Ind. Eng. Chem. Res. 2006, 45, 6393-6404
6393
REVIEWS Use of Poly(furfuryl alcohol) in the Fabrication of Nanostructured Carbons and Nanocomposites Huanting Wang* and Jianfeng Yao Department of Chemical Engineering, Monash UniVersity, Clayton, VIC3800, Australia
Poly(furfuryl alcohol) (PFA) is a thermally cross-linked polymer that is synthesized from furfuryl alcohol (FA) derived from renewable saccharidic resources. The monomer FA is liquid at room temperature and has high solubility in water and many organic solvents, and its polymerization can be carried out under flexible conditions. Therefore, PFA has been very attractive as a precursor for PFA nanocomposites and carbon materials. This paper reviews research works concerning the use of PFA in the preparation of nanostructured carbons, PFA-based nanocomposites, and carbon-based nanocomposites. Specifically, structural evolution and properties of PFA during carbonization, and fabrication of nanoporous carbons through nanocasting process, are introduced. PFA-derived nanostructured carbons used as molecular sieve adsorbents, membranes, catalysts, and components for electrochemical and electronic devices, etc., are covered. The recent development for the fabrication and application of PFA-based nanocomposites is highlighted. Finally, future research into this topic is briefly discussed. 1. Introduction Poly(furfuryl alcohol) (PFA) is a common thermosetting resin that is usually synthesized by the cationic condensation of its monomer furfuryl alcohol (FA).1-8 As one of the most important furanic derivatives, the characteristics of FA were first reported over a century ago. Only some scattered reports appeared on PFA and its uses as adhesives, binders, and glassy carbon precursors until around the 1970s, however.2,9-13 Since then, many studies have been devoted to the understanding of FA polymerization mechanisms and characterizations of PFA.1,4,6,8,12,14-16 FA has been industrially converted from furfural originating from renewable saccharidic biomasses.3 The polymer chemistry of FA and other furanic derivatives can be found in a paper reviewed by Gandini and Belgacem.5 In general, FA polymerizes exothermically in the presence of a catalyst such as acid and iodine in methylene chloride, producing black, amorphous, and branched and/or cross-linked structures.1,17 In addition to a flexible choice of catalysts, the FA polymerization process can be carried out at different temperatures and in various solvents. Importantly, PFA (FA) is compatible with many organic polymers and inorganic materials, and it gives high carbon yield when pyrolyzing. Therefore, PFA has been not only important for the use as adhesives and binders18-22 but also widely used to synthesize nanoporous carbons, glassy carbons, and polymer nanocomposites for a wide range of applications, such as adsorbents, separation membranes, catalysts, and electrodes of fuel cells, lithium batteries, and electric double-layer capacitors, etc. The methods for preparation of porous carbons and structure control at the nanometer scale by using different templates and organic precursors were very recently reviewed.23-25 * Corresponding author. Phone: +61 3 9905 3449. Fax: +61 3 9905 5686. E-mail:
[email protected].
This review is intended to summarize research works dealing with the use of PFA (FA) in the fabrication of nanostructured carbons and nanocomposites. Despite some other carbon precursors being mentioned, a comprehensive review of nanocarbons and nanocomposites dealing with other precursors falls beyond the scope of this review. This review is organized into the following sections: (1) Introduction; (2) PFA Carbonization, Structural Evolution, and Properties; (3) Nanoporous Carbons by Nanocasting; (4) Carbon Adsorbents and Membranes; (5) Nanocomposites; (6) Conclusion; Acknowledgment; and Literature Cited. 2. PFA Carbonization, Structural Evolution, and Properties Foley and others13,26-36 have devoted significant efforts in studying the PFA carbonization process and illustrated its structural evolution during carbonization and corresponding carbon properties. On heating under inert conditions, PFA reacted to produce water, methane, carbon dioxide, carbon monoxide, and hydrogen. As shown in the conceptual model (Figure 1), a highly chaotic structure consisting of amorphous carbon and some aromatic microdomains were formed at low temperatures of 200-500 °C, leading to a relatively large average pore size. As PFA was carbonized at higher temperatures and/or for a longer period of heat-treatment time, the aromatic microdomains became larger in size accompanied by formation of a slightly more ordered structure in the short range. But the carbonized PFA appeared to be highly disordered over the long range. As the carbonization process proceeded, aromatic microdomains continued to grow and rearranged to more ordered structures; meanwhile, the size of micropores resulting from the packing of aromatic microdomains decreased. Figure 2 shows the carbon nanostructures directly observed by high-resolution transmission electron microscopy (HRTEM).
