Catalytic pyrolysis of wood polymer composites over hierarchical mesoporous zeolites

Catalytic pyrolysis of wood polymer composites over hierarchical mesoporous zeolites

Energy Conversion and Management 195 (2019) 727–737 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www...

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Energy Conversion and Management 195 (2019) 727–737

Contents lists available at ScienceDirect

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

Catalytic pyrolysis of wood polymer composites over hierarchical mesoporous zeolites

T

Young-Min Kima,1, Jaehun Jeongb,1, Sumin Ryub,1, Hyung Won Leeb,1, Jung Sul Jungb, ⁎ Muhammad Zain Siddiquic, Sang-Chul Jungd, Jong-Ki Jeone, Jungho Jaef, Young-Kwon Parkb, a

Department of Environmental Engineering, Daegu University, Gyeongsan 38453, Republic of Korea School of Environmental Engineering, University of Seoul, Seoul 02504, Republic of Korea c Department of Environmental Sciences and Biotechnology, Hallym University, Chuncheon 24252, Republic of Korea d Department of Environmental Engineering, Sunchon National University, Suncheon 57922, Republic of Korea e Department of Chemical Engineering, Kongju National University, Cheonan 31080, Republic of Korea f School of Chemical and Biomolecular Engineering, Pusan National University, Busan 46241, Republic of Korea b

A R T I C LE I N FO

A B S T R A C T

Keywords: Hierarchical zeolites Catalytic pyrolysis Wood polymer composites Tandem micro reactor Aromatics

Hierarchical zeolites have superior catalytic properties over purely microporous zeolites, leading to the enhanced diffusivity of molecules and strong acidity of the catalyst. In this study, hierarchical desilicated mesoporous ZSM-5 and Beta were prepared by the desilication of commercial microporous zeolites and applied to the catalytic pyrolysis of wood polymer composites. Hierarchical desilicated mesoporous ZSM-5 and Beta showed the typical X-ray diffraction patterns of microporous ZSM-5 and Beta with higher mesoporosity compared to the parent materials. The activity of the desilicated zeolites for the catalytic pyrolysis of wood polymer composites was evaluated using a thermogravimetric analysis and tandem micro reactor-gas chromatography/mass spectrometry. Among the catalysts tested, the lowest decomposition temperatures of wood polymer composites were observed using hierarchical desilicated mesoporous Beta followed by hierarchical desilicated mesoporous ZSM-5 and ZSM-5. This trend correlated well with the mesoporosity of the catalysts. The formation efficiency of hierarchical desilicated mesoporous ZSM-5 was highest followed by microporous ZSM-5, hierarchical desilicated mesoporous Beta, and Beta, indicating that in addition to mesoporosity, the shape selectivity induced by microporosity and strong acidity are important for the aromatization of pyrolysis vapors. In addition, the aromatic formation efficiency of the catalysts differed according to the properties of wood polymer composites. Compared to wood polymer composite 2, wood polymer composite 1 produced a larger quantity of aromatics during catalytic pyrolysis over all the catalysts at 500 °C owing to its higher polyethylene content. Both wood polymer composites exhibited a similar aromatic formation efficiency during catalytic pyrolysis at 600 °C because the diffusion hindering effect of polypropylene molecules to the catalyst pores was lower at the higher temperature.

1. Introduction The depletion of fossil fuels and climate change due to their intensive use has prompted worldwide attention on the use of renewable energy [1]. Among the various renewable energies available, biomass is an important source for carbon-based fuel or chemical feedstock owing

to its abundance and cost effectiveness. Thermal conversion technologies, such as torrefaction, pyrolysis, and gasification, can be applied to produce various kinds of products and the technical suitability of each process is differentiated according to the target product [2]. Pyrolysis is a suitable thermal conversion technology of biomass to bio-oil and has been applied to many kinds of biomass, such as microalgae [3], woody

