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available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Carbon molecular sieve membranes for biofuel separation Pei Shi Tin
Huey Yi Lin a, Rui Chin Ong a, Tai-Shung Chung
a Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore b School of Chemical and Life Sciences, Singapore Polytechnic, 500 Dover Road, Singapore 139651, Singapore
A R T I C L E I N F O
A B S T R A C T
A feasibility study was conducted to explore the potential application of carbon molecular
Received 22 April 2010
sieve membranes (CMSMs) in biofuel separation through pervaporation. ‘‘Defect-free’’ car-
Accepted 15 September 2010
bon/ceramic composite membranes were prepared as the separation barrier for the dehy-
Available online 21 September 2010
dration of ethanol. The supported CMSM carbonized at 650 C showed a great total flux of 4 kg/m2 h with a reasonably high separation factor of 50 at a feed temperature of 60 C and did not swell in high temperature operations. These carbon membranes demonstrate outstanding separation performance and offer a viable alternative to current membranes for the separation and purification of biofuels. 2010 Elsevier Ltd. All rights reserved.
The depletion of non-renewable resources, oil price instability and global warming has raised general public awareness in the pursuit of alternative renewable energy sources. Biofuel, a sustainable energy offers a strategic solution in addressing current energy and environmental crises that emerge from a heavy dependence on fossil fuels. According to a study performed by Freedonia Group Inc., the world demand for biofuels will experience a strong annual growth of 20%, overall reaching 92 million metric tons by the year 2011.1 Biofuels like ethanol are currently produced from renewable processes such as the fermentation of glucose. Currently, conventional distillation techniques are employed to concentrate the ethanol extracted from the fermentation broth (3–8 w/w% ethanol) to an azeotropic limit of 95 w/w% [1,2]. However, a higher purity of ethanol (>99.5 w/w%) is required for combustible fuel application. Despite the advancements presented in the development of biofuel technology, many challenges in biofuel production and purification must be overcome before
it can be widely accepted as a worthy substitute for fossil fuels. The global focus on greener technologies also requires the development of separation techniques that are both energy efficient and environmentally friendly. Today, membrane processes have progressively matured and are industrially available for the large scale separation of various mixtures within the chemical and biochemical industries. They are recognized as being economical and a feasible substitute for conventional energy-intensive separations. Pervaporation is an energy efficient combination of membrane permeation and evaporation.2 A liquid stream containing two or more components is brought into contact with one side of a non-porous membrane while vacuum or gas purge is applied to the other side of the membrane.3 One or more components, due to the preferential sorption of the membrane to the components, will be adsorbed to the membrane surface. The adsorbed components then diffuse through the membrane and finally desorb at the permeate side of the membrane as vapor due to the applied vacuum or gas purge . Pervaporation appears to be a very promising and practical alternative
* Corresponding authors: Fax: +65 67791936. E-mail addresses: [email protected]
(P.S. Tin), [email protected]
(T.-S. Chung). 1 Davis J., Report: world biofuel demand will continue to skyrocket,
; [accessed 07.09]. 2 Mahesh Kumar S., Pervaporation: an overview,
; [accessed 12.09]. 3 Vane L., What is pervaporation?
; [accessed 12.09]. 0008-6223/$ - see front matter 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.09.031
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to facilitate biofuel separation and purification from biomass . This emerging membrane-based separation technique permits the separation of azeotrope, close-boiling solutions, thermally sensitive compounds, as well as the removal of dilute species in a mixture . In addition, pervaporation possesses unique features of low energy consumption and minimal contamination. Polymeric membranes are widely adopted as separation materials in the field of pervaporation for solvent dehydration and separation of organic mixtures. However, pervaporation processes conducted with polymeric membranes are prone to poor stability as well as low chemical and thermal resistances [5–7]. Their application in the adverse environment is further restricted by unfavorable swelling at high temperatures and high feed or water content, which subsequently results in the decline of their separation abilities with respect to time . Most modification attempts, namely polymer blending, grafting, cross-linking and mixed matrix membranes can only marginally improve the separation performance and durability of polymeric membranes [9–12], and it is an uphill task for polymeric membranes to breach the trade-off between productivity and selectivity. The shortcomings of polymeric membranes have spurred the development of new materials and technologies to accomplish the functional requirements of biofuel separation. In response to the above limitations, microporous inorganic membranes have emerged as promising materials to conquer the challenges and competition in current membrane-based separation technologies . Generally, besides having superior thermal stability and chemical resistance, they possess a unique feature of anti-swelling at high feed or water concentrations, elevated temperatures and long operation hours . Therefore, these membranes including zeolites, silica, alumina and carbon membranes with excellent separation capability (through molecular sieving) are suitable for a broad range of applications [5,14,15]. Nevertheless, there are drawbacks reported in the preparation and properties of the above-mentioned membranes. Particularly, in addition to the instability of silica against water , it is very tricky to synthesize crack-free zeolite membranes . Consequently, there is a growing research interest in using carbon molecular sieve membranes (CMSMs) for separation tasks, as they compete favorably with silica and zeolite membranes. CMSMs are highly porous materials and possess a dis˚. tribution of small selective pores in the order of 3–6 A Preparation of carbon composite membranes by depositing the carbon film on a macroporous substrate appears to be an effective solution to overcome the brittleness of CMSMs. Thus, CMSMs have been widely studied for precise discrimination between the gas molecules of similar molecular dimensions [18–20]. In addition, their microstructure and other properties can be tailored through controlling the pyrolysis conditions, as well as pre-/post-treatment. In general, despite their stability in aggressive environments, CMSMs attain exceptional productivity with high selectivity for separation tasks. In view of that, the CMSMs with molecular filtering properties show a great application potential for pervaporation that involves harsh solvent environments, although it is a rarely explored material for this separation technique. For example, it is definitely possible to fabricate a CMSM with
˚ for the dehydration of ethaeffective pore size of about 3–4 A ˚ ) (the molecular size of water = 2.8 A ˚ ). nol (4.4 A To the best of our knowledge, there were very few investigations have been conducted on the application of CMSMs supported on inorganic porous support for pervaporation. Recently, a preliminary relevant study was reported by Dong et al. , where their tetramethylamonium bromide (TMAB) carbon membrane showed a high separation factor in the pervaporation of water/ethanol and water/isopropanol mixtures at low temperature. Nonetheless, the fluxes obtained in their study are low as compared to those obtained by polymeric membranes. In the light of above-mentioned advantages, it is worthwhile to study, quantify and improve the separation performance of CMSMs for pervaporation. A feasibility study of conceptual demonstration was therefore carried out to examine the use of CMSMs as a medium for the dehydration of ethanol. Efforts were also focused in enhancing the separation performance of carbon membranes. Moreover, the influence of pyrolysis and operating temperature, as well as the effects of oxidation will be discussed in this pioneering research report.
Preparation of supported CMSMs
InocepTM a-alumina tube M20 supplied by Hyflux SIP Pte Ltd. was used as the porous support for CMSMs. The dimensions of the tube are: OD · ID = 3.7 · 2.7 ± 0.1 mm; and average pore size = 20 nm. The tubes were cut into lengths of 8 cm. The ceramic supports were ultrasonically washed in DI water for 30 min and vacuum dried at 120 C for at least 2 h before dip-coating in polymer solutions. Firstly, 5 w/w% polyethylene glycol (PEG) 4000 solution, a thermally labile polymer that completely decomposed at 500 C (Fig. 1) was chosen as a base coat solution to prevent the intrusion of polyimide solution during the coating step. The selective carbon layer was prepared by a solution of commercially available polyimide – Matrimid (from Ciba Polymers) in N-methyl-2-pyrrolidone. The supported CMSMs were then fabricated by dip-coating the 12 w/w% of Matrimid solution onto the porous support. The procedure was repeated once with 10 w/w% of Matrimid solution to achieve pin-hole free membrane after carbonization. The composite polymeric membranes were subjected to carbonization after solvent evaporation in vacuum oven at
Fig. 1 – Thermal degradation of PEG 4000 analyzed by TGA.
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Fig. 2 – Steps involved in the pyrolysis process for the final temperature of (a) 650 and (b) 750 C.
250 C overnight. The pyrolysis was performed using a Lenton horizontal vacuum tube furnace Model VTF 12/50/550. The detailed pyrolysis protocols are shown in Fig. 2.
X-ray pattern. The following Bragg’s equation is used to calculate the d-space:
where d is the d-space, h is the diffraction angle, k is the X-ray wavelength and n is an integral number.
A laboratory scale pervaporation unit was designed for membrane’s performance test and the details of the set-up were described elsewhere . A feed solution of 50 w/w% ethanol/water was used. It was found that the feed composition varied less than 0.5 w/w% during the entire experiment and can be therefore considered constant. The operational temperature was 60 C. The feed flow rate was maintained at 0.5 l/min for each module. The permeate pressure was maintained less than 5 mbar by a vacuum pump. Retentate and permeate samples were collected after 2 h conditioning. The flux and separation factor of CMSMs were calculated based on the equations reported elsewhere . The flux and separation factor are converted into permeance and selectivity using methods reported in the same literature.
