Steam Pyrolysis of Polyimides: Effects of Steam on Raw Material

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Steam Pyrolysis of Polyimides: Effects of Steam on Raw Material Recovery Shogo Kumagai, Tomoyuki Hosaka, Tomohito Kameda, and Toshiaki Yoshioka* Graduate School of Environmental Studies, Tohoku University, 6-6-07 Aoba, Aramaki-aza, Aoba-ku, Sendai, Miyagi 980-8579, Japan S Supporting Information *

ABSTRACT: Aromatic polyimides (PIs) have excellent thermal stability, which makes them difficult to recycle, and an effective way to recycle PIs has not yet been established. In this work, steam pyrolysis of the aromatic PI Kapton was performed to investigate the recovery of useful raw materials. Steam pyrolysis significantly enhanced the gasification of Kapton at 900 °C, resulting in 1963.1 mL g−1 of a H2 and CO rich gas. Simultaneously, highly porous activated carbon with a high BET surface area was recovered. Steam pyrolysis increased the presence of polar functional groups on the carbon surface. Thus, it was concluded that steam pyrolysis shows great promise as a recycling technique for the recovery of useful synthetic gases and activated carbon from PIs without the need for catalysts and organic solvents.

1. INTRODUCTION Aromatic polyimides (PIs) are lightweight and flexible and exhibit excellent thermal resistance and physical strength. Therefore, PIs are mainly used for flexible printed circuit boards. Moreover, their low weight and excellent stability make them viable as metal alternatives in the automobile and aeronautic industries. The PI film production in 2012 was 93 million m2, which is more than double of that in 2008.1 Therefore, significant demand of PI film is expected in the future. Unfortunately, the stability of PIs makes them difficult to recycle. Solvolysis processes, which break the polymer into monomers, have been previously reported for PI treatment. The use of subcritical water has been reported to enhance the hydrolysis of aromatic PIs.2,3 Also, alkaline hydrolysis at atmospheric pressure has been employed for the depolymerization of the imide rings.4,5 Thus, high temperature and pressure or alkaline catalysts are required for solvolysis. In addition, PI waste is often combined with other plastics and metals. These contaminants have a negative impact on the solvent, making its reuse problematic. Pyrolysis is a method that avoids the use of solvents and requires only heat to cleave the polymer chain, decomposing the plastic fraction into valuable gases and oils.6−9 In addition, other plastics, which must usually be removed before solvolysis, can be simultaneously decomposed into oil and gas compounds.10 However, pyrolysis of aromatic PIs produces low-value CO- and CO2-rich gas and more than 50 wt % carbonized solid.11−13 Therefore, the carbon produced is suitable as a precursor for carbon materials such as graphite films,14 activated carbons,15,16 and carbon foams.17,18 Steam pyrolysis is a promising decomposition method with the advantages of both pyrolysis and hydrolysis. Decomposition © 2015 American Chemical Society

is performed in a steam atmosphere at pyrolytic temperatures, accelerating hydrolysis at atmospheric pressure without a catalyst. We have previously reported that steam pyrolysis of poly(ethylene terephthalate) (PET) afforded terephthalic acid (TPA) in 72% yield, while pyrolytic conditions alone produced a lower yield of TPA.19,20 The effectiveness of steam pyrolysis on polycarbonates has also been reported, improving oil production significantly.21−23 Furthermore, steam pyrolysis allows incorporated inorganics to be separated as solids by volatilization of the organic material.24 On the basis of these reports, it is assumed that steam pyrolysis will promote the hydrolysis of imide rings and break down aromatic PIs into useful chemicals. Simultaneously, the positive effects of steam on the activation of the carbonized solid produced can be expected. Therefore, steam pyrolysis has excellent potential as a PI recycling technique. However, to the best of our knowledge, the effects of steam on the pyrolytic decomposition of PIs have not been investigated. Consequently, in this work we have investigated the steam pyrolysis of PIs using Kapton film (poly(4,4′-oxidiphenylenepyromellitimide)) as a typical PI. Steam pyrolysis was carried out using a tube reactor equipped with a steam generator, and the complex pathway of pyrolysis and hydrolysis was derived by analysis of the decomposition products. The effects of steam on the activation of the carbonized solids produced were evaluated through elemental composition, morphology, and surface chemistry analyses. Received: Revised: Accepted: Published: 13558

