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Synthesis of carbon nanotubes and porous carbons from printed circuit board waste pyrolysis oil
Journal of Hazardous Materials 179 (2010) 911–917
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Synthesis of carbon nanotubes and porous carbons from printed circuit board waste pyrolysis oil Cui Quan, Aimin Li ∗ , Ningbo Gao Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Linggong Road 2, Dalian 116024, PR China
a r t i c l e
i n f o
Article history: Received 16 September 2009 Received in revised form 3 February 2010 Accepted 22 March 2010 Available online 25 March 2010 Keywords: PCB waste pyrolysis oil Resource utilization CNTs Porous carbons
a b s t r a c t The possibility and feasibility of using pyrolysis oil from printed circuit board (PCB) waste as a precursor for advanced carbonaceous materials is presented. The PCB waste was first pyrolyzed in a laboratory scale fixed bed reactor at 600 ◦ C to prepare pyrolysis oil. The analysis of pyrolysis oil by gas chromatography–mass spectroscopy indicated that it contained a very high proportion of phenol and phenol derivatives. It was then polymerized in formaldehyde solution to synthesize pyrolysis oil-based resin which was used as a precursor to prepare carbon nanotubes (CNTs) and porous carbons. Scanning electron microscopy and transmission microscopy investigation showed that the resulting CNTs had hollow cores with outer diameter of ∼338 nm and wall thickness of ∼86 nm and most of them were filled with metal nanoparticles or nanorods. X-ray diffraction reveals that CNTs have an amorphous structure. Nitrogen adsorption isotherm analysis indicated the prepared porous carbons had a Brunauer–Emmett–Teller surface area of 1214 m2 /g. The mechanism of the formation of the CNTs and porous carbons was discussed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Advanced carbonaceous materials (ACMs) have drawn great attention throughout the world due to their particular microstructures, unique properties and great potential applications in many fields. Phenolic resin, which can be obtained from various combinations of phenols and aldehydes by using either a base or acid as the catalyst, is one of the well-used synthetic precursors for the production of ACMs such as glass-like carbons [1], carbon fibers [2], porous carbons [3,4], carbon nanotubes (CNTs) [5] and carbon membranes [6,7]. In order to develop a more competitive carbon material, it is necessary to consider developing carbon materials from the renewable resource. One of the methods to derive renewable resource is the conversion of waste substances to energy. Recently, studies from all over the world have pointed to Eucalyptus tar pitch, which is generated as a by-residue of Eucalyptus wood pyrolysis in charcoal production, as having one of the greatest potentialities as precursors of ACMs. Qiao et al. [8] successfully developed carbon fibers (maximum strength and modulus: 632 MPa and 44 GPa), activated carbon fibers (surface areas: 450–1600 m2 /g), and carbon films (thicknesses: 1–6 m) from biomass tar. Prauchner et al. [9] investigated the structural
∗ Corresponding author. Tel.: +86 0411 8470 7448; fax: +86 0411 8470 7448. E-mail address: [email protected] (A. Li). 0304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2010.03.092
evolution of Eucalyptus tar pitch processing towards carbon materials under carbonization. Printed circuit board (PCB) waste from waste electrical and electronic equipment is made of a copperclad laminate that mainly consists of 15% epoxy resin, 30% glass cloth filament, 22% copper coils, metal (Sn, Pb, Fe, Ni, etc.) and Br. One possible method of recycling PCB waste and recovering both the organic and non-organic fractions is pyrolysis. In the pyrolysis process, three products are typically obtained: liquids, gases and residues, which can be used as fuels or source of chemicals. A number of studies on the pyrolysis of PCB waste have been reported, most of which were dedicated to investigating the pyrolysis product characteristics [10–12], pyrolytic kinetics [13,14] and brominated compound formation and fate [15,16]. However, there are few studies on the use of PCB waste pyrolytic by-products, especially pyrolysis liquid. Therefore, advances in the knowledge of the characteristics and potential reuse routes of liquid product obtained in PCB waste pyrolysis are important. PCB waste pyrolysis liquid, which contained high concentrations of phenol and other aromatic compounds [10,11,16], may be used as chemical feedstock, whereas their separation and purification are complicated and costly because their contents are very limited. Hence, its application as a carbon source may be more feasible since phenol and phenol derivatives are the major components. Undoubtedly, it would be economically valuable and environmentally sound if PCB waste pyrolysis oil could be used to produce carbon materials with a high added value. To the best of our knowl-
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pyrolysis oil sample was polymerized with 6.5 g 37% formaldehyde solution in a heating magnetic whisk by using ammonium hydroxide as catalyst at 95 ◦ C for 4 h. After synthesis, the resin was cured by heat treatment for 2 h at 60 ◦ C, followed by 12 h at 120 ◦ C. The obtained resin was then ground into fine powder with a grinder (Retsch, ZM 200). 2.3. Preparation of CNTs and porous carbons
Fig. 1. Diagram of the fixed-bed reactor system. (1) electric furnace, (2) reactor tube, (3) PID controller, (4) thermocouple, (5) water condenser, (6) liquid collector and (7) glass wool filter.