10.1021/ie0602660 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/18/2006
6394
Ind. Eng. Chem. Res., Vol. 45, No. 19, 2006
Figure 1. Conceptual model for structural evolution of PFA-derived carbon.26
Figure 3. Schematic illustration of fabrication of nanoporous carbons through nanocasting processes.
stages of heat treatment. Carbon-carbon composites treated at ∼2000 °C possessed high flexural strength and low oxidation rates.30 3. Nanoporous Carbons by Nanocasting
Figure 2. High-resolution transmission electron microscopy (HRTEM) images of PFA-derived carbons: (a) 400 °C, (b) 500 °C, (c) 800 °C, and (d) 1200 °C.37
TEM revealed that significant structure reorganization of PFAderived carbon occurred as a function of synthesis temperature.37 It was obvious that, by controlling the pyrolysis conditions, the properties of carbon materials derived from PFA could be readily altered. For instance, PFA-derived carbon pyrolyzed at 600 °C consisting of both locally ordered and globally disordered carbon at the nanoscale exhibited the maximum molecular sieving property for oxygen and nitrogen separation. The sample pyrolyzed at 800 °C was paramagnetic and open, displaying the highest oxygen loading at equilibrium. The 1000 and 1200 °C samples were even more paramagnetic than the 800 °C sample, but their pores were too narrow for oxygen to diffuse in (Figure 2d),38 despite large values of paramagnetism.37 In the presence of metal catalyst, the formation of graphite and diamond was promoted under diamond-forming pressures and temperatures.39 Furthermore, the electrical conductivity of PFAderived carbons increased when heat-treatment temperature increased, and a trend of semiconductor-to-metal transition appears at or above a pyrolytic temperature of 850 °C. This suggested that PFA-derived carbons were potentially useful in electronic devices such as lithium ion batteries.34 Fiber-matrix bonding in carbon fiber reinforced PFA resin had a significant influence on the development of matrix microstructure at all
Nanocasting (or so-called templating) is an emerging technique for fabricating functional nanostructured materials. In the fabrication of nanostructured carbons through the nanocasting process, FA is particularly attractive since FA is liquid at room temperature and miscible with water and many organic solvents; it can be easily polymerized at various conditions such as in vapor, liquid, and solution under heating and/or in the presence of catalyst. It is noted that FA has a molecular dimension of 8.43 × 6.44 × 4.28 Å,40 which is smaller than the channel sizes of most of the hard templates. To prepare porous carbon nanocasts, FA or FA solution is generally impregnated into template pores and then polymerized in situ by heating FAfilled template and/or exposing it to acid. After carbonizing PFA-template in an inert gas atmosphere at high temperatures (e.g., >400 °C), the template is removed through dissolution, leaving replicated porous carbon structures (Figure 3 Route 1).41-70 Pore wall thickness and structure are adjustable at a certain extent by varying FA loading and repeating the nanocasting process. In addition to preformed hard templates, PFAbased hybrid structures can be directly formed by incorporating an inorganic component or a thermoplastic component into FA/ PFA, followed by in situ polymerization and/or solvent evaporation. Porous carbons will finally be obtained by thermal decomposition of the thermoplastic component during carbonization or dissolution of the inorganic component after carbonization (Figure 3 Route 2).71,72 However, such a fabrication method would normally give rise to disordered porous structures. Nanoporous carbons with different pore sizes (i.e., 50 nm, macropore) will be discussed as follows. 3.1. Microporous Carbons. Zeolites and clays are crystalline and microporous materials with ordered pore structures, and they have been widely used as the templates in the fabrication of microporous carbons. For instance, PFA was carbonized in zeolite channels resulting in carbon/zeolite composites, and then zeolites were removed by HF and HCl acid treatment.41-47 Zeolite Y templated carbon was first demonstrated by Kyotani and co-workers.41 The microscopic morphology of carbons obtained by FA-impregnated zeolite Y reflected that of corresponding zeolite Y; however, carbons in the channels did not have any regular stacking structure due to spatial limitation of zeolite Y channels.41 Later, microporous carbons with structural regularity of zeolite Y were successfully prepared by using a
