Preparation of C/CMS composite membranes derived from Poly ...

Desalination 249 (2009) 486–489Contents lists available at ScienceDirectDesalinationjournal homepage: www.elsevier.com/locate/desalPreparation of C/CMS composite membranes derived from Poly(furfuryl alcohol)polymerized by iodine catalystChengwen Song a,b , Tonghua Wang b, ⁎, Jieshan Qiu ba Environmental Information Institute, School of Environment Science and Engineering, Dalian Maritime University, 1 Linghai Road, Dalian 116026, Chinab State Key Laboratory of Fine Chemicals, Carbon Research Laboratory, Department of Materials Science and Chemical Engineering, School of Chemical Engineering,Dalian University of Technology, 158 Zhongshan Road, Dalian, 116012, ChinaarticleinfoabstractArticle history:Accepted 21 April 2009Available online 12 October 2009Keywords:Carbon membranesGas separationPoly(furfuryl alcohol)IodineC/CMS composite membranes derived from poly(furfuryl alcohol) (PFA) polymerized by iodine catalyst wereprepared. Gas separation performance was investigated by molecular probe study with pure gases (H 2 ,CO 2 ,O 2 , N 2 , and CH 4 ) at 25 °C. The pyrolysis behaviour of PFA was studied by TG and DTG. The surfacemorphology of C/CMS composite membranes was observed by SEM and HRTEM. The results show a C/CMScomposite membrane with uniform and defect-free thin top layer can be prepared by the PFA liquid in onlyone coating step. The C/CMS composite membranes have excellent gas separation properties for the gas pairssuch as H 2 /N 2 ,CO 2 /N 2 ,O 2 /N 2 and CO 2 /CH 4 , the permselectivities for above gas pairs in same sequence were124.72, 12.74, 9.12 and 15.91 respectively. Compared to carbon membranes derived from PFA polymerizedby acid catalyst, the carbon membranes obtained from PFA polymerized by iodine catalyst have slightlylower permselectivity, but higher permeance.© 2009 Elsevier B.V. All rights reserved.1. IntroductionCarbon molecular sieve (CMS) membranes have received muchattention as advanced materials for gas separation because of theirsuperior gas permeation performance, as well as their thermal andchemical stability compared to polymeric membranes [1–4]. Ingeneral, CMS membranes are prepared by pyrolysis of variouspolymers such as polyimide and derivatives [5,6], polyacrylonitrile(PAN) [7,8], phenol formaldehyde (PF) [9–11], polyvinylidenechloride–acrylate terpolymer (PVDC–AC) [12] and poly(furfurylalcohol) (PFA) [13–16]. Of these polymeric materials, the poly(furfuryl alcohol) (PFA), a cheap thermosetting resin with highcarbon content, has been regarded as a potential precursor for makingCMS composite membranes with high performance. As mentioned inour previous paper [17–21], the PFA has poor mechanics and elasticityproperties that are not favorable for forming a thin film. In addition,the commercial products of PFA featuring high viscosity are mostlychosen in preparation of CMS composite membranes. The commercialPFA needs to be dissolved in solvents to obtain a dilute solution withappropriate viscosity before coating onto the support, and because ofthis, the coating/carbonization procedures are usually needed to berepeated many times in order to obtain a CMS composite membranewith good gas separation performance. This makes the preparationtechnology more complex and costly, and often results in poorreproducible results. To tackle this problem, we propose that a poly⁎ Corresponding author. Fax: +86 0411 39893968.E-mail address: wangth@chem.dlut.edu.cn (T. Wang).(furfuryl alcohol) liquid made by polymerization of furfuryl alcohol isused as the dip-coating liquid to prepare the C/CMS compositemembrane, which is different from the commercial PFA process thatneeds to be conducted in solvents. In the polymerization of PFA, atraditional acid is generally chosen as catalyst, and we also preparedhigh performance gas separation carbon membranes successfullywith the acid catalyst [21]. Recently, we find that iodine catalyst canalso initialize the polymerization of furfuryl alcohol, and the PFApolymerized by iodine catalyst has a different chemical structurecompared to the PFA polymerized by acid catalyst. This may give us ahint that the PFA obtained by unique cross-linking style shouldcontribute another characteristic to separation performance of carbonmembranes. In this paper, we will adopt the PFA polymerized byiodine catalyst as precursor to prepare carbon membranes andinvestigate the performance of PFA-based carbon membranes.2. Experimental2.1. Preparation and characterization of supportThe coal-based carbon tubular supports were prepared by mixingthe coal particles with binder, and the mixture was extruded into atube with a 10 mm external diameter by a hydraulic extruder at 2.5–3.0 MPa. After drying at room atmosphere, the tubes were carbonizedup to 900 °C at a rate of 3 °C/min following Ar and were held for 1 h at900 °C before cooling back to room temperature naturally, whichleads to the final products, coal-based tubes that will be used as themembrane support.0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.desal.2009.04.006