July 5, 2015 October 6, 2015 October 21, 2015 October 21, 2015 DOI: 10.1021/acs.est.5b03253 Environ. Sci. Technol. 2015, 49, 13558−13565

Article

Environmental Science & Technology

Table 1. Thermal Decomposition Products of π at Different Temperatures in the Presence and Absence of 50 Vol % Steam temperature [°C]

500

700 steam

gases/wt % hydrogen carbon monoxide carbon dioxide hydrocarbons C1−C3 liquids/wt % non-N-containing compounds benzene toluene styrene phenylacetylene phenol indene 1-ethylideneindene naphthalene 2-methylnaphthalene fluorene anthracene phenanthrene fluoranthene pyrene N-containing compounds aniline benzonitrile 4-aminophenol 1,3-benzenedicarbonitrile phthalimide 4-phenoxybenzeneamine 4-phenylaminophenol 4,4′-oxidianiline N-(4-hydroxyphenyl)phthalimide not identif ied ethanol insoluble/wt % residue in sample holder/wt % total identified products/wt % not identified carbon/C% not identified nitrogen/N% a

1.5 − 1.5 + − 0.1 − − − − − − − − − − − − − − − − − − − − − − − − 0.1 0.1 93.5 95.2 3.6 3.9

22.5 − 3.3 19.1 + 4.7 0.1 − − − − 0.1 − − − − − − − − + 4.6 1.0 0.5 1.2 + 0.1 0.2 0.1 0.6 0.8 − + 64.1 91.3 15.4 22.6

900 steam

26.9 0.3 18.1 8.0 0.6 1.9 0.2 − − − − 0.2 − − − − − − − − − 1.5 0.5 1.0 − − − − − − − 0.2 0.4 57.8 86.6 9.8 48.6

34.7 1.3 20.7 10.2 2.4 1.7 0.6 + − − − 0.5 + − 0.1 − − + − − − 1.1 0.6 0.5 + − + − − − − − − 55.3 91.7 8.1 46.0

steam 35.5 0.9 25.2 8.3 1.1 0.1 + + − − − − − − + − − − − + − + + − − − − − − − 0.1 1.0 55.4 91.2 4.8 48.6

132.9 9.2 101.6 20.4 1.6 0.5 0.2 + + − − 0.1 + − − − − + − − − 0.3 0.2 0.1 − − − − − − − − − 15.3 148.7 35.3 96.3

: Not detected, + : < 0.05 wt %.

conditions, respectively. When constant helium flow was achieved, the reactor was heated to the desired temperature of 500, 700, or 900 °C. For steam pyrolysis, steam was fed into the reactor at 150 mL min−1 to achieve a steam concentration of 50 vol %. When a constant carrier gas flow was achieved, the sample holder was lowered into the heating zone where the sample was decomposed. The furnace temperature was maintained for 60 min. Liquid products were collected in two cooling traps cooled by ice and liquid nitrogen, respectively, while gaseous products were gathered into an aluminum gasbag. The inside of the reactor and traps were washed with ethanol. Then, ethanol-insoluble products were removed by filtration and weighed. The film solid remaining in the sample holder was weighed, and its elemental composition, morphology, and surface chemistry were analyzed. The products were defined as (1) gases collected in the gasbag with carbon numbers from 1 to 4; (2) liquids that were ethanol-soluble collected in the cooling traps, having a carbon number over 5; (3) ethanol insoluble that were ethanol-

2. EXPERIMENTAL SECTION 2.1. Materials. Kapton 200 V film was purchased from Du Pont-Toray Co., Ltd. (Tokyo, Japan), and cut into 8 mm × 8 mm pieces. The elemental composition of the material, derived using a Micro Corder JM10 (J-Science Lab Co., Ltd., Kyoto, Japan) was C: 67.8%, H: 3.0%, and N: 7.1%, with an O content of 22.0% assumed from the balance. Naphthalene and ethanol were obtained from Kanto Chemical Co., Inc. (Tokyo, Japan). Standard gases were supplied by GL Science, Inc.. 2.2. Steam Pyrolysis Experiments. The steam pyrolysis experiments were carried out using a tube reactor described in our previous work.24 The steam concentration was controlled by changing the furnace temperature of the steam generator. Details of the equipment and procedure have been published elsewhere.25 Briefly, 500 mg of PI film was fixed into a quartz sample holder, which was located at the top of the reactor, outside the heating zone. The air inside the reactor was replaced by a constant helium flow of 300 mL min−1 or 150 mL min−1 for 30 min under pyrolysis conditions or steam pyrolysis 13559

