Conversion of Polyethylene Terephthalate Based Waste Carpet to

Research Article pubs.acs.org/journal/ascecg

Conversion of Polyethylene Terephthalate Based Waste Carpet to Benzene-Rich Oils through Thermal, Catalytic, and Catalytic Steam Pyrolysis Shoucheng Du,† Julia A. Valla,‡ Richard S. Parnas,†,‡ and George M. Bollas*,‡ †

Department of Materials Science and Engineering and ‡Department of Chemical & Biomolecular Engineering, University of Connecticut, 97 North Eagleville Road, Unit 3136, Storrs, Connecticut 06269, United States S Supporting Information *

ABSTRACT: Management of carpet wastes has become a substantial environmental issue in the United States. Specifically, reutilization of polyethylene terephthalate (PET) from waste carpet is increasingly problematic because of the steadily growing market share of PET-based carpets and the very low value of their wastes. In this work, we investigate pyrolysis as an option for repurposing PET carpet wastes. In particular, slow and fast, thermal and catalytic pyrolyses, with and without the co-feeding of steam, are investigated in terms of their selectivity to monoaromatic products. It is seen that higher temperatures increase the conversion of PET to aromatic hydrocarbons. Pyrolysis at slow heating rates is very selective to benzene production. Thermal pyrolysis of waste carpet produces significant amounts of benzoic acid and acetylbenzoic acid as liquid products, whereas catalytic pyrolysis enhances the decarboxylation of these acids, producing aromatic hydrocarbons. ZSM-5 and CaO are effective catalysts for enhancing deoxygenation reactions during catalytic pyrolysis of waste carpet but with significantly different selectivities. Catalytic steam pyrolysis is seen to accomplish the highest selectivity to benzene among all the pyrolysis options studied, due to the enhancement of hydrolysis reactions. The essentially pure benzene organic liquid product from steam pyrolysis of carpet-originated PET presents a unique opportunity for the reutilization of this unsustainable waste. KEYWORDS: Pyrolysis, Catalytic steam pyrolysis, Waste carpet, PET, ZSM-5

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INTRODUCTION

Pyrolysis is one of the attractive methods to convert biomass, 7,8 biomass wastes, 9,10 and synthetic polymer wastes4,11−13 to useful chemicals. Depending on the heating rate, pyrolysis can be categorized as slow (0.1−1 K s−1) and fast pyrolysis (10−1000 K s−1 or higher). Generally, biomass slow pyrolysis generates large amounts of char, whereas fast pyrolysis enhances liquid selectivity.9,14 Pyrolysis of waste synthetic polymers is a win−win option for plastics recycling and waste volume reduction.2 Therefore, many lab-scale studies have analyzed the feasibility of pyrolysis of PET,13,15,16 polyethylene,17,18 polypropylene,19,20 and other plastics.21−23 Moreover, Pilot-scale facilities for plastics pyrolysis are developed worldwide.24 Contrary to other mechanical and chemical reprocessing options, pyrolysis can treat almost all types of synthetic polymers or mixtures of polymers with biomass.25,26 Although pyrolysis of PET-based waste carpet has not been studied in the past, previous research on PET thermal pyrolysis exists.15,27−30 Scheme 1a,b summarizes the most dominant reactions in the pyrolysis of PET, proposed by Grause et al.15 They suggested that PET first decomposes to terephthalic acid

About 1.8 billion kg of carpet is discarded annually in the United States as waste. Waste carpet accounts for over 3.5% of the waste disposed of in U.S. landfills, contributing to the loss of landfill space in an unsustainable manner. According to the 2013 annual report from the Carpet America Recovery Effort (CARE), only ∼5% of carpet waste was recycled and reused in 2013.1 Carpet generally consists of two parts: face fiber (45 wt % of the carpet) and backing (55 wt %). Face fiber can be generally categorized as nylon-based (typically nylon-6 and -66) or polyethylene terephthalate (PET)-based. The backing material is mainly made of resin of PVC with inorganic filler.2,3 Recycling and reusing nylon carpet has been extensively studied2,4−6 because of the relatively high value of nylon. However, the economics of PET recycling are not favorable,1 leading the nonrecyclable and nonbiodegradable PET waste to landfills. As a result, the cost of disposing PET-based carpets has tripled in the last couple of years.1 Reutilization of PET carpet has become increasingly important, as the fraction of PET-based carpet in the postconsumer carpet waste has been steadily increasing since 2008.1 PET-based carpet accounted for 34% of the total postconsumer carpets in 2013 and is forecasted to increase to 50% in 2016. © XXXX American Chemical Society