Abbreviations: H/Ceff, hydrogen to carbon effective; Tmax, maximum decomposition temperature; CaCO3, calcium carbonate; NaOH, sodium hydroxide; NH4NO3, ammonium nitrate; SiO2/Al2O3, silica to alumina; N2, nitrogen; PE, polyethylene; PP, polypropylene; ZSM-5, zeolite socony mobil-5; HDM, hierarchical desilicated mesoporous; WPCs, wood polymer composites; TGA, thermogravimetic analysis; TMR-GC/MS, tandem micro reactor-gas chromatography/mass spectrometry; DTG, differential thermogravimetry; BTEXs, benzene, toluene, ethylbenzene, and xylene; OMAHs, other mono aromatics; OPAHs, other poly aromatics; NH3-TPD, ammonia-temperature programmed desorption; XRD, X-ray diffraction ⁎ Corresponding author. E-mail address: [email protected] (Y.-K. Park). 1 Co-first authors. https://doi.org/10.1016/j.enconman.2019.05.034

0196-8904/ © 2019 Elsevier Ltd. All rights reserved.

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increase the cracking efficiency of synthetic polymers during the one step pyrolysis of WPCs. Recently, catalytic pyrolysis was suggested as the more efficient method for the production of value-added products, such as aromatic hydrocarbons, from WPCs. Lin et al. examined the catalytic pyrolysis of wood-plastics blended polymers over metal oxides [16] and ZSM-5 [19] to assess the potential of WPCs for the production of aromatics. Although their WPC sample was not a commercialized one containing CaCO3 and other additives, they demonstrated the synergistic production of aromatic hydrocarbons from the catalytic pyrolysis of WPCs due to the co-presence of wood and plastics in their sample. Recently, the catalytic pyrolysis of the commercialized WPCs containing CaCO3 and other additives was investigated using mesoporous MCM-41 [20] and various types of microporous zeolites as catalysts [15]. Although the catalysts could be applied to the catalytic pyrolysis of WPCs, the limited mass transfer of large molecular reactants to the micropores of zeolites [15] and the low acidity of the catalysts need to be overcome to increase the catalytic pyrolysis reaction efficiency. One possible way is the use of hierarchical desilicated mesoporous (HDM) zeolites because the mesopore size and acidity of zeolites can be increased by desilication, which can increase the catalytic reaction efficiency [21]. The decrease in overall reaction efficiency by the presence of large molecular plastics, e.g., PP, during the catalytic co-pyrolysis of biomass and plastics over ZSM-5 [22] can also be overcome using HDM zeolites. This suggests that HDM zeolites can be applied as efficient catalysts to the catalytic pyrolysis of organic wastes consisted with various kinds of polymeric components, such as WPCs. For the practical application of the catalyst on the commercial scale, the lifetime of the catalyst is also an important issue. However, the presence of meropores in HDM zeolites can facilitate the formation of coke, deactivating the catalyst [23]. Therefore, the sequential long- term reaction test of the HDM zeolites has to be performed to evaluate the overall lifetime of the HDM zeolites compared to patent zeolites. To the best of the authors’ knowledge, this work applied HDM zeolites to the catalytic pyrolysis of WPCs for the first time to overcome the low reaction efficiency of microporous zeolites. For this purpose, two types of HDM zeolites, HDMBeta and HDMZSM-5, were prepared by the desilication of commercial zeolites. Two types of WPCs with different compositional ratios of PE and PP, WPC1 (PE 18%; PP 9%) and WPC2 (PE 9%; PP 18%), were used to examine the effects of the properties of WPCs on the reaction performance. Thermogravimetric analysis (TGA) and tandem micro reactor-gas chromatography/mass spectrometry (TMR-GC/MS) were employed to compare the catalytic performance of the different catalysts in terms of the diffusion efficiency and aromatic formation efficiency. To compare the lifetime of HDMZSM-5 with that of ZSM-5, the consecutive catalytic pyrolysis of WPC was also evaluated by feeding a WPC sample seven times without changing the catalyst.