edge, up to now, there are no reports about the preparation of ACMs with PCB waste pyrolysis oil in literature. In this paper, the synthesis method of the CNTs and porous carbons from PCB waste pyrolysis oil was described in detail. Morphologies and structures of the resulting products were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and nitrogen (N2 ) adsorption isotherm techniques. Formation mechanism of products was also discussed. 2. Experimental 2.1. Preparation of PCB waste pyrolysis oil PCB waste used in this study was obtained from a local PCB factory and made from glass fiber reinforced epoxy resin (referred to commercially as FR-4). The pyrolysis experiment was performed under static atmospheric conditions in a laboratory scale fixed bed reactor (see Fig. 1). The reactor tube measured 375 mm in length by an internal diameter of 57 mm and was externally heated by a 2.5 kW electrical furnace. The temperature was measured by a K-type thermocouple placed at the center of the reactor tube and controlled by a proportional–integral–differential (PID) controller. 160 g PCB waste sample was placed in the reactor tube at the start of the experiment and then the reactor tube was sealed and purged by N2 to remove air inside. When the temperature of the electric furnace reached 600 ◦ C, the reactor tube was immediately inserted into the electric furnace. The temperature of the reactor tube was ramped to 600 ◦ C and maintained for 30 min to ensure that pyrolysis of the sample was complete. The product leaving the reactor passed through a water-cooled condenser and collected in a liquid trap. These liquid products contained aqueous and oil phases, which were separated and weighed. In addition, a glass wool filter was used to remove any oils that were not trapped by the condenser. After pyrolysis, the solid char was removed and weighed, and then the gas yield was calculated from the material balance. The experimental result of PCB waste pyrolysis, which was conducted at 600 ◦ C final temperature under static atmosphere, was obtained as follows: Yield of aqueous phase: 6.06 wt% Yield of residue: 63.06 wt% Yield of oil: 20.10 wt% Yield of gas: 10.78 wt% 2.2. Synthesis of pyrolysis oil-based resin The pyrolysis oil-based resin, which was used as a precursor in this work, was synthesized through the following procedure: 5.0 g
The synthesis process of the CNTs is described as follows: 1 g resin mixed with 20% ferrocene (w/w) and homogenized in ethanol in continuous stirring at 50 ◦ C until uniformity was reached. After alcohol evaporation, the mixture was ground into fine powder and loaded on a ceramic boat which was placed inside a stainless steel tubular reactor (I.D. = 57 mm). The mixture was heated to 200 ◦ C (5 ◦ C/min) in air with 1 h soaking time, and then up to 900 ◦ C with a heating rate of 3 ◦ C/min in a flow of N2 with holding periods for 1 h at 900 ◦ C. In order to prepare porous carbons, 1 g resin was soaked in a concentrated solution of KOH to yield a KOH/resin weight ratio of 4/1 for 3 h at 85 ◦ C and then the resin–KOH slurry was subjected to dry at 110 ◦ C for 24 h. The obtained solid product was first ground into powder, then transferred to a ceramic pan, and finally, carbonized in an oven under N2 atmosphere at a heating rate of 10 ◦ C/min from room temperature to 700 ◦ C and soaked for 2 h before cooling. After cooling, to remove the remaining KOH, the char was washed by stirring with 1 mol/L HCl solution at 85 ◦ C for 30 min, followed by filtration. The acid-washed sample was then washed by hot distilled water and filtered. This washing and filtration steps were repeated until the filtrate became neutral. At last, the washed char was dried at 110 ◦ C for 24 h. So, porous carbon sample was prepared. Here in this paper the yield of CNTs and porous carbons was simply defined as the weight ratio of final carbons to the initial raw materials used. 2.4. Characterization 2.4.1. Ultimate analysis The oil from pyrolysis of PCB waste was kept in a closed brown glass bottle in refrigerator. The carbon (C), hydrogen (H) and N contents of the pyrolysis oil were performed on a CHN analyzer (Elementar, VarioEL III). 2.4.2. Gas chromatography–mass spectroscopy (GC–MS) analysis The obtained pyrolysis oil was distilled at atmospheric pressure, and the fraction whose boiling point was lower than 300 ◦ C was subjected to GC–MS analysis. The GC–MS was an Agilent GC/6890N coupled with MSD/5975. EI mode was adopted. The analytical column was an HP-5MS capillary column (30 m × 0.32 mm × 0.25 m). Helium was used as carrier gas at a constant flow of 1.0 ml/min. The injector temperature was 280 ◦ C and the oven was held at 40 ◦ C for 3 min, then ramped to 280 ◦ C at 5 ◦ C/min, and then held for 20 min. Typical operating conditions were ionization energy, 70 eV; ion source temperature, 230 ◦ C; and scans per second oven mass range electron (m/z), 50–550. Identification of the GC–MS spectral features was achieved with the use of a built-in library. All library-matched species exhibited the degree of match better than 90%. 2.4.3. Thermogravimetric (TG) analysis In order to simulate the carbonization property of the pyrolysis oil-based resin, the resin was subjected to a thermal degradation on a WCT-1C thermobalance under N2 atmosphere. Approximately 5 mg resin powder was placed in an alumina pan, and then heated from room temperature to 800 ◦ C at a heating rate of 10 ◦ C/min
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Fig. 2. SEM photographs of the prepared pyrolysis oil-based resin.
and kept isothermal for 10 min. The carrier gas (N2 ) flow rate was maintained at 30 ml/min during experiment. 2.4.4. SEM analysis A JEOL JSM-5600LV was employed for SEM observation of the prepared resin, CNTs and porous carbons. Each powder sample was mounted onto a double-side sticky tape over aluminum stubs and coated with gold under vacuum prior to the study. 2.4.5. TEM analysis The morphology of the prepared CNTs was also examined with TEM using a Tecnai G220 S-Twin microscope operated at 200 kV. Specimens for TEM observations were sonicated in ethanol for 3 min and then loaded onto a copper micro-grid covered with carbon film. 2.4.6. XRD analysis Structural properties of the CNTs were determined by XRD (Shimadu LaBX XRD-6000) using a diffractometer with Cu K␣ radiation over the 2 range of 20–100◦ . The accelerating voltage and the applied current were 40 kV and 30 mA, respectively. 2.4.7. N2 adsorption isotherm analysis The N2 adsorption of the porous carbons was measured at −196 ◦ C on an ASAP 2020N instrument. The sample was degassed at 300 ◦ C for 12 h to obtain a residual pressure of less than 1 × 10−6 mmHg. The Brunauer–Emmett–Teller surface area (SBET ) of the sample was calculated from the adsorption date in the relative pressure interval from 0.04 to 0.2. Mesopore surface area (Smes ), micropore surface area (Smic ) and volume (Vmic ) were calculated using the t-plot method. The total pore volume (Vtot ) was estimated from nitrogen adsorption at a relative pressure of 0.99, and the mesopore volume (Vmes ) was calculated as the difference between Vmic and Vtot . 3. Results and discussion 3.1. Compositional analysis of the pyrolysis oil The pyrolysis oil obtained from PCB waste was dark brown in colour. Ultimate analysis and H/C molar ratio of the oil are listed in Table 1. It could be seen that pyrolytic oil is a C-rich hydrocarbon mixture containing little N. The H/C molar ratio of the oil is 1.14, which indicated that the components in the oil have more aromatic structures. A detailed analysis of the oil was carried out by GC–MS. A summary of the main components as well as the quantified area was presented in Table 2. GC–MS analysis of the oil showed that the pyrolysis oil contained many organic species, over 30 and phenol was by far the most prominent compound (58.58%), followed
by 4-(1-methylethyl)-phenol (19.66%), which might originate from the splitting of bisphenol A structure. 4-Methylphenol (2.41%) was also presented in the pyrolysis oil, presumably from the further breakdown of 4-(1-methylethyl)-phenol. As well as bisphenol A decomposition products, the oil also contained 2-methylphenol (5.47%) and 2,4-dimethylphenol (1.92%), which have been reported to be present during the pyrolysis of epoxy novolac [17]. Besides, the pyrolysis oil contained many other phenolic compounds with extremely lower concentrations. All of these phenol and phenol derivatives in the oil accounted for 90.63%. 3.2. Polymerization of the pyrolysis oil into resin As the PCB waste pyrolysis oil contained a mixture of phenolic compounds, including tri-functional monomer (phenol), difunctional monomer (4-(1-methylethl)-phenol, 2-methylphenol and 4-methylphenol, etc.) and mono-functional monomer (2,4dimethyphenol, etc.) which, respectively, can form building and linear polymeric chains with formaldehyde under basic conditions. Firstly, formaldehyde would be added to the activated ortho and para positions of phenolic rings, forming substituted methyl phenols. As the reaction continues, these condense to form ether linkages or methylene bridges giving a low-molecular weight (300–700 g/mol) branched polymer [18]. Whereas, other unreactive components (ethylbenzene, m-xylene, styrene, etc.) in the pyrolysis oil may not take part in the polymerization. The polymerization of pyrolysis oil with formaldehyde provided resin with yield of 66.18%. Fig. 2 shows surface micrographs of the prepared pyrolysis oilbased resin. From the 100× magnifications (Fig. 2a) it could be seen that the shape of the resin is angular blocky structure. The external surface of the resin on length scale of ∼10 m, as seen in the 1000× magnifications (Fig. 2b and c), is more or less smooth-although wavy or slightly grooved and its surface is dense. The result of the thermal dissociation investigation of the resin was shown in Fig. 3. This curve was quite similar to the one that observed by Kishore et al. [6] and could be divided into five stages as shown in Fig. 3. The weight loss during degradation takes place predominantly in the temperature range from 280 to 696 ◦ C. The reactions occurring in this stage may be: cracking, dehydration, dehydrogenation, etc. The decomposition products of the resin may
Table 1 Main characteristics of the PCB waste pyrolysis oil. Ultimate analysis (wt%) C 64.05 a
By difference.
