Ex-situ catalytic upgrading of vapors from microwave-assisted pyrolysis of low-density polyethylene with MgO

Ex-situ catalytic upgrading of vapors from microwave-assisted pyrolysis of low-density polyethylene with MgO

Energy Conversion and Management 149 (2017) 432–441 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www...

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Energy Conversion and Management 149 (2017) 432–441

Contents lists available at ScienceDirect

Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Ex-situ catalytic upgrading of vapors from microwave-assisted pyrolysis of low-density polyethylene with MgO Liangliang Fan a,b,c, Yaning Zhang c,d, Shiyu Liu c, Nan Zhou c, Paul Chen c, Yuhuan Liu a,b, Yunpu Wang b, Peng Peng c, Yanling Cheng c, Min Addy c, Hanwu Lei e, Roger Ruan c,⇑ a

State Key Laboratory of Food Science and Technology, Nanchang University, Nanchang 330047, China Engineering Research Center for Biomass Conversion, Ministry of Education, Nanchang University, Nanchang 330047, China Center for Biorefining and Department of Bioproducts and Biosystems Engineering, University of Minnesota, 1390 Eckles Ave., St. Paul, MN 55108, USA d School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China e Department of Biological Systems Engineering, Washington State University, 2710 Crimson Way, Richland, WA 99354, USA b c

a r t i c l e

i n f o

Article history: Received 9 June 2017 Received in revised form 11 July 2017 Accepted 18 July 2017

Keywords: Ex-situ upgrading LDPE MgO Microwave-assisted pyrolysis Mechanism

a b s t r a c t Ex-situ catalytic upgrading of vapors from microwave-assisted pyrolysis of LDPE using MgO as the base catalyst was investigated and the effects of catalyst to reactant ratio, pyrolysis temperature, and catalytic reaction temperature on the yields and chemical profiles of products were examined. 24.2–38.5 wt.% of ex-situ upgraded liquid yield were obtained under varied reaction conditions. Productive gas yield (higher than 56.6 wt.%) and low solid residue yield (less than 7.1 wt.%) were obtained as long as the pyrolysis temperature was over 500 °C during the ex-situ upgrading process. The coke yield was negligible, ranging from 0.14 wt.% to 1.94 wt.% based on LDPE mass. The conversion of alkenes to aromatics was improved at higher catalyst to feedstock ratios, higher pyrolysis temperatures, and higher catalytic reaction temperatures. The total percentage of gasoline fraction in the upgraded pyrolysis oil ranged from 79.5% (37.3% mono-aromatics and 42.2% C5-C12 aliphatics) to 96.0% (39.7% mono-aromatics and 56.3% C5-C12 aliphatics) under various conditions, compared with 10.5% mono-aromatics and 30.2% C5-C12 aliphatics from non-catalytic pyrolysis. The main composition of gas product was hydrogen, C1-C3 olefins and paraffins, varying with reaction condition. Both free radical and carbanion mechanisms for the conversion of LDPE with MgO were proposed. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Plastics touch every aspect of our life, such as packaging, construction, transportation, agriculture, healthcare, and electronics. Furthermore, they are important materials in many other manufacturing industries [1]. An estimated 299 million tons of plastics were produced in the world in 2013 [2]. The increasing use of plastics contributes to the large amount of municipal solid waste, which is one of the main culprits of environmental contamination. Several methods including incineration, landfill, and recycling have been investigated and implemented for plastic waste disposal. However, the application of these methods is limited because they are less environmentally friendly or costly [1,2]. Recently, there is an increasing interest in the recovery of energy and chemicals from plastics to satisfy the increased consumption of energy [3–5]. Pyrolysis is one of the effective methods to convert plastic waste ⇑ Corresponding author. E-mail address: [email protected] (R. Ruan). http://dx.doi.org/10.1016/j.enconman.2017.07.039 0196-8904/Ó 2017 Elsevier Ltd. All rights reserved.

to valuable energy and the liquid oil from pyrolysis can be directly used in various fields such as boilers, diesel engines and turbines [6]. In recent years, microwave-assisted pyrolysis (MAP) has become a popular and reliable technology applied in chemicals and fuels conversion [7]. MAP process provides volumetric and instantaneous heating to sample, leading to many advantages over conventional pyrolysis including uniform internal heating, high energy efficacy, and low cost [8]. Besides, the heating characteristics may contribute to a narrow chemical profile of pyrolysis products. In MAP process, microwave absorbents are usually used to assist pyrolysis due to the poor dielectric property of some samples, such as LDPE [9]. Low-density polyethylene (LDPE) waste is recognized as the second largest plastic wastes [2]. Conversion of LDPE waste to valuable fuels and chemicals is potentially profitable because of its weakness of intermolecular force [2]. Liquid oil from pyrolysis of LDPE is of high calorific value, acids free, and water free because of the absence of oxygen during the conversion process and high content of carbon and hydrogen in LDPE. However, LDPE pyrolytic

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oil has a broad and complex chemical profile [10]. To obtain homogeneous and refined chemical fuels from LDPE waste, the crude pyrolytic oil must be further upgraded usually through postconversion catalytic processes with adaptable catalysts which could narrow the spectrum of product distribution and elevate the selectivity for valuable chemical products [11]. In addition, catalytic cracking reduces energy demand for pyrolysis due to the reduction of activation energy induced by the catalysts, lowering the required pyrolysis temperature dramatically [12]. Alternatively, in-situ or ex-situ catalytic upgrading can be used in the pyrolytic process [13,14]. For in-situ catalytic upgrading, the catalyst is mixed evenly with the feedstock and is introduced into the same reactor for thermal decomposition. For ex-situ catalytic upgrading, the catalyst is placed in a secondary reactor and the pyrolysis vapors undergo the second upgrading process. Compared with the in-situ catalytic upgrading, the ex-situ catalytic upgrading presents more advantages. In the ex-situ catalytic upgrading process, the pyrolysis and catalytic reaction temperature can be regulated separately for optimal performance and the vapors have a higher probability of being exposed to the catalyst. In addition, ex-situ catalytic upgrading avoids the difficult separation of catalyst from char and enables easier reactivation with less catalyst loading. Liu et al. [15] have successfully applied the ex-situ catalytic technology in the reforming of pubescens pyrolytic intermediates and found that ex-situ catalytic process improved the production of phenols. Various catalysts were also developed by Liu et al. [16– 18] for the ex-situ catalytic co-pyrolysis of pubescens and LDPE. They indicated that ex-situ catalytic process improved the selectivity of aromatics compared to in-situ catalytic process [16]. A few studies were also investigated on the ex-situ catalytic upgrading of LDPE pyrolytic vapors. In the study conducted by Uemichi et al. [19], ex-situ catalytic conversion of polyethylene into gasoline-range fuels was investigated and resulted in higher gasoline yield than in-situ catalytic process. Xue et al. [20] performed the study of in-situ and ex-situ catalytic upgrading of different types of waste plastics and found in-situ catalytic process improved aromatic production compared to ex-situ catalytic process. In the present study, a microwave-assisted pyrolysis reactor followed by an ex-situ catalytic bed was designed for investigation of LDPE conversion. In general, solid acid catalysts such as zeolites are used in catalytic pyrolysis or upgrading process because they are commonly used in petroleum industry [21]. Zeolite catalyst, such as ZSM-5, has been effectively applied in polyethylene degradation due to its strong acidity [16] and mainly exhibited high efficiency on aromatization [22]. In contrast, base catalysts received much less attention [21,23]. MgO is a cost-effective base catalyst which is a sustainable alternative to solid acid catalyst for the catalytic upgrading of pyrolysis vapors [24]. The objective of this study was to investigate the ex-situ catalytic upgrading of vapors from MAP of LDPE and obtain more valuable and better quality products in the presence of MgO. In the present work, LDPE was decomposed in a microwave reactor with the assistance of the microwave absorbent, silicon carbide (SiC). The pyrolysis vapors were upgraded through a separate catalyst bed loaded with MgO. The pyrolysis products were characterized to evaluate the effect of the base catalyst and effects of catalyst to feedstock ratio, pyrolysis temperature, and catalytic reaction temperature were investigated.