biomass [4], and fruit peel [5]. The actual use of biomass pyrolysis oil is limited by its low stability, resulting in a large content of oxygenates in bio-oil [6]. To increase the value of the biomass pyrolysis process, the catalytic pyrolysis of biomass is being investigated widely for various types of biomass. Many researchers have attempted to produce aromatic hydrocarbons during the catalytic pyrolysis of biomass because of their high stability and heating value. Zeolite Socony Mobil-5 (ZSM-5) is the most efficient catalyst for the production of aromatic hydrocarbons which can be used directly as a fuel additive because of the high heating value [7], chemical feedstock for polymer synthesis [8], or solvent after additional distillation [9]. On the other hand, the short lifetime of ZSM5 due to rapid catalyst deactivation caused by severe coke deposition and low aromatic yields limit its use in the catalytic pyrolysis of biomass [10]. Accordingly, several researchers have attempted to increase the aromatic production efficiency and reduce coke formation by modifying the properties of the catalytic pyrolysis feedstock through biomass torrefaction [11] and by the additional use of cost-effective catalysts, such as red mud and natural zeolites [12]. Recently, the use of mesoporous zeolite with a hierarchical structure was suggested to be an efficient way of increasing the yields of aromatics because the presence of a mesoporous structure can provide enhanced mass transfer and cracking of large molecular reaction intermediates, which do not diffuse easily to the micropores of ZSM-5 [5]. The catalytic co-pyrolysis of biomass and hydrogen-sufficient plastics is a simple and effective way of increasing the production efficiency of aromatics with reduced coke formation. The effective role of cofeeding plastics as a hydrogen donor to biomass [13] and an interaction of pyrolysis intermediates of biomass and plastics [14] can lead to the synergistic formation of aromatics. Wood polymer composites (WPCs) are materials made from wood, synthetic polymers, such as polyethylene (PE) and polypropylene (PP), calcium carbonate (CaCO3), and other additives [15]. Owing to their higher mechanical durability than wood, WPCs are used widely as feedstock for the manufacture of outdoor benches and floors, and an estimated 5.6 million metric tons were produced in 2019 [16]. Although the production amount is increasing, the recycling ratio is relatively low because of the decreased material properties caused by macromolecular decomposition during its use. Therefore, a desirable treatment, such as pyrolysis, needs to be developed to decrease the environmental burden and recover the renewable energy. Non-catalytic and catalytic pyrolysis of WPCs were investigated by many researchers as summarized in Table 1. Schwarzinger et al. [17] suggested to employ different temperatures, i.e., 550 °C for the 1st step and 700 °C for the 2nd step, for the isolation of pyrolysis products from biomass and synthetic polymers in WPCs because of their different chemical properties. Meanwhile, Sun et al. [18] suggested a one-step high-temperature pyrolysis at 625 °C to obtain the larger amount of light hydrocarbons because the radicals formed from wood decomposition can Table 1 Previous literatures for the non-catalytic and catalytic pyrolysis of WPCs. No

WPC components

Catalyst

Temperature

Refs.

1

No Catalyst

1st step: 550 °C 2nd step: 700 °C

[17]

2

Spruce + PP Beech + PP Commercial WPC Polar wood + HDPE

No catalyst

[18]

3

Poplar wood + PP

[16]

4 5

Poplar wood + HDPE Poplar wood + PP Sawdust + PE + PP + CaCO3 + Other additives

ZnO, CaO, Fe2O3, and MgO HZSM-5

475, 550, and 625 °C 500, 550, 600, and 650 °C 550 °C 525 °C

[20]

6

Sawdust + PE + PP + CaCO3 + Other additives

600 °C

[15]

Si-MCM-41, H-V-MCM-41 HY, HBeta, HZSM-5

728

[19]

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measured by nitrogen (N2) adsorption–desorption analysis. The acidity of the catalyst was analyzed by ammonia-temperature programmed desorption (NH3-TPD). X-ray diffraction (XRD) was also performed to determine the structural properties of the catalysts.