DOI: 10.1021/acs.est.5b03253 Environ. Sci. Technol. 2015, 49, 13558−13565

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Environmental Science & Technology insoluble deposited in the reactor and the cooling traps; and (4) film solid remaining in the sample holder. 2.3. Analytical Methods for Gaseous and Liquid Products. Gaseous and liquid products were identified and quantified by GC-MS (Agilent technology, GC: HP6890, column for gas: CP-PoraBOND Q, column for liquid: InertCap 5MS/Sil, MS: HP5973), GC-flame ionization detector (GCFID, for gases: GL Science, GC4000, column: CP-PoraBOND Q; for liquids: GL Science, GC390, column: InertCap 5MS/ Sil), and GC-thermal conductivity detector (GC-TCD, for gases, GL Science, GC323, column: molecular sieve 30−60 mesh size). Quantification of gaseous and liquid products by GC-TCD and GC-FID was carried out by the internal standard method using CO2 and naphthalene, respectively. Detailed analytical conditions and procedures have been reported previously.24 2.4. Analysis of Film Solids. The elemental composition of film solids remaining in the sample holder was determined using a Micro Corder JM10. A BELLSORP-mini II automatic surface analyzer (MicrotracBELL, Japan) was used to determine the BET specific surface area and pore size of the film solids. Micropore volume was determined by the Dubinin−Radushkevich (DR) method. Total pore volume was calculated from N2 adsorption amount at the relative pressure (P/P0) = 0.99. Mesopore volume was determined by subtracting the micropore volume from the total pore volume. The surface chemistry of the film solids was investigated by Xray photoelectron spectroscopy (XPS) (Axis Ultra, Shimadzu Corporation, Japan) using a MgKα line. The XPS spectra were analyzed and processed with CasaXPS 2.3.16 software. The XPS spectra were calibrated for a carbon C 1s excitation at a binding energy of 284.6 eV. In addition, charging compensation was applied assuming that the C 1s carbon peak appeared at a binding energy of 284.6 eV. The morphology of the char was observed using a scanning electron microscope (SEM) (S-4800, Hitachi, Ltd.). 2.5. Definition of Product Yields. All products were standardized based on the weight of sample input. The weight fraction of the decomposition products was defined as follows:

NID fraction [C%] = 100 [C%] − × 100%

product weight [g] × 100% weight of sample input [g]

= 100 [N%] − × 100%

nitrogen weight of the product [g] × 100% nitrogen weight of sample input [g]

(5)

3. RESULTS AND DISCUSSION 3.1. Influence of Steam on the Degradation Products and Pathway. The influence of steam on the degradation of the PI film was investigated at a temperature range between 500 and 900 °C. Although pyrolysis of PI has been previously reported,11−13,26,27 normal pyrolysis was carried out for comparison with steam pyrolysis. The product distribution is summarized in Table 1, and the gas volumes standardized by sample input are summarized in Figure 1. Only 1.5 wt % of CO

Figure 1. Composition and volume of gases standardized by material input at different temperatures in the presence and absence of 50 vol % steam. (1)

(13.1 mL g−1 sample) and a negligible amount of CO2 were identified at 500 °C in the absence of steam. Therefore, 93.5 wt % of the sample remained in the sample holder. This behavior is a result of partial imide ring cleavage, indicated by the light orange arrows in Scheme 1. This is consistent with previous results.12,26 Steam addition significantly enhances gas yield, resulting in 22.5 wt % (135.7 mL g−1 sample). CO2 production is significantly increased to 19.1 wt % (106.4 mL g−1 sample), and the presence of CO is also confirmed. In addition, Ncontaining liquid products, such as aniline, benzonitrile, phthalimide, 4-phenoxybenzeneamine, 4-aminophenol, 4-phenylaminophenol, 4,4′-oxidianiline, and N-(4-hydroxyphenyl) phthalimide, are produced at a total of 4.6 wt %. This indicates the complex pyrolysis pathway (light orange arrows) and hydrolysis (blue arrows) suggested in Scheme 1. Hydrolysis of imide rings and the subsequent decarboxylation by pyrolysis results in CO2 and phthalimide. Also, this hydrolysis produces amino groups, leading to the production of 4-phenoxybenzene-

