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Catalytic co-pyrolysis of epoxy-printed circuit board and plastics over HZSM-5 and HY
Journal of Cleaner Production 168 (2017) 366e374
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Catalytic co-pyrolysis of epoxy-printed circuit board and plastics over HZSM-5 and HY Young-Min Kim a, b, 1, Tae Uk Han b, 1, Seungdo Kim b, Jungho Jae c, d, Jong-Ki Jeon e, Sang-Chul Jung f, Young-Kwon Park a, * a
School of Environmental Engineering, University of Seoul, Seoul, 02504, Republic of Korea Department of Environmental Sciences and Biotechnology, Hallym University, Chunchon, 24252, Republic of Korea Clean Energy Research Center, Korea Institute of Science and Technology, Seoul, 02792, Republic of Korea d Division of Energy & Environment Technology, KIST School, Korea University of Science and Technology, Seoul, 02792, Republic of Korea e Department of Chemical Engineering, Kongju National University, Cheonan, 31080, Republic of Korea f Department of Environmental Engineering, Sunchon National University, Suncheon, 57922, Republic of Korea b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 December 2016 Received in revised form 27 August 2017 Accepted 30 August 2017 Available online 3 September 2017
The catalytic co-pyrolysis of epoxy-printed circuit board (e-PCB) and thermoplastics, high density polyethylene (HDPE) and polypropylene (PP), over HZSM-5 and HY was performed using a thermogravimetric analyzer and pyrolyzer-gas chromatography/mass spectrometry. The catalytic pyrolysis of e-PCB over both HZSM-5 and HY eliminated the brominated compounds, mainly bromo-phenols and -bisphenol As, to some extent. The co-feeding of HDPE and PP on the catalytic pyrolysis of e-PCB over both HZSM-5 and HY revealed different debromination efficiencies due to the properties of the thermoplastics and catalysts. A comparison of HY(80) and HZSM-5(80) with the same SiO2/Al2O3 ratio (80) revealed HY(80) had a better elimination efficiency of brominated compounds during the catalytic copyrolysis of e-PCB and HDPE or PP because of its larger pore size than HZSM-5(80). The lowest bromine content was achieved when HDPE and HY(30) were used as the co-feeding reactant and catalyst on the pyrolysis of e-PCB due to the large pore size and high acidity of HY(30), allowing the easier diffusion of large molecular brominated bisphenol A and HDPE molecules into the pores of the catalyst and efficient catalytic intermolecular reactions in the pores of the catalyst. The catalytic co-pyrolysis of ePCB and HDPE over HY(30) also produced large amounts of mono-aromatic hydrocarbons and monophenol, which can be used as fuels or chemical feedstock. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Catalytic co-pyrolysis Bromine fixation Epoxy-printed circuit board High density polyethylene Polypropylene HY
1. Introduction United Nations Environment Program (UNEP, 2009) estimates the annual amount of waste electrical and electronic equipment (WEEE) generated to be 20e50 million tons, which is increasing rapidly with the accelerated replacement cycle of electrical and electronic equipment. Therefore, the development of a desirable treatment technology for WEEE is an important research area. The necessity of recycling of WEEE is emphasized more than other municipal wastes because of its potential harmful effects on human health due to the brominated frame retardants (Terakado et al.,
* Corresponding author. E-mail address: [email protected] (Y.-K. Park). 1 Co-first authors. http://dx.doi.org/10.1016/j.jclepro.2017.08.224 0959-6526/© 2017 Elsevier Ltd. All rights reserved.
2011), such as brominated epoxy resin or tetrabromobisphenol A (TBBA) in printed circuit boards (PCBs) (Deng et al., 2016) as well as plastics, such as brominated high impact polystyrene (HIPS) in WEEE (Jakab et al., 2003). Although large amounts of WEEE is being recycled after being exported from developed countries to developing countries, considerable amounts are still being discarded in landfill, which can cause additional contamination of water and soil (Sepulveda et al., 2010). PCBs are the main parts of electrical and electronic equipment. Owing to the presence of valuable metals, such as copper, calcium, iron, nickel, zinc, aluminum, silver, gold, etc (Hall and Williams, 2007)., in PCBs, resource recovery from PCBs is considered as a valuable recycling process. To this end, various physical and mechanical separation methods have been suggested by many researchers (Eswaraiah et al., 2008). On the other hand, the high cost of separation methods is a significant limitation. Therefore, large
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amounts of PCBs are still treated by conventional methods, such as landfill and incineration, in which the contamination of soil and ground water by the leachate and large amounts of dioxins and furan emissions are serious problems (Chien et al., 2000). Proper treatment and recycling of non-metallic fraction is also being emphasized because the weight fraction of non-metallic fraction is higher than metallic fraction in PCBs (Duan et al., 2016). To recycle non-metallic parts of PCBs, Rajagopal et al. (2017) synthesized sustainable composite panels by hot-pressing the mixture of nonmetallic waste PCBs and automotive plastics. Recently, the pyrolysis of PCBs, which is the thermal decomposition of PCBs at high temperatures under a non-oxygen atmosphere, was investigated and is considered a promising technology to recover high-value metals from PCB without harmful gas emissions (Hall and Williams, 2007). The recovery of product gas and oils, which can be used as fuels or chemical feedstock, is an additional merit of the e-PCB pyrolysis process and its value is increasing due to the worldwide energy issues (Jin et al., 2011). On the other hand, the use of PCB pyrolysis oil is limited because it contains large amounts of brominated compounds (Ghosh et al., 2015), such as bromophenols and bromobisphenol-As (Kim et al., 2015). High contents of brominated compounds in pyrolysis oil from PCBs can have the harmful effects due to the toxicity of brominated chemicals. Therefore, additional process, such as the catalytic upgrading of PCB pyrolysis oil is being investigated to minimize the contents of brominated compounds in PCB pyrolysis oil. Various catalysts, such as metal oxides (Shen et al., 2016) or zeolites (Zhao et al., 2017) have been assessed for the catalytic pyrolysis of PCBs. For example, Terakado et al. (2013) investigated the bromine fixation effect of metal oxides in the pyrolysis of e-PCB and observed the elimination of hydrogen bromide and brominated organic compounds through the formation of metal bromides or oxybromide. Blazso (2005) also reduced the concentration of brominated compounds in the PCB pyrolysis oil by the catalytic pyrolysis of flame-retarded PCBs and phthalic polyester containing brominated polystyrene over zeolite catalysts. Co-pyrolysis of PCB and other polymeric materials such as plastics and biomass were also suggested as the potential treatment method for PCBs. Hornung et al. (2005) reported the synergistic debromination effect on the pyrolysis of brominated compounds by co-feeding with polypropylene (PP) in a pyrolysis reactor. Recently, Wu and Qiu (2014) also performed the vacuum co-pyrolysis of waste PCBs with Chinese fir sawdust. Although they obtained the increased yields of value-added brominated aromatic compounds due to the effective interactions between the pyrolyzates of PCBs and the Chinese fir sawdust, the reduced bromine content in the product oil was not reported. One of the candidate co-feeding materials on the catalytic pyrolysis of PCBs is waste plastics, such as high density polyethylene (HDPE) and polypropylene (PP) because large amounts of HDPE and PP are released into the environment and need to be treated properly. The reaction efficiency of the catalytic pyrolysis of a polymer can be influenced largely by the properties of the reactant polymers and catalysts (Muhammad et al., 2015). Therefore, the use of an appropriate catalyst and co-feeding plastics on the pyrolysis of PCBs can not only decrease the contents of brominated compounds, but also increase the amounts of value-added chemicals in the product oil; however, its actual application has not been reported in the literature. Additionally, the effects of co-feeding materials and catalytic properties, such as acidity and pore size, on the debromination efficiency need to be investigated prior to its actual application to a commercial plant. In this study, the in-situ catalytic co-pyrolysis of PCB and thermoplastics, HDPE and PP, over different catalysts, HZSM-5 and HY, was performed using a thermogravimetric (TG) analyzer and
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analytical pyrolyzer-gas chromatography/mass spectrometry (PyGC/MS). The co-feeding effect of the thermoplastics and catalyst on the pyrolysis of PCB was estimated according to the amounts of residual brominated compounds and value-added chemicals in the product oil. 2. Experiments 2.1. Materials Waste e-PCB (FR-4 type, epoxy resin reinforced by glass fibers), which was obtained from a local PCB recycling company in Korea, was ground and cryo-milled after copper removal using a gravity separation device. The metal removed part of the PCB was dried at 80 C for 3 h, sieved to a particle size less than 300 mm, and used as the sample for the pyrolysis of e-PCB. Physico-chemical properties of e-PCB are shown in Table S1 (Supplementary Information, Kim et al., 2013). Metal removed e-PCB still contained large amounts of inorganic compounds (68.1%). Owing to the presence of Brcontaining epoxy resin, Br content in e-PCB was especially higher than other elements (H, O, and N) except C. The powder forms of HDPE and PP were purchased from Samsung Chemical Co., Korea. 2.2. Catalysts Three kinds of catalysts, HZSM-5 (SiO2/Al2O3 80), HY (SiO2/Al2O3 80), and HY (SiO2/Al2O3 30), were purchased from Zeolyst international. The physico-chemical properties of the catalysts were measured by N2 adsorption-desorption (Table 1), and the acidities of the catalysts were determined by temperature-programmed desorption of ammonia (NH3-TPD) analysis (Fig. 1, Table S2). A comparison of HY(80) and HZSM-5(80) revealed HY(80) has a larger pore size and surface area. NH3-TPD curves of three catalysts (Fig. 1) showed that NH3 desorption temperature of HZSM-5(80) is higher than those of HY(80) and HY(30). This indicates that the acid strength of HZSM-5 is highest among the catalysts. Meanwhile, both the total number of acid site and the number of strong acid site decreased in the order HY (30) > HZSM-5 (80) > HY (80), as shown in Table S2. Scanning electron microscopy (SEM) analysis of the catalysts (Fig. S1) revealed that the particles of HZSM-5 and HY catalysts exhibited significantly different shape and size. HY catalysts had the cubic particles having average size between 0.6 and 0.8 mm, and HZSM-5 had the sphere-like crystals having the smaller particle size than HYs. Prior to TG and Py-GC/MS analysis, all catalysts were calcined at 550 C in air. 2.3. TG analysis Individual e-PCB, HDPE, and PP (1 mg) and their mixtures (2 mg), e-PCB/PP or e-PCB/HDPE, were heated non-isothermally in a TG analyzer (TGA Pyris 1, PerkinElmer Co., USA) from room
Table 1 Physico-chemical properties of HZSM-5 and HY. Catalyst
HZSM-5 (80)
HY (80)
HY (30)
SiO2/Al2O3 mole ratio Surface area (m2/g) Micropore surface area a (m2/g) Pore volumeb (cm3/g) Micropore volumec (cm3/g) Pore size (Å)
80 456 376 0.270 0.159 5.1 5.5, 5.3 5.6
80 743 573 0.571 0.296 7.4 7.4
30 903 742 0.531 0.229 7.4 7.4
a b c
Pore volume at p/p0 ¼ 0.99. By t-plot. By t-plot.