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from Alfa Aesar (USA) was used as the catalyst. Prior to use, LDPE and MgO were air dried. The carbon content and hydrogen content of LDPE were determined in the previous study [8]. 2.2. Experimental setup Fig. 1 shows the schematic diagram of the microwave-assisted pyrolysis system coupled with an ex-situ upgrading reactor outside of the microwave. The pyrolysis system mainly consists of a microwave oven (Mars 6, CEM microwave technology Ltd., USA), a Ktype thermocouple for temperature measurement, a pyrolysis reactor, a condenser for liquid product collection and a vacuum pump for air purging. The microwave oven has a maximum power of 3 kW and a magnetic frequency of 2.45 GHz. The ex-situ catalytic upgrading reactor is composed of a quartz column filled with the catalyst sandwiched between a layer of quartz wool and a polyporous fritted disc, whose pore size was 90–150 mm. The quartz wool and fritted disc help secure the catalyst powder in place and facilitate even passing of the pyrolysis vapors through the column. A heating tape was used to heat the catalyst bed and the catalytic reaction temperature was measured using a K-type thermocouple. SiC was used as microwave absorbent to enhance the heating of LDPE powder. For each run of the experiment, around 15.0 g (weighed accurately) LDPE powder mixed with about 500 g SiC were added together into a 500 mL quartz flask reactor before being placed in the microwave cavity. The quartz reactor, catalyst bed, and liquid collector were connected together using quartz connecters. The external of the connecting pipeline was wrapped by quartz wool to avoid energy dissipation as well as the condensation of pyrolysis vapors. Prior to pyrolysis, the flask was vacuumed for 10 min to purge the air, assuring an oxygen-free atmosphere in the pyrolysis system. The pyrolysis bed was heated at a heating rate of around 80–100 °C/min with the assistance of microwave radiation because of the strong microwave absorbing ability of SiC. Once the pyrolysis bed reached the desired temperature, the temperature was maintained for 20 min by turning on and off the microwave oven manually. The temperature of the catalytic upgrading reactor was controlled by a temperature

2. Materials and methods 2.1. Materials LDPE powder (1000 mm) obtained from Alfa Aesar (USA) was used as the feedstock. The fine magnesium oxide (MgO) powder

Fig. 1. Schematic diagram of ex-situ catalytic microwave-assisted pyrolysis system. (1) Microwave oven; (2) quartz reactor; (3) K-type thermocouple; (4) quartz wool; (5) catalyst bed; (6) fritted disc; (7) liquid collector; (8) Condensing system; (9) connection with a vacuum pump.

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controller. After undergoing MAP and ex-situ catalytic upgrading, the vapors were condensed to form liquid oil and collected in a 500 mL flask while the non-condensable gaseous product was collected with a 1 L gas bag. Inadequate ratio of catalyst to reactant may results in rapid deactivation of the catalyst and poor catalytic effect, so various ratios of catalyst to reactant (1/15, 1/10, 1/5 and 1/3) were firstly investigated to predict the effect of catalyst on the products yields and chemical profiles. Besides, other parameter conditions including various pyrolysis temperatures (350, 400, 450, 500, and 550 °C), and catalytic reaction temperatures (350, 400, 450, 500, and 550 °C) were studied. The obtained liquid product was weighed to calculate the liquid yield based on the mass of LDPE. The yield of solid residue was calculated by mass difference of the quartz reactor before and after reaction. The content of coke deposited on the catalyst was calculated by mass difference. The coke yield was determined based on reactant mass and catalyst mass, respectively. The gas yield was calculated according to mass balance.

70

50 40 30 20 10 0 No catalyst 1/15

80

1/3

60

Liquid Solid residue Gas

50 40 30 20 10 0 350

400

450

500

550

o

Pyrolysis temperature ( C) (b)

2.4. Characterization of gaseous product

80 70

Product yield (wt.%)

The collected gaseous product was determined by a CP-4900 micro-GC (Varian Inc., USA) coupled with a thermal conductivity detector (TCD). Two separate column were loaded on the microGC system to characterize gaseous product. H2, O2, N2, and CO2 were analyzed on an MS5A column with helium carrier gas while hydrocarbons (C1-C3) were analyzed on a PoraPLOT Q (PPQ) column with argon carrier gas. Prior to analysis, the temperatures of both column and injector were all set at 80 °C. Standard gases (H2, O2, N2, CO2, CH4, C2H4, C2H6, C3H6 and C3H8) were used for calibration. The gaseous product was measured according to the calibration curve of each standard gas. The volume percentage of C4+ hydrocarbon gases were determined by difference.

1/5

(a)

Product yield (wt.%)

The chemical composition of the liquid product was analyzed using an Agilent 7890-5975C GC/MS coupled with an HP-5 capillary column. The test condition of GC/MS was described in the previous study [8]. The chemical compounds were identified according to the spectral data from NIST library. The chromatographic area percentage of each compound was calculated to characterize the product selectivity.

1/10

Catalyst to LDPE ratio

70 2.3. Characterization of liquid product

Liquid Solid residue Gas

60

Product yield (wt.%)

434

Liquid Solid residue Gas

60 50 40 30 20 10 0

2.5. Statistical analysis Each run of the experiment was performed in three replicates, and the data were presented as mean accompanied by a standard deviation (SD). If variation analysis was needed, the significant difference was determined using SPSS Version 13.0 (SPSS Inc., USA) with Duncan test (p < 0.05).

350

400

450

500

550

Catalytic reaction temperature (°C) (c) Fig. 2. Product yield as a response of (a) catalyst to LDPE ratio at 500 °C of pyrolysis temperature and 450 °C of catalytic reaction temperature, (b) pyrolysis temperature at 1/5 of catalyst to LDPE ratio and 450 °C of catalytic reaction temperature, and (c) catalytic reaction temperature at 1/5 of catalyst to LDPE ratio and 500 °C of pyrolysis temperature.