H/C molar ratio
H
N
6.09
2.86
Othersa 27.00
1.14
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Table 2 Identification and yield (area%) of pyrolysis oil (<300 ◦ C subfraction). No.
RTa (min)
Name
MWb
Formula
GC peak area (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
5.647 5.859 6.431 7.547 9.138 11.381 11.724 12.027 12.972 13.229 13.961 14.219 14.774 15.266 15.758 15.810 15.941 16.170 16.330 16.473 17.440 17.692 18.350 18.459 18.711 18.825 19.048 20.181 20.307 21.738 23.969 25.205 27.431 27.843
Ethylbenzene m-Xylene Styrene Bromobenzene Phenol 2-Methyl-phenol 2-Bromo-phenol 4-Methyl-phenol 2-Methyl-benzofuran 2,3-Dihydro-2-methyl-benzofuran 3-Ethylphenol 2,4-Dimethylphenol 2-Ethylphenol 2-Allylphenol 1-Bromo-4-(1-methylethyl)-benzene 3-Ethyl-5-methyl-phenol 4,7-Dimethyl-benzofuran 5,8-Dihydro-1-naphthalenol 2-(2-Hydroxyphenyl)buta-1,3-diene 4-(1-Methylethyl)-phenol 3,3-Dimethylindan-1-one 2,2,4-Trimethyl-2H-chromene 1-Tert-butyl-3-ethyl-5-methylbenzene 2-Bromo-p-cymene 4-Isopropyl-1,6-dimethyl-1,2,3,4-tetrahydronaphthalene 1,3-Dimethylnaphthalene 1-Methylenepentyl-benzene 2,2 ,5,5 -Tetramethyl-1,1 -biphenyl 1,2,3,4-Tetrahydro-1,1,6-trimethyl-naphthalene 1,3-Dimethyl-naphthalene 2,4-di-tert-Butyl-6-methylphenol Methyl 3,5-dibromo-4-methylbenzoate 3,3 ,4,4 -tetramethyl-1,1 -biphenyl 2,2 ,5,5 -tetramethyl-1,1 -biphenyl
106 106 104 156 94 108 172 108 132 134 122 122 122 134 198 136 146 146 146 136 160 174 142 212 202 160 160 176 174 156 220 306 210 210
C8 H10 C8 H10 C8 H8 C6 H5 Br C6 H6 O C7 H8 O C6 H5 BrO C7 H8 O C9 H8 O C9 H10 O C8 H10 O C8 H10 O C8 H10 O C9 H10 O C9 H11 Br C9 H12 O C10 H10 O C10 H10 O C10 H10 O C9 H12 O C11 H12 O C12 H14 O C11 H10 C10 H13 Br C15 H22 C12 H16 C12 H16 C13 H20 C13 H18 C12 H12 C15 H24 O C9 H8 Br2 O2 C16 H18 C16 H18
0.33 0.25 0.37 0.39 58.58 5.47 1.02 2.41 1.88 0.47 0.56 1.92 1.03 0.18 0.10 0.09 0.12 0.07 0.19 19.66 0.34 0.18 0.14 0.47 0.08 0.25 0.15 0.13 0.80 0.58 0.28 0.41 0.30 0.76
a b
Retention time. Molecule weight.
include higher molecular weight condensable volatiles such as benzene, toluene, phenol, cresol, xylenol and other heavy compounds, low-molecular weight gas such as H2 O, CO, CO2 , CH4 , and a carbonaceous residue (char) [19]. The char yield of the synthesis resin obtained from the TG curve is about 58.98%. 3.3. Characteristics of CNTs and porous carbons The SEM micrographs of the carbon materials obtained from catalytic pyrolysis of the pyrolysis oil-based resin with ferrocene under N2 atmosphere are shown in Fig. 4. SEM images show that
Fig. 3. TG curve of the prepared resin under N2 atmosphere at a heating rate of 10 ◦ C/min.
the as-formed products contained tubule structures and most of them were filled with metal nanoparticles or nanorods (see Fig. 4 indicated by arrows A and B). As indicated in Fig. 4, the length of the prepared tubules reaches up to several micrometers. These tubules were further observed by TEM (Fig. 5), which revealed that the products were hollow-centered CNTs with outer diameter of ∼338 nm and wall thickness of ∼86 nm. There are metal nanorods embedded in the body of CNTs and the length of these nanorods varies, as shown in Fig. 5a, indicating that the metal particles played the important role for CNTs growth. The yield of the obtained CNTs was 56.82% and they had larger diameter and longer length than those observed by other researchers [5]. Liu et al. [20] also observed such Fe-filled CNTs in the decomposition of cyclohexane over Fe2 O3 /SiO2 –Al2 O3 catalysts. Metal-filled CNTs provide several possible applications, e.g. as catalyst in heterogeneous catalysis and as recording media due to their novel magnetic properties, therefore, increasing attention has been paid to CNTs [21]. However, further research is necessary to optimize the preparation parameters of the pyrolysis oil-based resin and CNTs, and investigate systematically the magnetic properties of the Fe-filled CNTs. The XRD pattern of the CNTs is shown in Fig. 6, from which a sharp peak at 25.92◦ can be seen clearly, corresponding to the (0 0 2) reflection of carbon. The value of d0 0 2 for this peak is 3.433 Å, higher than that of perfect graphite (d0 0 2 = 3.354 Å). The reflection around 43.46◦ corresponds to the (1 0 0) plane and indicates the presence of honeycomb structure formed by sp2 hybridized carbons. In addition to these two prominent graphite peaks, C(0 0 4), C(1 1 0) and C(1 1 2) peaks could also be seen in Fig. 6. XRD result revealed that catalytic pyrolysis of the pyrolysis oil-based resin at 900 ◦ C led to the formation of amorphous carbon.