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Fig. 1. NH3-TPD curves of HZSM-5 and HY catalysts.
temperature to 700 C at a heating rate of 20 C/min under 60 mL/ min of nitrogen gas. To assess the catalytic pyrolysis, 5 mg of catalyst was also mixed together with the sample and pyrolyzed under the same condition. 2.4. Py-GC/MS Individual e-PCB, HDPE, PP (1 mg), and their mixtures (2 mg) in a pyrolyzer sample cup was inserted into the center position of the pre-heated pyrolyzer (Py-3030D, Frontier Laboratories Ltd., Japan) at 600 C under free fall. In the case of catalytic pyrolysis, 5 or 10 mg of catalyst was also added to the sample cup and blended manually. The pyrolysis vapor emitted from the pyrolyzer was introduced to a metal capillary column (UA-5, 30 m 0.25 mm i.d. X 0.25 mm film) via a GC inlet after GC inlet splitting (Split ratio, 100:1) in a GC/MS (Agilent 7890A/7975C inert). To increase the GC/MS peak sharpness, the chemicals in the column were cryo-focused at the front part of the column using a Micro Jet Cryo-Trap (MJT-1030E, Frontier Laboratories Ltd., Japan), separated by a GC oven program, from 40 C (5 min) to 340 C (10 min) at a heating rate of 20 C/min, and detected using a MS detector. Chemical identification in the chromatogram was performed using the NIST 8th library (National Institute of Standard and Technology, USA) and F-Search pyrolyzate library (F-Search, Frontier Laboratories Ltd., Japan). Integration of each peak on the total ion chromatogram (scan mode) and Sim ion chromatogram (SIM mode) was also performed to compare the amounts of each compound produced from each experiment by comparing the integrated peak areas. 3. Results and discussion 3.1. TG analysis Fig. 2 shows the DTG curves obtained from the thermal and catalytic pyrolysis of the individual samples and their mixtures over HZSM-5(80), HY(80), and HY(30). Table 2 lists the maximum decomposition temperatures (Tmaxs) on the DTG curves. The main non-catalytic decomposition of e-PCB was performed between 250 and 400 C. The organic part of e-PCB consisted of brominated and non-brominated epoxy resin that they were decomposed independently during non-isothermal pyrolysis (Kim et al., 2013). The Tmax values of e-PCB were not shifted to a lower temperature, even when large amounts (5 mg) of HY(80) or HZSM-5(80) were applied.
Fig. 2. DTG curves obtained from thermal and catalytic pyrolysis of e-PCB, HDPE, PP, and their mixtures over HZSM-5 and HY catalysts.
This can be explained by the thermal property of the epoxy resin. Thermosetting plastics, such as epoxy resin, having a cross-linking structure, do not melt when they are heated instead they get
Y.-M. Kim et al. / Journal of Cleaner Production 168 (2017) 366e374 Table 2 Maximum decomposition temperatures on the DTG curves obtained from the thermal and catalytic co-pyrolysis of e-PCB, HPDE, and PP over HZSM-5 and HY catalysts. Maximum decomposition temperature (Tmaxa/Tmaxb, C)
e-PCB HDPE PP e-PCB/HDPE e-PCB/PP a b
No Catalyst
HZSM-5(80)
HY(80)
333 490 464 331/486 328/470
333 397 363 335/410 335/491
333 398 343 331/471 330/431
Tmax on the decomposition DTG curve of e-PCB. Tmax on the decomposition DTG curve of HDPE or PP.
decomposed directly into volatile species. Therefore, the catalysts cannot promote the decomposition of e-PCB due to mass transfer limitations (solid-solid contact) and Tmax values on the DTG curves of e-PCB are not influenced by the catalysts. The molecular sizes of the main pyrolyzates of e-PCB, such as brominated phenols and bisphenol As, are also considerably larger than the pore sizes of HZSM-5(80) and HY(80), limiting the diffusion of these pyrolysis intermediates to the pore of the catalysts. In contrast, the Tmax values of e-PCB were moved to a lower temperature when e-PCB was pyrolyzed together with HDPE or PP. This indicates the presence of mutual interactions between the e-PCB and HDPE or PP, resulting in the easier decomposition of e-PCB. Czegeny et al. (2012) reported that the H-abstraction by bromine radicals can initiate the decomposition of high impact polystyrene (HIPS) when brominated epoxy oligomer is pyrolyzed together with HIPS. When ePCBs are co-pyrolyzed with thermoplastics, such as HDPE and PP having 137 and 176 C of melting point, respectively (Yu et al., 2016), the melted molecules of the thermoplastics probably penetrate into the network structure of the epoxy resin and can be decomposed by the abstracted hydrogen and bromine radicals. Radical intermediates of these thermoplastics will also be able to assist in the decomposition of the epoxy resin. As shown in Table 2, lower Tmax values of HDPE were also achieved when it was pyrolyzed together with e-PCB, which confirms the mutual interaction between e-PCB and HDPE. The catalytic pyrolysis of HDPE over HZSM-5(80) and HY(80) revealed lower decomposition temperatures than their thermal decomposition DTG curves, indicating that the HDPE molecules can be diffused into the pores of HZSM-5(80) and HY(80). The catalytic pyrolysis of PP over HZSM-5(80) and HY(80) showed much lower temperatures than those of PP over the same catalysts due to the lower thermal stability of PP molecules than HDPE. In addition, the catalytic pyrolysis of PP over HY(80) had a much lower Tmax (343 C) than that over HZSM-5(80), which indicates that the diffusion of PP molecules into HY(80) is easier than that into HZSM-5(80). This can be due to the fact that HY(80) has the larger pore size than HZSM-5(80), although its acidity is much lower than that of HZSM-5 (Table 1, Table S2 and Fig. 1). Compared to the catalytic pyrolysis of the individual samples, the catalytic co-pyrolysis of e-PCB/HPDE or e-PCB/PP revealed higher Tmax values of HDPE and PP decomposition over both catalysts. This suggests that the decomposition of HDPE and PP is hindered when e-PCB is co-pyrolyzed over the catalysts. These hindering effects for the decompositions of HDPE and PP in the catalytic co-pyrolysis of e-PCB/PP or e-PCB/HDPE can be explained by coke or char produced from the catalytic pyrolysis of e-PCB. In the case of the catalytic pyrolysis of e-PCB, large molecular pyrolyzates, such as brominated phenols and bisphenol A, can act as coke precursors that can block the pores of the catalysts and deactivate the catalysts. The diffusion of PP and HDPE molecules will be more difficult if coke fills and blocks the pores. Similar
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results were also reported in the study that examined the catalytic co-pyrolysis of biomass and thermoplastics (Kim et al., 2016). They found that the coke produced from the catalytic pyrolysis of cellulose hinders the catalytic pyrolysis of thermoplastics. Between PP and HDPE, PP had a more severe hindering effect than HDPE when co-pyrolyzed with e-PCB, especially over HZSM-5(80). The Tmax values of HDPE(430 C) and PP(373 C) on the DTG curves obtained from the catalytic co-pyrolysis of e-PCB/PP and e-PCB/HDPE over HY(30) were lower than those over HY(80) and HZSM-5(80). This suggests that HY(30) is the most effective catalyst in reducing this hindering effect on the catalytic co-pyrolysis of e-PCB/PP and ePCB/HDPE because of its higher acidity than HY(80) and larger pore size than HZSM-5(80) (Table 1, Table S2 and Fig. 1). From the results obtained from TG analysis, it can be inferred that the debromination efficiency and the formation of valueadded compounds from catalytic co-pyrolysis can be influenced by the type of co-feeding plastic and the pore size/structure of the catalysts. 