The morphology of supported tubular CMSMs prepared in this work is studied using a Jeol JSM-5600LV scanning electron microscope (SEM). Wide angle X-ray diffraction was performed to determine the interchain spacing (d-space) of CMSMs using Bruker X-ray Diffractometer (Bruker D8 advanced diffractometer) at room temperature. The d-space can be determined from the peak center obtained on each
nk ¼ 2d sin h
Results and discussion
Fig. 3 shows the SEM images of a supported tubular CMSM pyrolyzed at 650 C. The integrally symmetric carbon layer is coated uniformly on the porous ceramic support with no visible delamination. The thickness of the selective CMSM layer is approximately 5 lm. The scanning electron microphotograph of outer surface (3b) also reveals that the continuous carbon film is free of cracks and pin-holes, a desirable membrane structure required for separation, although it does not appear as a smooth surface. Supported CMSMs prepared from the carbonization at 750 C also displayed similar morphologies with thinner selective carbon layer slightly less than 5 lm. This is due to the higher weight loss or greater thermal decomposition of the polymer during heating process at the higher pyrolysis temperature. It has been observed that the polymer concentration and number of layers of coating is one of the crucial parameters in controlling the morphology and thickness of the separating layer in order to achieve ‘‘defect-free’’ composite membranes. Table 1 summarizes the pervaporation performance of composite CMSMs prepared at various conditions. The CMSMs from this preliminary study exhibit promising separation
Fig. 3 – Scanning electron microphotograph of supported CMSMs prepared through carbonization at 650 C.
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Table 1 – Pervaporation performance of supported tubular CMSMs prepared from carbonization of matrimid. Membrane ID
Supported tubular polymeric membrane CMSM-650b
Total flux (g/m2 h)
EtOH permeance (g/m2 hr kPa)
Permeate (H2O w/w%)
60 85 60
98.02 98.42 95.23
50 62 21
72.47 133.21 29.70
3907 4131 634
223.22 87.92 35.34
3.08 0.66 1.19
Water permeance (g/m2 hr kPa)
Matrimid precursor. Supported CMSM pyrolyzed at 650 C. Supported CMSM pyrolyzed at 750 C.
capability. A separation factor of 50 coupled with a high flux of about 4 kg/m2 h is achieved by the supported CMSMs carbonized at 650 C, under an operating temperature of 60 C. The flux and separation factor of the CMSMs are significantly higher than its polymeric precursor (Table 1) and many other polymide precursors . Similarly, the permeance and selectivity achieved by carbon membranes are very promising for the dehydration of ethanol. This implies that the separation capability of membranes was greatly improved after carbonization. It can produce microporous CMSMs with molecular sieving properties to resolve the trade-off problem between the permeation flux and separation efficiency for pervaporation.
Literature data of pervaporation performance for ethanol dehydration is collected from different studies and summarized in Table 2. It can be seen from Table 2 that CMSMs exhibit comparable or even better ethanol dehydration performance than other types of membranes. Furthermore, CMSMs exhibits superior flux performance compared to other polymeric membranes. This promising trend makes CMSMs worth for any further research in alcohol dehydration applications. A more extensive literature summary on various membrane performances for alcohol dehydration via pervaporation can be found in a review paper done by Chapman et al. .
Table 2 – Summary of literature data from various studies on pervaporation performance of ethanol dehydration. Membrane
Polyaniline Poly vinyl alcohol (PVA) PVA
Zeolite NaA Silicalite/ polydimethylsiloxane (PDMS) Sodium-polysulfone (NaPsf) Sodium sulfonate polysulfone CMSMs-650 TMAB carbon membrane PVA/polyacrylamide (PAAM) Aromatic polyimide PI2080 PVA/poly(acrylic acid) (PAA) Natural rubber/ crosslinked PVA a
Feed Ethanol temperature concentration (C) in feed (w/w%)
Total flux (g/m2 h)
Not mentioned 40
30 50 50
95 95 95
16.4 30.8 45–53
500 38 5900–28,300
Crosslinked by glutaraldehyde Crosslinked by glutaraldehyde and modified by monochloroacetic acid
Supported on tubular aalumina Supported on a-alumina Interpenetrating network (IPN) membrane
IPN on poly(ether sulfone) support Semi-IPN embedded with zeolite 4A
Estimated values from graphs.