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Fig. 4. SEM photographs of the CNTs synthesized by catalytic pyrolysis of the prepared resin with ferrocene at 900 ◦ C (arrows A and B point nanoparticles and nanorods, respectively).
Fig. 5. TEM images of the prepared CNTs (arrow C point nanorod inside the CNT body).
Beside the CNTs, Fig. 6 also indexed the presence of Fe (present as several strong peaks at 44.70◦ and 50.83◦ ) and Fex C (a strong peak at ca. 40.40◦ ) in the products. Based on the above experimental results, the growing mechanism of CNT synthesis can be envisioned as follows: nanometer iron produced from ferrocene decomposition accumulates carbon on its surface with the formation of Fe–C solid solution. After a very short period of time, the Fe–C solid solution reached a supersaturated point and carbon atom started to nucleate and grow in a nanotube shape. During the slow growth, the shape of the Fe catalyst may be changed, e.g. from nanoparticle to nanorod by carbon forming a cylinder around it. If the catalyst is
Fig. 6. The XRD pattern of the obtained CNTs.
reduced before the decomposition of resin, the size of the Fe catalyst does not increase during the growth of the nanotubes. In this condition, the decomposition of pyrolysis oil-based resin will result in the formation of nanotubes free of metal. The prepared resin carbonized to porous carbons at yield of 38.05%. Fig. 7 represents surface micrographs of the porous carbons activated by KOH at a magnification of 1000. The SEM images show that the surface of the sample is rich in pores. The N2 adsorption and desorption isotherms of the prepared porous carbons were displayed in Fig. 8. According to the classification by IUPAC, A type I isotherm is observed, that demonstrates the generation of micropores, due to KOH activation. This phenomenon has also been found for the KOH–porous carbons prepared from other materials [22,23]. The curve started at a certain absorbed volume of gas molecules at very low pressure. It indicated a strong interaction between the N2 gas molecules and the surface of the sample. The initial (steepest) part of the isotherm represents the micropore filling (rather than surface coverage) and a low slope of the plateau is indicative of multilayer adsorption on the external surface. Meanwhile, a rather broad hysteresis loop was observed for the prepared porous carbons as indicated in Fig. 8, ascribed to the development of mesoporosity. The pore texture parameters, including SBET , Smic , Smes , Vtot , Vmic and Vmes , which could be calculated from Fig. 8 were gathered in Table 3. From Table 3, it could be observed that KOH activation of resin produced porous carbons with a BET surface area of 1214 m2 /g, while the total pore volume, micropore area and volume were 0.64 cm3 /g, 892 m2 /g and 0.41 cm3 /g, respectively. By contrast with the SEM images of the pristine resin and porous carbons obtained, it could be seen that the structure of the pris-
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Fig. 7. SEM photographs of porous carbons prepared from carbonization of KOH-treated resin at 700 ◦ C.
Table 3 Characteristics of the prepared porous carbons. Smic SBET
Surface area (m2 /g) SBET
Smic
Smes
1214
892
322
Smes SBET
Vtot 0.73
0.27
tine resin is very dense and there are almost no micropores in it (Fig. 2). After the activation process, the structure becomes different; it is less dense and pores of different size and shapes appear as shown in Fig. 8. The pores of porous carbons formed mainly from the removal of impregnated KOH and KOH-derived compounds, leaving the space previously occupied by the compounds [24]. Kim et al. [25] considered that the mechanism of pore developed by KOH activation is a kind of microexplosion system. When the pristine resin was soaked in a large amount of KOH, a thin film of KOH should be coated on its surface and the interior of the resin should be covered with KOH completely. During the activation in an inert atmosphere, the K+ ions on the resin make small micropores (size < 2 nm) in the initial reaction, and in the propagation state some walls between micropores are demolished by further reaction of K+ ions, resulting in the formation of mesopores (2–50 nm). In our research, the KOH activation method produced porous carbons with a Smes /SBET ratio of 0.27 and a Vmes /Vtot ratio of 0.36, while the fractions of Smic /SBET and Vmic /Vtot were 0.73 and 0.64, also indicating that all existing pores are predominance of microporosity. However, the presence of mesopores in microporous carbons could improve transport of molecules within porous network and facilitate adsorption of larger molecules [3].
Vmic Vtot
Pore volume (cm3 /g)
0.64
Vmic 0.41
Vmes Vtot
Vmes 0.23
0.64
0.36
4. Conclusions It is unambiguously demonstrated that PCB waste pyrolysis oil may be a good precursor for making ACMs, which develops a new route for the application of the PCB waste pyrolysis oil. Analyses of molecular compositions of the PCB waste pyrolysis oil reveal that it contained high concentrations of phenol-group species. The components may be polymerized with formaldehyde into pyrolysis oil-based resin, and then CNTs and porous carbons were successfully developed from the pyrolysis oil-based resin. Hollow-centered and straight CNTs with outer diameter of ∼338 nm could be directly obtained by pyrolysis of the resin using ferrocene as catalyst precursor at 900 ◦ C in nitrogen flow. The length of the CNTs was several microns. The carbon in the CNTs is amorphous. The external surface of the porous carbons prepared from carbonization of KOH-treated resin at 700 ◦ C is full of cavities. The BET surface area and micropore volume of the porous carbons were 1214 m2 /g and 0.41 cm3 /g, respectively. Compared with other ACMs, ACMs derived from the PCB waste pyrolysis oil may be more competitive because of their cheap material and simple processing. Acknowledgements This project was supported by the Geping GreenAid Project—Environmental Scientific Research “123 Project” of Liaoning Provine, China (No. CEPF2008-123-2-14) and China Postdoctoral Science Foundation (No. 20090451264). References
Fig. 8. The adsorption and desorption isotherms of N2 on prepared porous carbons.