Ind. Eng. Chem. Res., Vol. 45, No. 19, 2006 6395
Figure 4. Ordered mesoporous carbon obtained by using SBA-15 as the template and FA as the carbon precursor: (a) transmission electron microscopy (TEM) image viewed along the direction of the ordered mesoporous carbon and the corresponding Fourier diffractogram; (b) schematic model for the carbon structure.56
two-step method involving PFA impregnation and chemical vapor deposition of propylene or acetonitrile.42-45,48 The microporous carbons had a three-dimensional nanoarray structure, whose arrangement was identical to that of supercages of zeolite Y. Surprisingly, the Brunauer-Emmett-Teller (BET) surface area was found to be as high as 3600 m2/g.43,44 Besides Zeolite Y, several other types of zeolites (zeolite β, ZSM-5, mordenite, and zeolite L) were employed as templates to the synthesis of high-surface-area microporous carbons. It was found that the carbon precursor filling method had a significant effect on the degree of regularity of long-range ordering in the carbons; the optimum filling procedure was strongly dependent on zeolite type. To obtain porous carbons with higher regularity, template zeolites should have a larger three-dimensional channel size (>0.6-0.7 nm), and posttreatment using chemical vapor deposition (CVD) was essential.45,48 Clay-templated carbons were prepared by introducing FA into the lamellae of clays such as montmorillonite (MONT), taeniolite (TAEN), saponite (SAPO), and smectite.49-55 Microporous carbons obtained after clay removal had nearly the same characteristics, e.g., film-like shape and highly stacked structure with small interplanar spacing. The spacing of carbons from MONT was as small as 0.34 nm. It was noted that the graphitizability of PFA-derived carbons was promoted in the presence of clays; the order of graphitizability was TAENcarbon > MONT-carbon > SAPO-carbon.49 Moreover, carbons prepared by carbonization of PFA within the smectite and TAEN matrixes possessed the sieving effects for molecular sizes between 0.36 and 0.6 nm,53,54 whereas high carbon content and high micropore surface area were observed in carbonTAEN nanocomposites.55 3.2. Mesoporous Carbon Structures. 3.2.1. Ordered Mesoporous Carbons. Joo and co-workers first demonstrated the fabrication of ordered mesoporous carbons by using mesoporous aluminosilicate SBA-15 as a template.56 SBA-15 mesopores were filled with FA by the incipient wetness technique, followed by acid-catalyzed polymerization of FA, forming a PFA layer on pore walls. The PFA layer was then converted to carbon inside SBA-15 template by pyrolysis under vacuum at a temperature up to 1100 °C. The mesoporous carbons with wellordered pipelike pores (CMK-5, Figure 4) were formed after the template was removed with hydrofluoric acid or NaOH aqueous solution. To study the effect of organic precursors on carbon structures and adsorption properties, FA along with other precursors, such as sucrose, xylose, acenaphthene, mesophase pitch, and petroleum pitch, were used to obtain inverse carbon replicas of SBA-15 and KIT-6. It was revealed that the structures of resulting ordered mesoporous carbons were mainly determined by the template used, and their adsorption properties
varied with the type of carbon precursor.57 In addition, facile carbonization of PFA was observed in the aluminosilicate form of the template.58 By changing the amount of the PFA inside the template pores, the inside diameter (the wall thickness) was slightly varied.56,57,59,60 CMK-5 carbons obtained after SBA-15 template removal exhibited very high nitrogen BET specific surface areas (∼2000 m2 g-1) and total pore volumes (∼1.5 cm3 g-1).59,61 Ordered mesoporous carbon (CMK-5) was synthesized via a nanocasting process by using SBA-15, instead of AlSBA-15, as the template, and by using FA as the carbon source and oxalic acid as the catalyst.62 Other mesoporous silicas and aluminosilicalites such as LPFDU-12,63 SBA-16,64,65 MCM-41 and MCM-48,46,65,66 and KIT657 were also employed as templates to prepare ordered mesoporous carbons using FA as the carbon source. The