3.2. Non-catalytic pyrolysis Fig. 3(a) presents the non-catalytic pyrogram of e-PCB at 600 C. Large amounts of phenol and bisphenol A were produced by the thermal decomposition of e-PCB together with alkylated or brominated phenols and bisphenol As (Blazso and Czegeny, 2006). High phenol and bisphenol A contents in the pyrolysis oil of e-PCB are meaningful because they can be used as important chemical feedstocks for the production of industrial polymers, such as polycarbonates, epoxy resin, phenolic resins, etc. (Weber et al., 2004). In addition, the direct use of e-PCB pyrolysis oil as a chemical feedstock for petrochemical process appears to be difficult because considerable amounts of brominated compounds, such as bromophenol, dibromophenol, dibromo-methyl-dihydrofuran, dibromobisphenol A, 2-(3-Bromophenyl)-2-(3,5-dibromo-4-hydroxyphenyl)propane, tribromobisphenol A, and tetrabromobisphenol A (TBBA), are contained in the pyrolysis oil of e-PCB (Kim et al., 2013). Fig. 3(b) and (c) show the non-catalytic pyrogram of the e-PCB/ PP and e-PCB/HDPE mixture, respectively. Many different compounds were formed due to the additional decomposition of HDPE and PP. Alkylated olefins with a wide range of carbon numbers, which are typical pyrolyzates of PP (Tsuge et al., 2011), were detected together with the same types of e-PCB pyrolyzates on the pyrogram of e-PCB/PP. Typical pyrolyzates of HDPE, triplet peaks (alkadiene/alkene/alkane) having a wide range of carbon numbers (Tsuge et al., 2011), were also detected on the pyrogram of the ePCB/HDPE mixture together with the pyrolyzates of e-PCB. Although the peak heights of the e-PCB pyrolyzates were not changed dramatically by co-pyrolysis with PP or HDPE, those of phenol were increased together with the decreased peak heights of bisphenol As. These phenomena can be explained by the mutual interactions between the pyrolyzates of e-PCB and those of PP or HDPE. Although the peak intensities of the e-PCB pyrolyzates were changed, the changes in the peak heights of brominated phenols and bisphenol As were insignificant, particularly when PP was copyrolyzed with e-PCB. This suggests that the co-pyrolysis of ePCB with PP does not lead to the additional debromination of brominated phenols and biophenol As. 3.3. Catalytic pyrolysis Fig. 4 presents the pyrograms obtained from the catalytic pyrolysis of e-PCB over HZSM-5(80) and HY(80). Compared to the non-catalytic pyrolysis of e-PCB, the amounts of phenol and monoaromatics, such as benzene, toluene, ethylbenzene, and xylenes, were higher together with a lower bisphenol As content over both
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Fig. 3. Pyrograms obtained from the thermal decomposition of e-PCB, e-PCB/PP, and e-PCB/HDPE.
catalysts. The catalytic pyrolysis of e-PCB also produced smaller amounts of brominated phenols and bisphenol As than its noncatalytic pyrolysis. Between HY(80) and HZSM-5(80), HY(80) produced smaller amounts of brominated phenols and bisphenol As during the catalytic pyrolysis of e-PCB. Using HY(80), most of the brominated bisphenol As, having larger molecular sizes than brominated phenols, were eliminated, whereas considerable amounts of brominated bisphenol A still remained when HZSM-5(80) was applied. The lower efficiency of HZSM-5(80) for the elimination of brominated bisphenol A can be explained by its smaller pore size than HY(80). The higher debromination efficiency of the larger pore zeolites on the catalytic pyrolysis of brominated polymer has been reported by Blazso and Czegeny (2006). They found that the larger pore size of molecular sieves allows the effective trapping of brominated aromatic compounds during the catalytic pyrolysis of
tetrabromobisphenol A (TBBA). Hall and Williams (2008) also found that the amounts of debrominated oils produced with HY(80) were larger than those with HZSM-5(80). They explained this phenomenon by the larger pore size of HY(80). A higher selectivity toward aromatic hydrocarbons was achieved when HZSM-5(80) was used as the catalyst on the catalytic pyrolysis of ePCB because of its higher acidity than HY(80) (Fig. 1), leading to the additional dehydration of phenols to aromatics. The pore size (5.5e5.6 Å) and structure (MFI) of HZSM-5 is also effective in aromatic hydrocarbon formation because of its shape selectivity (Elordi et al., 2011). Recently, Ma et al. (2016a) performed the catalytic pyrolysis of TBBA containing acrylonitrile-styrene-butadiene (TBBA-ABS) over microporous catalysts (HZSM-5, HY, and Hb) and mesoporous catalysts (silica MCM-41 and Al2O3). They also applied the same catalysts on the catalytic pyrolysis of decabromodiphenyl oxide containing high impact polystyrene (Br-HIPS) (Ma et al.,
Fig. 4. Pyrograms obtained from catalytic pyrolysis of e-PCB over (a) HZSM-5(80) and (b) HY(80).
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2017). They reported a higher selectivity toward aromatic hydrocarbons, such as toluene and ethylbenzene, over catalysts with higher acidity. In addition, a higher debromination efficiency was achieved over the catalysts with a larger pore size and surface area. As shown in Table 1, HY(80) has a larger pore size and surface area than HZSM-5(80). Hence, the efficient elimination of brominated compounds on the catalytic pyrolysis of e-PCB over HY(80) can be explained by its larger pore size and surface area. 3.4. Catalytic co-pyrolysis of e-PCB and plastics (HDPE and PP) Fig. 5 shows the pyrograms obtained from the catalytic copyrolysis of e-PCB/PP or e-PCB/HDPE over HZSM-5(80) or HY(80). Similar to the results of the catalytic pyrolysis of e-PCB, the catalytic co-pyrolysis of both e-PCB/PP and e-PCB/HDPE over HZSM-5(80) produced larger amounts of aromatic hydrocarbons than those over HY(80). In addition, the amounts of aromatic hydrocarbons obtained from the catalytic co-pyrolysis of e-PCB/PP and e-PCB/ HDPE over HZSM-5(80) were much larger than those of e-PCB. This can be explained by the high acidity of HZSM-5 (Sakata et al., 1999) and the co-feeding effect of PP or HDPE. By co-feeding PP or HDPE, more intermediate species for aromatic formation, e.g., olefins, can be produced, which will lead to the formation of aromatics in larger
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quantity. The catalytic co-pyrolysis of e-PCB with PP or HDPE over HY(80) and HZSM-5(80) also decreased the amounts of brominated phenols and bisphenol As. In particular, it was difficult to find brominated bisphenol As peaks on the total ion chromatogram obtained from the catalytic co-pyrolysis of e-PCB/PP and e-PCB/HPDE over HY(80). To compare the amounts of residual brominated compounds in more detail, the sim ion chromatograms for all reactions performed in this study were also analyzed (Fig. S2). Fig. 6 presents their integrated peak areas obtained from each reaction. As shown in Fig. 6, the catalytic co-pyrolysis of e-PCB/PP or ePCB/HDPE over both HY(80) and HZSM-5(80) produced smaller amounts of brominated compounds than the non-catalytic pyrolysis of e-PCB. The catalytic co-pyrolysis of e-PCB/HDPE over HZSM5(80) and HY(80) produced smaller amounts of brominated phenols and bisphenol As, which confirmed the synergistic debromination by the catalytic co-pyrolysis of e-PCB with HDPE. In addition, the peak intensities for the brominated phenols and bisphenol As obtained from the catalytic co-pyrolysis of the e-PCB/PP mixture were similar to those from the catalytic pyrolysis of e-PCB over the same catalyst. HY(80) was more effective in debromination during the catalytic co-pyrolysis of both e-PCB/PP and e-PCB/HDPE than HZSM-5(80). This can be explained by the easier diffusion of HDPE
Fig. 5. Pyrograms obtained from catalytic co-pyrolysis of e-PCB and plastics (PP and HDPE) over HZSM-5(80) and HY(80).