Received: March 3, 2016 Revised: April 8, 2016

A

DOI: 10.1021/acssuschemeng.6b00450 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Scheme 1. Reactions Leading to Major Products of PET Thermal Pyrolysis (a,b) and Catalytic Steam Pyrolysis (c,d) over CaO (Adapted from Grause et al.15)

different properties on the product distribution. We consider slow and fast, thermal (noncatalytic) and catalytic pyrolyses, with and without the co-feeding of steam. Specifically, the effect of temperature on product selectivity was first studied with thermal pyrolysis to scope out the temperature that produces the largest yield to acids. Catalytic pyrolysis over ZSM-5 and CaO catalysts was then performed with the purpose of upgrading the acids and other oxygenates observed from thermal pyrolysis. Different heating rates were explored as an option to improve the liquid product quality. Lastly, steam catalytic pyrolysis over CaO was compared with nonsteam catalytic pyrolysis and thermal pyrolysis. The overall objective of this structured experimental work was the maximization of liquid products of high purity to one or only few components. It is shown that steam pyrolysis over CaO is the most effective approach to meet this goal.

and benzoic acid vinyl ester. Then, benzoic acid vinyl ester undergoes rearrangement and decarbonylation reactions, forming acetophenone. Artetxe et al.27 identified benzoic acid and acetylbenzoic acid as the most abundant compounds in the liquid product after pyrolysis of PET at 600 °C. They also observed increased yields to acetophenone at higher temperature. The addition of catalyst in the pyrolysis of synthetic polymers is known to enhance deoxygenation reactions. ZSM-5 has been widely used as an effective catalyst in catalytic pyrolysis of synthetic polymers, due to its unique shapeselective pores and relatively strong acidic characteristics.31−33 However, relatively high amounts of polyaromatic hydrocarbons (PAHs) and coke are produced from pyrolysis over ZSM-5, resulting in oils of low quantity and quality. Thus, other types of catalysts, such as calcium oxide (CaO), were studied in an effort to improve the oil quality.29,30 As shown in Scheme 1d, in pyrolysis over CaO, aromatic acids can be selectively converted to aromatic hydrocarbons. The effect of steam on the conversion and selectivity of PET via steam pyrolysis (hightemperature hydrolysis) has been studied by Yoshioka and coworkers.28−30 Co-feeding steam in the catalytic pyrolysis of PET enhances hydrolysis reactions, which is beneficial to the quality of oil produced. As shown in Scheme 1c,d, the decomposition of PET via catalytic steam pyrolysis proceeds via two steps. In the first step, the hydrolysis of PET leads to formation of terephthalic acid at high yields. In the second step, decarboxylation of terephthalic acid occurs over a CaO catalyst. Driven by prior work on PET pyrolysis, the research presented here focuses on the most promising options for the pyrolysis of waste carpet to chemicals. We present the results of pyrolysis of the components of carpet waste (face fiber and backing material) separately to distinguish the impact of their

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EXPERIMENTAL SECTION

Feedstock and Catalysts. Carpet waste was provided by CARE and was from a standard residential-grade carpet. The carpet face fiber and backing material were received separated. The proximate and ultimate analyses of the carpet face fiber and backing material are shown in Table 1. Overall, the two materials were quite similar in elemental composition, with high carbon content and relatively low oxygen content, close to the theoretical values for pure PET (C, 62.5 wt %; H, 4.2 wt %; O, 33.3 wt %). Bulk ZSM-5 zeolite (CBV 8014) from Zeolyst International, Inc. and calcium oxide from Fisher Scientific were used as catalysts in the experiments. The catalysts were calcined in air at 550 °C for 5 h prior to experiments. The physicochemical characterization of the ZSM-5 and CaO catalysts is presented in Table 2. Experimental Setup and Procedure. Fast pyrolysis experiments were performed in a 5200HP PyGC system from CDS Analytical Inc. For the catalytic experiments, waste carpet was packed between two catalyst beds, with 10 mg of catalyst on each side. In this configuration, B


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5973N mass selective detector and an Agilent DB-5 column. The yield to solids was determined by oxidation ex situ in a tube furnace. Characterization of the spent CaO (a mixture of CaO and CaCO3) for quantifying the carbon content in CaCO3 was performed in a Q-500 thermogravimetric analyzer from TA Instruments and a TG 209 F1 Libra thermogravimetric analyzer coupled to a QMS 403C quadrupole mass spectrometer from NETZSCH. Identification of the solid pyrolysis product was accomplished using an Agilent 6210 time-offlight mass spectrometer (TOF MS) and an Elementar Americas Inc. Vario MICRO elemental analyzer. The repeatability of the experiments was confirmed by performing each catalytic and catalytic steam pyrolysis experiment at least three times. The experimental standard deviation is reported in the experimental results in the form of error bars.