Table 2 Compositional properties of different WPCs (Unit: wt. %). Sample

Wood powder

PE

PP

CaCO3

Othersa

WPC1 WPC2

58 58

18 9

9 18

8.5 8.5

6.5 6.5

a

2.3. Thermogravimetric analysis

Lubricant, coupling agent, UV blocking agent, pigment

The weight loss of the WPCs during the non-isothermal TGA was measured by increasing the temperature of the WPCs (4 mg) from ambient to 800 °C at a heating rate of 20 °C/min under N2 at a flow rate of 50 mL/min. To determine the change in WPCs decomposition temperature by catalyst use, the catalytic TGA of the WPCs was also performed by additional adding and mixing of 4 mg of catalyst (catalyst to sample: 1 to 1) and heated under the same conditions.

2. Materials and methods Selected materials and designed experiments to know the catalytic pyrolysis behavior of WPCs are summarized in Fig. S1 (Supplementary Material). 2.1. Wood polymer composites

2.4. Pyrolysis and catalytic pyrolysis using micro reactor

Two types of commercialized WPCs with different PE and PP contents were used as the feeding materials for thermal and catalytic pyrolysis (Table 2). Prior to the experiments, both WPCs were cryomilled, sieved, and dried at 80 °C for 4 h. Both WPCs contain wood powder, PE, PP, and CaCO3 as the main materials together with other additives. On the other hand, WPCs have different amounts of PE and PP, which can result in different thermal behaviors and product distributions during their thermal and catalytic pyrolysis over different catalysts. Although both WPCs have different PE and PP compositional ratios, the proximate and ultimate analysis results of both biopolymers were similar because PE and PP have similar C and H contents (Table 3). Owing to the presence of PE and PP in the WPCs, the calculated hydrogen to carbon effective (H/Ceff) ratios of both WPCs were much higher than that of wood powder [15], suggesting the synergistic production of aromatics from the WPCs.

TMR-GC/MS, as shown in Fig. S2, consisted of vertical two furnaces with a GC/MS, and was used for qualitative and quantitative analysis of the products of the thermal and catalytic pyrolysis of WPCs [24]. For the non-catalytic pyrolysis of WPCs, 1 mg of WPCs was free-fallen into the 1st furnace of TMR preheated at 500 or 600 °C to provide the flash pyrolysis condition. The product vapor was transferred to the separation column (UA-5, 30 m length × 0.25 mm inner diameter × 0.25 μm film thickness) via the 2nd furnace of TMR (320 °C), and separated by the GC oven temperature program (from 40 °C (3 min hold) to 320 °C (5 min) at 20 °C/min), after a GC inlet flow splitting (320 °C, split ratio 200: 1) and cryo-focusing using liquid N2 (−195 °C, 3 min). After the separation of the products in the column, each product was analyzed by MS in scan mode (scan range: 10–550, scan speed 5.14 scans/sec). The peaks on the chromatogram were identified using NIST MS searching software and integrated to compare the amounts of each product using the MS peak area. For the catalytic pyrolysis of the WPCs, 1 mg of the catalyst was blended with 1 mg of WPCs (catalyst to sample ratio of 1:1) and analyzed using the same procedure. The effect of catalyst to sample ratio was also tested by using different amounts of HDMZSM-5 (0.25, 0.5, 1.0, 2.0, and 4.0 mg) mixed with the same amount of WPC1 (1 mg) as the sample for TMR-GC/MS analysis. To estimate the catalyst lifetime of HDMZSM-5 during its continuous use, ex-situ sequential catalytic pyrolysis of WPC1 was additionally carried out by loading 3 mg of ZSM-5 or HDMZSM-5 to the 2nd furnace of the TMR reactor and feeding 1 mg of WPC1 to the 1st furnace. During the test, 1 mg of WPC1 was fed continuously up to seven times without changing the catalyst.