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Research studies have revealed that the carbonization temperature has remarkable effects on the microstructure of resultant CMSMs [18–20]. Hence, an experiment was carried out to study the effect of pyrolysis temperature on the pervaporation performance of CMSMs. As shown in Table 1, the permeation flux and permeance decreases unselectively when the pyrolysis temperature increases from 650 to 750 C. This is probably due to the densification of carbon selective layer at an elevated pyrolysis temperature. Many studies have confirmed that pyrolysis at high temperatures may cause pore shrinkage and the conversion of amorphous microdomains to aromatic microdomains. The alignment of graphitic structures shifts the pore size distribution towards smaller micropores. Consequently, the effective pore size may be too narrow for the dehydration of ethanol, triggering a decline in the permeation flux as well as the separation factor. This can be verified from the d-space of CMSMs obtained from XRD in Table 3 as the CMSM pyrolyzed at 750 C has a smaller average d-space than CMSM pyrolyzed at 650 C. The XRD results as seen in Table 3 have further proved that ˚ CMSM pyrolyzed at 650 C has an average d-space of 3.75 A which is capable of separating water from ethanol solution by size exclusion. Nonetheless, CMSMs derived from the pyrolysis of polymeric precursors are well-known with distinctive features, where the final membrane’s morphology and microstructure can be tailored to obtain desirable permeation properties. Since this is a preliminary study on supported CMSMs for pervaporation, it is reasonable to speculate that the membranes’ separation efficiency can be substantially improved after process optimization. Besides refining the coating steps to obtain a thinner selective layer, the performance optimization can be also achieved by tailoring the microstructure and pore size of the carbon selective layer through controlled pyrolysis, preand post-treatment, to allow for precise discrimination between the penetrating molecules. In view of the fact that carbon membranes are hydrophobic in nature, modification to improve its hydrophilicity is recommended to further enhance the dehydration performance. The advancing contact angle (88 ± 4) measured by a KSV Sigma 701 Tensiometer with distilled water confirms the hydrophobic properties of CMSMs. Oxidation, an established posttreatment method for CMSMs, seems to be a simple but effective way to alter the hydrophobicity of carbon membranes . Oxygen (O2) would readily chemisorb to the carbon surface and form carbon–oxygen complexes, which is the usual case when a carbon membrane is exposed to air/O2. This oxygen-containing surface acts as primary sites for water sorption and attracts more water molecules. On the other hand, a major drawback of oxidation is that it broadens the pore
Table 3 – D-space of supported tubular CMSMs prepared from carbonization of matrimid obtained from XRD. Membrane ID ˚) d-Space (A a b
Supported CMSM pyrolyzed at 650 C. Supported CMSM pyrolyzed at 750 C.
size of resultant CMSMs. Thus, the careful control of oxidation of parameters, e.g., the final temperature and dwelling duration is necessary. In brief, high performance pervaporation membranes, with superior separation efficiency and stability can be prepared through controlled pyrolysis and posttreatment [36–38]. On top of these, the influence of operating temperature on pervaporation performance was also examined, since the swelling of polymeric membranes at elevated temperatures has been identified as one of the major problems in pervaporation. The general performance trend for polymeric membranes in pervaporation demonstrates an increase in the permeation flux accompanied by a decline in selectivity when the operating temperature increases [39,40]. This phenomenon is not surprising, as high operating temperature enhances the segmental motion of polymer chains, as well as increases the free volume of the polymer. The thermally-induced motion of polymer chains facilitates the permeation of molecules across the membranes, but undermines its ability to discriminate between the penetrants. In contrast with the observation in polymeric membranes, a higher operating temperature results in an increment in the separation factor and selectivity of CMSMs without compromising the permeation flux (Table 1). This finding is very encouraging as it indicates the anti-swelling properties of CMSMs at high temperature operations. Nevertheless, the permeance for both ethanol and water declined with increasing pervaporation temperature. It is believed that besides greater kinetic energies of penentrants at an elevated temperature, other effects such as increased vapor pressure driving force on the feed side of membranes may also have significant influence on the transport of molecules through membranes. Therefore, more studies are required to investigate the effect of operating (pervaporation) temperature on the separation performance of carbon membranes. On the other hand, further research experiments are also mandatory for subsequent confirmation on the thermal stability of CMSMs.
Supported CMSMs were prepared from dip-coating of polyimide solution on the microporous ceramic support. This is a conceptual demonstration study to investigate the feasibility of using CMSMs for pervaporation. The preliminary results show the CMSMs with molecular-filtering feature exhibiting impressive separation capability for ethanol dehydration. Moreover, CMSMs stand out from organic membranes for their excellent thermal and chemical stability, as well as good processability. Hence, it appears that carbon membranes may be considered as a viable material in pervaporation processes for biofuel separation.
Acknowledgements The authors would like to express sincere gratitude to A*Star for funding this research with the Grant number of R-398-000044-305 (Nanoscience and Nanotechnology Initiative) and R279-000-288-305. Special appreciation is due to Dr. Wang
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Yan, Mr. Sukitpaneenit Panu and Mr. Yong Kah Jin Joel for their valuable suggestions and kind assistance. Thanks are also due to Dr. Teoh May May for her helps on contact angle measurements.
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