3. Results and discussion 3.1. Products yields 3.1.1. Effect of catalyst to feedstock ratio Products yields under different MgO to LDPE ratios are shown in Fig. 2(a). The pyrolysis temperature and catalytic reaction temperature were kept constant, at 500 °C and 450 °C, respectively. As a blank control group, pyrolysis of LDPE without catalyst was also studied. For the non-catalytic pyrolysis, the secondary reactor temperature was kept at 350 °C to prevent the condensation of pyrolysis vapors on the catalytic tube. As shown in Fig. 2(a), as the ratio of MgO to LDPE increased from no catalyst to 1/10, the liquid yield

decreased from 46.3 wt.% to 30.3 wt.% while the gas yield increased from 52.8 wt.% to 67.0 wt.%, indicating that the MgO catalyst enhanced the cracking of the macromolecular volatiles. This was because MgO reduced carbon number of aliphatic hydrocarbons [25] and more volatiles were converted to gaseous molecules. However, there was no dramatic change in the liquid yield when the ratio of MgO to LDPE increased from 1/10 to 1/3, suggesting that appropriate ratio of MgO to LDPE showed enough catalytic effect for the cracking of the volatile compounds into gas components. The solid residue was also produced during the pyrolysis process. As shown in Fig. 2(a), the solid residue yield was relatively

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low, ranging from 1.0 wt.% to 7.1 wt.%, showing the high LDPE conversion rate. Besides, an increasing trend was observed in the solid residue yield when MgO to LDPE ratio increased from no catalyst to 1/3 because higher loading of catalyst in the packed catalyst bed increased the residence time of the volatiles from pyrolysis, lowering the decomposition rate of LDPE on the pyrolysis bed as a result of more solid residue formed during the process. 3.1.2. Effect of pyrolysis temperature Pyrolysis temperature plays an important role in the cracking reaction of LDPE. Volatiles are released away from LDPE when the energy provided by the pyrolysis temperature is higher than the enthalpy of CAC in the polymer chain, resulting in the rupture of carbon chain. It was reported that the thermal degradation of LDPE began at around 300 °C according to the thermogravimetry (TG) study [26]. Marcilla et al. [27] indicated that the liquid fraction formed initially at approximate 360–385 °C. Thus, the study of pyrolysis temperature effect started at 350 °C. The catalytic reaction temperature was kept at 450 °C and the ratio of MgO to LDPE was kept at 1/5. Fig. 2(b) shows product yields at different pyrolysis temperatures (350–550 °C). The liquid yield increased slightly from 24.2 wt.% at 350 °C to 30.1 wt.% at 500 °C because more energy was provided to weaken the bond link of carbon chain at higher temperatures, leading to increasing cracking of polymer chains. Thus, more vapors were volatilized from LDPE pyrolysis and condensed to organic liquid product. However, a further increase in pyrolysis temperature lowered the liquid yield because higher decomposition reaction induced by increased temperature, converting more liquid product to gas compounds. This also indicated that the optimal pyrolysis temperature for liquid production was around 500 °C. It is noteworthy that the solid residue yield at 350 °C was very high, at 49.5 wt.%. This could be attributed to the low energy provided by low pyrolysis temperature, which was not enough for the complete decomposition of LDPE to volatiles. The partially decomposed LDPE presented as solid waxes, which showed high boiling point and were difficult to be volatilized. Nevertheless, the solid residue yield was still much lower than that from the previous study [23], in which catalytic pyrolysis of LDPE was investigated and up to 66.43 wt.% of solid was obtained at 400 °C of pyrolysis temperature. Lower yield of solid residue in the present work could ascribe to the even and effective heating provided by microwave radiation. The solid residue yield decreased dramatically with the increasing pyrolysis temperature because more LDPE and oligomer macromolecules were cracked to smaller molecules at higher pyrolysis temperature. It was also reason for the significant increasing of gas product yield. 3.1.3. Effect of catalytic reaction temperature The effect of catalytic reaction temperature on the yields of products from ex-situ upgrading process was also investigated. The pyrolysis temperature and catalyst to LDPE ratio were kept constant, at 500 °C and 1/5, respectively. Interestingly, compared with non-catalytic pyrolysis, which produced 46.3 wt.% of liquid and 52.8 wt.% of gas, the ex-situ catalytic upgrading processes presented lower liquid yield and higher gas yield, ranging from 24.4 to 38.5 wt.%, and 56.6 wt.% to 72.8 wt.%, respectively. This indicates that the existence of MgO catalyst strengthened the decomposition of macromolecules volatiles. As shown in Fig. 2(c), the gas yield increased significantly as the catalytic reaction temperature increased from 350 to 550 °C while the liquid yield presented an opposite trend. It should be noted that the volatile compounds from LDPE pyrolysis could undergo further cracking in the catalyst bed, especially in which the catalytic reaction temperature was higher than pyrolysis temperature, as a result of higher gas product production at higher. Besides, the increasing catalytic reaction temperature could also improve the catalytic activity of MgO and

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promote the degradation of long chain hydrocarbon macromolecules. The solid residue yield at the varied conditions was negligible and remained unchanged, at around 2.5 wt.%, for the reason that the formation of solid residue was mainly affected by aforementioned pyrolysis temperature. 3.1.4. Coke yield The coke yield under different reaction conditions is shown in Table 1. It was determined based on catalyst and reactant mass, respectively. The coke yield was negligible based on LDPE mass, most ranging from 0.14 wt.% to 0.61 wt.%, except for that at 350 °C of catalytic reaction temperature (1.94 wt.%). The low coke yield could be attributed to the advantage of the ex-situ catalytic upgrading system, which reduced the possibility of direct contact between catalyst and waxes [22]. Besides, another possible reason is the large pore diameter of MgO [28], which makes pyrolysis vapors from LDPE easily passing through. However, when the catalytic reaction temperature was 350 °C, the coke yield reached as high as 1.94 wt.%. This is probably because many long-carbonchain aliphatic hydrocarbons were not easy to be further decomposed at low catalytic reaction temperature and still remained during passing through the catalyst bed, resulting in that a lot of long chain macromolecules were trapped in the pores of MgO. It also indicates that adequate condition is necessary to ensure the catalytic activity of MgO and the reduction of coke deposition. By further comparison, it is observed that the coke yield increased slightly with the increasing ratio of catalyst to LDPE. It is attributed to more contacting possibilities between pyrolysis vapors and catalyst induced by the increased catalyst loading, resulting in more deposit on the catalyst. Generally, the coke yield based on reactant was lower at higher pyrolysis and catalytic reaction temperatures. As the pyrolysis or catalytic reaction temperature increased, the macromolecule volatiles were prone to be further cracked to small molecules, which were not easy to be deposited on the catalyst. For better understanding of catalyst deposition by coke, the coke yield was also calculated based on catalyst mass. As shown in Table 1, the coke yield based on catalyst decreased with the increasing loading of catalyst, suggesting that small amount of catalyst was much easier to be deposited by coke. This also indicates that appropriate ratio of catalyst to reactant is needed to guarantee the catalytic effect. It was also observed that the coke yield based on catalyst reduced as the increasing pyrolysis temperature and catalytic reaction temperature. It was indicated that waxes were the main coke precursors [29]. Higher temperatures enhanced the decomposition of waxes to short-chain hydrocarbons, reducing the coke formation on the catalyst. 3.2. Chemical composition of liquid product 3.2.1. Effect of catalyst to feedstock ratio Generally, mono-aromatics, poly-aromatics, alkenes, and alkanes were produced as the liquid product during the ex-situ catalytic upgrading process. Specifically, the aliphatic hydrocarbons can be divided into gasoline fraction (C5-C12 hydrocarbons) and diesel fraction (C12+ hydrocarbons) [30]. The effect of catalyst to LDPE ratio on the chemical selectivity was investigated and shown in Fig. 3(a). The liquid product derived from non-catalytic microwave-assisted pyrolysis of LDPE mainly consisted of aliphatic hydrocarbons, which accounted for 84.1%. Besides, a few of aromatics (10.5% of mono-aromatic and 5.3% of poly-aromatics) were also produced from the non-catalytic pyrolysis. This is different from the other studies [31,32], in which almost no aromatics were produced from the thermal cracking of LDPE without catalyst. This could be attributed to the difference of pyrolysis system. Unlike conventional thermal pyrolysis, microwave-assisted pyrolysis in the presence of microwave absorbent provided evenly heating to