comparisons among carbons templated by clays, zeolites, and mesoporous materials were made by Meyers and co-workers.67 Templated carbons could replicate over long distances the periodicity of inorganic matrixes for the cases of zeolite and mesoporous materials routes. However, the carbons were not simply an exact negative of the template. The carbon porosity may be affected by carbonization, demineralization, and the extent of filling of template’s pore network. In terms of their properties, the porous carbons prepared from zeolite Y, beta, ZSM-5, and montmorillonite clay displayed significant voltage hysteresis on charge/discharge, and carbons prepared from zeolite Y displayed unique voltage curvers.67 3.2.2. Disordered Mesoporous Carbons. Lee and coworkers reported mesocellular carbon foams with large cellular pores (∼20 nm) and disordered uniform pores (∼4 nm) synthesized by using MSU-F silica as the template and PFA as the carbon source.68 MSU-1 was also used as the template to fabricate spherical wormlike mesoporous carbon particles whose pore size and porosity were easily tailored by changing FA impregnation and polymerization conditions.69 Such templates were prepared under neutral conditions using cheap sodium silicate as the silica source. Zarbin and co-workers synthesized glass-carbon composites by pyrolysis of PFA inside the pores of a commercial Vycor glass with a pore size of 8 nm.70 The glass-PFA nanocomposites could be prepared by polymerizing FA with or without oxalic acid as the catalyst, since acidic silanol groups of Vycor glass pores were able to catalyze FA polymerization. Mesoporous carbons were obtained by removing SiO2 from the glass-carbon composites with HF, and they possessed a low crystallinity which laid between graphite and the glassy carbon resulting from plain PFA resin.70 Another mesoporous carbon was prepared by removing silica from a carbon/silica nanocomposite obtained by hydrolysis of tetraethyl silicate (TEOS) co-condensed with FA. The pore size distribution of such a carbon strongly depended on the gelation conditions and the molar ratio of FA/TEOS. The mesoporous carbon with a narrow size distribution centered at 4 nm could be prepared by optimizing the sol-gel process.71 Strano and co-workers used poly(ethylene glycol), which has a negligible carbon yield upon pyrolysis, as the template to generate mesopores.72 The nanocasting process was dominated by both the molecular size of template and the rate of expulsion of decomposed template material during solid formation.72 3.3. Hierarchical Porous Carbons. The carbon monoliths with a hierarchical, fully interconnected pore structure were obtained from FA through nanocasting of hierarchical silicas with adjustable pore sizes (2-4 nm and 0.5-30 µm for mesopore and macropore, respectively).73 The replication of
6396
Ind. Eng. Chem. Res., Vol. 45, No. 19, 2006
Figure 5. Schematic illustration of the formation of carbon nanopillar arrays (left) and field emission scanning electron microscopy (FESEM) images of (a) top and (b) side views of a monolith of nanopillar arrays, (c) the individual pillars of glassy carbon, and (d) an EDS spectrum of the nanopillar carbon monolith.79
hierarchical porous silica enabled the preparation of hierarchical porous carbons with various shapes such as particles, rods, tablets, and membranes.73-75 A one-step impregnation procedure involving the use of a solvent for FA and the parallel incorporation of a catalyst was developed for fabricating carbon monoliths that possess micropores, one or two modes of mesopores and macropores.73 In addition, a mixture of FA and trimethylbenzene was used to infiltrate the silica templates during the nanocasting. The amount of FA introduced inside the pores of the silica was readily adjusted by varying the concentration of FA, and thus, pore structure and pore size distribution (PSD) of the resulting carbons could be conveniently controlled.76 When the silica pores were fully filled with FA, the carbon mesopores with a narrow unimodal PSD were derived exclusively from the silica framework. If the pores of silica template were partially infiltrated, the carbon obtained would possess two kinds of mesopores (bimodal PSD) since some unfilled silica pores coalesced after removal of silica walls.60,69 In this regard, ordered