2.2. Catalysts Two types of parent microporous catalysts, ZSM-5(30) and HBeta (38), which have different structures and silica to alumina (SiO2/Al2O3) ratios (30 and 38), were purchased from Zeolyst. The two catalysts were desilicated using sodium hydroxide (NaOH) solution (0.2 M, 60 °C, and 30 min), centrifuged (5,000 rpm, more than 10 times), and dried (110 °C, overnight). The dried catalysts were ion-exchanged with ammonium nitrate (NH4NO3) (0.1 M, 12 h) as the ammonium form at ambient temperature, and called HDMZSM-5 and HDMBeta. The parent and HDM catalysts were calcined (550 °C, 3 h) prior to use in catalytic pyrolysis. The pore size and surface area of all catalysts used in this study were

3. Results and discussion To optimize the catalyst and reaction conditions for aromatics production via the catalytic pyrolysis of WPCs, catalyst property analysis, non-isothermal catalytic TG analysis, and isothermal catalytic TMR-GC/MS analysis were systematically conducted together with investigation of the effect of catalyst to sample ratio and the long-term stability through a continuous feed injection.

Table 3 Physico-chemical characteristics of WPC and woody biomass (Unit: wt. %). Sample Proximate analysis

Ultimate analysisa (wt. %)

H/Ceffc

WPC1

WPC2

Water Volatiles Fixed carbon Ash Sum

2.0 86.4 4.7 6.9 100.0

2.3 83.7 6.5 7.5 100.0

C H Ob N S Sum

55.6 8.0 35.9 0.5 0 100 0.722

54.0 8.1 37.5 0.4 0.0 100 0.726

3.1. Hierarchical desilicated mesoporous zeolites Fig. 1 presents XRD patterns of the HDM zeolites prepared by the desilication of parent zeolites. After desilication, HDMBeta and HDMZSM-5 still had the typical XRD peaks of BEA [25] and MFI structure [26], respectively, indicating that the crystal structures of the zeolites were maintained after the desilication treatment. The N2 sorption results of the catalysts (Table 4) indicated that the pore diameters (Dp) and pore volumes of the desilicated zeolites, HDMZSM-5 and HDMBeta, were larger than those of parent ZSM-5 and Beta. Both HDMZSM-5 and HDMBeta had large meso surface areas and mesopore volumes, indicating mesopore formation caused by their desilication

C, carbon; H, hydrogen; O, oxygen; N, nitrogen; S, sulfur. a On a dry basis. b By difference. c H/Ceff = (H-2O-3N-2S)/C. 729

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Fig. 2. NH3-TPD curves of the catalysts.

weight loss regions, zones A (200–400 °C), B (401–520 °C), and C (660–750 °C), which can be assigned to the decomposition of wood powder, plastics (PE and PP), and CaCO3, respectively [15]. Although two WPCs had different PP and PE contents, their DTG curves were similar. Table 5 lists the maximum decomposition temperature (Tmax) for each peak on the DTG curves. The Tmax values of wood (Zone A) and CaCO3 (Zone C) on the DTG curves of both WPCs were 371 ± 1 °C and 722 ± 1 °C regardless of the catalyst used (Table 5), suggesting a minor catalytic effect on the decomposition temperature during TGA. The Tmax values of the PE/PP mixture (Zone B) on the DTG curves of both WPCs were lowered using the catalysts. Compared to Beta and ZSM-5, HDMBeta and HDMZSM-5 showed the lower Tmax values of the PE/PP mixture on the DTG curves of both WPCs, highlighting the importance of the mass transfer rate for WPC decomposition. Lee et al. [21] reported that the overall conversion of reactant molecules can be increased by applying the desilicated ZSM-5, particularly when a large molecular reactant was used because of the increased mesoporosity induced by the desilication treatment. Although the decomposition temperatures of the PE/PP mixture of both WPCs were decreased further using catalysts with a larger pore size, the extent of the decrease in decomposition temperatures of each polymer varied according to the catalyst used. The Tmax values of the PE/PP mixture of WPC1 with a smaller PP content over ZSM-5 and HDMZSM-5, 469 °C and 465 °C, respectively, were lower than that of WPC2, which has a higher PP content (472 °C and 469 °C, respectively). The Tmax values of the PE/PP mixture of WPC1 over HDMBeta (450 °C) and Beta (466 °C) were higher than those of WPC2 (446 °C over HDMBeta and 458 °C over Beta). This suggests that the decomposition temperature of WPC was differentiated by the catalyst properties and the PP content in each WPC. The higher decomposition temperatures of the PE/PP mixture in WPC2 over HDMZSM-5 and ZSM-5 compared to those of WPC1 suggest that the diffusion of PP to the catalyst pores is limited severely over ZSM-5 and moderately over HDMZSM-5. The lower decomposition temperature of the PE/PP mixture of WPC2 over HDMBeta and Beta suggests that the diffusion limitation of PP molecules to the micro pore can be minimized using HDMBeta and Beta due to the larger micropore size (0.7 nm) of HDMBeta and Beta than that (0.5 nm) of HDMZSM-5 and ZSM-5. When PP diffusion to the catalyst pore is not limited, PP can be decomposed at lower temperatures than PE during catalytic pyrolysis because the tertiary carbon of PP has lower stability than the secondary carbon, which is the main backbone of PE molecules. Kim et al. [22] reported that PP molecules do not diffuse as easily into the micropores of ZSM-5 than PE molecules because of their larger kinetic molecular diameter than PE. Although HDMZSM-5 has higher and stronger acidity than HDMBeta (Fig. 2), it had higher decomposition temperatures of the PE/PP mixture for both