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Fig. 6. Sim ion peak intensities for the brominated phenol and biophenol As obtained from the thermal and catalytic pyrolysis of e-PCB and their mixtures (PCB/HDPE or e-PCB/PP) over HZSM-5 and HY catalysts. The values in the parenthesis indicate the amount of the catalyst used for catalytic pyrolysis.
and PP into the pores of HY than HZSM-5(80) due to the larger pore size of HY than HZSM-5. The enhanced diffusion of reactant molecules into the catalyst pore can lead to the more effective debromination of e-PCB pyrolyzates. Although more effective debromination was achieved on the catalytic co-pyrolysis of e-PCB/HDPE over HY(80), the amounts of brominated gas (bromomethane) were not increased dramatically compared to the catalytic pyrolysis of e-PCB alone over HY(80). This suggests that most of the bromine was fixed into the coke. The debromination by fixation into coke can be also rationalized by the higher debromination efficiency over HY than that over HZSM-5. The large pore size facilitates the growth of heavier coke compounds due to the additional hydrogen transfer, oligomerization, and condensation (Ma et al., 2017). To determine the effect of the catalyst amounts, 10 mg of HY was applied to the catalytic co-pyrolysis of e-PCB/PP or e-PCB/HDPE. Much larger amounts of brominated compounds were eliminated by applying 10 mg of HY instead of 5 mg of catalysts. This indicates that an increase in the total acidity can also improve the debromination efficiency during the catalytic co-pyrolysis of e-PCB/PP or e-PCB/HDPE. Compared to the e-PCB/PP mixture, that of e-PCB/ HDPE showed higher debromination efficiency on the catalytic copyrolysis of e-PCB with thermoplastics over HY(80). Additional debromination was also achieved by applying 10 mg of HY(30) catalyst, which has higher acidity than HY(80), as the catalyst on the catalytic co-pyrolysis of e-PCB/HDPE. Interestingly, the use of HY(30) was also effective on debromination during the catalytic copyrolysis of e-PCB and PP. This suggests that the impeded diffusion of PP molecules or its reaction intermediates into the pores of HY can be minimized by applying a high acid catalyst; even e-PCB and PP were co-pyrolyzed. To examine the Br-elimination pathways of brominated compounds during the catalytic co-pyrolysis of e-PCB and plastics over HY and HZSM-5 catalysts, additional experiments using a fixed-bed reactor was performed. Fig. 7 shows the yields (wt.%) of gas, liquid, and solid residue obtained from the catalytic co-pyrolysis of e-PCB and plastics (HDPE or PP) over HZSM-5(80) and HY(30/80). The yields of oil and solid were obtained by measuring the weights of
condensed oil and remained solid residue after the pyrolysis reaction, and the yield of gas was calculated by subtracting the summed yields of oil and solid from 100%. Compared to HZSM-5(80), the larger amount of solid residue was produced over HY(80) during the catalytic co-pyrolysis of e-PCB/PP and e-PCB/HDPE and this can be understood as a result of the larger amount of coke formed on HY(80). Importantly, the largest amount of solid residue (char and coke) was obtained from the catalytic co-pyrolysis of e-PCB/HDPE over HY(30) which showed the highest debromination efficiency (Fig. 6). This indicates that the amount of char and/or coke are also increased by the catalytic reaction over HY(30).
Fig. 7. Yields of gas, liquid, and solid residue obtained from the catalytic co-pyrolysis of e-PCB/HDPE or e-PCB/PP over HY and HZSM-5 catalysts using a fixed bed reactor.
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Bromine contents fixed in catalyst were also analyzed using a combustion ion chromatography (CIC) according to the method in the literature (Ma et al., 2017). As expected, HY(80) had the larger bromine content than HZSM-5(80) after the catalytic co-pyrolysis of e-PCB and plastics (Fig. 8). The largest amount of bromine was obtained from HY(30) used for the catalytic co-pyrolysis of e-PCB and HDPE. Overall, these results demonstrate that the eliminated bromine is fixed as a solid coke in the catalyst. Fig. 9 shows the amounts of phenol and BTEXs produced from the thermal and catalytic pyrolysis of e-PCB alone and the catalytic co-pyrolysis of e-PCB/HDPE and e-PCB/PP over HZSM-5 and HYs. Compared to the non-catalytic pyrolysis of e-PCB, the amounts of
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phenol and BTEXs were increased by catalytic pyrolysis due to the effective catalytic reactions, such as the dehydroxylation of phenol, secondary cracking of bisphenol As, and debromination of brominated phenols and bisphenol As. Between HY(80) and HZSM-5(80), HY(80) revealed higher selectivity toward mono-phenol due to the easier diffusion of brominated compounds into the larger pores of HY and their effective debromination. In addition, higher selectivity toward BTEXs was achieved over HZSM-5(80) because of its stronger acidity and shape selectivity for aromatic formation. By cofeeding PP or HDPE in the catalytic pyrolysis of e-PCB, much larger amounts of aromatic hydrocarbons were obtained over both HY(80) and HZSM-5(80) because the added PP and HDPE also induces the formation of aromatics. Between HZSM-5(80) and HY(80), HZSM-5(80) produced larger amounts of BTEXs than HY during the catalytic co-pyrolysis of e-PCB/HDPE or e-PCB/PP. In contrast, HY(80) provided higher selectivity toward mono-phenol. Although HZSM-5(80) produced the larger cumulative amount of BTEXs and phenol than HY(80) owing to its higher acidity (Lee et al., 2016), it also produced larger amount of brominated compounds than HY(80) during the catalytic co-pyrolysis of e-PCB/HDPE (Fig. 6). This suggests that the catalytic co-pyrolysis of e-PCB/HDPE over HY is an efficient method to achieve not only increased debromination efficiency, but also large amounts of phenol and BTEXs. As expected, the cumulative amounts of BTEXs and phenol were also increased by increasing the amounts of HY(80) and they were increased further by applying HY(30) with high acidity. Overall, the catalytic co-pyrolysis of e-PCB/HDPE over 10 mg of HY(30) was the most effective condition for not only debromination, but also the formation of valuable products in the final product oil.
4. Conclusions
Fig. 8. Bromine content deposited in the catalyst after the catalytic co-pyrolysis of ePCB/HDPE or e-PCB/PP over HY and HZSM-5 catalysts using a fixed bed reactor.
Fig. 9. Total ion peak intensities for phenol and BTEXs obtained from the thermal and catalytic pyrolysis of e-PCB and the catalytic co-pyrolysis of e-PCB/HDPE and e-PCB/PP) over HZSM-5 and HY catalysts.
Effective debromination during the pyrolysis of e-PCB was achieved by the additional use of catalysts (HY and HZSM-5) and the co-feeding of thermoplastics, such as HDPE and PP. The copyrolysis of e-PCB with HDPE or PP involved intermolecular reactions between the pyrolyzates of e-PCB and thermoplastics. On the other hand, a distinct debromination effect could not be achieved during the non-catalytic co-pyrolysis of e-PCB with HDPE or PP. Compared to the non-catalytic pyrolysis of e-PCB, its catalytic pyrolysis over HY(80) and HZSM-5(80) produced smaller amounts of brominated phenols and bisphenol As. HY(80) had a better debromination effect than HZSM-5(80). In addition, catalytic pyrolysis of e-PCB over HY(80) was quite effective in eliminating the large molecular brominated bisphenol As due to the easier diffusion of these large molecular brominated bisphenol As into the pores of HY(80) having a larger pore size than HZSM-5. The catalytic copyrolysis of e-PCB/HDPE over HY(80) led to enhanced debromination due to the synergistic bromine fixation on the catalyst. Among the various kinds of reactions applied in this study, the extent of debromination in pyrolysis oil was maximized on the catalytic copyrolysis of e-PCB and HDPE over HY(80) because of the easier diffusion of reactant molecules into the pores of HY(80) and the more efficient bromine fixation into coke. The total debromination efficiency was also increased using larger amounts of HY catalyst and applying highly acidic HY(30). Large amounts of valuable compounds, such as phenol and BTEXs, were also produced from the catalytic co-pyrolysis of e-PCB and HDPE over HZSM-5 and HY catalysts. This unique approach provides the solution not only for the proper treatment of waste PCBs and plastics but also for the production of value-added products from those wastes, thus contributing to the development of cleaner processes for the energy production.