mesoporous carbon with a bimodal pore size distribution was fabricated by using SBA-15 as the template and diluted FA solution as the carbon source.77,78 3.4. Carbon Nanopillar Arrays, Nanotubes, and Patterns. Rahman and Yang used anodic alumina membranes on aluminum, which consisted of an array of parallel straight nanopores, as templates to fabricate nanopillar arrays of glassy carbon (Figure 5).79 Specifically, anodic alumina nanopores were filled with FA, followed by the polymerization of FA using zinc chloride solution as the catalyst at 80 °C. PFA was then pyrolyzed inside the nanopores. The solid or hollow carbon nanopillar arrays with a diameter of 50-60 nm were finally obtained by dissolving the templates with NaOH solution.79 By controlling the amount of FA infiltrated into straight pores of anodic alumina membranes, carbon tubes and tube arrays with controllable wall thicknesses were fabricated by the nanocasting approach (Figure 6).80,81 The carbon nanotubes asobtained exhibited an outer diameter of 30 or 230 nm and a length of 60 or 75 µm, which precisely reflected the channel diameter and the thickness of the template, respectively.80 The carbon nanotubes loaded with Pt, Ag, and Fe were also fabricated by such a template technique. Additionally, “tubewithin-tubes” structures were grown, and they showed a high
Figure 6. SEM and TEM images of the original carbon prepared by carbonizing PFA at 900 °C ((a) SEM, (c) TEM) and the sample further treated at 2800 °C ((b) SEM, (d) TEM).80
lithium ion intercalation capacity and had a high prospect for high-performance Li+ batteries.81 Schueller and others reported the fabrication of free-standing high-carbon microstructures by soft-lithographic techniques for glassy carbon microelectromechanical systems (MEMS).82-85 Polymeric precursors (PFA or copolymers of FA-phenol) were used to fabricate micropatterns using poly(dimethylsiloxane) (PDMS) molds (Figure 7).82-85 Microstructures pyrolyzed at 900 °C became electrically conductive, and they had a conductivity of close to 10-2 Ω cm. Elementary microelectromechanical functions using the glassy carbon microstructures were thus demonstrated.85 4. Carbon Adsorbents and Membranes 4.1. Adsorbents. 4.1.1. Modification of Porous Carbons by PFA Deposition. PFA-derived microporous carbons are a class of important carbogenic molecular sieves with a narrow
Ind. Eng. Chem. Res., Vol. 45, No. 19, 2006 6397
Figure 7. Scanning electron micrographs of carbon structures prepared by carbonization at 1000 °C of a copolymer of FA-phenol patterned by µTM. (A) and (B) The inner frame can be electrostatically actuated. This type of structure represents the sensing unit for an accelerometer. (C) Precursor to an interdigitated capacitor: the interdigitated comb structures become electrically isolated once the supporting frame is broken. (D) Optical deflector: the initial carbonization step at 1000 °C (in a tube furnace) was followed by heat treatment under Ar up to 1800 °C (in an induction furnace). Electrostatic actuation can induce angular deflection of the central plate. The angle of deflection of a light beam reflected off the surface of the central plate can, therefore, be controlled electrostatically.85
size distribution that are capable of distinguishing molecules on the basis of their sizes and shapes.26-28,86 PFA-derived carbons in powder form have been extensively studied for industrial gas adsorption and separation.24,25,27 To date, a number of methods have been available for preparing carbon adsorbents. The methods, including direct carbonization of bulk PFA resin and the nanocasting process, have been discussed in the previous two sections. Another common method to make carbon adsorbents is to modify other porous carbons using PFA to achieve better performances. For example, composite carbon monoliths were prepared by pyrolysis of PFA-impregnated compressed expanded graphite (CEG) blocks and further activation. The resultant materials composed of a graphite backbone and a thin microporous carbon layer derived from FA exhibited improved adsorption property.87,88 The stiffness of adsorbents could be improved by increasing pyrolysis temperature.89 The surface properties and microstructure of porous carbons prepared from PFA were likely modified by treating with oxygen and carbon dioxide