Table 1. Proximate and Ultimate Analyses of the Carpet Face Fiber and Backing Material

ultimate analysis

proximate analysis

a

C H N S Oa moisture volatile fixed carbon ash

face fiber (wt %)

backing material (wt %)

61.91 4.25 0.07 0.12 24.36 0.00 87.79 2.91 9.29

61.45 4.61 0.12 0.13 19.09 1.39 81.89 2.12 14.60

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Calculated by difference.

RESULTS AND DISCUSSION Thermal Fast Pyrolysis of Carpet Waste. Table 3 shows the product distribution from pyrolysis of carpet face fiber at different temperatures. Higher temperatures promote the production of gas and liquid over solid. Specifically, the production of CO and CO2 was enhanced by increasing the temperature. Higher carbon yields to monocyclic and polycyclic aromatic hydrocarbons (MAH and PAH) were also obtained at higher temperature. The yield to acids reached a maximum value at the temperature of 600 °C and then dropped. This means that at relatively low temperature carpet face fiber mainly underwent depolymerization and decomposition reactions, leading to the formation of acids, whereas at higher temperature, decarboxylation of the acids dominated. Good carbon balance was obtained at the temperature of 400 °C. At higher temperature, only about 75% of the total carbon in the feedstock could be identified due to the presence of heavier compounds in the liquid product, which the gas chromatography/mass spectroscopy (GC-MS) could not quantify. The distribution of the products from pyrolysis of the carpet backing material at different temperatures is also shown in Table 3. Pyrolysis of carpet backing material produced less liquid product and more solid product compared to pyrolysis of face fiber. The total yields of MAHs from pyrolysis of the carpet backing material were generally slightly higher and the total yield to acids was relatively lower than that from pyrolysis of face fiber. The temperature effect on product distribution was similar in pyrolysis of face fiber and pyrolysis of backing material. Figure 1 presents a comparison of the major liquid products from pyrolysis of the carpet face fiber and backing material.

a catalyst to carpet weight ratio of 20 was used for all the catalytic pyrolysis experiments. The heating rate in the microreactor was set to 1000 °C/s. The experimental setup has been described previously,34 and the detailed procedure is provided in the Supporting Information. In brief, pyrolysis gas products were measured online with an Agilent 5975C mass spectrometer, calibrated for H2, CO, CO2, CH4, and Ar. The condensable vapor products were captured using an adsorbent downstream of the pyrolysis reactor and then quantified by thermal desorption to an Agilent 6890 gas chromatographer, equipped with an Agilent DB-5 column and a 5973 N mass selective detector. Quantification of benzene, toluene, ethylbenzene, xylene, styrene, phenol, benzofuran, indane, and naphthalene was feasible via direct calibration, while semiquantification was used for the remaining compounds. The yield to solid products was measured by weighing the mass of the reaction capsule after each experiment. All experiments were performed in triplicate to ensure precision. Pyrolysis of carpet face fiber and backing material at slow heating rates, including thermal, catalytic, and catalytic steam pyrolysis, was performed in a specially designed fixed-bed reactor, also described in the Supporting Information. In this setup, the reactor was heated to 750 °C at a heating rate of 5 °C/min. In the catalytic experiments, the catalyst to feed weight ratio was maintained at 10:1. For the steam pyrolysis experiments, steam was provided by a steam generator, connected to a high-performance liquid chromatography pump, entrained via Ar flow. The water flow rate was set to 0.11 sccm for all the steam pyrolysis experiments, which provided about 60 vol % steam in the gas stream. Gas products were analyzed online with the mass spectrometer, calibrated for H2, CH4, CO, CO2, and Ar. The condensable products were collected in two impingers, each filled with 40 mL of methanol placed in a dry ice bath. In experiments with steam, one additional empty impinger in the water−ice bath was used to collect water downstream of the reactor. The liquid products were analyzed in an Agilent 6890N gas chromatographer equipped with a