Fig. 1. XRD peak patterns of HDMBeta and HDMZSM-5. Table 4 The N2 sorption results of catalysts. Catalyst

SBETa (m2/ g)

Smicroa (m2/g)

Smesoa,c (m2/g)

Vtotalb (cm3/ g)

Vmicrob (cm3/g)

Vmesob,c (cm3/g)

Dp (nm)

HDMBeta Beta HDMZSM-5 ZSM-5

713 735 465 396

656

57

0.40

0.18

302

163

0.58 0.50 0.33 0.25

0.13

0.20

3.9 0.7 2.1 0.5

a,b c

Evaluated by t-plot method. Vmeso = Vtotal − Vmicro.

treatment. HDMZSM-5 had a larger mesopore to micropore volume ratio (1.5) than HDMBeta (0.5), suggesting the HDMZSM-5 had a large amount of mesopores, even after its desilication treatment. Fig. 2 shows the NH3-TPD curves of the catalysts. Both HDMZSM-5 and HDMBeta contained a larger amount of weak acid sites, which appeared at approximately 200 °C, than ZSM-5 and Beta. In contrast, HDMZSM-5 and ZSM-5 contained larger amounts of strong acid sites, which appeared at approximately 420 °C, than HDMBeta and Beta. The total acid site amount of HDMBeta and HDMZSM-5 were larger than those of Beta and ZSM-5, suggesting that the number of acid sites of parent zeolites was increased by desilication.

3.2. Thermogravimetric analysis Fig. 3 presents the differential thermogravimetry (DTG) curves of the thermal and catalytic TGA of the WPCs over different catalysts at 20 °C/min. The non-catalytic DTG curves of both WPCs showed three 730

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Fig. 3. Thermal and catalytic DTG curves of (a) WPC1 and (b) WPC2 over different catalysts.

WPCs, confirming the relationship between the pore size and plastic decomposition temperature during the catalytic TGA of WPCs.

Table 5 Maximum decomposition temperatures of each decomposition zone on the thermal and catalytic TG analysis of WPCs over the catalysts.

Catalyst No Catalyst ZSM-5 HDMZSM-5 Beta HDMBeta

Tmax values of WPC1 (oC)

Tmax values of WPC2 (oC)

A 371 372 372 372 372

A 371 371 371 372 371

B 485 469 465 466 450

C 721 722 722 722 722

B 483 472 469 458 446

3.3. Pyrolysis and catalytic pyrolysis using micro reactor

C 722 722 722 722 722

Fig. 4 shows the TMR-GC/MS chromatograms of the non-catalytic pyrolysis of WPCs at 500 °C. Owing to the compositional properties of WPCs, the non-catalytic pyrolysis of both WPCs had the typical pyrolyzates of wood, PE, and PP [27]. Oxygen-containing pyrolyzates, including acetic acid, furfural, levoglucosan, and phenols, such as guaiacol, engenol, and syringols, observed on the non-catalytic pyrogram of the WPCs can be classified as the typical pyrolyzates of the lignocellulosic components of wood biomass, consisting of hemicellulose, cellulose, and lignin [28]. Isoalkadienes (Cx,iae) and isoalkadienes (Cx,iade), such as C9,iae, C15,iae C21,iae C22,iade C25,iade, C28,iade, C31,iade, C34,iade, C37,iade, C40,iade, and C43,iade, are typical 731