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Table 1 Coke yield based on catalyst and reactant under various conditions. Run

Catalyst to LDPE ratio

Pyrolysis temperature (°C)

Catalytic reaction temperature (°C)

Coke yield (wt.% based on catalyst)

Coke yield (wt.% based on reactant)

1 2 3 4 5 6 7 8 9 10 11 12

1/15 1/10 1/5 1/3 1/5 1/5 1/5 1/5 1/5 1/5 1/5 1/5

500 500 500 500 550 450 400 350 500 500 500 500

450 450 450 450 450 450 450 450 550 500 400 350

2.15 ± 0.05 1.50 ± 0.00 1.20 ± 0.10 1.20 ± 0.15 1.15 ± 0.10 1.55 ± 0.20 1.45 ± 0.35 3.05 ± 0.15 0.73 ± 0.13 1.85 ± 0.15 1.85 ± 0.05 9.70 ± 0.10

0.14 ± 0.01 0.15 ± 0.00 0.24 ± 0.02 0.40 ± 0.05 0.23 ± 0.02 0.31 ± 0.04 0.29 ± 0.07 0.61 ± 0.03 0.15 ± 0.03 0.37 ± 0.03 0.37 ± 0.01 1.94 ± 0.02

c de e e e de de b f cd cd a

e e de c de cd cd b e c c a

Note: Different letters behind mean values in the same column suggest significance of differences (p < 0.05).

reactants, promoting the formation of light olefins, which are reactants for the production of aromatics through Diels-Alder reaction [33]. It was observed that C12+ alkenes were dominant (38.3%) in the liquid fraction from non-catalytic pyrolysis. However, as the ratio of MgO to LDPE increased, the selectivity of C12+ alkenes decreased significantly until all C12+ alkenes were converted completely at 1/5 of MgO to LDPE ratio. This was because the addition of MgO facilitated the cracking of long chain aliphatic hydrocarbons to smaller molecules [25], resulting in the increasing production of C5-C12 alkenes from 23.3% (non-catalytic pyrolysis) to 37.5% (1/15 of MgO to LDPE ratio). However, as the MgO to LDPE ratio increased from 1/15 to 1/3, a dramatic downward trend was observed in the selectivity of C5-C12 alkenes since more addition of MgO with more conspicuous catalytic effect was conducive to the conversion of light olefins to aromatics through Diels-Alder reaction [22]. Thus, the selectivity of mono-aromatics increased significantly with the increasing ratio of MgO to LDPE. The ratio of catalyst to LDPE also affected the selectivity of alkanes. Compared with non-catalytic process, the addition of catalyst initially increased the selectivity of C5-C12 alkanes due to the cracking of long chain olefins over MgO. However, unlike C5-C12 alkenes, there was no significant change in the selectivity of C5-C12 alkanes when the ratio of catalyst to LDPE continued to increase for the reason that C5-C12 alkanes cannot be converted to aromatics as a result of little influence made by the catalyst. Interestingly, although the selectivity of C12+ alkanes decreased as the catalyst to LDPE ratio increased from no catalyst to 1/15 because of secondary cracking reaction during catalytic process, a slight upward trend was observed when the ratio of catalyst to LDPE kept increasing from 1/15 to 1/3. This could be the reason that an increasing amount of hydrogen radicals were produced through the increasing aromatization of light olefins, resulting in more hydrogenation of alkenes over MgO [34] and then more production of alkanes. Besides, a small amount of poly-aromatics were also produced during the non-catalytic or catalytic processes and it was observed that MgO made no contribution to selectivity of poly-aromatics, which have detrimental biological effects and are harmful to environment due to the toxicity, mutagenicity and carcinogenicity of poly-aromatics. As a result, the MgO catalyst improved the quality of liquid product from LDPE by increasing the production of monoaromatics and alkenes.

3.2.2. Effect of pyrolysis temperature The effect of pyrolysis temperature on the chemical selectivity of liquid product is shown in Fig. 3(b). In comparison to noncatalytic pyrolysis process at 500 °C of pyrolysis temperature, the upgrading of pyrolysis oil produced higher proportion of C5-C12 alkenes (50.4%) and lower proportion of C12+ alkenes (8.1%) even

at low pyrolysis temperature (350 °C) because of the cracking effect induced by MgO placed in the catalyst bed. Besides, the proportion of C12+ alkenes decreased from 8.1% at 350 °C to 0% at 500 °C due to the enhanced decomposition reaction induced by higher pyrolysis temperatures. Also, the proportion of C5-C12 alkenes decreased dramatically with the increasing pyrolysis temperature, indicating the enhanced aromatization of light olefins at higher pyrolysis temperatures. Several reports [35,36] have also certified that higher temperatures are in favor of the formation of aromatic compounds because of the triggering of Diels-Alder reaction. As a result, a steady increase in the proportion of mono-aromatics was observed at higher temperatures. Short chain paraffin makes no influence on the aromatization, however, as pyrolysis temperature increased, more paraffins were converted to non-condensable gas fraction. Thus, lower proportion of C5-C12 alkanes were observed at higher pyrolysis temperatures. It should be noted that C12+ alkanes showed an upward trend in selectivity when the pyrolysis temperature increased from 350 to 450 °C, which could be attributed to the increasing production of in-situ hydrogen induced by more aromatic production at higher pyrolysis temperatures, improving the hydrogenation of alkenes for alkanes formation in the presence of MgO [34]. Besides, the non-polar hydrocarbons (straight-chain alkanes) were more difficult to be cracked than those polar hydrocarbons (alkenes). As mentioned before, more aromatics were produced at higher pyrolysis temperatures, resulting in more hydrogenation of olefins. Thus, the selectivity of C12+ alkanes increased as pyrolysis temperature increased from 350 to 450 °C. However, the cracking of C12+ alkanes also occurred simultaneously at higher pyrolysis temperatures. When the pyrolysis temperature increased from 450 °C to 550 °C, the effect of cracking reaction outweighed that of hydrogenation, leading to lower proportion of C12+ alkanes. Low selectivity of polyaromatics (0–6.3%) was also observed in the pyrolysis temperature range of 350–550 °C, which further certified that the presence of MgO promoted the production of mono-aromatics but hindered the formation of poly-aromatics. The gradual increase in the proportion of poly-aromatics in higher temperatures was attributed to the conversion of light olefins produced in higher temperatures.