C. Song et al. / Desalination 249 (2009) 486–489487The pore structure which is a characteristic of support werestudied at room temperature by bubble–pressure method withisopropanol as a wetting liquid and nitrogen as porometry gas. Theaverage pore size and pore size distribution were calculated accordingto the method recommended by Venkataraman [22]. The porosity wasmeasured following the criterion method of the China NationalStandards. The surface morphology of support was examined usingscanning electron microscopy (KYKY2800B).2.2. The preparation procedure of C/CMS composite membrane2.2.1. Synthesis of poly(furfuryl alcohol)The polymeric reaction of furfuryl alcohol was carried out in areactive vessel, in which the mixture of furfuryl alcohol and iodinecatalyst was added and stirred for 11 h at 35 °C to make a viscous poly(furfuryl alcohol) liquid. The polymerization reaction of poly(furfurylalcohol) is shown in Fig. 1.Fig. 2. TG and DTG curves of PFA.2.2.2. Preparation and characterization of C/CMS composite membraneThe viscous poly(furfuryl alcohol) liquid obtained was directlyused as a dip-coating liquid to prepare C/CMS composite membrane. Acoal-based carbon tubular support dried at 90 °C for 2 h was dippedinto the poly(furfuryl alcohol) liquid for 5min and removed from thecoating liquid at 10 cm/min. The coated membrane was first dried at40 °C for 12 h, and then dried at 80 °C for 24 h. The dried PFA-basedpolymeric membrane was carbonized in Ar up to 600 °C–900 °C at therate of 1 °C/min and held for 4 h.TG analysis (TGA, TGA/SDTA851 e Mettler-Toledo, Switzerland) inflowing nitrogen was conducted with a heating rate of 10 °C/min andat temperatures ranging from 100 °C to 800 °C to evaluate the thermalstability of poly(furfuryl alcohol) samples. The surface morphologiesof C/CMS composite membranes were observed by scanning electronmicroscopy (KYKY2800B). The microstructures of carbon membraneswere examined by high resolution transmission electron microscopy(HRTEM, JEM-2000EX operated at 100 kV).2.3. Molecular probe study of C/CMS composite membraneTo evaluate the capability of C/CMS composite membranes for gasseparation, molecular probe studies were respectively conducted at25 °C with a variable volume–constant pressure method to measurethe permeability of single component gas through the membranes.Tested gases included H 2 (2.89 Å), CO 2 (3.3 Å), O 2 (3.46 Å), N 2(3.64 Å), and CH 4 (3.8 Å). To ensure good reproducibility, all of theresults reported here are averaged based on the measurements of atleast 3 membranes prepared and tested under the same conditions.3. Result and discussion3.1. Thermal decomposition behavior of poly(furfuryl alcohol)TG is used to ascertain the thermal decomposition behavior of PFA(Fig. 2). It was found that the thermal decomposition temperature ofPFA is about 180 °C, and then PFA showed rapid thermal decomposition.The remaining weight is nearly 37.2% of its initial weight up to atemperature of 900 °C. DTG curves were also integrated to investigatethermal decomposition behavior of PFA. As observed, the decompositionprocess of PFA can be divided into three steps. The first steptakes place at 180–250 °C. Elimination of oxygen atoms from furanrings gives dominant contribution to this weight loss [23]. The majorFig. 3. SEM of C/CMS composite membrane.thermal degradation occurred from 250 to 700 °C, the mass loss of48.5% is ascribed to the transformation from furan rings to condensedaromatic ring [24]. It is meaningful to investigate the thermalbehavior and reactive dynamics for the second stage. After 700 °C,the phenomenon of weight loss is not obvious and 7.6% weight losstakes place in this stage.3.2. Morphology and microstructure of PFA-based carbon membraneFig. 3 shows the SEM microphotograph of the cross section of theC/CMS composite membrane. The resulting carbon membraneprocesses an asymmetric structure that consists of the top thin layerand the macroporous support. The top thin layer with a thickness ofaround 10 μm is very smooth and almost defect-free, whichdetermines the separation properties of carbon membrane, whereasthe macroporous support provides high mechanical strength ofcarbon membrane. A close adherence between the top thin layerand the support can be clearly seen. This evidences that a defect-freethin top layer over the C/CMS composite membrane can be preparedby coating the coal-based support using the viscous poly(furfurylalcohol) liquid polymerized by iodine catalyst in one single step.Fig. 1. Polymerization reactions of poly(furfuryl alcohol) polymerized by iodine catalyst.