(2)

nitrogen fraction [N%] =

nitrogen weight of AQP [g] nitrogen weight of the sample input [g]

NID consisted of coke deposited on the reactor wall, highboiling compounds that could not be detected by GC analysis, and nitrogen compounds such as N2, NH3, NOx, and HCN.

The carbon fraction (C%) and nitrogen fraction (N%) of decomposition products were defined as follows: carbon fraction [C%] carbon weight of the product [g] = × 100% carbon weight of sample input [g]

(4)

NID fraction [N%]

weight fraction [wt%] =

carbon weight of AQP [g] carbon weight of the sample input [g]

(3)

Because the total weight fraction of all products can be more than 100 wt % as a result of the reaction with steam, the fraction of nonidentified products (NID) was calculated by subtracting the carbon and nitrogen content of all quantified products (AQP) from those of the initial sample input: 13560

DOI: 10.1021/acs.est.5b03253 Environ. Sci. Technol. 2015, 49, 13558−13565

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Environmental Science & Technology Scheme 1. Pyrolysis and Hydrolysis Reaction Pathway of π into Liquid Compounds during Steam Decomposition

The influence of steam at 700 °C is very small, resulting in a slight enhancement of gas production to 34.7 wt % (400.2 mL g−1 sample). The amounts of liquid and char produced are comparable with those obtained under normal pyrolytic conditions. These results suggest that pyrolysis is much faster than hydrolysis at 700 °C. Therefore, the influence of hydrolysis may be hindered by the strong influence of pyrolysis. Pyrolytic imide ring cleavage is further enhanced by increasing the temperature to 900 °C, resulting in 25.2 wt % (219.8 mL g−1 sample) of CO, although the CO2 yield is comparable to that observed at 700 °C. This might be due to the Boudouard reaction (eq 6), which is thermodynamically preferred at 900 °C.28

amine and 4,4′-oxybisbenzeneamine. The cleavage of ether bonds in 4-phenoxybenzeneamine and 4,4′-oxybisbenzeneamine by pyrolysis produces aniline, phenol, and 4-aminophenol. The formation of 4-phenylaminophenol may proceed via the condensation of 4-aminophenol and phenol. N-(4hydroxyphenyl)phthalimide is the result of ether bond cleavage without imide ring cleavage. Enhancement of hydrolysis by steam drastically reduces the amount of char to 64.1 wt %, although this is still high. This result suggests that carbonization is much faster than the hydrolysis reaction. Significant increase in the NID nitrogen fraction suggests that nitrogen is released as gaseous nitrogen compounds, such as N2, NH3, HCN, and NOx.12 By increasing the temperature to 700 °C in the absence of steam, we dramatically increased CO and CO2 yields to 18.1 wt % (157.7 mL g−1 sample) and 8.0 wt % (44.4 mL g−1 sample), respectively. In addition, H2 (0.3 wt %, 17.6 mL g−1 sample) and hydrocarbons (0.6 wt %, 3.4 mL g−1 sample) are also produced. A total of 1.9 wt % liquid is obtained, which mainly consists of 0.2 wt % of phenol, 0.5 wt % of aniline, and 1.0 wt % of benzonitrile. These products are produced via an additional pyrolysis pathway, presented as vivid orange arrows in Scheme 1. The C−N bond between the imide ring and the benzene ring is radically cleaved.12 Phenol is produced by the fission of the ether bond between two benzene rings.26 Furthermore, an imide ring that contains a radical can rearrange and decarboxylate, producing CO2 and benzonitrile.27 Aniline may be produced by the reaction between the isocyanate unit and H2O contained in the sample.12 Temperature increase did not enhance PI decomposition because the rigid aromatic PI structure is rapidly converted into char, resulting in 57.8 wt %. NID nitrogen increases to 48.6 N% due to the emission of gaseous nitrogen compounds, as discussed above.