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Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Synthesis of carbon nanotubes and porous carbons from printed circuit board waste pyrolysis oil Cui Quan, Aimin Li ∗ , Ningbo Gao Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Linggong Road 2, Dalian 116024, PR China
a r t i c l e
i n f o
Article history: Received 16 September 2009 Received in revised form 3 February 2010 Accepted 22 March 2010 Available online 25 March 2010 Keywords: PCB waste pyrolysis oil Resource utilization CNTs Porous carbons
a b s t r a c t The possibility and feasibility of using pyrolysis oil from printed circuit board (PCB) waste as a precursor for advanced carbonaceous materials is presented. The PCB waste was first pyrolyzed in a laboratory scale fixed bed reactor at 600 ◦ C to prepare pyrolysis oil. The analysis of pyrolysis oil by gas chromatography–mass spectroscopy indicated that it contained a very high proportion of phenol and phenol derivatives. It was then polymerized in formaldehyde solution to synthesize pyrolysis oil-based resin which was used as a precursor to prepare carbon nanotubes (CNTs) and porous carbons. Scanning electron microscopy and transmission microscopy investigation showed that the resulting CNTs had hollow cores with outer diameter of ∼338 nm and wall thickness of ∼86 nm and most of them were filled with metal nanoparticles or nanorods. X-ray diffraction reveals that CNTs have an amorphous structure. Nitrogen adsorption isotherm analysis indicated the prepared porous carbons had a Brunauer–Emmett–Teller surface area of 1214 m2 /g. The mechanism of the formation of the CNTs and porous carbons was discussed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Advanced carbonaceous materials (ACMs) have drawn great attention throughout the world due to their particular microstructures, unique properties and great potential applications in many fields. Phenolic resin, which can be obtained from various combinations of phenols and aldehydes by using either a base or acid as the catalyst, is one of the well-used synthetic precursors for the production of ACMs such as glass-like carbons [1], carbon fibers [2], porous carbons [3,4], carbon nanotubes (CNTs) [5] and carbon membranes [6,7]. In order to develop a more competitive carbon material, it is necessary to consider developing carbon materials from the renewable resource. One of the methods to derive renewable resource is the conversion of waste substances to energy. Recently, studies from all over the world have pointed to Eucalyptus tar pitch, which is generated as a by-residue of Eucalyptus wood pyrolysis in charcoal production, as having one of the greatest potentialities as precursors of ACMs. Qiao et al. [8] successfully developed carbon fibers (maximum strength and modulus: 632 MPa and 44 GPa), activated carbon fibers (surface areas: 450–1600 m2 /g), and carbon films (thicknesses: 1–6 m) from biomass tar. Prauchner et al. [9] investigated the structural
∗ Corresponding author. Tel.: +86 0411 8470 7448; fax: +86 0411 8470 7448. E-mail address: [email protected] (A. Li). 0304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2010.03.092
evolution of Eucalyptus tar pitch processing towards carbon materials under carbonization. Printed circuit board (PCB) waste from waste electrical and electronic equipment is made of a copperclad laminate that mainly consists of 15% epoxy resin, 30% glass cloth filament, 22% copper coils, metal (Sn, Pb, Fe, Ni, etc.) and Br. One possible method of recycling PCB waste and recovering both the organic and non-organic fractions is pyrolysis. In the pyrolysis process, three products are typically obtained: liquids, gases and residues, which can be used as fuels or source of chemicals. A number of studies on the pyrolysis of PCB waste have been reported, most of which were dedicated to investigating the pyrolysis product characteristics [10–12], pyrolytic kinetics [13,14] and brominated compound formation and fate [15,16]. However, there are few studies on the use of PCB waste pyrolytic by-products, especially pyrolysis liquid. Therefore, advances in the knowledge of the characteristics and potential reuse routes of liquid product obtained in PCB waste pyrolysis are important. PCB waste pyrolysis liquid, which contained high concentrations of phenol and other aromatic compounds [10,11,16], may be used as chemical feedstock, whereas their separation and purification are complicated and costly because their contents are very limited. Hence, its application as a carbon source may be more feasible since phenol and phenol derivatives are the major components. Undoubtedly, it would be economically valuable and environmentally sound if PCB waste pyrolysis oil could be used to produce carbon materials with a high added value. To the best of our knowl-
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pyrolysis oil sample was polymerized with 6.5 g 37% formaldehyde solution in a heating magnetic whisk by using ammonium hydroxide as catalyst at 95 ◦ C for 4 h. After synthesis, the resin was cured by heat treatment for 2 h at 60 ◦ C, followed by 12 h at 120 ◦ C. The obtained resin was then ground into fine powder with a grinder (Retsch, ZM 200). 2.3. Preparation of CNTs and porous carbons
Fig. 1. Diagram of the fixed-bed reactor system. (1) electric furnace, (2) reactor tube, (3) PID controller, (4) thermocouple, (5) water condenser, (6) liquid collector and (7) glass wool filter.