3.2.3. Effect of catalytic reaction temperature The chemical selectivity as a response of catalytic reaction temperature is shown in Fig. 3(c). A very small amount of C12+ alkenes together with high proportion of C5-C12 alkenes were observed at 350 °C of catalytic reaction temperature, implying a significant cracking reaction over MgO occurred at even low catalytic reaction temperature. In addition, higher catalytic reaction temperatures facilitated the cracking reactions of long chain hydrocarbons, resulting in the reduction of C12+ alkenes selectivity until complete

Chemical selectivity (%)

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Mono-aromatics Alkenes C5-C12 Alkanes C5-C12 Alkenes C12+ Alkanes C12+ Poly-aromatics

100 90 80 70 60 50 40 30 20 10 0 No catalyst 1/15

1/10

1/5

1/3

Catalyst to LDPE ratio

Chemical selectivity (%)

(a) Mono-aromatics Alkenes C5-C12 Alkanes C5-C12 Alkenes C12+ Alkanes C12+ Poly-aromatics

100 90 80 70 60 50 40 30 20 10 0 350

400

450

500

550

o

Pyrolysis temperature ( C)

Chemical selectivity (%)

(b) Mono-aromatics Alkenes C5-C12 Alkanes C5-C12 Alkenes C12+ Alkanes C12+ Poly-aromatics

100 90 80 70 60 50 40 30 20 10 0 350

400

450

500

550

Catalytic reaction temperature (°C)

(c) Fig. 3. Chemical selectivity of liquid products as a response of (a) catalyst to LDPE ratio at 500 °C of pyrolysis temperature and 450 °C of catalytic reaction temperature, (b) pyrolysis temperature at 1/5 of catalyst to LDPE ratio and 450 °C of catalytic reaction temperature, and (c) catalytic reaction temperature at 1/5 of catalyst to LDPE ratio and 500 °C of pyrolysis temperature.

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appropriate catalytic reaction temperature of MgO is necessary for the production of C12+ alkanes. Interestingly, as the catalytic reaction temperature increased from 500 to 550 °C, the selectivity of C5-C12 alkenes decreased slightly. This was because the lower and lower proportion of C12+ alkanes made less contribution to the production of C5-C12 alkenes. On the other hand, a further enhanced pyrolysis could occur as the catalytic reaction temperature was higher than pyrolysis temperature, resulting in more cracking of C5-C12 alkenes into intermediates for the formation of aromatics at higher temperatures. Also, the more severity of cracking reactions at higher catalytic reaction temperatures favored the decomposition of C5-C12 alkanes, leading to the slight decreasing of C5-C12 alkanes production. Although MgO presented marginal effect in the selectivity of poly-aromatics at various catalytic reaction temperatures, it should be noted that higher temperatures are conducive to poly-aromatics formation, which is attributed to the conversion of light olefins. From Fig. 3, the gasoline fraction from ex-situ catalytic process included mono-aromatics and C5-C12 aliphatic hydrocarbons, which ranged from 15.2% to 50.3%, and from 30.2% to 68.7%, respectively, depending on the reaction conditions. The total percentage of gasoline fraction in the upgraded pyrolysis oil were calculated and ranged from 79.5% (37.3% mono-aromatics and 42.2% C5-C12 aliphatics) to 96.0% (39.7% mono-aromatics and 56.3% C5C12 aliphatics) under various conditions, which were much higher than that from non-catalytic pyrolysis (10.5% mono-aromatics and 30.2% C5-C12 aliphatics), revealing the high catalytic effect of MgO on the conversion of LDPE to gasoline-range hydrocarbons. The MgO induced ex-situ catalytic process significantly improved the quality of liquid product in terms of gasoline selectivity. However, it should be noted that the decreasing liquid yield induced by exsitu catalytic process was also a barrier to the application of the pyrolytic oil. In the study, the gasoline yield based on LDPE mass obtained from various ex-situ catalytic processes under 500 °C of pyrolysis temperatures were calculated and ranged from 21.6 wt. % to 36.3 wt.%, which was higher than 18.8 wt.% from noncatalytic process. The oil quality was also much higher than that from other studies. Sriningsih et al. [37] investigated the conversion from LDPE waste to fuel over zeolite supported catalysts and found that Co-Mo/Z resulted in the highest selectivity of gasoline (71.49%) because of its strong acidity among the catalysts. However, the synthetic catalyst was costly and required extra preparation techniques. Also, the metal supported catalysts were much easier to be deactivated by coke. Zhang et al. [22] studied catalytic pyrolysis of LDPE over ZSM-5 and got 74.73–88.49% of gasolinerange hydrocarbons in the oil product. The composition of the gasoline-range hydrocarbons was mainly mono-aromatics, which was different from the result obtained in the present work. This was due to the aromatic-shape-selectivity of ZSM-5 [38]. 3.3. Gas composition

conversion at 400 °C. Higher catalytic reaction temperatures also favored hydrogen transfer reactions [22], enhancing the aromatization of light olefins. As a result, a downward trend in C5-C12 alkenes selectivity while an upward trend was observed in monoaromatics selectivity from 350 to 450 °C. Besides, an increasing catalytic reaction temperature (350–450 °C) improved the selectivity of C12+ alkanes because of hydrogenation over MgO by consumption of in-situ hydrogen, which was formed increasingly at higher catalytic reaction temperatures. However, when the catalytic reaction temperature increased from 450 to 500 °C, the selectivity of C5-C12 alkenes increased while that of C12+ alkanes decreased, which could be explained by more severity of cracking of long chain paraffin at higher catalytic reaction temperature, converting more C12+ alkanes to C5-C12 alkenes. This also implies that

Fig. 4 shows the gas composition under different reaction conditions. The pyrolysis of LDPE mainly produced gaseous compounds containing hydrogen and C1-C4 hydrocarbons. In this study, hydrogen and C1-C3 hydrocarbons (methane, ethylene, ethane, propylene, and propane) were detected with micro GC. The difference in the gaseous fraction noted as C4+ hydrocarbons. The effect of catalyst to LDPE ratio is illustrated in Fig. 4(a). It was found that the increasing addition of catalyst lowered the amount of hydrogen, methane, and ethylene while improved significantly the amount of C4+ hydrocarbons, implying the addition of catalyst favored the production of hydrocarbons with higher carbon number. This may be ascribed to the secondary decomposition of long-chain hydrocarbons during passing through the catalyst bed. Thus, a large amount of gaseous products with relatively high

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No catalyst 1/15 1/10 1/5 1/3

Volume percentage (vol.%)

35 30 25 20 15 10 5 0

ene thane ylene opane gen ane p E dro Meth Ethyl Pr Hy Pro

C4+

(a)

Volume percetage (vol.%)

40 35 30

P-350 °C P-400 °C P-450 °C P-500 °C P-550 °C

25 20 15 10 5 0 en ne ne ne ne ne rog etha thyle Etha ropyle Propa M E Hyd P

C4+

(b)

