurement of concentration of GO dispersions.The correlation ship between the absorbances of GO dispersions at a wave length of 400 nm and the times of pre-treatment was also showed in Fig.7(upper).It could be clearly observed that the absorbance decreased with the increase of pre-treatment times.At the beginning 30 min,a significant decrease of the absorbance could be observed,while a slight decline was contributed by the further pre-treatment after 40 min.In addition,some precipitates could be observed clearly at the first 10 min from the photos of GO dispersions in Fig.7(left).While,only little precipitation was found after 4 times of centrifugal treatment.This result suggests that most GO sheets with large size or multi-layer could be removed by centrifugal pre-treated for 3–4 times.

The particle size distributions of GO dispersions before and after 40 min centrifugal treatment are showed in Fig.8.It could be found that GO sheets with particle size of 4–6 μm had been removed from the dispersion after the high-speed centrifugal treatment,while the centrifugal treatment made slight difference for the GO sheets with particle size of200–400 nm.By the way,the particle size distribution determined by DLS here is a qualitative analysis but not a quantitative analysis.Here,40 min was applied for the centrifugal treatment of the GO dispersions.

Fig.6.(a)and(b)TEM micrographs of GO sheets,(c)AFM micrograph of GO sheets.

Fig.7.UV–vis absorption spectrum of GO dispersions(right),photos of GO dispersions(left)and effect of centrifuge time on the absorbances of GO dispersions(upper).

Fig.8.Effect of centrifugal treatment on the particle size distributions of GO dispersions.

The purified GO dispersion was diluted and used for the preparation of GO membranes.As-prepared GO membranes were thermal treated at 60°C for 12 h and then applied for the dehydration of butanol in PV process.When the water mass content in feed was 10%and the PV temperature was 50°C,the performance of GO membranes derived from GO dispersions before and after centrifugal treatment was tested and showed in Fig.9.It could be found that after the centrifugal treatment,the separation factor of GO membrane for water and butanol was improved to 230,which was almost 7 times than that of GO membrane without pre-treatment,while the permeate decreased from 5 kg·m-2·h-1to 3.1 kg·m-2·h-1only with a ratio of 38%.This was likely due to the reason that a uniform d-spacing between GO sheets could be obtained by removing the large size GO sheets.

Fig.9.Effect of centrifugal treatment on the performance of GO membranes.

3.3.Effect of thermal treatment temperature on the contact angle of GO membranes

The correlation of contact angle(CA)with thermal treatment temperature is showed in Fig.10.The CA increased with the thermal treatment temperature.When the temperature increased from 30°C to 60 °C,CA of GO membrane increased from 10°to 70°.In contrast,the CA was almost constant when the temperature varies from 60°C to 90°C.Combining with the results of TG analyses,it could be explained by two reasons.One reason is that both the free water and trapped water between GO sheets escaped further when the thermal treatment temperature increased.With the loss of water,the d-spacing of GO sheets decreased and lead to an increased resistance for the permeation of water in GO membrane.The other reason is that the bonded water could also be removed at a high temperature and the hydrophilic groups were destroyed,which could weaken the water-absorbing ability of GO.As a result,the GO color turned from light yellow to black after being thermal treated at 90°C for 12 h.With respect to the prevention of excessive dehydration of GO and the destruction of hydrophilic groups,the thermal treatment should be controlled below 60 °C.Hence,60 °C was chosen as the thermal treatment temperature.

GO membranes treated and untreated at 60°C for 12 h were tested in PV processes at 50 °C,60 °C and 70 °C,respectively.Then,the GO membranes were immersed in their corresponding feeds at the initial PV temperature for 24 h.Subsequently,the PV performances of these GO membranes were tested again to estimate the stabilities of GO membranes.The details in change are showed in Fig.11.It could be found that thermal treated GO membranes showed smaller change ratios in permeabilities and separation factors than that of GO membranes without thermal treatment.It indicates that the stabilities of GO membranes were improved by the thermal treatment at 60°C for 12 h.This is likely due to the fact that the interaction force between GO sheets was enhanced by the removal of trapped water in GO during the thermal treatment process.Hence,all the GO membranes used below were thermal treated at 60°C for 12 h.

Additionally,the activation energy values for permeation were calculated by using Arrhenius equation[38].When the water mass content in feed was 10%,the calculated apparent activation energies of water and butanol permeation were 11.3 and 21.9 kJ·mol-1,respectively.It means that the permeation of butanol through the membrane is more temperature-sensitive than water.

Fig.10.Effect of thermal treatment temperature on the contact angle of GO membranes,and appearance of GO membranes thermal treated at30 °C(left)and 90 °C(right)for 12 h.

Fig.11.Effect of thermal treatment on the stability of GO membranes.