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Acknowledgement This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2015R1A2A2A11001193). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jclepro.2017.08.224. References Blazso, M., 2005. In situ modification of pyrolysis products of macromolecules in an analytical pyrolyser. J. Anal. Appl. Pyrol. 74, 344e352. http://dx.doi.org/10.1016/ j.jaap.2004.11.034. Blazso, M., Czegeny, Z., 2006. Catalytic destruction of brominated aromatic compounds studied in a catalyst microbed coupled to gas chromatography/mass spectrometry. J. Chromatogr. A 1130, 91e96. http://dx.doi.org/10.1016/ j.chroma.2006.05.009. Chien, Y.-C., Wang, H.P., Lin, K.-S., Huang, Y.-J., Yang, Y.W., 2000. Fate of bromine in pyrolysis of printed circuit board wastes. Chemosphere 40, 383e387. http:// dx.doi.org/10.1016/S0045-6535(99)00251-9. Czegeny, Z., Jakab, E., Blazso, M., Bhaskar, T., Sakata, Y., 2012. Thermal decomposition of polymer mixtures of PVC, PET and ABS containing brominated flame retardant: formation of chlorinated and brominated organic compounds. J. Anal. Appl. Pyrolysis 96, 69e77. http://dx.doi.org/10.1016/j.jaap.2012.03.006. Deng, Y., Paraskevas, D., Tian, Y., Acker, K.V., Dewulf, W., Duflou, J.R., 2016. Life cycle assessment of flax-fiber reinforced epoxidized linseed oil composite with a flame retardant for electronic applications. J. Clean. Prod. 133, 427e438. http:// dx.doi.org/10.1016/j.jclepro.2016.05.172. Duan, H., Hu, J., Yuan, W., Wang, Y., Yu, D., Song, Q., 2016. Characterizing the environmental implications of the recycling of non-metallic fractions from waste printed circuit boards. J. Clean. Prod. 137, 546e554. http://dx.doi.org/ 10.1016/j.jclepro.2016.07.131. Elordi, G., Olazar, M., Lopez, G., Castano, P., Bilbao, J., 2011. Role of pore structure in the deactivation of zeolites (HZSM-5, Hb and HY) by coke in the pyrolysis of polyethylene in a conical spouted bed reactor. Appl. Catal. B Environ. 224e231. http://dx.doi.org/10.1016/j.apcatb.2010.12.002. Eswaraiah, C., Kavitha, T., Vidyasagar, S., Narayanan, S.S., 2008. Classification of metals and plastics from printed circuit boards (PCB) using air classifier. Chem. Eng. Process 47, 565e576. http://dx.doi.org/10.1016/j.cep.2006.11.010. Ghosh, B., Chosh, M.K., Parhi, P., Mukherjee, B.K., 2015. Waste printed circuit boards recycling: an extensive assessment of current status. J. Clean. Prod. 94, 5e19. http://dx.doi.org/10.1016/j.jclepro.2015.02.024. Hall, W.J., Williams, P.T., 2007. Separation and recovery of materials from scrap printed circuit boards. Resour. Conserv. Recy. 51, 691e709. http://dx.doi.org/ 10.1016/j.resconrec.2006.11.010. Hall, W.J., Williams, P.T., 2008. Removal of organobromine compounds from the pyrolysis oils of flame retarded plastics using zeolite catalysts. J. Anal. Appl. Pyrol. 81, 139e147. http://dx.doi.org/10.1016/j.jaap.2007.09.008. Hornung, A., Donner, S., Balabanovich, A., Seifert, H., 2005. Polypropylene as a reductive agent for dehalogenation of brominated organic compounds. J. Clean. Prod. 13, 525e530. http://dx.doi.org/10.1016/j.jclepro.2003.09.001. Jakab, E., Uddin, Md.A., Bhaskar, T., Sakata, Y., 2003. Thermal decomposition of flame-retarded high-impact polystyrene. J. Anal. Appl. Pyrol. 68e69, 83e99. http://dx.doi.org/10.1016/S0165-2370(03)00075-5. Jin, Y.-Q., Tao, L., Chi, Y., Yan, J.-H., 2011. Conversion of bromine during thermal
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Catalytic co-pyrolysis of epoxy-printed circuit board and plastics over HZSM-5 and HY Young-Min Kim a, b, 1, Tae Uk Han b, 1, Seungdo Kim b, Jungho Jae c, d, Jong-Ki Jeon e, Sang-Chul Jung f, Young-Kwon Park a, * a
School of Environmental Engineering, University of Seoul, Seoul, 02504, Republic of Korea Department of Environmental Sciences and Biotechnology, Hallym University, Chunchon, 24252, Republic of Korea Clean Energy Research Center, Korea Institute of Science and Technology, Seoul, 02792, Republic of Korea d Division of Energy & Environment Technology, KIST School, Korea University of Science and Technology, Seoul, 02792, Republic of Korea e Department of Chemical Engineering, Kongju National University, Cheonan, 31080, Republic of Korea f Department of Environmental Engineering, Sunchon National University, Suncheon, 57922, Republic of Korea b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 December 2016 Received in revised form 27 August 2017 Accepted 30 August 2017 Available online 3 September 2017
The catalytic co-pyrolysis of epoxy-printed circuit board (e-PCB) and thermoplastics, high density polyethylene (HDPE) and polypropylene (PP), over HZSM-5 and HY was performed using a thermogravimetric analyzer and pyrolyzer-gas chromatography/mass spectrometry. The catalytic pyrolysis of e-PCB over both HZSM-5 and HY eliminated the brominated compounds, mainly bromo-phenols and -bisphenol As, to some extent. The co-feeding of HDPE and PP on the catalytic pyrolysis of e-PCB over both HZSM-5 and HY revealed different debromination efficiencies due to the properties of the thermoplastics and catalysts. A comparison of HY(80) and HZSM-5(80) with the same SiO2/Al2O3 ratio (80) revealed HY(80) had a better elimination efficiency of brominated compounds during the catalytic copyrolysis of e-PCB and HDPE or PP because of its larger pore size than HZSM-5(80). The lowest bromine content was achieved when HDPE and HY(30) were used as the co-feeding reactant and catalyst on the pyrolysis of e-PCB due to the large pore size and high acidity of HY(30), allowing the easier diffusion of large molecular brominated bisphenol A and HDPE molecules into the pores of the catalyst and efficient catalytic intermolecular reactions in the pores of the catalyst. The catalytic co-pyrolysis of ePCB and HDPE over HY(30) also produced large amounts of mono-aromatic hydrocarbons and monophenol, which can be used as fuels or chemical feedstock. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Catalytic co-pyrolysis Bromine fixation Epoxy-printed circuit board High density polyethylene Polypropylene HY
1. Introduction United Nations Environment Program (UNEP, 2009) estimates the annual amount of waste electrical and electronic equipment (WEEE) generated to be 20e50 million tons, which is increasing rapidly with the accelerated replacement cycle of electrical and electronic equipment. Therefore, the development of a desirable treatment technology for WEEE is an important research area. The necessity of recycling of WEEE is emphasized more than other municipal wastes because of its potential harmful effects on human health due to the brominated frame retardants (Terakado et al.,
* Corresponding author. E-mail address: [email protected] (Y.-K. Park). 1 Co-first authors. http://dx.doi.org/10.1016/j.jclepro.2017.08.224 0959-6526/© 2017 Elsevier Ltd. All rights reserved.
2011), such as brominated epoxy resin or tetrabromobisphenol A (TBBA) in printed circuit boards (PCBs) (Deng et al., 2016) as well as plastics, such as brominated high impact polystyrene (HIPS) in WEEE (Jakab et al., 2003). Although large amounts of WEEE is being recycled after being exported from developed countries to developing countries, considerable amounts are still being discarded in landfill, which can cause additional contamination of water and soil (Sepulveda et al., 2010). PCBs are the main parts of electrical and electronic equipment. Owing to the presence of valuable metals, such as copper, calcium, iron, nickel, zinc, aluminum, silver, gold, etc (Hall and Williams, 2007)., in PCBs, resource recovery from PCBs is considered as a valuable recycling process. To this end, various physical and mechanical separation methods have been suggested by many researchers (Eswaraiah et al., 2008). On the other hand, the high cost of separation methods is a significant limitation. Therefore, large