Table 2. Physicochemical Characterizations of the ZSM-5 and CaO Catalystsa porosityb

physical property ρbulk ZSM-5 CaO

ρ

500 3300

acid site densityc

basic site densityd

dp

Stotal

Sext

Bs

Ls

Bs/Ls

0.5−2

419 2

126 3

115.7

100.2

1.15

UCO2 0.11

ρbulk is bulk density (kg/m3); ρ, density (kg/m3); dp, particle size (μm); Stotal, BET surface area (m2/g); Sext, t-plot external surface area (m2/g); Bs, Brønsted acid site density (μmol/g); Ls, Lewis acid site density (μmol/g); UCO2, CO2 uptake (mmol/g). bThe porosity analysis was performed in a Micromeritics ASAP 2020 accelerated surface area and porosimetry system. Samples were degassed at 250 °C under vacuum for 12 h before analysis. c Analysis of acid site density was performed by diffuse reflectance infrared Fourier transform spectroscopy. The catalyst was calcined in nitrogen at 500 °C for 1 h. The catalyst was cooled to 130 °C and then dosed with enough pyridine (99 wt %) to achieve saturation. Physisorbed pyridine was removed by slowly heating the samples to 230 °C and remaining isothermal for 1 h. Final spectra were taken after cooling to 130 °C, using the unexposed sample as the background. Extinction coefficients of 1.67 and 2.22 cm/μmol were used for concentration calculation of Brønsted and Lewis acid sites, respectively. dCO2 uptake was analyzed using temperature-programmed desorption (TPD). Prior to analysis, the catalyst was degassed in nitrogen at 1000 °C for 30 min. The catalyst was cooled to 100 °C and then exposed to CO2 until saturation. Argon was purged to remove the excess CO2 in the chamber. TPD was performed by heating the catalyst from 100 to 1000 °C at 15 °C/min. a

C


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Table 3. Product Distribution from Pyrolysis of Carpet Face Fiber and Backing Material at Different Temperatures carbon yield (C%)

400 °C

total solid total gas CO CO2 CH4 C3H6 total liquid MAHs PAHs acids Other oxygenates carbon balance

77.93 ± 2.48 9.21 ± 0.64 2.85 ± 0.11 4.39 ± 0.32 1.17 ± 0.09 0.81 ± 0.13 14.65 ± 1.49 0.21 ± 0.02 0.09 ± 0.01 13.70 ± 1.41 0.66 ± 0.05 101.79

total solid total gas CO CO2 CH4 C3H6 total liquid MAHs PAHs acids other oxygenates carbon balance

77.53 ± 3.34 8.39 ± 1.36 2.28 ± 0.16 4.20 ± 0.27 0.91 ± 0.06 1.01 ± 0.02 9.61 ± 0.52 0.56 ± 0.12 0.06 ± 0.01 13.05 ± 1.16 0.57 ± 0.07 95.52

500 °C carpet face fiber 42.14 ± 0.76 12.32 ± 0.87 3.97 ± 0.12 5.95 ± 0.62 1.43 ± 0.07 0.98 ± 0.07 20.58 ± 1.57 0.78 ± 0.07 0.76 ± 0.07 18.08 ± 1.29 0.96 ± 0.14 75.05 carpet backing material 52.76 ± 3.49 9.89 ± 2.07 2.38 ± 0.25 5.20 ± 0.24 1.04 ± 0.04 1.26 ± 0.03 11.63 ± 0.56 1.22 ± 0.14 0.58 ± 0.05 14.85 ± 1.80 0.75 ± 0.08 74.28

600 °C

900 °C

39.00 ± 1.11 13.47 ± 1.01 4.59 ± 0.51 6.76 ± 0.27 1.29 ± 0.13 0.83 ± 0.10 23.40 ± 0.82 1.38 ± 0.06 1.04 ± 0.04 19.68 ± 0.67 1.30 ± 0.05 75.87

28.70 ± 2.37 21.66 ± 2.14 9.86 ± 0.94 9.44 ± 0.95 1.97 ± 0.09 0.40 ± 0.16 24.58 ± 1.53 3.93 ± 0.21 2.79 ± 0.31 15.13 ± 0.65 2.74 ± 0.36 74.94

46.79 ± 3.29 11.5 ± 2.31 3.58 ± 0.30 5.46 ± 0.22 1.04 ± 0.07 1.42 ± 0.11 8.54 ± 0.69 1.83 ± 0.15 1.08 ± 0.09 15.12 ± 1.98 0.78 ± 0.10 66.83