CO2 + C ⇄ 2CO

(6)

Total liquid production is drastically reduced to 0.1 wt %, and contains mainly polycyclic aromatic compounds, such as naphthalene and fluoranthene, due to the benzene ring aggregation29 indicated by the red arrows in Scheme 1. The char amount (55.4 wt %) is comparable to that observed at 700 °C. By increasing the temperature to 900 °C in the presence of steam, we significantly enhanced char decomposition to 15.3 wt %. The yields of H2 and CO are significantly increased to 9.2 wt % (1023.8 mL per gram of sample) and 101.6 wt % (812.9 per gram of sample), respectively. The volume ratio of H2/CO is 1.3. The H2- and CO-rich gas can be directly obtained by the water−gas reaction (eq 7) between steam and char: C + H 2O ⇄ H 2 + CO

(7)

This reaction is thermodynamically favored at 900 °C (Figure S1), which was calculated by using FactSage 6.2 thermodynamic calculation software. Highly selective synthetic 13561

DOI: 10.1021/acs.est.5b03253 Environ. Sci. Technol. 2015, 49, 13558−13565

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Environmental Science & Technology

Table 2. Elemental Composition, BET Surface Area, and Pore Volume of Film Solids after Processing under Different Conditions samples untreated 500 °C 700 °C 900 °C a

BET surface area [m2 g−1]

elemental composition [wt %]

pyrolysis steam pyrolysis steam pyrolysis steam

a

C

H

N

O

67.8 69.0 73.8 84.9 85.9 90.1 92.1

3.0 3.0 3.5 2.0 2.0 0.9 0.9

7.1 7.3 7.8 7.3 6.7 6.6 2.4

22.0 20.7 15.0 5.8 5.5 2.4 4.6

n.d.b n.d. n.d. n.d. 570 n.d. 1874

pore volume [cm3 g−1] micropore

mesopore

n.d. n.d. n.d. n.d. 0.235 n.d. 0.824

n.d. n.d. n.d. n.d. 0.008 n.d. 0.038

Balance. bBelow measurable limit.

Figure 2. XPS spectra of film solids after processing under different conditions: (a) C 1s after pyrolysis, (b) O 1s after pyrolysis, (c) N 1s after pyrolysis, (d) C 1s after steam pyrolysis, (e) O 1s after steam pyrolysis, and (f) N 1s after steam pyrolysis.

carbonized aromatic PI is suitable for producing synthetic gas directly. 3.2. Influence of Steam on the Morphology and Surface Chemistry of Film Solids. The elemental composition, BET surface area, and pore volume of film solids remaining in the sample holder are summarized in Table 2. The elemental compositions of the untreated sample and the film solids obtained at 500 °C under normal pyrolytic conditions are comparable, and the sample retains its flexibility. The carbon content increases with increasing temperature due to the

gas recovery (94 vol %) can be achieved without steam reforming catalysts because the aromatic PI structure rapidly converted into char even though it is in a steam atmosphere. However, it has reported that olefin plastics such as polyethylene and polypropylene do not produce char in pyrolytic conditions.8 Although synthetic gas can be recovered from these polymers in the presence of steam without reforming catalysts, main products are C1−C4 hydrocarbons.30,31 Thus, the present work demonstrates the easily 13562

DOI: 10.1021/acs.est.5b03253 Environ. Sci. Technol. 2015, 49, 13558−13565

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Figure 3. Relative intensity of specific bonds obtained under several conditions: (a) C 1s, (b) O 1s, and (c) N 1s.