edge, up to now, there are no reports about the preparation of ACMs with PCB waste pyrolysis oil in literature. In this paper, the synthesis method of the CNTs and porous carbons from PCB waste pyrolysis oil was described in detail. Morphologies and structures of the resulting products were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and nitrogen (N2 ) adsorption isotherm techniques. Formation mechanism of products was also discussed. 2. Experimental 2.1. Preparation of PCB waste pyrolysis oil PCB waste used in this study was obtained from a local PCB factory and made from glass fiber reinforced epoxy resin (referred to commercially as FR-4). The pyrolysis experiment was performed under static atmospheric conditions in a laboratory scale fixed bed reactor (see Fig. 1). The reactor tube measured 375 mm in length by an internal diameter of 57 mm and was externally heated by a 2.5 kW electrical furnace. The temperature was measured by a K-type thermocouple placed at the center of the reactor tube and controlled by a proportional–integral–differential (PID) controller. 160 g PCB waste sample was placed in the reactor tube at the start of the experiment and then the reactor tube was sealed and purged by N2 to remove air inside. When the temperature of the electric furnace reached 600 ◦ C, the reactor tube was immediately inserted into the electric furnace. The temperature of the reactor tube was ramped to 600 ◦ C and maintained for 30 min to ensure that pyrolysis of the sample was complete. The product leaving the reactor passed through a water-cooled condenser and collected in a liquid trap. These liquid products contained aqueous and oil phases, which were separated and weighed. In addition, a glass wool filter was used to remove any oils that were not trapped by the condenser. After pyrolysis, the solid char was removed and weighed, and then the gas yield was calculated from the material balance. The experimental result of PCB waste pyrolysis, which was conducted at 600 ◦ C final temperature under static atmosphere, was obtained as follows: Yield of aqueous phase: 6.06 wt% Yield of residue: 63.06 wt% Yield of oil: 20.10 wt% Yield of gas: 10.78 wt% 2.2. Synthesis of pyrolysis oil-based resin The pyrolysis oil-based resin, which was used as a precursor in this work, was synthesized through the following procedure: 5.0 g
The synthesis process of the CNTs is described as follows: 1 g resin mixed with 20% ferrocene (w/w) and homogenized in ethanol in continuous stirring at 50 ◦ C until uniformity was reached. After alcohol evaporation, the mixture was ground into fine powder and loaded on a ceramic boat which was placed inside a stainless steel tubular reactor (I.D. = 57 mm). The mixture was heated to 200 ◦ C (5 ◦ C/min) in air with 1 h soaking time, and then up to 900 ◦ C with a heating rate of 3 ◦ C/min in a flow of N2 with holding periods for 1 h at 900 ◦ C. In order to prepare porous carbons, 1 g resin was soaked in a concentrated solution of KOH to yield a KOH/resin weight ratio of 4/1 for 3 h at 85 ◦ C and then the resin–KOH slurry was subjected to dry at 110 ◦ C for 24 h. The obtained solid product was first ground into powder, then transferred to a ceramic pan, and finally, carbonized in an oven under N2 atmosphere at a heating rate of 10 ◦ C/min from room temperature to 700 ◦ C and soaked for 2 h before cooling. After cooling, to remove the remaining KOH, the char was washed by stirring with 1 mol/L HCl solution at 85 ◦ C for 30 min, followed by filtration. The acid-washed sample was then washed by hot distilled water and filtered. This washing and filtration steps were repeated until the filtrate became neutral. At last, the washed char was dried at 110 ◦ C for 24 h. So, porous carbon sample was prepared. Here in this paper the yield of CNTs and porous carbons was simply defined as the weight ratio of final carbons to the initial raw materials used. 2.4. Characterization 2.4.1. Ultimate analysis The oil from pyrolysis of PCB waste was kept in a closed brown glass bottle in refrigerator. The carbon (C), hydrogen (H) and N contents of the pyrolysis oil were performed on a CHN analyzer (Elementar, VarioEL III). 2.4.2. Gas chromatography–mass spectroscopy (GC–MS) analysis The obtained pyrolysis oil was distilled at atmospheric pressure, and the fraction whose boiling point was lower than 300 ◦ C was subjected to GC–MS analysis. The GC–MS was an Agilent GC/6890N coupled with MSD/5975. EI mode was adopted. The analytical column was an HP-5MS capillary column (30 m × 0.32 mm × 0.25 m). Helium was used as carrier gas at a constant flow of 1.0 ml/min. The injector temperature was 280 ◦ C and the oven was held at 40 ◦ C for 3 min, then ramped to 280 ◦ C at 5 ◦ C/min, and then held for 20 min. Typical operating conditions were ionization energy, 70 eV; ion source temperature, 230 ◦ C; and scans per second oven mass range electron (m/z), 50–550. Identification of the GC–MS spectral features was achieved with the use of a built-in library. All library-matched species exhibited the degree of match better than 90%. 2.4.3. Thermogravimetric (TG) analysis In order to simulate the carbonization property of the pyrolysis oil-based resin, the resin was subjected to a thermal degradation on a WCT-1C thermobalance under N2 atmosphere. Approximately 5 mg resin powder was placed in an alumina pan, and then heated from room temperature to 800 ◦ C at a heating rate of 10 ◦ C/min
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Fig. 2. SEM photographs of the prepared pyrolysis oil-based resin.
and kept isothermal for 10 min. The carrier gas (N2 ) flow rate was maintained at 30 ml/min during experiment. 2.4.4. SEM analysis A JEOL JSM-5600LV was employed for SEM observation of the prepared resin, CNTs and porous carbons. Each powder sample was mounted onto a double-side sticky tape over aluminum stubs and coated with gold under vacuum prior to the study. 2.4.5. TEM analysis The morphology of the prepared CNTs was also examined with TEM using a Tecnai G220 S-Twin microscope operated at 200 kV. Specimens for TEM observations were sonicated in ethanol for 3 min and then loaded onto a copper micro-grid covered with carbon film. 2.4.6. XRD analysis Structural properties of the CNTs were determined by XRD (Shimadu LaBX XRD-6000) using a diffractometer with Cu K␣ radiation over the 2 range of 20–100◦ . The accelerating voltage and the applied current were 40 kV and 30 mA, respectively. 2.4.7. N2 adsorption isotherm analysis The N2 adsorption of the porous carbons was measured at −196 ◦ C on an ASAP 2020N instrument. The sample was degassed at 300 ◦ C for 12 h to obtain a residual pressure of less than 1 × 10−6 mmHg. The Brunauer–Emmett–Teller surface area (SBET ) of the sample was calculated from the adsorption date in the relative pressure interval from 0.04 to 0.2. Mesopore surface area (Smes ), micropore surface area (Smic ) and volume (Vmic ) were calculated using the t-plot method. The total pore volume (Vtot ) was estimated from nitrogen adsorption at a relative pressure of 0.99, and the mesopore volume (Vmes ) was calculated as the difference between Vmic and Vtot . 3. Results and discussion 3.1. Compositional analysis of the pyrolysis oil The pyrolysis oil obtained from PCB waste was dark brown in colour. Ultimate analysis and H/C molar ratio of the oil are listed in Table 1. It could be seen that pyrolytic oil is a C-rich hydrocarbon mixture containing little N. The H/C molar ratio of the oil is 1.14, which indicated that the components in the oil have more aromatic structures. A detailed analysis of the oil was carried out by GC–MS. A summary of the main components as well as the quantified area was presented in Table 2. GC–MS analysis of the oil showed that the pyrolysis oil contained many organic species, over 30 and phenol was by far the most prominent compound (58.58%), followed
by 4-(1-methylethyl)-phenol (19.66%), which might originate from the splitting of bisphenol A structure. 4-Methylphenol (2.41%) was also presented in the pyrolysis oil, presumably from the further breakdown of 4-(1-methylethyl)-phenol. As well as bisphenol A decomposition products, the oil also contained 2-methylphenol (5.47%) and 2,4-dimethylphenol (1.92%), which have been reported to be present during the pyrolysis of epoxy novolac [17]. Besides, the pyrolysis oil contained many other phenolic compounds with extremely lower concentrations. All of these phenol and phenol derivatives in the oil accounted for 90.63%. 3.2. Polymerization of the pyrolysis oil into resin As the PCB waste pyrolysis oil contained a mixture of phenolic compounds, including tri-functional monomer (phenol), difunctional monomer (4-(1-methylethl)-phenol, 2-methylphenol and 4-methylphenol, etc.) and mono-functional monomer (2,4dimethyphenol, etc.) which, respectively, can form building and linear polymeric chains with formaldehyde under basic conditions. Firstly, formaldehyde would be added to the activated ortho and para positions of phenolic rings, forming substituted methyl phenols. As the reaction continues, these condense to form ether linkages or methylene bridges giving a low-molecular weight (300–700 g/mol) branched polymer [18]. Whereas, other unreactive components (ethylbenzene, m-xylene, styrene, etc.) in the pyrolysis oil may not take part in the polymerization. The polymerization of pyrolysis oil with formaldehyde provided resin with yield of 66.18%. Fig. 2 shows surface micrographs of the prepared pyrolysis oilbased resin. From the 100× magnifications (Fig. 2a) it could be seen that the shape of the resin is angular blocky structure. The external surface of the resin on length scale of ∼10 m, as seen in the 1000× magnifications (Fig. 2b and c), is more or less smooth-although wavy or slightly grooved and its surface is dense. The result of the thermal dissociation investigation of the resin was shown in Fig. 3. This curve was quite similar to the one that observed by Kishore et al. [6] and could be divided into five stages as shown in Fig. 3. The weight loss during degradation takes place predominantly in the temperature range from 280 to 696 ◦ C. The reactions occurring in this stage may be: cracking, dehydration, dehydrogenation, etc. The decomposition products of the resin may
Table 1 Main characteristics of the PCB waste pyrolysis oil. Ultimate analysis (wt%) C 64.05 a
By difference.