34.91 ± 1.82 17.94 ± 1.72 6.78 ± 0.35 7.84 ± 0.41 1.72 ± 0.08 1.59 ± 0.10 10.67 ± 0.94 3.88 ± 0.28 2.41 ± 0.33 12.78 ± 0.89 1.69 ± 0.21 63.52

of face fiber produced significantly higher amounts of acetylbenzoic acid at the temperature of 500 and 600 °C than pyrolysis of backing material. This might be related to the higher volatile content of the face fiber (Table 1). Catalytic Fast Pyrolysis of Carpet Waste. ZSM-5 has been widely used as an effective catalyst for deoxygenation reactions in catalytic pyrolysis of biomass, especially decarbonylation and decarboxylation.7,9,34−36 Utilization of ZSM-5 in catalytic pyrolysis of synthetic polymers has also been investigated previously.18,32,33,37 Moreover, CaO has been used as an effective catalyst for catalytic decarboxylation reactions during pyrolysis of PET.15,28−30 Therefore, here, we focused on these two catalytic systems in terms of their decarboxylation capability and differences in distribution of products from catalytic pyrolysis of carpet waste. As shown in Figure 2, thermal pyrolysis of PET carpet face fiber produced oxygenated oils, mainly acids (benzoic acid and acetylbenzoic acid). Both ZSM-5 and CaO are excellent catalysts in enhancing decarboxylation reactions. However, the reaction pathways with each catalyst are different. Pyrolysis of carpet face fiber over ZSM-5 produced relatively high yields to benzene, benzene derivatives, and indenes/naphthalenes, whereas pyrolysis over CaO enhanced the production of benzene, with concomitant lower yields to benzene derivatives and indenes/naphthalenes. The hydrocarbon pool mechanism appeared to dominate the reactions over ZSM-5, wherein primary pyrolysis products undergo alkylation and hydrogen transfer reactions on the ZSM-5 acid sites, leading to monoand polyaromatic hydrocarbons.36,38 The catalytic pyrolysis over CaO mainly follows the reaction mechanism of Scheme 1c,d. However, undesirable ketones (mainly acetophenone) were also promoted by CaO. This was attributed to the heat generated in the CaO bed due to exothermic reactions and,

Figure 1. Major liquid products from thermal fast pyrolysis of carpet (a) face fiber and (b) backing material in a microreactor.

Benzoic acid and acetylbenzoic acid were the two most abundant compounds in the liquid from pyrolysis of both face fiber and backing material. This is consistent with Artetxe et al.,27 who performed fast pyrolysis of PET in a conical spoutedbed reactor. Generally, the conversion to benzene, biphenyl, acetophenone, benzoic acid, and vinylbenzoic acid from pyrolysis of backing material was close to that from pyrolysis of face fiber. This is attributed to the similarity of the elemental composition of the two materials (Table 1). However, pyrolysis D


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Figure 2. Liquid product yields from thermal and catalytic fast pyrolysis of carpet face fiber in a microreactor at 600 °C.

possibly, the enhanced catalytic production of acetophenone by CaO. The latter does not agree with the literature, where acetophenone formation has been accepted as a thermal reaction.15 It seems that ZSM-5 enhances all the deoxygenation reactions, including decarboxylation and decarbonylation reactions, whereas CaO is efficient only for the removal of the carboxyl group, due mainly to its basic sites.39 This does not mean that ZSM-5 is a better catalyst than CaO for carpet pyrolysis, in the sense that the carpet carbon is not selectively converted to just benzene and its derivatives but instead to polyaromatic hydrocarbons such as indenes/naphthalenes. Overall, catalytic fast pyrolysis demonstrated a much better product selectivity to aromatic hydrocarbons, compared with that of thermal pyrolysis. Both the ZSM-5 and CaO catalysts were able to effectively remove the carboxyl group from the acids, but the oil quality after catalytic upgrading was lowered due to the presence of PAHs and/or ketones. Thermal Slow Pyrolysis of Carpet Waste. The production of benzene-rich oil was investigated through slow pyrolysis of carpet in the fixed-bed reactor. The idea of utilizing slow heating rate in pyrolysis originates in the work of Grause et al.,15 who performed pyrolysis of PET at different heating rates (2−10 °C/min). They found that slower heating rates resulted in better selectivity to benzene. Therefore, we explored the applicability of the findings of Grause et al. on waste carpet pyrolysis in a much wider temperature and heating rate range. The impact of the heating rate is assessed here by comparing pyrolysis in the microreactor with that in a bench-scale fixedbed reactor with a heating rate of 5 °C/min. A comparison of the liquid product distribution from slow and fast pyrolysis of carpet face fiber is shown in Figure 3. The yield to hydrocarbons in thermal fast pyrolysis increased linearly with temperature. The yield to hydrocarbons in thermal slow pyrolysis follows the same trend. The heating rate did not change the yield to hydrocarbons in carpet face fiber pyrolysis. However, significantly higher amounts of oxygenates, mostly acids, were produced with the faster heating rate, whereas at slow heating rate, deoxygenation reactions were enhanced. Overall, the faster pyrolysis heating rate enhanced the yields to organic liquid products, with similar yields to hydrocarbons and higher yields to oxygenates, compared with the slow heating