promotion of carbonization, reaching 90.1 wt % at 900 °C. The nitrogen content is relatively constant over the investigated temperature range. The oxygen content dramatically decreases with increasing temperature due to CO and CO2 production. The porosity of the film solids could not be determined because it was below the measuring limit of the apparatus used. Steam enhances the hydrolysis of the imide rings and promotes further decarboxylation, resulting in a decrease in the oxygen content of the film solids at 500 °C. The nitrogen content of the solids from steam pyrolysis at 700 °C are slightly decreased compared with that obtained at pyrolysis at 700 °C while carbon content slightly increased. This suggests that the nitrogen in the film solids is preferentially removed by steam. The BET surface area is significantly increased to 570 m2 g−1. Micropore is dominant in the film solids, resulting in 0.235 cm3 g−1 that accounts for 97% of total pore volume. The carbon and oxygen contents at 900 °C in the presence of steam are slightly higher than those obtained under normal pyrolytic conditions at 900 °C, while the nitrogen content decreases as well as steam pyrolysis at 700 °C. This indicates that oxygen derived from steam is incorporated into the structure of the film solids at 900 °C. The porosity of film solids is dramatically improved at 900 °C, resulting in a BET surface area maximum of 1874 m2 g−1. Furthermore, 0.824 cm3 g−1 of micropore is observed, accounting for 96% of the total pores. The pore-size distribution of film solids obtained after steam pyrolysis at 700 and 900 °C are summarized in Figure S2. Nahil et al.32 have reported that micropore-rich activated carbon is produced from acrylic textile by steam activation for 1 h, which is consistent with the present results. In addition, Sato et al.16 have reported the formation of 1200 m2 g−1 surface-area carbon from PIs by processing at 700 °C for 10 h under a CO2 atmosphere, and Laušević et al.33 achieved more than 2000 m2 g−1 through CO2 activation. The pores reported in both of these works were predominantly smaller than 1 nm. Therefore, the present work indicates that steam activation of PIs makes it possible to produce high-surface-area activated carbon with micropores employing comparatively short treatment times. The pictures and SEM images of film solids obtained after experiments are summarized in Figures S3 and S4, respectively. The surface morphology is significantly changed, and pores are increasingly formed with increasing temperature in the presence of steam. The XPS spectra of film solids after processing at different temperatures in the presence and absence of steam are presented in Figure 2. The XPS spectra of the C 1s excitation (Figure 2a) of the untreated sample show several carbon species. The peak at 284.5 eV is assigned to aromatic C−C and

C−H bonds, the peak at 285.1 eV is attributed to the C−N bond, the peak at 286.1 eV is assigned to the C−O bond of the ether group, and the peak at 288.3 eV is attributed to the CO bond in the imide ring. In addition, there is a shakeup satellite due to π−π* transitions in the aromatic rings. These peak assignments are in agreement with previous works.32,34−36 A new small peak at 290.4 eV is observed at 500 °C, which is attributed to the NCO bond37 in isocyanate groups produced by pyrolytic cleavage of the imide ring (Scheme 1). In addition, we cannot ignore the possible contribution of C−OH bonds to the peak at 286.8 eV observed at 700 °C, which is assigned to C−O bonds. The relative intensity of C−N peak decreases with increasing temperature (Figure 3a). In addition, the relative CO peak intensity decreases while relative C−O intensity increases at 500 °C. These behaviors indicate that ether bond is more stable than the imide ring, which is in agreement with Scheme 1. The O 1s excitation XPS spectra of the film solid obtained under pyrolytic conditions are presented in Figure 2b. The main CO bond peak in the imide ring at 531.8 eV, the C−O bond in ether group at 533.3 eV, and the weak chemisorbed oxygen signal at 535.6 eV are present in the untreated sample. The increase and decrease behavior of relative intensity of C O and C−O peaks shown in Figure 3b is consistent with these peaks shown in Figure 3a. The N 1s excitation XPS spectra derived from normal pyrolytic conditions are presented in Figure 2c. The main Kapton-type C−N bond peak at 400.3 eV and the very weak pyridine-type C−N bond peak at 398.6 eV are identified in the untreated sample. Konno et al.35 reported that the Kapton-type structure gradually changed to an intermediate pyridine-type structure between 500 and 600 °C and was further transformed into a pyrrole-type structure above 600 °C. Therefore, the relative intensity of Kapton-type peaks decreases with an increase in pyridine-type peaks at 500 °C (Figure 3c). The peak at 400.3 eV gradually shifts with increasing temperature, reaching 400.8 eV at 900 °C. At the same time, the relative intensity of the pyridine-type peak decreases, suggesting that the pyridine-type structure is converted to pyrrole, pyridone, and quaternary N type structures by ring expansion at 900 °C, behavior that is in agreement with the previous works.38,39 Additionally, weak peaks for pyridine-N-oxide/ammonia at 402.7 eV and chemisorbed N-oxide at 404.1 eV are observed above 700 °C. The XPS spectra obtained after steam pyrolysis at different temperatures are presented in Figure 2d−f. The relative peak intensities and positions of C−C/C−H, C−N, and π−π* transitions in aromatic rings obtained after pyrolysis at all temperatures are comparable. The peak at 290.0−290.6 eV is 13563