H/C molar ratio
H
N
6.09
2.86
Othersa 27.00
1.14
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Table 2 Identification and yield (area%) of pyrolysis oil (<300 ◦ C subfraction). No.
RTa (min)
Name
MWb
Formula
GC peak area (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
5.647 5.859 6.431 7.547 9.138 11.381 11.724 12.027 12.972 13.229 13.961 14.219 14.774 15.266 15.758 15.810 15.941 16.170 16.330 16.473 17.440 17.692 18.350 18.459 18.711 18.825 19.048 20.181 20.307 21.738 23.969 25.205 27.431 27.843
Ethylbenzene m-Xylene Styrene Bromobenzene Phenol 2-Methyl-phenol 2-Bromo-phenol 4-Methyl-phenol 2-Methyl-benzofuran 2,3-Dihydro-2-methyl-benzofuran 3-Ethylphenol 2,4-Dimethylphenol 2-Ethylphenol 2-Allylphenol 1-Bromo-4-(1-methylethyl)-benzene 3-Ethyl-5-methyl-phenol 4,7-Dimethyl-benzofuran 5,8-Dihydro-1-naphthalenol 2-(2-Hydroxyphenyl)buta-1,3-diene 4-(1-Methylethyl)-phenol 3,3-Dimethylindan-1-one 2,2,4-Trimethyl-2H-chromene 1-Tert-butyl-3-ethyl-5-methylbenzene 2-Bromo-p-cymene 4-Isopropyl-1,6-dimethyl-1,2,3,4-tetrahydronaphthalene 1,3-Dimethylnaphthalene 1-Methylenepentyl-benzene 2,2 ,5,5 -Tetramethyl-1,1 -biphenyl 1,2,3,4-Tetrahydro-1,1,6-trimethyl-naphthalene 1,3-Dimethyl-naphthalene 2,4-di-tert-Butyl-6-methylphenol Methyl 3,5-dibromo-4-methylbenzoate 3,3 ,4,4 -tetramethyl-1,1 -biphenyl 2,2 ,5,5 -tetramethyl-1,1 -biphenyl
106 106 104 156 94 108 172 108 132 134 122 122 122 134 198 136 146 146 146 136 160 174 142 212 202 160 160 176 174 156 220 306 210 210
C8 H10 C8 H10 C8 H8 C6 H5 Br C6 H6 O C7 H8 O C6 H5 BrO C7 H8 O C9 H8 O C9 H10 O C8 H10 O C8 H10 O C8 H10 O C9 H10 O C9 H11 Br C9 H12 O C10 H10 O C10 H10 O C10 H10 O C9 H12 O C11 H12 O C12 H14 O C11 H10 C10 H13 Br C15 H22 C12 H16 C12 H16 C13 H20 C13 H18 C12 H12 C15 H24 O C9 H8 Br2 O2 C16 H18 C16 H18
0.33 0.25 0.37 0.39 58.58 5.47 1.02 2.41 1.88 0.47 0.56 1.92 1.03 0.18 0.10 0.09 0.12 0.07 0.19 19.66 0.34 0.18 0.14 0.47 0.08 0.25 0.15 0.13 0.80 0.58 0.28 0.41 0.30 0.76
a b
Retention time. Molecule weight.
include higher molecular weight condensable volatiles such as benzene, toluene, phenol, cresol, xylenol and other heavy compounds, low-molecular weight gas such as H2 O, CO, CO2 , CH4 , and a carbonaceous residue (char) [19]. The char yield of the synthesis resin obtained from the TG curve is about 58.98%. 3.3. Characteristics of CNTs and porous carbons The SEM micrographs of the carbon materials obtained from catalytic pyrolysis of the pyrolysis oil-based resin with ferrocene under N2 atmosphere are shown in Fig. 4. SEM images show that
Fig. 3. TG curve of the prepared resin under N2 atmosphere at a heating rate of 10 ◦ C/min.
the as-formed products contained tubule structures and most of them were filled with metal nanoparticles or nanorods (see Fig. 4 indicated by arrows A and B). As indicated in Fig. 4, the length of the prepared tubules reaches up to several micrometers. These tubules were further observed by TEM (Fig. 5), which revealed that the products were hollow-centered CNTs with outer diameter of ∼338 nm and wall thickness of ∼86 nm. There are metal nanorods embedded in the body of CNTs and the length of these nanorods varies, as shown in Fig. 5a, indicating that the metal particles played the important role for CNTs growth. The yield of the obtained CNTs was 56.82% and they had larger diameter and longer length than those observed by other researchers [5]. Liu et al. [20] also observed such Fe-filled CNTs in the decomposition of cyclohexane over Fe2 O3 /SiO2 –Al2 O3 catalysts. Metal-filled CNTs provide several possible applications, e.g. as catalyst in heterogeneous catalysis and as recording media due to their novel magnetic properties, therefore, increasing attention has been paid to CNTs [21]. However, further research is necessary to optimize the preparation parameters of the pyrolysis oil-based resin and CNTs, and investigate systematically the magnetic properties of the Fe-filled CNTs. The XRD pattern of the CNTs is shown in Fig. 6, from which a sharp peak at 25.92◦ can be seen clearly, corresponding to the (0 0 2) reflection of carbon. The value of d0 0 2 for this peak is 3.433 Å, higher than that of perfect graphite (d0 0 2 = 3.354 Å). The reflection around 43.46◦ corresponds to the (1 0 0) plane and indicates the presence of honeycomb structure formed by sp2 hybridized carbons. In addition to these two prominent graphite peaks, C(0 0 4), C(1 1 0) and C(1 1 2) peaks could also be seen in Fig. 6. XRD result revealed that catalytic pyrolysis of the pyrolysis oil-based resin at 900 ◦ C led to the formation of amorphous carbon.