gases90 and microwave plasmas of the same gases.91,92 Moreira and co-workers reported the modification of activated carbons by depositing PFA and carbonization under an inert atmosphere.93,94 The carbon molecular sieves obtained were examined to separate CO2/air in a fixed bed at room temperature, and they exhibited a high capacity for separating CO2 from air.93 The separation of CO2 from CO2/air mixtures was expected to be useful for capturing CO2 and, thus, lowering CO2 emission. The modified activated carbons used in the pressure-swingadsorption process showed a high O2/N2 selectivity in air separation.94 4.1.2. Microporous Carbon Nanoparticles and Spheres. Microporous carbon particles are of great interest because the diffusion of guest species through microporous carbons can be significantly manipulated by changing their particle sizes and shapes; microporous carbon particles would be useful as molecular sieves for developing carbon-polymer composite membranes that have shown high potential for industrial gas separations. Additionally, microporous spheres with function-
alized shells could provide them with specific catalytic, magnetic, electronic, optical, or optoelectronic properties and, thus, broaden their uses. Spherical porous carbon particles were first prepared by using the PFA atomization process a decade ago.74 The particles with diameters of 20-150 µm were easily achieved by adjusting the parameters of the aerosol generation technique. Wang and co-workers have very recently reported a novel approach to the synthesis of microporous carbon nanoparticles and spheres.95 An amphiphilic triblock copolymer was used to control formation of PFA particles in the course of polymerization of FA. A temporary silica barrier was employed to prevent the PFA particles from irreversibly aggregating during drying and carbonization. Finally, the silica was completely removed by using HF or NaOH solution. With this method, microporous carbon nanoparticles with a mean size of 45 nm were prepared and exhibited good dispersibility in various solvents (Figure 8a).95 When a silica temporary barrier was replaced by sulfuric acid treatment, “nonstick” PFA spheres with controllable sizes were formed. Colloidal microporous carbon spheres with an average size of 260 nm-1.5 µm were prepared by carbonization of such “nonstick” PFA spheres. The particle size distributions of two samples were exemplified in Figure 8b. The high dispersibility of the microporous carbon spheres was simultaneously realized by removing surface functional groups of the PFA spheres with the evaporation-induced concentrated sulfuric acid.96 4.2. Carbon Membranes. The selection of polymeric precursors is a key factor for preparing high-quality carbon membranes. PFA is among the most commonly used polymeric precursors such as polyimide and derivatives, polyacrylonitrile (PAN), phenolic resin, polyvinylidene chloride-acrylate terpolymer (PVDC-AC), and phenol formaldehyde.97 PFA is an amorphous polymer with a nongraphitizable structure and is suitable for depositing a thin layer on porous substrates by applying PFA solution or vapor deposition polymerization of FA. PFA-derived carbon membranes usually exhibit a narrow pore size distribution and, thus, high gas selectivity. Chen and Yang reported crack-free carbon molecular sieve membrane supported on a macroporous graphite substrate formed by repeatedly coating a layer of PFA and controlled pyrolysis.98 Steady-state diffusion fluxes of single-component and binary mixtures of CH4/C2H6 through the membrane were analyzed.98 Foley and co-workers reported high-selectivity carbon membranes fabricated by deposition of PFA on a tubular macroporous stainless steel support.99-103 The ultrasonic deposition was used to coat a thin and continuous layer of PFA in a precisely controlled manner. The crack-free carbon membranes having the nanopores with a narrow size distribution centered ∼0.45-0.5 nm100,104 exhibited high permeance of small molecules and high ideal separation factors (e.g., 30:1 for O2/N2, 178:1 for He/N2, and 331:1 for H2/N2).99 The ultra- and nanofiltration carbon membranes were also produced using spray deposition of PFA/poly(ethylene glycol) mixtures on macroporous stainless steel. Pores in the ultrafiltration range were found to vary with the average molecular weight of poly(ethylene glycol). 90% cutoffs of 2 × 104, 3.5 × 104, and 6 × 104 g mol-1 of dextran were measured for the membranes prepared by employing 2000, 3400, and 8000 g mol-1 of poly(ethylene glycol), respectively.105 Tsotsis and co-workers described the preparation of carbon molecular sieve membranes, their separation, and transport characteristics with gas mixtures.106 They used PFA as the polymeric precursor in the preparation of carbon films. Separation factors for CO2/CH4 in the range of 34-37 were obtained