DOI: 10.1021/acs.est.5b03253 Environ. Sci. Technol. 2015, 49, 13558−13565

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Environmental Science & Technology assigned to not only the NCO bond but also to the ester and carboxyl O−CO bond in the presence of steam due to hydrolysis of the imide ring, as previously discussed (Scheme 1). The relative intensity of the CO peak decreases with increasing temperature after steam pyrolysis (Figure 3a,b). The relative intensity of CO at 500 °C in the presence of steam is smaller than that obtained at pyrolytic conditions. It is due to the enhancement of imide ring hydrolysis and further decarboxylation, as discussed previously (Scheme 1). Conversely, the C−O/C−OH signal increases with increasing temperature, which is not observed at pyrolytic conditions. It suggests that steam H2O oxidized carbon on the char surface,40 increasing OH groups. The peak positions (Figure 2f) and relative intensities (Figure 3c) in the N 1s spectra due to Kapton-type and pyridine-type species are comparable with those obtained after pyrolysis until 700 °C. However, the relative intensity of pyridine-type is drastically decreased comparing with that obtained at pyrolytic condition at 900 °C. The first reason for this behavior is conversion from pyridine-type to pyrrole, pyridone, and quaternary N types by ring expansion as same as pyrolytic conditions. The second reason is the oxidation of pyridine-type structure by steam. In fact, the relative intensity of pyrrole−pyridone−quaternary N and pyridine-N-oxide is higher than those observed at pyrolytic conditions. The increase of pyrrole−pyridone−quaternary N types is mainly due to the increase of pyridone-type structure by accelerating oxidation of carbon by steam H2O.40 The significant increase of pyridine-N-oxide is due to the oxidation of pyridine-type structure, which behavior is consistent with previous works carried out steam activation of N-containing char.32,38 These results imply that steam H2O attack carbon near nitrogen atoms, enhancing nitrogen removal from char. This is linked to the fact that nitrogen content in char is decreased by steam activation. In summary, the viability of PI decomposition by steam pyrolysis was investigated in this work. Analysis of gaseous and liquid products obtained under different conditions allowed us to suggest a decomposition pathway for the steam pyrolysis of PIs, which progressed via a complex combination of pyrolysis and hydrolysis. Steam significantly enhanced the water−gas reaction at 900 °C, resulting in a 1963.1 mL g−1 sample of a H2and CO-rich gas. The H2‑ and CO-rich gas is useful as a H2 source and feedstock for industrial processes such as ammonia synthesis and the Fischer−Tropsch process. Simultaneously, steam activated film solids, resulting in micropore-rich activated carbon with a BET surface area of 1874 m2 g−1. Steam activation increased pyridone-type and pyridine-N-oxide structures by steam H2O oxidation. Therefore, the obtained activated carbon may be applicable to the adsorption of polar organics in polluted water and gas. Thus, this work indicated that steam pyrolysis shows great promise as a method to convert PI waste into valuable H2−CO synthetic gas and activated carbon, simultaneously.

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SEM images of film solids obtained after experiments. (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel: +81-22-795-7211; fax: +81-22-795-7212; e-mail: [email protected], Funding

This work was supported by JSPS KAKENHI grant numbers 25241022 and 15H06010. Notes

The authors declare no competing financial interest.

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REFERENCES

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b03253. The results of thermodynamic calculation of the water− gas reaction, pore size distribution of film solids obtained after steam pyrolysis at 700 and 900 °C, and pictures and 13564

DOI: 10.1021/acs.est.5b03253 Environ. Sci. Technol. 2015, 49, 13558−13565

Article

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DOI: 10.1021/acs.est.5b03253 Environ. Sci. Technol. 2015, 49, 13558−13565