Figure 3. Comparison of the hydrocarbons and oxygenates (a) yields and (b) liquid product selectivity from thermal pyrolysis of carpet face fiber under slow and fast heating rates. The lines and circles are used to guide the eyes.

rate. Although the slow heating rate did not contribute much to the yields to hydrocarbons, it improved their selectivity in the liquid. At the same time, the slower heating rate enhanced the deoxygenation reactions, leading to lower selectivity to oxygenates. The enhancement of deoxygenation reactions and the improved selectivity to hydrocarbons in slow pyrolysis is opposite to claims for pyrolysis of biomass that production of volatiles is enhanced and solid char formation is reduced at fast heating rates.35,40 Catalytic and Steam Catalytic Slow Pyrolysis of Carpet Waste. Comparison of ZSM-5 and CaO in the catalytic fast pyrolysis showed that CaO is a better catalyst than ZSM-5 for steam co-fed experiments because of the relatively good selectivity of CaO to benzene and its derivatives. Thus, we performed catalytic and steam catalytic slow pyrolysis of carpet waste over CaO to explore better ways of converting the waste carpet to benzene-rich oils. Catalytic steam pyrolysis of PET over ZSM-5 was not conducted because of the known tendency of zeolites to deactivate and deteriorate in terms of crystal structure in the presence of steam.41,42 Table 4 shows the product distribution from slow thermal, catalytic, and catalytic steam pyrolysis of the carpet face fiber and backing material. The different product distribution, especially solid and gas yields, between face fiber and backing material is attributed to the different components contained in the two. The face fiber contains mostly PET, whereas the backing material contains nylon, polypropylene, PVC, some PET, styrene butadiene latex, and inorganic filler. Moreover, fire retardant dispersed at considerably different concentrations in the face and backing materials could be responsible for the different product distributions.43,44 Thermal pyrolysis of both the face fiber and backing material generated smaller amounts of liquid and gas products, compared with those from catalytic and catalytic steam pyrolysis. The solid yield was high in thermal pyrolysis, E


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ACS Sustainable Chemistry & Engineering Table 4. Product Distribution in Slow Pyrolysis of Carpet Face Fiber and Backing Material (C%) face fiber

backing material

carbon yield (C%)

thermal

CaO

CaO + steam

thermal

CaO

CaO + steam

a

28.73 8.50 6.03 1.88

18.63 ± 0.94 22.31 ± 1.43 15.01 ± 0.92 0.46 ± 0.04 5.98 ± 0.32 0.86 ± 0.15 37.03 ± 6.42 37.02 ± 6.42 0.01 ± 0.00

1.07 ± 0.20 13.85 ± 0.62 0.76 ± 0.24 0.61 ± 0.03 11.66 ± 0.31 0.82 ± 0.04 54.69 ± 3.44 54.28 ± 3.20 0.41 ± 0.24

20.81 3.39 1.95 1.18

5.10 ± 0.27 17.10 ± 0.82 6.11 ± 0.27 0.38 ± 0.24 10.03 ± 0.25 0.58 ± 0.06 33.43 ± 2.42 33.42 ± 2.42 0.01 ± 0.00

0.81 ± 0.12 9.73 ± 1.01 0.29 ± 0.01 0.49 ± 0.06 8.61 ± 0.88 0.34 ± 0.06 38.15 ± 3.72 38.15 ± 3.72

22.03

30.39

44.37

51.31

total solid total gas CO CO2 CO2 in CaCO3b CH4 total liquid identifiedc hydrocarbons oxygenates yellow compoundd unidentified carbone

0.59 23.56 3.93 1.91 17.72 39.21

0.26 25.98 7.34 1.60 17.04 49.82

a

The total solid yield is measured by collecting and burning the char products and measuring the weight change. Char is assumed to be pure carbon, as relatively high temperature is reached for slow pyrolysis. bThe released CO2 can be captured by CaO, becoming CaCO3 during pyrolysis. The yields of the captured CO2 are measured by TPO analysis of the spent CaCO3/CaO mixture. cThe identified total liquid product carbon yield for catalytic steam pyrolysis does not include minor amounts of organics in the aqueous phase (in the first impinger of the condenser train) because of the potential harm that water can do to the GC-MS. dYellow heavy organic compound is observed on the reactor wall and in the transfer line between catalyst bed and condenser. The weight of the compound is measured after collection. The carbon fraction of the compound is obtained from elemental analysis. eThe unidentified contains organic compounds that cannot be confidently (>80% quality) identified by GC-MS and/or are beyond the analysis limitations of the GC-MS. They are calculated by the carbon balance.