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amounts of PCBs are still treated by conventional methods, such as landfill and incineration, in which the contamination of soil and ground water by the leachate and large amounts of dioxins and furan emissions are serious problems (Chien et al., 2000). Proper treatment and recycling of non-metallic fraction is also being emphasized because the weight fraction of non-metallic fraction is higher than metallic fraction in PCBs (Duan et al., 2016). To recycle non-metallic parts of PCBs, Rajagopal et al. (2017) synthesized sustainable composite panels by hot-pressing the mixture of nonmetallic waste PCBs and automotive plastics. Recently, the pyrolysis of PCBs, which is the thermal decomposition of PCBs at high temperatures under a non-oxygen atmosphere, was investigated and is considered a promising technology to recover high-value metals from PCB without harmful gas emissions (Hall and Williams, 2007). The recovery of product gas and oils, which can be used as fuels or chemical feedstock, is an additional merit of the e-PCB pyrolysis process and its value is increasing due to the worldwide energy issues (Jin et al., 2011). On the other hand, the use of PCB pyrolysis oil is limited because it contains large amounts of brominated compounds (Ghosh et al., 2015), such as bromophenols and bromobisphenol-As (Kim et al., 2015). High contents of brominated compounds in pyrolysis oil from PCBs can have the harmful effects due to the toxicity of brominated chemicals. Therefore, additional process, such as the catalytic upgrading of PCB pyrolysis oil is being investigated to minimize the contents of brominated compounds in PCB pyrolysis oil. Various catalysts, such as metal oxides (Shen et al., 2016) or zeolites (Zhao et al., 2017) have been assessed for the catalytic pyrolysis of PCBs. For example, Terakado et al. (2013) investigated the bromine fixation effect of metal oxides in the pyrolysis of e-PCB and observed the elimination of hydrogen bromide and brominated organic compounds through the formation of metal bromides or oxybromide. Blazso (2005) also reduced the concentration of brominated compounds in the PCB pyrolysis oil by the catalytic pyrolysis of flame-retarded PCBs and phthalic polyester containing brominated polystyrene over zeolite catalysts. Co-pyrolysis of PCB and other polymeric materials such as plastics and biomass were also suggested as the potential treatment method for PCBs. Hornung et al. (2005) reported the synergistic debromination effect on the pyrolysis of brominated compounds by co-feeding with polypropylene (PP) in a pyrolysis reactor. Recently, Wu and Qiu (2014) also performed the vacuum co-pyrolysis of waste PCBs with Chinese fir sawdust. Although they obtained the increased yields of value-added brominated aromatic compounds due to the effective interactions between the pyrolyzates of PCBs and the Chinese fir sawdust, the reduced bromine content in the product oil was not reported. One of the candidate co-feeding materials on the catalytic pyrolysis of PCBs is waste plastics, such as high density polyethylene (HDPE) and polypropylene (PP) because large amounts of HDPE and PP are released into the environment and need to be treated properly. The reaction efficiency of the catalytic pyrolysis of a polymer can be influenced largely by the properties of the reactant polymers and catalysts (Muhammad et al., 2015). Therefore, the use of an appropriate catalyst and co-feeding plastics on the pyrolysis of PCBs can not only decrease the contents of brominated compounds, but also increase the amounts of value-added chemicals in the product oil; however, its actual application has not been reported in the literature. Additionally, the effects of co-feeding materials and catalytic properties, such as acidity and pore size, on the debromination efficiency need to be investigated prior to its actual application to a commercial plant. In this study, the in-situ catalytic co-pyrolysis of PCB and thermoplastics, HDPE and PP, over different catalysts, HZSM-5 and HY, was performed using a thermogravimetric (TG) analyzer and
367
analytical pyrolyzer-gas chromatography/mass spectrometry (PyGC/MS). The co-feeding effect of the thermoplastics and catalyst on the pyrolysis of PCB was estimated according to the amounts of residual brominated compounds and value-added chemicals in the product oil. 2. Experiments 2.1. Materials Waste e-PCB (FR-4 type, epoxy resin reinforced by glass fibers), which was obtained from a local PCB recycling company in Korea, was ground and cryo-milled after copper removal using a gravity separation device. The metal removed part of the PCB was dried at 80 C for 3 h, sieved to a particle size less than 300 mm, and used as the sample for the pyrolysis of e-PCB. Physico-chemical properties of e-PCB are shown in Table S1 (Supplementary Information, Kim et al., 2013). Metal removed e-PCB still contained large amounts of inorganic compounds (68.1%). Owing to the presence of Brcontaining epoxy resin, Br content in e-PCB was especially higher than other elements (H, O, and N) except C. The powder forms of HDPE and PP were purchased from Samsung Chemical Co., Korea. 2.2. Catalysts Three kinds of catalysts, HZSM-5 (SiO2/Al2O3 80), HY (SiO2/Al2O3 80), and HY (SiO2/Al2O3 30), were purchased from Zeolyst international. The physico-chemical properties of the catalysts were measured by N2 adsorption-desorption (Table 1), and the acidities of the catalysts were determined by temperature-programmed desorption of ammonia (NH3-TPD) analysis (Fig. 1, Table S2). A comparison of HY(80) and HZSM-5(80) revealed HY(80) has a larger pore size and surface area. NH3-TPD curves of three catalysts (Fig. 1) showed that NH3 desorption temperature of HZSM-5(80) is higher than those of HY(80) and HY(30). This indicates that the acid strength of HZSM-5 is highest among the catalysts. Meanwhile, both the total number of acid site and the number of strong acid site decreased in the order HY (30) > HZSM-5 (80) > HY (80), as shown in Table S2. Scanning electron microscopy (SEM) analysis of the catalysts (Fig. S1) revealed that the particles of HZSM-5 and HY catalysts exhibited significantly different shape and size. HY catalysts had the cubic particles having average size between 0.6 and 0.8 mm, and HZSM-5 had the sphere-like crystals having the smaller particle size than HYs. Prior to TG and Py-GC/MS analysis, all catalysts were calcined at 550 C in air. 2.3. TG analysis Individual e-PCB, HDPE, and PP (1 mg) and their mixtures (2 mg), e-PCB/PP or e-PCB/HDPE, were heated non-isothermally in a TG analyzer (TGA Pyris 1, PerkinElmer Co., USA) from room
Table 1 Physico-chemical properties of HZSM-5 and HY. Catalyst
HZSM-5 (80)
HY (80)
HY (30)
SiO2/Al2O3 mole ratio Surface area (m2/g) Micropore surface area a (m2/g) Pore volumeb (cm3/g) Micropore volumec (cm3/g) Pore size (Å)
80 456 376 0.270 0.159 5.1 5.5, 5.3 5.6
80 743 573 0.571 0.296 7.4 7.4
30 903 742 0.531 0.229 7.4 7.4
a b c
Pore volume at p/p0 ¼ 0.99. By t-plot. By t-plot.
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Fig. 1. NH3-TPD curves of HZSM-5 and HY catalysts.
temperature to 700 C at a heating rate of 20 C/min under 60 mL/ min of nitrogen gas. To assess the catalytic pyrolysis, 5 mg of catalyst was also mixed together with the sample and pyrolyzed under the same condition. 2.4. Py-GC/MS Individual e-PCB, HDPE, PP (1 mg), and their mixtures (2 mg) in a pyrolyzer sample cup was inserted into the center position of the pre-heated pyrolyzer (Py-3030D, Frontier Laboratories Ltd., Japan) at 600 C under free fall. In the case of catalytic pyrolysis, 5 or 10 mg of catalyst was also added to the sample cup and blended manually. The pyrolysis vapor emitted from the pyrolyzer was introduced to a metal capillary column (UA-5, 30 m 0.25 mm i.d. X 0.25 mm film) via a GC inlet after GC inlet splitting (Split ratio, 100:1) in a GC/MS (Agilent 7890A/7975C inert). To increase the GC/MS peak sharpness, the chemicals in the column were cryo-focused at the front part of the column using a Micro Jet Cryo-Trap (MJT-1030E, Frontier Laboratories Ltd., Japan), separated by a GC oven program, from 40 C (5 min) to 340 C (10 min) at a heating rate of 20 C/min, and detected using a MS detector. Chemical identification in the chromatogram was performed using the NIST 8th library (National Institute of Standard and Technology, USA) and F-Search pyrolyzate library (F-Search, Frontier Laboratories Ltd., Japan). Integration of each peak on the total ion chromatogram (scan mode) and Sim ion chromatogram (SIM mode) was also performed to compare the amounts of each compound produced from each experiment by comparing the integrated peak areas. 3. Results and discussion 3.1. TG analysis Fig. 2 shows the DTG curves obtained from the thermal and catalytic pyrolysis of the individual samples and their mixtures over HZSM-5(80), HY(80), and HY(30). Table 2 lists the maximum decomposition temperatures (Tmaxs) on the DTG curves. The main non-catalytic decomposition of e-PCB was performed between 250 and 400 C. The organic part of e-PCB consisted of brominated and non-brominated epoxy resin that they were decomposed independently during non-isothermal pyrolysis (Kim et al., 2013). The Tmax values of e-PCB were not shifted to a lower temperature, even when large amounts (5 mg) of HY(80) or HZSM-5(80) were applied.
Fig. 2. DTG curves obtained from thermal and catalytic pyrolysis of e-PCB, HDPE, PP, and their mixtures over HZSM-5 and HY catalysts.
This can be explained by the thermal property of the epoxy resin. Thermosetting plastics, such as epoxy resin, having a cross-linking structure, do not melt when they are heated instead they get
Y.-M. Kim et al. / Journal of Cleaner Production 168 (2017) 366e374 Table 2 Maximum decomposition temperatures on the DTG curves obtained from the thermal and catalytic co-pyrolysis of e-PCB, HPDE, and PP over HZSM-5 and HY catalysts. Maximum decomposition temperature (Tmaxa/Tmaxb, C)
e-PCB HDPE PP e-PCB/HDPE e-PCB/PP a b
No Catalyst
HZSM-5(80)
HY(80)
333 490 464 331/486 328/470
333 397 363 335/410 335/491
333 398 343 331/471 330/431
Tmax on the decomposition DTG curve of e-PCB. Tmax on the decomposition DTG curve of HDPE or PP.