3.4.Effect of water content in feed on the properties of GO membrane

Controlling the PV temperature at 50°C,the effects of water content in feed on the properties of GO membranes prepared form centrifugal treated GO dispersion were studied.As shown in Fig.12,the permeate increased with the increase of water content in feed.Meanwhile,the separation factor for water and butanol came down with the increase of water content.When the water mass content in feed was 4.8%,the separation factor was 440 and the permeate was 1.3 kg·m-2·h-1.On the contrary,the separation factor came down to 124 and the permeate increased to 5.9 kg·m-2·h-1when the water mass content in the feed increased to 16.7%.This might be due to the change in d-spacing with the increase of water content in feed.

In addition,we measured the d-spacing of GO membranes soaked in different butanol solutions with various water contents by using XRD[44].Samples of GO membrane supported by AAO were loaded into different butanol solutions at ambient temperature for 6 h and examined with XRD.The water mass contents of butanol were controlled at 5%,10%and 20%,respectively.As shown in Fig.13,a significant shift in the(001)reflection with the increase of water content was observed,which corresponds to an increase in d-spacing.According to the Bragg equation[43],the d-spacing was calculated and showed in Fig.13.For GO membrane soaked in butanol solution with 20%water mass content,the d-spacing was 1.473 nm,which is 1.7 times larger than that of dry GO membrane(0.885 nm).These results suggest that the d-spacing between GO sheets is sensitive to the water content in the feed,which are in good accordance with literature[16].The higher water content means the larger the d-spacing,which was of benefit for the transfer of water in graphene membrane.Contrarily,the larger d-spacing means the more butanol molecules could pass through the GO membrane.As a result,the separation factor decreased and the permeate increased with the increase of water content in feed.

Fig.12.Effect of water content in feed on the performance of GO membrane.

Fig.13.XRD patterns of GO membranes soaked in butanol solution with various water contents.

The optical and FESEM images of GO membrane are showed in Fig.14.Optical images of AAO disk and AAO supported GO membrane[Fig.14(a)]show that both the AAO disk and GO membrane were so thin.We could easily distinguish the characters behind them by naked eyes.The surface morphologies of the GO membrane characterized by FESEM and AFM are presented in Fig.14(b)and(c),respectively.It could be found that all the pores on the surface of AAO were completely covered by GO sheets and formed a lot of wrinkles.This indicates that a defect-free GO separation layer was formed on the top ofAAO substrate.The cross-section of supported GO membrane is showed in Fig.14(d).Cylindrical pore structure of AAO substrate and layered structure of GO membrane could be clearly observed.The thickness of GO membrane with a value of 600 nm could also be obtained.

The performance of various membranes reported in literature and the GO membranes we prepared for the dehydration of butanol via PV are listed in Table 1.Compared with organic membranes,it could be found that the GO membranes we prepared for butanol dehydration showed high flux at a relatively low PV temperature.Although the separation factor of such GO membranes should be further improved,the water content in permeate was up to 95%.It indicates that the GO membranes we prepared showed a relatively high efficiency for the dehydration of butanol via PV.Compared with inorganic membranes,the fluxes of GO membranes we prepared were at a similar magnitude,while a further improvement in separation factor should be carried out for our GO membranes.However,considering the cheap raw materials(graphite)and facile membrane formation process,GO shows a good potential to be one kind of low-cost membrane materials.Additionally,a relatively low PV temperature we applied also means a low cost for the process of butanol dehydration.In general,the GO PV membranes we prepared are suitable candidates for butanol dehydration processes.

The stability ofGO membranes was measured at the PV temperature of 50°C and with the water mass content in feed of 10%.As shown in Fig.15,the separation factor of water and butanol was stable at the value up 200 and the permeate was almost at a constant value of 3.1 kg·m-2·h-1.The PSI was calculated to be 710 kg·m-2·h-1.The GO membrane still showed a good and stable performance after running for 1800 min.

Fig.14.(a)Optical images of AAO disk(left)and AAO supported GO membrane(right),(b)FESEM micrograph of GO membrane,(c)AFM image of the surface morphology of GO membrane,and(d)FESEM image of the cross-sectional morphology of GO membrane.

Table 1 PV performance of various membranes for butanol dehydration

4.Conclusions

Fig.15.The stability of GO membrane.

GO sheets with 2–4 layers and a typical 2D structure were prepared via a modified Hummers method.A lot of hydrophilic groups were detected on the surface and the edge of GO sheets.As-prepared GO dispersion was used to prepare GO membrane on the surface of ceramic micro filtration substrate via a facile vacuum-assisted filter method.Such supported GO membrane without any detected defects showed a good performance for the dehydration of butanol via a PV process.A centrifugal per-treatment at 10000 r·min-1for 40 min could remove the large size GO sheets efficiently and improve the separation properties.The water content in feed could influence the d-spacing of GO sheets and affect the performance of GO membrane.The GO membrane showed a high permeate of 3.1 kg·m-2·h-1and a separation factor of 230 at the conditions with a PV temperature of 50°C and a feed water content of 10%(by mass).After running for 1800 min,the GO membrane still showed a good stability for the dehydration of butanol in the PV process.Such GO membranes are potential candidates for the dehydration of butanol.