reflecting the limited extent of depolymerization reactions. In all the experiments, CO was the most abundant component in the gas products. In catalytic pyrolysis over CaO, liquid product yields, particularly hydrocarbons yields, for both face fiber and backing material were significantly improved, compared with that of thermal pyrolysis. A significant fraction of the gas product was still CO, but production of CO2 was enhanced by the catalyst. In catalytic steam pyrolysis, only minor amounts of solids were produced. The hydrolysis reactions promoted the production of hydrocarbons, leading to the highest hydrocarbon yield among the three pyrolysis configurations. The production of CO2 outweighed that of CO, indicating enhanced decarboxylation reactions during catalytic steam pyrolysis. The yield to CO2 should include both the gaseous CO2 and the CO2 captured by CaO in the form of CaCO3. Thus, analysis of the spent CaO was necessary for identification and quantification of the adsorbed CO2. Temperature-programmed oxidation (TPO) of the spent CaO/CaCO3 was performed in TGA and TGA-MS, and the results are shown in Figure 4. In Figure 4a, two peaks at a temperature of around 400 and 600 °C are seen in the TGA of the spent CaO/CaCO3. The temperature of decomposition of CaCO3 has been reported to be at around 600 °C.45 So, the 400 °C peak represents the decomposition/devolatilization of char or other adsorbed organic compounds and/or the dehydration of Ca(OH)2.46 Identification of the reactions that could contribute to the weight loss at 400 °C was achieved by using TGA-MS. A typical plot is shown in Figure 4b, where only water and CO2 are observed during the ion scanning (1−50 m/z) at 400 and 600 °C, respectively. This means that the weight loss at 400 and 600 °C was due to the dehydration of Ca(OH)2 and the decomposition of CaCO3, respectively. The formation of Ca(OH)2 was partly due to the moisture in air,47 which was verified by performing TGA of the pure CaO (not shown here). In conclusion, only the weight loss from the second peak was considered in the calculation of the carbon yield to CO2. A yellow heavy organic solid substance was formed in the thermal pyrolysis experiments. Identification of this substance

Figure 4. (a) Temperature-programmed oxidation (TPO) of the spent CaO from catalytic and catalytic steam pyrolysis of carpet face fiber and backing material in TGA; (b) TPO of spent CaO from catalytic pyrolysis of carpet face fiber in TGA-MS, a typical plot for identification of the emitted volatiles.

was not successful through GC-MS due to the high boiling point of the substance. Thus, other characterization methods, such as TOF MS and elemental analysis, were explored. Direct infusion of the tar substance was performed in TOF MS with the electrospray ionization. The TOF MS spectrum of the tar substance is shown in the Supporting Information. The two highest peaks identified corresponded to accurate mass (m/z) of 907.3498 and 885.3646. The difference of the two accurate mass peaks matches that of the mass of an ionized sodium subtracted by a proton, meaning that one proton in the yellow F


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reaction intermediates, and the subsequent catalytically upgrading on the CaO catalyst, downstream from the thermal reaction zone. The benzene yield and selectivity shown here are comparable to those reported by Grause et al.,15 who performed catalytic steam pyrolysis of PET for the production of benzene-rich oil. The produced benzene-rich oil can be potentially used as high-quality liquid fuel or a precursor for production of useful chemicals since the purity of benzene is very high. A more detailed comparison of the liquid product distribution between catalytic pyrolysis and catalytic steam pyrolysis is shown in Figure 6. Besides benzene, other major

tar substance was replaced by the ionized sodium during the TOF MS analysis. This means that the actual molecular weight of the tar substance was about 885. By elemental analysis, the C/H and C/O molar ratios of the yellow tar substance were analyzed to be 1.08 and 2.34, respectively. From these results, we concluded that the yellow tar substance had the formula C42H42O21. Further identification of the compound in terms of elucidating its chemical structure was not feasible. This finding agrees with Ç it et al.,48 who performed thermal pyrolysis of PET at 700 °C. They analyzed the pyrolysis tar with gel permeation chromatography and showed that the pyrolysis tar contained mostly components of molecular weight of about 500. This unidentified substance is also consistent with Kumagai et al.,28 who performed steam pyrolysis of various PET-based composite wastes and found poor carbon balances due to the presence of a heavy solid compound in the liquid product. Pyrolysis versus Hydrolysis. Benzene yields and selectivity from different pyrolysis experiments are compared in Figure 5. Thermal pyrolysis only generated minor amounts