6398
Ind. Eng. Chem. Res., Vol. 45, No. 19, 2006
Figure 8. Particle size distribution of microporous carbon nanoparticles and spheres from FA: (a) microporous nanoparticles prepared by using silica temporary barrier; their particle size distribution was measured by photon correlation spectroscopy in ethanol, toluene, and tetrahydrofuran;95 (b) microporous carbon spheres by polymerization of FA at room temperature, followed by concentrated sulfuric acid treatment. Samples C-F-RT and C-P-RT were prepared by using F127 and P123 as surfactant, respectively. Their particle size distributions were measured by photon correlation spectroscopy in ethanol.96 Table 1. PFA (FA) and Other Common Precursors Used in the Fabrication of Nanostructured Carbons precursor PFA
source commercial resin monomer FA monomer FA
vapor deposition polymerization
monomer FA
acid-catalyzed polymerization in surfactant solution acid-catalyzed copolymerized with TEOS acid catalyzed polymerization dissolved in methanol, dip-coating acid- or base-catalyzed polycondensation from gas phases base-catalyzed polycondensation in surfactant solution pyrolyzed under vacuum
monomer FA phenolic resin
polyimide & derivatives polyvinylidene chloride & poly(vinyl chloride) polyacrylonitrile
processing dissolved in acetone, and coated by ultrasonic spray infiltrated and polymerized under acid catalysis or heating
monomer FA commercial resin monomers phenol & formaldehyde monomers phenol & formaldehyde commercial aromatic polyamide hollow fibers monomers commercial resin copolymers of acrylonitrile & methyl methacrylate monomer acrylonitrile
sucrose
commercial product
propylene acenaphthene
commercial product commercial product
mesophase pitch
commercial product
petroleum pitch
commercial product
coal tar pitch
commercial product
example of products
ref
supported carbon membranes
99
templated microporous carbon and mesoporous carbon, cabon nanopillar arrays, carbon nanotubes supported carbon membranes, zeolite-carbon membranes microporous carbon particles
41,42,44,46
mesoporous silica, silica-carbon composites, mesoporous carbons, carbon supported catalysts supported carbon membranes templated carbon
108,109 95,96 71,110,111 112-117 118 119
ordered mesoporous polymers and carbons hollow fiber gas separation membrane microporous carbon adsorbents, and membranes supported gas separation membranes, carbon adsorbents
120
dissolved in DMSO, spinning
hollow fiber carbon membranes
125
vapor deposition in template pores, polymerized by γ-ray radiation treated with sulfuric acid before carbonization, low carbon yield chemical vapor deposition a mixture of acenaphthene and mesoporous aluminosilicate (template) was placed in an autoclave and heated infiltrated at temperatures above softening point infiltrated at temperatures above softening point carbonization and coating
templated microporous carbons
41
templated mesoporous carbons
66,126,127
templated microporous carbons templated mesoporous carbons
41 57
templated mesoporous carbons
57
templated mesoporous carbons
57
supported gas separation membranes
128
dissolved in dimethylform-amide and polymerized at elevated temperatures dissolved in N-methyl pyrrolidone
for binary mixtures of CO2 and CH4 and four-gas mixtures consisting of CO2, CO, H2, and CH4.106 Gordon and Cussler conducted theoretical studies on transport mechanisms of air separation through porous membranes, which include binary diffusion, Knudsen diffusion, Poiscuille flow, surface diffusion, capillary condensation, and critical flow.107 Zhang and coworkers coated microporous carbon layer on supported silicalite-1 membranes using PFA prepolymer solution.108 Carbon/ silicalite-1 composite membranes thus formed showed improved gas-separation selectivities, indicating the intercrystalline defects were effectively eliminated. No permeation of n-butane and i-butane through the composite membrane was detected at the