Volume percentage (vol.%)

40 35 30

C-350 °C C-400 °C C-450 °C C-500 °C C-550 °C

from 1/5 to 1/3. This is because the residence time of hydrocarbons in the catalyst bed increased with the continued addition of catalyst, facilitating the further cracking of C4+ hydrocarbons as a result of the increasing percentage of hydrogen, methane, and ethylene. Overall, the production and the cracking of gaseous hydrocarbons occurred simultaneously, which could be the reason for the change in the percentage of ethane, propylene, and propane. The gas composition under different pyrolysis temperature is presented in Fig. 4(b). The percentage of C4+ hydrocarbons was observed to be decreased significantly with an increase in pyrolysis temperature from 350 to 450 °C because higher pyrolysis temperature favored the chain scission of C4+ hydrocarbons, producing more methane and ethylene. However, the percentage of C4+ hydrocarbons increased significantly as the pyrolysis temperature increased from 450 to 550 °C. As shown in Fig. 2(b), almost all of the solid residue was converted to gaseous product with the increasing pyrolysis temperature from 450 to 550 °C. A large amount of C4+ hydrocarbons were produced before converted to smaller molecules, resulting in the increasing percentage of C4+ hydrocarbons in the gaseous product. As the pyrolysis temperature increased from 350 to 400 °C, the production of ethane and propylene increased firstly and then because of the chain scission of C4+ hydrocarbons. However, more severe chain scission occurred at higher pyrolysis temperatures (over 400 °C), resulting in a decreasing production of ethane and propylene. The effect of catalytic reaction temperature on the gas composition was also studied. As shown in Fig. 4(c), the percentage of C4+ hydrocarbons decreased significantly with the increasing catalytic reaction temperature because of the secondary pyrolysis enhanced by higher catalytic reaction temperatures. As a result, other compounds in the gaseous product presented an upward trend. The improved aromatization induced by higher catalytic reaction temperature may be also another reason for the increasing percentage of hydrogen. 3.4. Plausible mechanism for catalytic upgrading of LDPE derived oil

25 20 15 10 5 0

en ne ne ne ne ne rog etha thyle Etha ropyle Propa M E Hyd P

C4+

(c) Fig. 4. Gas composition as a response of (a) catalyst to LDPE ratio at 500 °C of pyrolysis temperature and 450 °C of catalytic reaction temperature, (b) pyrolysis temperature at 1/5 of catalyst to LDPE ratio and 450 °C of catalytic reaction temperature, and (c) catalytic reaction temperature at 1/5 of catalyst to LDPE ratio and 500 °C of pyrolysis temperature. Note: P-350 °C represents 350 °C of pyrolysis temperature while C-350 °C represents 350 °C of catalytic reaction temperature. The same as the other expressions.

molecular weight were produced before further cracked to smaller molecules while the percentage of the existed hydrogen, methane and ethylene reduced accordingly. Besides, the decreasing in the percentage of hydrogen may be attributed to the hydrogenation of olefins with the consumption of hydrogen over MgO. As mentioned before, the aromatization by consuming olefins increased with the increasing ratio of catalyst to LDPE. This also gave rise to the reduction of ethylene, which is one of the main reactants for aromatization. However, the percentage of C4+ hydrocarbons decreased dramatically when the catalyst to LDPE ratio increased

It was observed that the aromatics and gasoline hydrocarbons were dominant in the organic liquid from ex-situ catalytic upgrading of LDPE pyrolysis vapors over MgO. A plausible mechanism for the LDPE conversion during the catalytic process was proposed in the study. It was generally reported that the thermal cracking of polyethylene was supported by free radical mechanism [39,40]. In the pyrolysis bed, the energy provided by microwave heating resulted in the breakage of CAC bond in LDPE. Because LDPE has plenty of branching, two types of free radicals were produced during the cracking of LDPE. One was radicals with long-chain hydrocarbon induced by random scission while the other was radical fragments with low-molecular weight due to branched chain scission [22]. Consequently, the free radicals were converted to alkanes and alkenes through hydrogen transfer reaction (Fig. 5(a)) [41]. The volatiles from microwave-assisted pyrolysis were upgraded in the catalyst bed and the relevant reactions over MgO occurred. The catalytic cracking of hydrocarbons proceeds via the formation of either carbocations or carbanions [42]. It was suggested that the mechanism for the conversion of polymer into small molecules in the presence of MgO includes hydrogen abstraction, b-scission, and termination [21]. When the microwave pyrolysis volatiles passed through the catalyst bed, a hydrogen proton attached to a backbone carbon of long-chain hydrocarbon could be abstracted by the base catalyst, resulting in the formation of carbanion (Fig. 5(b)). The newly formed anions were unstable and b-scission was improved with the assistance of MgO, producing a large amount of alkenes with shorter carbon chain. The b-scission was the main factor that led to the production of short-chain alkenes, and the further formation of aromatics through Diels-Alder reaction. As a result, lots of gasoline-range

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439

Fig. 5. Mechanism for ex-situ catalytic microwave-assisted pyrolysis of LDPE over MgO.

hydrocarbons were produced. This was consistent with the previous study [21], in which MgO was used for polystyrene conversion and resulted in the degradation of polystyrene because of MgO-

induced b-scission. However, the continued b-scission of long carbanion chain may be terminated by capturing the protons from the surface of MgO as a result of the formation of long-chain alkanes

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Fig. 5 (continued)

(Fig. 5(c)). It should be noted that the free radical mechanism also proceeded in catalytic cracking of polymer. It is supported by Kumar et al. [43], who proposed that free radicals are firstly formed during the catalytic cracking of polymer. Besides the formation of anions, the intermediate radicals are also generated during the catalytic process. However, these radicals can also be terminated by recombination as indicated in Fig. 5(d), resulting in the increase in the formation of long-chain alkanes. Besides, lots of aromatics were produced in the catalytic process. As shown in Fig. 5(e), the free radicals could be further cracked into smaller fragments carrying radicals at both ends of chain hydrocarbon. The fragment radicals may undergo cyclization and dehydrogenation to form aromatics [44]. Besides, the carbanions formed through MgOinduced hydrogen abstraction could also undergo cyclization to form aromatics. On the other hand, Diels-Alder reaction of light olefins produced from the degradation of long chain hydrocarbons was another important pathway for the production of aromatics [22,45].

process. Hydrogen, C1-C3 olefins and paraffins were the main constitutes of gas product and varied with different reaction conditions. A plausible mechanism related to free radicals and carbanions was proposed for the LDPE conversion. Acknowledgements The present work is supported in part by the International Science and Technology Cooperation Program of China (2014DFA61040), Program of Natural Science Foundation of China (21466022), Key Project of Jiangxi Provincial Department of Science and Technology (20161BBF60057; GCXZ2014-124), the Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative-Citizen Commission on Minnesota Resources (LCCMR), Xcel Energy, and University of Minnesota Center for Biorefining. References