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Fig. 4. SEM photographs of the CNTs synthesized by catalytic pyrolysis of the prepared resin with ferrocene at 900 ◦ C (arrows A and B point nanoparticles and nanorods, respectively).
Fig. 5. TEM images of the prepared CNTs (arrow C point nanorod inside the CNT body).
Beside the CNTs, Fig. 6 also indexed the presence of Fe (present as several strong peaks at 44.70◦ and 50.83◦ ) and Fex C (a strong peak at ca. 40.40◦ ) in the products. Based on the above experimental results, the growing mechanism of CNT synthesis can be envisioned as follows: nanometer iron produced from ferrocene decomposition accumulates carbon on its surface with the formation of Fe–C solid solution. After a very short period of time, the Fe–C solid solution reached a supersaturated point and carbon atom started to nucleate and grow in a nanotube shape. During the slow growth, the shape of the Fe catalyst may be changed, e.g. from nanoparticle to nanorod by carbon forming a cylinder around it. If the catalyst is
Fig. 6. The XRD pattern of the obtained CNTs.
reduced before the decomposition of resin, the size of the Fe catalyst does not increase during the growth of the nanotubes. In this condition, the decomposition of pyrolysis oil-based resin will result in the formation of nanotubes free of metal. The prepared resin carbonized to porous carbons at yield of 38.05%. Fig. 7 represents surface micrographs of the porous carbons activated by KOH at a magnification of 1000. The SEM images show that the surface of the sample is rich in pores. The N2 adsorption and desorption isotherms of the prepared porous carbons were displayed in Fig. 8. According to the classification by IUPAC, A type I isotherm is observed, that demonstrates the generation of micropores, due to KOH activation. This phenomenon has also been found for the KOH–porous carbons prepared from other materials [22,23]. The curve started at a certain absorbed volume of gas molecules at very low pressure. It indicated a strong interaction between the N2 gas molecules and the surface of the sample. The initial (steepest) part of the isotherm represents the micropore filling (rather than surface coverage) and a low slope of the plateau is indicative of multilayer adsorption on the external surface. Meanwhile, a rather broad hysteresis loop was observed for the prepared porous carbons as indicated in Fig. 8, ascribed to the development of mesoporosity. The pore texture parameters, including SBET , Smic , Smes , Vtot , Vmic and Vmes , which could be calculated from Fig. 8 were gathered in Table 3. From Table 3, it could be observed that KOH activation of resin produced porous carbons with a BET surface area of 1214 m2 /g, while the total pore volume, micropore area and volume were 0.64 cm3 /g, 892 m2 /g and 0.41 cm3 /g, respectively. By contrast with the SEM images of the pristine resin and porous carbons obtained, it could be seen that the structure of the pris-
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Fig. 7. SEM photographs of porous carbons prepared from carbonization of KOH-treated resin at 700 ◦ C.
Table 3 Characteristics of the prepared porous carbons. Smic SBET
Surface area (m2 /g) SBET
Smic
Smes
1214
892
322
Smes SBET
Vtot 0.73
0.27
tine resin is very dense and there are almost no micropores in it (Fig. 2). After the activation process, the structure becomes different; it is less dense and pores of different size and shapes appear as shown in Fig. 8. The pores of porous carbons formed mainly from the removal of impregnated KOH and KOH-derived compounds, leaving the space previously occupied by the compounds [24]. Kim et al. [25] considered that the mechanism of pore developed by KOH activation is a kind of microexplosion system. When the pristine resin was soaked in a large amount of KOH, a thin film of KOH should be coated on its surface and the interior of the resin should be covered with KOH completely. During the activation in an inert atmosphere, the K+ ions on the resin make small micropores (size < 2 nm) in the initial reaction, and in the propagation state some walls between micropores are demolished by further reaction of K+ ions, resulting in the formation of mesopores (2–50 nm). In our research, the KOH activation method produced porous carbons with a Smes /SBET ratio of 0.27 and a Vmes /Vtot ratio of 0.36, while the fractions of Smic /SBET and Vmic /Vtot were 0.73 and 0.64, also indicating that all existing pores are predominance of microporosity. However, the presence of mesopores in microporous carbons could improve transport of molecules within porous network and facilitate adsorption of larger molecules [3].
Vmic Vtot
Pore volume (cm3 /g)
0.64
Vmic 0.41
Vmes Vtot
Vmes 0.23
0.64
0.36
4. Conclusions It is unambiguously demonstrated that PCB waste pyrolysis oil may be a good precursor for making ACMs, which develops a new route for the application of the PCB waste pyrolysis oil. Analyses of molecular compositions of the PCB waste pyrolysis oil reveal that it contained high concentrations of phenol-group species. The components may be polymerized with formaldehyde into pyrolysis oil-based resin, and then CNTs and porous carbons were successfully developed from the pyrolysis oil-based resin. Hollow-centered and straight CNTs with outer diameter of ∼338 nm could be directly obtained by pyrolysis of the resin using ferrocene as catalyst precursor at 900 ◦ C in nitrogen flow. The length of the CNTs was several microns. The carbon in the CNTs is amorphous. The external surface of the porous carbons prepared from carbonization of KOH-treated resin at 700 ◦ C is full of cavities. The BET surface area and micropore volume of the porous carbons were 1214 m2 /g and 0.41 cm3 /g, respectively. Compared with other ACMs, ACMs derived from the PCB waste pyrolysis oil may be more competitive because of their cheap material and simple processing. Acknowledgements This project was supported by the Geping GreenAid Project—Environmental Scientific Research “123 Project” of Liaoning Provine, China (No. CEPF2008-123-2-14) and China Postdoctoral Science Foundation (No. 20090451264). References
Fig. 8. The adsorption and desorption isotherms of N2 on prepared porous carbons.
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