488 C. Song et al. / Desalination 249 (2009) 486–489Fig. 4. HRTEM of C/CMS composite membrane.gases in carbon membranes. It can be seen that the gas permeance ofthe carbon membranes is in the order of H 2 >CO 2 >O 2 >N 2 >CH 4 ,which exactly follows the reverse order of kinetic diameter of thesefive molecules. Actually, the permeance of small gases through amolecular sieving material is following the above order. This alsoindicates unambiguously that for carbon membranes made in thepresent work, the mechanism involved in the gas permeation is amolecular sieving mechanism.Influence of pyrolysis temperature on gas separation performanceof PFA-based carbon membranes was investigated (as shown inTable 1). As the pyrolysis temperature increased from 600 °C to900 °C, the permeance of the selected gases sharply decreased, whilegas permselectivity increased. These results seem to be in agreementwith the trade-off relationship between gas permeance and permselectivityof membranes. The reasons for this phenomenon are thathigh temperature pyrolysis leads to carbon membranes to form morecompact amorphous carbon structure, in which the ultramicroporestructure is shrunk and becomes more favorable to the separation ofsmaller gases.3.4. Comparison in gas separation performanceSeparation performances of PFA-based carbon membranes polymerizedat acid catalyst and iodine catalyst are listed in Table 1 (PFAsynthesized by acid and iodine catalyst can be noted as PFA_A andPFA_I). As shown, the permeance of the carbon membrane derivedfrom PFA_I is higher than that derived from PFAA, whereas thepermselectivity is lower that derived from PFA-acid. The phenomenonmay attribute to the difference in chemical structure of two PFAs. Thecase is complex and needs further investigation.4. ConclusionsFig. 5. Variation of permeance with the kinetic diameter of penetrant gases.Fig. 4 reveals that the HRTEM images of the thin top layers of theC/CMS composite membranes were prepared by the pyrolysis of poly(furfuryl alcohol). As shown, the thin top layers are amorphousturbostratic carbon that is rich in ultramicropores, in which the layerplanesgraphite-like microcrystallines are embedded or dispersed innoncrystalline carbons [25]. The layer-planes in graphite-like microcrystallinesare oriented randomly, and are associated with each otherin an edge-to-edge way to form the walls of entangled pores thatare believed to function as the diffusion channels of gas molecules(the “white” regions represent the pore structure, while the “black”regions represent the carbon matrix) [26].3.3. Separation performance of PFA-based carbon membraneThe permeation performance of the carbon membranes wasmeasured at 25 °C. Fig. 5 shows the permeation results of pureA C/CMS composite membrane has been prepared by directlycoating a layer of PFA polymerized by iodine catalyst onto a porouscoal-based carbon tubular support followed by controlled pyrolysis.The as-obtained composite membranes show good permselectivitiesfor gas pairs such as H 2 /N 2 ,CO 2 /N 2 ,O 2 /N 2 and CO 2 /CH 4 .Thegastransport through C/CMS composite membrane is found to be anactivated diffusion process in which a molecular sieving mechanismis involved. The separation performance of C/CMS compositemembranes obtained using the novel process indicates that the C/CMS composite membranes with coal-based support prepared fromthe PFA polymerized by iodine catalyst are of potential for gasseparation.AcknowledgementThis work was supported by the National Natural Science Foundationof China (No. 20276008 and 20776024), the Visiting Scholar Foundationof State Key Laboratory of Fine Chemicals in Dalian University ofTechnology, Education Department of Liaoning Province (2009A098),and Young Teacher Foundation of Dalian Maritime University (DLMU-ZL-200817).Table 1Separation performance of PFA-based carbon membranes at different catalysts.Sample Permeance (mol/m 2 s Pa ×10 − 10 ) Permselectivity ReferenceH 2 CO 2 O2 N 2 CH 4 H 2 /N 2 CO 2 /N 2 O 2 /N 2 CO 2 /CH 4PFA_A_600 68.29 6.22 2.14 0.25 0.064 272.3 24.8 8.5 97.9 [19]PFA_I_600 108.45 11.08 7.93 0.87 0.50 124.72 12.74 9.12 22.23 This workPFA_I_700 88.68 8.09 5.85 0.61 0.14 146.50 13.36 9.67 57.57 This workPFA_I_800 27.64 2.82 1.63 0.15 0.04 183.62 18.73 10.82 64.85 This workPFA_I_900 12.48 1.33 0.76 0.06 0.02 196.47 20.95 11.89 79.60 This work

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