4. Conclusions Volatiles obtained from microwave-assisted pyrolysis were further ex-situ catalytic upgraded with MgO and effects of catalyst to reactant ratios, pyrolysis temperature, and catalytic reaction temperature were investigated. Compared with non-catalytic pyrolysis (46.3 wt.%), ex-situ catalytic upgrading with MgO resulted in lower yield of liquid, ranging from 24.2 to 38.5 wt.% under varied conditions. Higher catalyst to reactant ratios (0–1/10), pyrolysis temperatures (350–550 °C), and catalytic reaction temperatures (350– 550 °C) favored the production of gas product. The solid residue yield was mainly affected by pyrolysis temperature and low than 7.1 wt.% when the pyrolysis temperature was over 500 °C. The yield of coke deposited on MgO was negligible, varying from 0.14 to 1.94 wt.% based on LDPE mass. In terms of chemical composition of liquid product, more catalyst loading together with higher pyrolysis temperature and catalytic reaction temperature favored the conversion of alkenes to mono-aromatics. Besides, the addition of MgO promoted the hydrogenation of alkenes for alkanes formation as well as catalytic conversion of diesel fraction (C12+ hydrocarbons) to gasoline fraction, which accounted for 79.5–96.0% of liquid oil under varied conditions of the ex-situ catalytic upgrading

[1] Kunwar B, Cheng H, Chandrashekaran SR, Sharma BK. Plastics to fuel: a review. Renew Sust Energ Rev 2016;54:421–8. http://dx.doi.org/10.1016/j. rser.2015.10.015. [2] Sharuddin SDA, Abnisa F, Daud WMAW, Aroua MK. A review on pyrolysis of plastic wastes. Energ Convers Manage 2016;115:308–26. http://dx.doi.org/ 10.1016/j.enconman.2016.02.037. [3] Sørum L, Grønli M, Hustad JE. Pyrolysis characteristics and kinetics of municipal solid wastes. Fuel 2001;80:1217–27. http://dx.doi.org/10.1016/j. resconrec.2006.12.002. [4] Stelmachowski M. Thermal conversion of waste polyolefins to the mixture of hydrocarbons in the reactor with molten metal bed. Energ Convers Manage 2010;51:2016–24. http://dx.doi.org/10.1016/j.enconman.2010.02.035. [5] Obeid F, Zeaiter J, Ala’a H, Bouhadir K. Thermo-catalytic pyrolysis of waste polyethylene bottles in a packed bed reactor with different bed materials and catalysts. Energ Convers Manage 2014;85:1–6. http://dx.doi.org/10.1016/j. enconman.2014.05.075. [6] Bridgwater AV. Review of fast pyrolysis of biomass and product upgrading. Biomass Bioenerg 2012;38:68–94. http://dx.doi.org/10.1016/j. biombioe.2011.01.048. [7] Russell AD, Antreou EI, Lam SS, Ludlow-Palafox C, Chase HA. Microwaveassisted pyrolysis of HDPE using an activated carbon bed. RSC Adv 2012;2:6756–60. http://dx.doi.org/10.1039/C2RA20859H. [8] Fan L, Chen P, Zhang Y, Liu S, Liu Y, Wang Y, et al. Fast microwave-assisted catalytic co-pyrolysis of lignin and low-density polyethylene with HZSM-5 and MgO for improved bio-oil yield and quality. Bioresour Technol 2017;225:199–205. http://dx.doi.org/10.1016/j.biortech.2016.11.072. [9] Li T, Remón J, Shuttleworth P, Jiang Z, Fan J, Clark J, et al. Controllable production of liquid and solid biofuels by doping-free, microwave-assisted,

L. Fan et al. / Energy Conversion and Management 149 (2017) 432–441

[10]

[11]

[12]

[13]

[14]

[15]

[16]

[17]

[18]

[19]

[20]

[21]

[22]

[23]

[24]

[25]

[26]

pressurised pyrolysis of hemicellulose. Energ Convers Manage 2017;144:104–13. http://dx.doi.org/10.1016/j.enconman.2017.04.055. San Miguel G, Serrano DP, Aguado J. Valorization of waste agricultural polyethylene film by sequential pyrolysis and catalytic reforming. Ind Eng Chem Res 2009;48:8697–703. http://dx.doi.org/10.1021/ie900776w. López A, De Marco I, Caballero B, Adrados A, Laresgoiti M. Deactivation and regeneration of ZSM-5 zeolite in catalytic pyrolysis of plastic wastes. Waste Manage 2011;31:1852–8. http://dx.doi.org/10.1016/j.wasman.2011.04.004. Elordi G, Olazar M, Lopez G, Artetxe M, Bilbao J. Product yields and compositions in the continuous pyrolysis of high-density polyethylene in a conical spouted bed reactor. Ind Eng Chem Res 2011;50:6650–9. http://dx.doi. org/10.1021/ie200186m. Gamliel DP, Du S, Bollas GM, Valla JA. Investigation of in situ and ex situ catalytic pyrolysis of miscanthus  giganteus using a PyGC–MS microsystem and comparison with a bench-scale spouted-bed reactor. Bioresour Technol 2015;191:187–96. http://dx.doi.org/10.1016/j.biortech.2015.04.129. Luo G, Resende FLP. In-situ and ex-situ upgrading of pyrolysis vapors from beetle-killed trees. Fuel 2016;166:367–75. http://dx.doi.org/10.1016/ j.fuel.2015.10.126. Wenwu L, Changwei H, Yu Y, Liangfang Z, Dongmei T. Effect of the interference instant of zeolite HY catalyst on the pyrolysis of pubescens. Chinese J Chem Eng 2010;18:351–4. http://dx.doi.org/10.1016/S1004-9541(08)60364-X. Liu W, Hu C, Yang Y, Tong D, Li G, Zhu L. Influence of ZSM-5 zeolite on the pyrolytic intermediates from the co-pyrolysis of pubescens and LDPE. Energ Convers Manage 2010;51:1025–32. http://dx.doi.org/10.1016/j. enconman.2009.12.005. Liu WW, Hu CW, Yang Y, Tong DM, Zhu LF, Zhang RN, et al. Study on the effect of metal types in (Me)-Al-MCM-41 on the mesoporous structure and catalytic behavior during the vapor-catalyzed co-pyrolysis of pubescens and LDPE. Appl Catal B-Enviro 2013;129:202–13. http://dx.doi.org/10.1016/j. apcatb.2012.09.002. Liu W, Hu C, Tong D, Yang Y, Li G, Zhu L, et al. Catalytic effect of KF-846 on the reforming of the primary intermediates from the co-pyrolysis of pubescens and LDPE. Energ Convers Manage 2014;88:565–72. http://dx.doi.org/10.1016/ j.enconman.2014.08.028. Uemichi Y, Nakamura J, Itoh T, Sugioka M, Garforth AA, Dwyer J. Conversion of polyethylene into gasoline-range fuels by two-stage catalytic degradation using silica alumina and HZSM-5 zeolite. Ind Eng Chem Res 1999;38:385–90. http://dx.doi.org/10.1021/ie980341+. Xue Y, Johnston P, Bai X. Effect of catalyst contact mode and gas atmosphere during catalytic pyrolysis of waste plastics. Energ Convers Manage 2017;142:441–51. http://dx.doi.org/10.1016/j.enconman.2017.03.071. Woo OS, Ayala N, Broadbelt LJ. Mechanistic interpretation of base-catalyzed depolymerization of polystyrene. Catal Today 2000;55:161–71. http://dx.doi. org/10.1016/S0920-5861(99)00235-7. Zhang X, Lei H, Yadavalli G, Zhu L, Wei Y, Liu Y. Gasoline-range hydrocarbons produced from microwave-induced pyrolysis of low-density polyethylene over ZSM-5. Fuel 2015;144:33–42. http://dx.doi.org/10.1016/ j.fuel.2014.12.013. Shah J, Jan MR, Mabood F, Jabeen F. Catalytic pyrolysis of LDPE leads to valuable resource recovery and reduction of waste problems. Energ Convers Manage 2010;51:2791–801. http://dx.doi.org/10.1016/j. enconman.2010.06.016. Stefanidis S, Karakoulia S, Kalogiannis K, Iliopoulou E, Delimitis A, Yiannoulakis H, et al. Natural magnesium oxide (MgO) catalysts: a costeffective sustainable alternative to acid zeolites for the in situ upgrading of biomass fast pyrolysis oil. Appl Catal B-Environ 2016;196:155–73. http://dx. doi.org/10.1016/j.apcatb.2016.05.031. Pütün E. Catalytic pyrolysis of biomass: effects of pyrolysis temperature, sweeping gas flow rate and MgO catalyst. Energy 2010;35:2761–6. http://dx. doi.org/10.1016/j.energy.2010.02.024. Aboulkas A, El Bouadili A. Thermal degradation behaviors of polyethylene and polypropylene. Part I: pyrolysis kinetics and mechanisms. Energ Convers Manage 2010;51:1363–9. http://dx.doi.org/10.1016/j.enconman.2009.12.017.