Figure 6. Hydrolysis versus pyrolysis in catalytic slow pyrolysis of carpet.

compounds, lumped as naphthalenes, biphenyls, and heavy PAHs, also appeared in the liquid product from catalytic pyrolysis of the carpet face fiber and backing material. However, when steam was co-fed, their quantities were essentially eliminated. Pyrolysis and hydrolysis reactions might coexist in steam pyrolysis. Quantification of the extent of pyrolysis and hydrolysis reactions during the steam pyrolysis of polyesters, including PET, has been performed by Kumagai et al.49 using 18 O-labeled steam. It was found that lower reaction temperature and higher steam concentration enhanced the extent of hydrolysis reactions during steam pyrolysis of PET. Bednas et al.50 also studied the pyrolysis reactions of PET. They suggested a rearrangement of benzoic acid vinyl ester, formed as a primary product of the pyrolytic fission of PET, with a subsequent decarboxylation (Scheme 1a,b). Grause et al.15 claimed that PET does not depolymerize by an unzip mechanism, forming constituent monomers. Instead, it undergoes a one-step reaction, in which each ester group is hydrolyzed independently (Scheme 1c). After hydrolysis, decarboxylation of the resulting terephthalic acid occurred in the presence of CaO, producing benzene (Scheme 1d). Combination of the schemes for pyrolysis and hydrolysis from the literature15,50 with the experimental results of this study (Figure 6) leads to the conclusion that benzoic acid vinyl ester is a major precursor for the formation of the subsequent naphthalenes, biphenyls, and heavy PAHs but not carboxylic acids. This is consistent with Kumagai et al.,49 who claimed that during pyrolysis vinyl esters were byproducts that, once formed,

Figure 5. Benzene yield and selectivity in liquid products (organic phase for the catalytic steam pyrolysis) in thermal, catalytic, and steam catalytic slow pyrolysis of carpet (a) face fiber and (b) backing material in a fixed-bed reactor. Benzene selectivity is calculated as the amount of carbon in benzene divided by the total amount of carbon in the organic liquid product identified.

of benzene at low liquid product selectivities. The addition of CaO improved the benzene yield and selectivity significantly. Steam pyrolysis over CaO produced the highest benzene yield and liquid product selectivity. In catalytic steam pyrolysis (or high-temperature catalytic hydrolysis) of face fiber and backing material, about 54 and 38 C% benzene yields were measured, respectively. The selectivity to benzene in the organic phase of the liquid product collected from pyrolysis of face fiber and backing material reached 98 and 99 C%, respectively. The enhanced production of benzene in the presence of CaO and steam is mainly attributed to the thermal depolymerization of PET, forming terephthalic acid, and the subsequent decarboxylation reactions promoted by CaO (Scheme 1c,d). The effect of steam in the absence of CaO was studied by Grause et al.29 Hydrolysis of PET was shown to produce benzene at very low yields. The high selectivity obtained in this work was feasible due to the high-temperature steam, enhancing hydrolysis G


Research Article


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could not be further pyrolyzed to carboxylic acids. Acetophenone is a unique pyrolysis product, meaning that it is only observed in the liquid products from pyrolysis experiments, as shown in Scheme 1b.

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CONCLUSIONS In summary, pyrolysis of PET-based waste carpet was shown as a viable waste management option with simple chemistry. At high temperature, benzoic acid and acetylbenzoic acid were the most abundant liquid products. Deoxygenation of these acids was favorable at slow heating rates with either ZSM-5 or CaO catalysts. ZSM-5 enhanced polymerization reactions to form polyaromatic hydrocarbons, whereas pyrolysis over CaO generated unfavorable ketones. Addition of steam on CaOcatalyzed slow pyrolysis of carpet waste PET-favored hydrolysis and deoxygenation reactions resulted in ∼100% benzene purity in the organic product. This was feasible for both carpet waste components, which can eliminate several expensive separation steps, and allows for the efficient and profitable reutilization of PET carpet wastes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b00450. Physicochemical properties of ZSM-5 and CaO catalysts and detailed experimental procedures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: +1-860-486-4602. E-mail: [email protected]. Funding

This work was supported in part by the National Science Foundation under Grant No. CBET 1236738. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Supply of the carpet waste by Carpet America Recovery Effort is gratefully acknowledged. S.D. thanks Dr. Anthony Provatas and Dr. You-Jun Fu for critical discussions of the MS TOF results.

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REFERENCES

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Conversion of Polyethylene Terephthalate Based Waste Carpet to

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