decomposed directly into volatile species. Therefore, the catalysts cannot promote the decomposition of e-PCB due to mass transfer limitations (solid-solid contact) and Tmax values on the DTG curves of e-PCB are not influenced by the catalysts. The molecular sizes of the main pyrolyzates of e-PCB, such as brominated phenols and bisphenol As, are also considerably larger than the pore sizes of HZSM-5(80) and HY(80), limiting the diffusion of these pyrolysis intermediates to the pore of the catalysts. In contrast, the Tmax values of e-PCB were moved to a lower temperature when e-PCB was pyrolyzed together with HDPE or PP. This indicates the presence of mutual interactions between the e-PCB and HDPE or PP, resulting in the easier decomposition of e-PCB. Czegeny et al. (2012) reported that the H-abstraction by bromine radicals can initiate the decomposition of high impact polystyrene (HIPS) when brominated epoxy oligomer is pyrolyzed together with HIPS. When ePCBs are co-pyrolyzed with thermoplastics, such as HDPE and PP having 137 and 176 C of melting point, respectively (Yu et al., 2016), the melted molecules of the thermoplastics probably penetrate into the network structure of the epoxy resin and can be decomposed by the abstracted hydrogen and bromine radicals. Radical intermediates of these thermoplastics will also be able to assist in the decomposition of the epoxy resin. As shown in Table 2, lower Tmax values of HDPE were also achieved when it was pyrolyzed together with e-PCB, which confirms the mutual interaction between e-PCB and HDPE. The catalytic pyrolysis of HDPE over HZSM-5(80) and HY(80) revealed lower decomposition temperatures than their thermal decomposition DTG curves, indicating that the HDPE molecules can be diffused into the pores of HZSM-5(80) and HY(80). The catalytic pyrolysis of PP over HZSM-5(80) and HY(80) showed much lower temperatures than those of PP over the same catalysts due to the lower thermal stability of PP molecules than HDPE. In addition, the catalytic pyrolysis of PP over HY(80) had a much lower Tmax (343 C) than that over HZSM-5(80), which indicates that the diffusion of PP molecules into HY(80) is easier than that into HZSM-5(80). This can be due to the fact that HY(80) has the larger pore size than HZSM-5(80), although its acidity is much lower than that of HZSM-5 (Table 1, Table S2 and Fig. 1). Compared to the catalytic pyrolysis of the individual samples, the catalytic co-pyrolysis of e-PCB/HPDE or e-PCB/PP revealed higher Tmax values of HDPE and PP decomposition over both catalysts. This suggests that the decomposition of HDPE and PP is hindered when e-PCB is co-pyrolyzed over the catalysts. These hindering effects for the decompositions of HDPE and PP in the catalytic co-pyrolysis of e-PCB/PP or e-PCB/HDPE can be explained by coke or char produced from the catalytic pyrolysis of e-PCB. In the case of the catalytic pyrolysis of e-PCB, large molecular pyrolyzates, such as brominated phenols and bisphenol A, can act as coke precursors that can block the pores of the catalysts and deactivate the catalysts. The diffusion of PP and HDPE molecules will be more difficult if coke fills and blocks the pores. Similar
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results were also reported in the study that examined the catalytic co-pyrolysis of biomass and thermoplastics (Kim et al., 2016). They found that the coke produced from the catalytic pyrolysis of cellulose hinders the catalytic pyrolysis of thermoplastics. Between PP and HDPE, PP had a more severe hindering effect than HDPE when co-pyrolyzed with e-PCB, especially over HZSM-5(80). The Tmax values of HDPE(430 C) and PP(373 C) on the DTG curves obtained from the catalytic co-pyrolysis of e-PCB/PP and e-PCB/HDPE over HY(30) were lower than those over HY(80) and HZSM-5(80). This suggests that HY(30) is the most effective catalyst in reducing this hindering effect on the catalytic co-pyrolysis of e-PCB/PP and ePCB/HDPE because of its higher acidity than HY(80) and larger pore size than HZSM-5(80) (Table 1, Table S2 and Fig. 1). From the results obtained from TG analysis, it can be inferred that the debromination efficiency and the formation of valueadded compounds from catalytic co-pyrolysis can be influenced by the type of co-feeding plastic and the pore size/structure of the catalysts. 3.2. Non-catalytic pyrolysis Fig. 3(a) presents the non-catalytic pyrogram of e-PCB at 600 C. Large amounts of phenol and bisphenol A were produced by the thermal decomposition of e-PCB together with alkylated or brominated phenols and bisphenol As (Blazso and Czegeny, 2006). High phenol and bisphenol A contents in the pyrolysis oil of e-PCB are meaningful because they can be used as important chemical feedstocks for the production of industrial polymers, such as polycarbonates, epoxy resin, phenolic resins, etc. (Weber et al., 2004). In addition, the direct use of e-PCB pyrolysis oil as a chemical feedstock for petrochemical process appears to be difficult because considerable amounts of brominated compounds, such as bromophenol, dibromophenol, dibromo-methyl-dihydrofuran, dibromobisphenol A, 2-(3-Bromophenyl)-2-(3,5-dibromo-4-hydroxyphenyl)propane, tribromobisphenol A, and tetrabromobisphenol A (TBBA), are contained in the pyrolysis oil of e-PCB (Kim et al., 2013). Fig. 3(b) and (c) show the non-catalytic pyrogram of the e-PCB/ PP and e-PCB/HDPE mixture, respectively. Many different compounds were formed due to the additional decomposition of HDPE and PP. Alkylated olefins with a wide range of carbon numbers, which are typical pyrolyzates of PP (Tsuge et al., 2011), were detected together with the same types of e-PCB pyrolyzates on the pyrogram of e-PCB/PP. Typical pyrolyzates of HDPE, triplet peaks (alkadiene/alkene/alkane) having a wide range of carbon numbers (Tsuge et al., 2011), were also detected on the pyrogram of the ePCB/HDPE mixture together with the pyrolyzates of e-PCB. Although the peak heights of the e-PCB pyrolyzates were not changed dramatically by co-pyrolysis with PP or HDPE, those of phenol were increased together with the decreased peak heights of bisphenol As. These phenomena can be explained by the mutual interactions between the pyrolyzates of e-PCB and those of PP or HDPE. Although the peak intensities of the e-PCB pyrolyzates were changed, the changes in the peak heights of brominated phenols and bisphenol As were insignificant, particularly when PP was copyrolyzed with e-PCB. This suggests that the co-pyrolysis of ePCB with PP does not lead to the additional debromination of brominated phenols and biophenol As. 3.3. Catalytic pyrolysis Fig. 4 presents the pyrograms obtained from the catalytic pyrolysis of e-PCB over HZSM-5(80) and HY(80). Compared to the non-catalytic pyrolysis of e-PCB, the amounts of phenol and monoaromatics, such as benzene, toluene, ethylbenzene, and xylenes, were higher together with a lower bisphenol As content over both
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Fig. 3. Pyrograms obtained from the thermal decomposition of e-PCB, e-PCB/PP, and e-PCB/HDPE.
catalysts. The catalytic pyrolysis of e-PCB also produced smaller amounts of brominated phenols and bisphenol As than its noncatalytic pyrolysis. Between HY(80) and HZSM-5(80), HY(80) produced smaller amounts of brominated phenols and bisphenol As during the catalytic pyrolysis of e-PCB. Using HY(80), most of the brominated bisphenol As, having larger molecular sizes than brominated phenols, were eliminated, whereas considerable amounts of brominated bisphenol A still remained when HZSM-5(80) was applied. The lower efficiency of HZSM-5(80) for the elimination of brominated bisphenol A can be explained by its smaller pore size than HY(80). The higher debromination efficiency of the larger pore zeolites on the catalytic pyrolysis of brominated polymer has been reported by Blazso and Czegeny (2006). They found that the larger pore size of molecular sieves allows the effective trapping of brominated aromatic compounds during the catalytic pyrolysis of
tetrabromobisphenol A (TBBA). Hall and Williams (2008) also found that the amounts of debrominated oils produced with HY(80) were larger than those with HZSM-5(80). They explained this phenomenon by the larger pore size of HY(80). A higher selectivity toward aromatic hydrocarbons was achieved when HZSM-5(80) was used as the catalyst on the catalytic pyrolysis of ePCB because of its higher acidity than HY(80) (Fig. 1), leading to the additional dehydration of phenols to aromatics. The pore size (5.5e5.6 Å) and structure (MFI) of HZSM-5 is also effective in aromatic hydrocarbon formation because of its shape selectivity (Elordi et al., 2011). Recently, Ma et al. (2016a) performed the catalytic pyrolysis of TBBA containing acrylonitrile-styrene-butadiene (TBBA-ABS) over microporous catalysts (HZSM-5, HY, and Hb) and mesoporous catalysts (silica MCM-41 and Al2O3). They also applied the same catalysts on the catalytic pyrolysis of decabromodiphenyl oxide containing high impact polystyrene (Br-HIPS) (Ma et al.,
Fig. 4. Pyrograms obtained from catalytic pyrolysis of e-PCB over (a) HZSM-5(80) and (b) HY(80).
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2017). They reported a higher selectivity toward aromatic hydrocarbons, such as toluene and ethylbenzene, over catalysts with higher acidity. In addition, a higher debromination efficiency was achieved over the catalysts with a larger pore size and surface area. As shown in Table 1, HY(80) has a larger pore size and surface area than HZSM-5(80). Hence, the efficient elimination of brominated compounds on the catalytic pyrolysis of e-PCB over HY(80) can be explained by its larger pore size and surface area. 3.4. Catalytic co-pyrolysis of e-PCB and plastics (HDPE and PP) Fig. 5 shows the pyrograms obtained from the catalytic copyrolysis of e-PCB/PP or e-PCB/HDPE over HZSM-5(80) or HY(80). Similar to the results of the catalytic pyrolysis of e-PCB, the catalytic co-pyrolysis of both e-PCB/PP and e-PCB/HDPE over HZSM-5(80) produced larger amounts of aromatic hydrocarbons than those over HY(80). In addition, the amounts of aromatic hydrocarbons obtained from the catalytic co-pyrolysis of e-PCB/PP and e-PCB/ HDPE over HZSM-5(80) were much larger than those of e-PCB. This can be explained by the high acidity of HZSM-5 (Sakata et al., 1999) and the co-feeding effect of PP or HDPE. By co-feeding PP or HDPE, more intermediate species for aromatic formation, e.g., olefins, can be produced, which will lead to the formation of aromatics in larger
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quantity. The catalytic co-pyrolysis of e-PCB with PP or HDPE over HY(80) and HZSM-5(80) also decreased the amounts of brominated phenols and bisphenol As. In particular, it was difficult to find brominated bisphenol As peaks on the total ion chromatogram obtained from the catalytic co-pyrolysis of e-PCB/PP and e-PCB/HPDE over HY(80). To compare the amounts of residual brominated compounds in more detail, the sim ion chromatograms for all reactions performed in this study were also analyzed (Fig. S2). Fig. 6 presents their integrated peak areas obtained from each reaction. As shown in Fig. 6, the catalytic co-pyrolysis of e-PCB/PP or ePCB/HDPE over both HY(80) and HZSM-5(80) produced smaller amounts of brominated compounds than the non-catalytic pyrolysis of e-PCB. The catalytic co-pyrolysis of e-PCB/HDPE over HZSM5(80) and HY(80) produced smaller amounts of brominated phenols and bisphenol As, which confirmed the synergistic debromination by the catalytic co-pyrolysis of e-PCB with HDPE. In addition, the peak intensities for the brominated phenols and bisphenol As obtained from the catalytic co-pyrolysis of the e-PCB/PP mixture were similar to those from the catalytic pyrolysis of e-PCB over the same catalyst. HY(80) was more effective in debromination during the catalytic co-pyrolysis of both e-PCB/PP and e-PCB/HDPE than HZSM-5(80). This can be explained by the easier diffusion of HDPE
Fig. 5. Pyrograms obtained from catalytic co-pyrolysis of e-PCB and plastics (PP and HDPE) over HZSM-5(80) and HY(80).