441

[27] Marcilla A, Beltrán M, Navarro R. Evolution of products during the degradation of polyethylene in a batch reactor. J Anal Appl Pyrol 2009;86:14–21. http://dx. doi.org/10.1016/j.jaap.2009.03.004. [28] Li X, Xiao W, He G, Zheng W, Yu N, Tan M. Pore size and surface area control of MgO nanostructures using a surfactant-templated hydrothermal process: high adsorption capability to azo dyes. Colloid Surface A 2012;408:79–86. http:// dx.doi.org/10.1016/j.colsurfa.2012.05.034. [29] Artetxe M, Lopez G, Amutio M, Elordi G, Bilbao J, Olazar M. Light olefins from HDPE cracking in a two-step thermal and catalytic process. Chem Eng J 2012;207:27–34. http://dx.doi.org/10.1016/j.cej.2012.06.105. [30] Önal E, Uzun BB, Pütün AE. Bio-oil production via co-pyrolysis of almond shell as biomass and high density polyethylene. Energ Convers Manage 2014;78:704–10. http://dx.doi.org/10.1016/j.enconman.2013.11.022. [31] Aguado J, Serrano D, San Miguel G, Castro M, Madrid S. Feedstock recycling of polyethylene in a two-step thermo-catalytic reaction system. J Anal Appl Pyrol 2007;79:415–23. http://dx.doi.org/10.1016/j.jaap.2006.11.008. [32] Williams PT, Williams EA. Fluidised bed pyrolysis of low density polyethylene to produce petrochemical feedstock. J Anal Appl Pyrol 1999;51:107–26. http:// dx.doi.org/10.1016/S0165-2370(99)00011-X. [33] Cypres R. Aromatic hydrocarbons formation during coal pyrolysis. Fuel Process Technol 1987;15:1–15. http://dx.doi.org/10.1016/0378-3820(87)90030-0. [34] Hattori H, Tanaka Y, Tanabe K. A novel catalytic property of magnesium oxide for hydrogenation of 1,3-butadiene. J Am Chem Soc 1976;98:4652–3. http:// dx.doi.org/10.1021/ja00431a056. [35] del Remedio Hernández M, Gómez A, García ÁN, Agulló J, Marcilla A. Effect of the temperature in the nature and extension of the primary and secondary reactions in the thermal and HZSM-5 catalytic pyrolysis of HDPE. Appl Catal AGen 2007;317:183–94. http://dx.doi.org/10.1016/j.apcata.2006.10.017. [36] López G, Olazar M, Artetxe M, Amutio M, Elordi G, Bilbao J. Steam activation of pyrolytic tyre char at different temperatures. J Anal Appl Pyrol 2009;85:539–43. http://dx.doi.org/10.1016/j.jaap.2008.11.002. [37] Sriningsih W, Saerodji MG, Trisunaryanti W, Triyono, Armunanto R, Falah II. Fuel production from LDPE plastic waste over natural zeolite supported Ni, NiMo, Co and Co-Mo metals. Procedia Environ Sci 2014;20:215–24. http://dx.doi. org/10.1016/j.proenv.2014.03.028. [38] Liu S, Xie Q, Zhang B, Cheng Y, Liu Y, Chen P, et al. Fast microwave-assisted catalytic co-pyrolysis of corn stover and scum for bio-oil production with CaO and HZSM-5 as the catalyst. Bioresour Technol 2016;204:164–70. http://dx. doi.org/10.1016/j.biortech.2015.12.085. [39] Rahimi N, Karimzadeh R. Catalytic cracking of hydrocarbons over modified ZSM-5 zeolites to produce light olefins: a review. Appl Catal A-Gen 2011;398:1–17. http://dx.doi.org/10.1016/j.apcata.2011.03.009. [40] Artetxe M, Lopez G, Elordi G, Amutio M, Bilbao J, Olazar M. Production of light olefins from polyethylene in a two-step process: pyrolysis in a conical spouted bed and downstream high-temperature thermal cracking. Ind Eng Chem Res 2012;51:13915–23. http://dx.doi.org/10.1021/ie300178e. [41] Serrano D, Aguado J, Escola J, Rodríguez J, San Miguel G. An investigation into the catalytic cracking of LDPE using Py–GC/MS. J Anal Appl Pyrol 2005;74:370–8. http://dx.doi.org/10.1016/j.jaap.2004.11.026. [42] Wong S, Ngadi N, Abdullah T, Inuwa I. Conversion of low density polyethylene (LDPE) over ZSM-5 zeolite to liquid fuel. Fuel 2017;192:71–82. http://dx.doi. org/10.1016/j.fuel.2016.12.008. [43] Kumar S, Panda AK, Singh R. A review on tertiary recycling of high-density polyethylene to fuel. Resour Conserv Recy 2011;55:893–910. http://dx.doi. org/10.1016/j.resconrec.2011.05.005. [44] Undri A, Rosi L, Frediani M, Frediani P. Efficient disposal of waste polyolefins through microwave assisted pyrolysis. Fuel 2014;116:662–71. http://dx.doi. org/10.1016/j.fuel.2013.08.037. [45] Koo JK, Kim SW, Seo YH. Characterization of aromatic hydrocarbon formation from pyrolysis of polyethylene-polystyrene mixtures. Resour Conserv Recy 1991;5:365–82. http://dx.doi.org/10.1016/0921-3449(91)90013-E.