121 122,123 27,124
measurement temperatures up to 453 K, suggesting that the pore size of the composite membrane was ∼0.4 nm. By carefully oxidizing the carbon layer in air at 623 K, the pore size of the composite membrane was enlarged to ∼0.5 or 0.55 nm. Therefore, selective oxidation of the carbon layer was useful for modifying the pore size of the composite membrane.108 Wang and co-workers prepared supported carbon membranes by vapor deposition polymerization of FA at 90 °C on γ-Al2O3/ R-Al2O3 or glass/R-Al2O3 support tubes.109 After the second polymerization/carbonization cycle, the membranes had ideal selectivities of 10-13 for O2/N2, 80-90 for CO2/CH4, and 90350 for H2/N2 at room temperature. The permeances were
Ind. Eng. Chem. Res., Vol. 45, No. 19, 2006 6399
Figure 10. AFM cross-sectional images of Nafion 115 and Nafion-PFA nanocomposite membranes: (a) plain Nafion 115; (b) 3.9 wt % PFA; (c) 5.4 wt % PFA; (d) 12.4 wt % PFA.131
Figure 9. Scanning electron microscopy images of porous alumina supported zeolite A layer (a, b) and zeolite A-PFA nanocomposite membrane (c, d, e), and schematic structure of zeolite A layer (f): (a) top view of zeolite A layer; (b) cross-sectional view of zeolite A layer (3-5 µm thick) on porous alumina; (c) top view of zeolite A-PFA nanocomposite layer; (d) cross-sectional view of zeolite A-PFA nanocomposite layer on porous alumina; (e) cross-sectional view of zeolite A-PFA nanocomposite layer at higher magnification.40
dramatically increased when the permeation temperature was increased from room temperature to 150 °C; however, the selectivities dropped, e.g., one of the membranes had H2/N2 ideal selectivity of 30.109 In addition to PFA (FA), a large number of other precursors have been used to fabricate nanoporous carbon materials. Table 1 briefly summarizes PFA (FA) and other common precursors used in the fabrication of nanostructured carbons. PFA(FA) is obviously a complementary carbon precursor. 5. Nanocomposites 5.1. Zeolite-PFA Nanostructured Membranes. Very recently, Wang and co-workers developed a novel strategy that could potentially bypass the difficulty of preparing a defectfree pure zeolite membrane using in situ crystallization.129,130 A gas impermeable PFA99 was employed to fill up nonzeolitic mesopores of a nanocrystal-derived hierarchical porous zeolite layer on the substrate (Figure 9). Continuous zeolitic transport pathways for guest molecules were presumably provided by well-connected zeolitic channels in the hierarchical porous zeolite layer (Figure 9f). Deposition of PFA inside interparticle mesopores preloaded with a catalyst was carried out using vapor deposition polymerization (VDP) of FA. A zeolite A-PFA nanocomposite membrane thus fabricated exhibited an O2/N2 separation selectivity of as high as 8.2. The technique combining dip-coating of zeolite nanocrystals and VDP potentially provided a powerful tool to manufacturing large-scale, high-performance zeolite-based composite membranes. On the other hand, the methodology could be useful for studying the separation property of the intrinsic zeolite membranes.40 5.2. Polymer Nanocomposites. Cross-linked PFA is hydrophobic and chemically stable, while FA is soluble in water and many common solvents. PFA has been recently employed to
modify commercial Nafion (a perfluorosulfonic polymer) membranes by in situ polymerization of FA inside Nafion structures to lower methanol permeability through plain Nafion membranes, because high methanol crossover through plain Nafion membranes significantly lowered fuel efficiency and cell performance and, thus, impeded commercialization of direct methanol fuel cells. Atomic force microscopy (AFM) images (Figure 10) showed hydrophilic domain size decreased and cluster-like hydrophobic domains appeared, implying that chemically stable and hydrophobic domains were introduced into the hydrophilic zones of the Nafion structure. Such uniform Nafion-PFA nanocomposite membranes possessed substantially increased resistance to methanol permeation.131,132 The effect of the quantity of PFA incorporated on the properties of the Nafion-PFA nanocomposite membranes was also studied. The results showed that, at an appropriate PFA loading (e.g.,