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Fig. 6. Sim ion peak intensities for the brominated phenol and biophenol As obtained from the thermal and catalytic pyrolysis of e-PCB and their mixtures (PCB/HDPE or e-PCB/PP) over HZSM-5 and HY catalysts. The values in the parenthesis indicate the amount of the catalyst used for catalytic pyrolysis.
and PP into the pores of HY than HZSM-5(80) due to the larger pore size of HY than HZSM-5. The enhanced diffusion of reactant molecules into the catalyst pore can lead to the more effective debromination of e-PCB pyrolyzates. Although more effective debromination was achieved on the catalytic co-pyrolysis of e-PCB/HDPE over HY(80), the amounts of brominated gas (bromomethane) were not increased dramatically compared to the catalytic pyrolysis of e-PCB alone over HY(80). This suggests that most of the bromine was fixed into the coke. The debromination by fixation into coke can be also rationalized by the higher debromination efficiency over HY than that over HZSM-5. The large pore size facilitates the growth of heavier coke compounds due to the additional hydrogen transfer, oligomerization, and condensation (Ma et al., 2017). To determine the effect of the catalyst amounts, 10 mg of HY was applied to the catalytic co-pyrolysis of e-PCB/PP or e-PCB/HDPE. Much larger amounts of brominated compounds were eliminated by applying 10 mg of HY instead of 5 mg of catalysts. This indicates that an increase in the total acidity can also improve the debromination efficiency during the catalytic co-pyrolysis of e-PCB/PP or e-PCB/HDPE. Compared to the e-PCB/PP mixture, that of e-PCB/ HDPE showed higher debromination efficiency on the catalytic copyrolysis of e-PCB with thermoplastics over HY(80). Additional debromination was also achieved by applying 10 mg of HY(30) catalyst, which has higher acidity than HY(80), as the catalyst on the catalytic co-pyrolysis of e-PCB/HDPE. Interestingly, the use of HY(30) was also effective on debromination during the catalytic copyrolysis of e-PCB and PP. This suggests that the impeded diffusion of PP molecules or its reaction intermediates into the pores of HY can be minimized by applying a high acid catalyst; even e-PCB and PP were co-pyrolyzed. To examine the Br-elimination pathways of brominated compounds during the catalytic co-pyrolysis of e-PCB and plastics over HY and HZSM-5 catalysts, additional experiments using a fixed-bed reactor was performed. Fig. 7 shows the yields (wt.%) of gas, liquid, and solid residue obtained from the catalytic co-pyrolysis of e-PCB and plastics (HDPE or PP) over HZSM-5(80) and HY(30/80). The yields of oil and solid were obtained by measuring the weights of
condensed oil and remained solid residue after the pyrolysis reaction, and the yield of gas was calculated by subtracting the summed yields of oil and solid from 100%. Compared to HZSM-5(80), the larger amount of solid residue was produced over HY(80) during the catalytic co-pyrolysis of e-PCB/PP and e-PCB/HDPE and this can be understood as a result of the larger amount of coke formed on HY(80). Importantly, the largest amount of solid residue (char and coke) was obtained from the catalytic co-pyrolysis of e-PCB/HDPE over HY(30) which showed the highest debromination efficiency (Fig. 6). This indicates that the amount of char and/or coke are also increased by the catalytic reaction over HY(30).
Fig. 7. Yields of gas, liquid, and solid residue obtained from the catalytic co-pyrolysis of e-PCB/HDPE or e-PCB/PP over HY and HZSM-5 catalysts using a fixed bed reactor.
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Bromine contents fixed in catalyst were also analyzed using a combustion ion chromatography (CIC) according to the method in the literature (Ma et al., 2017). As expected, HY(80) had the larger bromine content than HZSM-5(80) after the catalytic co-pyrolysis of e-PCB and plastics (Fig. 8). The largest amount of bromine was obtained from HY(30) used for the catalytic co-pyrolysis of e-PCB and HDPE. Overall, these results demonstrate that the eliminated bromine is fixed as a solid coke in the catalyst. Fig. 9 shows the amounts of phenol and BTEXs produced from the thermal and catalytic pyrolysis of e-PCB alone and the catalytic co-pyrolysis of e-PCB/HDPE and e-PCB/PP over HZSM-5 and HYs. Compared to the non-catalytic pyrolysis of e-PCB, the amounts of
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phenol and BTEXs were increased by catalytic pyrolysis due to the effective catalytic reactions, such as the dehydroxylation of phenol, secondary cracking of bisphenol As, and debromination of brominated phenols and bisphenol As. Between HY(80) and HZSM-5(80), HY(80) revealed higher selectivity toward mono-phenol due to the easier diffusion of brominated compounds into the larger pores of HY and their effective debromination. In addition, higher selectivity toward BTEXs was achieved over HZSM-5(80) because of its stronger acidity and shape selectivity for aromatic formation. By cofeeding PP or HDPE in the catalytic pyrolysis of e-PCB, much larger amounts of aromatic hydrocarbons were obtained over both HY(80) and HZSM-5(80) because the added PP and HDPE also induces the formation of aromatics. Between HZSM-5(80) and HY(80), HZSM-5(80) produced larger amounts of BTEXs than HY during the catalytic co-pyrolysis of e-PCB/HDPE or e-PCB/PP. In contrast, HY(80) provided higher selectivity toward mono-phenol. Although HZSM-5(80) produced the larger cumulative amount of BTEXs and phenol than HY(80) owing to its higher acidity (Lee et al., 2016), it also produced larger amount of brominated compounds than HY(80) during the catalytic co-pyrolysis of e-PCB/HDPE (Fig. 6). This suggests that the catalytic co-pyrolysis of e-PCB/HDPE over HY is an efficient method to achieve not only increased debromination efficiency, but also large amounts of phenol and BTEXs. As expected, the cumulative amounts of BTEXs and phenol were also increased by increasing the amounts of HY(80) and they were increased further by applying HY(30) with high acidity. Overall, the catalytic co-pyrolysis of e-PCB/HDPE over 10 mg of HY(30) was the most effective condition for not only debromination, but also the formation of valuable products in the final product oil.
4. Conclusions
Fig. 8. Bromine content deposited in the catalyst after the catalytic co-pyrolysis of ePCB/HDPE or e-PCB/PP over HY and HZSM-5 catalysts using a fixed bed reactor.
Fig. 9. Total ion peak intensities for phenol and BTEXs obtained from the thermal and catalytic pyrolysis of e-PCB and the catalytic co-pyrolysis of e-PCB/HDPE and e-PCB/PP) over HZSM-5 and HY catalysts.
Effective debromination during the pyrolysis of e-PCB was achieved by the additional use of catalysts (HY and HZSM-5) and the co-feeding of thermoplastics, such as HDPE and PP. The copyrolysis of e-PCB with HDPE or PP involved intermolecular reactions between the pyrolyzates of e-PCB and thermoplastics. On the other hand, a distinct debromination effect could not be achieved during the non-catalytic co-pyrolysis of e-PCB with HDPE or PP. Compared to the non-catalytic pyrolysis of e-PCB, its catalytic pyrolysis over HY(80) and HZSM-5(80) produced smaller amounts of brominated phenols and bisphenol As. HY(80) had a better debromination effect than HZSM-5(80). In addition, catalytic pyrolysis of e-PCB over HY(80) was quite effective in eliminating the large molecular brominated bisphenol As due to the easier diffusion of these large molecular brominated bisphenol As into the pores of HY(80) having a larger pore size than HZSM-5. The catalytic copyrolysis of e-PCB/HDPE over HY(80) led to enhanced debromination due to the synergistic bromine fixation on the catalyst. Among the various kinds of reactions applied in this study, the extent of debromination in pyrolysis oil was maximized on the catalytic copyrolysis of e-PCB and HDPE over HY(80) because of the easier diffusion of reactant molecules into the pores of HY(80) and the more efficient bromine fixation into coke. The total debromination efficiency was also increased using larger amounts of HY catalyst and applying highly acidic HY(30). Large amounts of valuable compounds, such as phenol and BTEXs, were also produced from the catalytic co-pyrolysis of e-PCB and HDPE over HZSM-5 and HY catalysts. This unique approach provides the solution not only for the proper treatment of waste PCBs and plastics but also for the production of value-added products from those wastes, thus contributing to the development of cleaner processes for the energy production.
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