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Fig. 4. TMR-GC/MS chromatograms for the non-catalytic pyrolysis of (a) WPC1 and (b) WPC2 at 500 °C.

typical non-catalytic pyrolyzates, i.e., oxygenates from wood, heavy branched hydrocarbons from PP, and heavy linear hydrocarbons from PE. The catalytic pyrolysis of WPCs produced larger amounts of aromatic hydrocarbons than their non-catalytic pyrolysis. These results suggest that all catalysts used in this study can provide a higher deoxygenation and cracking efficiency during the catalytic pyrolysis of WPCs. During the catalytic pyrolysis of WPCs over zeolites, wood is decomposed initially to oxygenates, then diffuses into the zeolite pores, where they are further converted to deoxygenated reaction intermediates, such as light hydrocarbons, furans, ketones, and phenols, over the catalysts via dehydration, dealkylation, dealkoxylation, decarbonylation, and decarboxylation reactions over acid sites located mainly inside the zeolite pores. These light hydrocarbons form hydrocarbon pool species trapped inside the small pore of the zeolites and act as a catalytic center to produce aromatic hydrocarbons [32]. The phenols derived from the lignin fraction of wood can be converted to aromatic hydrocarbons and coke via a phenolic pool mechanism. Therefore, the overall aromatic hydrocarbon formation efficiency from biomass via catalytic pyrolysis is controlled by the mass transfer rate to the catalyst active sites, shape selectivity induced by microporosity, and the acidity of catalysts [33]. In contrast to wood pyrolysis, the catalytic

pyrolyzates of isotactic PP, and normal triplets, alkadienes (Cx,ade), alkenes (Cx,ae), alkanes (Cx,aa), with wide carbon range were generated from the flash pyrolysis of PE [29]. Although the peak intensities for the pyrolyzates of woody biomass were similar in the chromatograms of WPC1 and WPC2, those of PE and PP were different due to the different PE:PP compositional ratios in each WPC. Compared to WPC1, WPC2 has higher intensities for isoalkenes and isoalkadienes and lower peak intensities for normal alkenes and alkanes because of the higher PP content in WPC2. Both WPCs produced many products over a wide range of carbon numbers. In particular, the large amount of oxygen-containing pyrolyzates of biomass and heavy hydrocarbons can cause thermal instability [30] and clogging of the oil condensing line of the pyrolysis plant [31]. Although clogging of the oil condensing line by heavy wax produced from PE and PP can be alleviated by the presence of oxygenates in the pyrolysis oil of WPCs, the additional use of a catalyst is a desirable approach for the production of value-added stable oil from WPCs. Fig. 5 presents the TMR-GC/MS chromatograms for the catalytic pyrolysis of WPCs over different catalysts at 500 °C. Compared to the non-catalytic pyrolysis of WPCs shown in Fig. 4, the catalytic pyrolysis of WPCs over the catalysts led to a decrease in peak intensity for the 732

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Fig. 5. TMR-GC/MS chromatograms obtained from the catalytic pyrolysis of WPCs over the catalysts at 500 °C: (a) WPC1 + HDMBeta; (b) WPC1 + HDMZSM-5; (c) WPC1 + ZSM-5; (d) WPC2 + HDMBeta; (e) WPC2 + HDMZSM-5; (f) WPC2 + ZSM-5. 733

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Fig. 5. (continued)

734

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pyrolysis of PE and PP over zeolites produces mainly light olefins via intramolecular hydrogen transfer and beta-scission of the C–C bond [34]. These hydrocarbons can be converted to aromatic hydrocarbons via oligomerization, cyclization, and dehydrogenation reactions or can react with furans, the catalytic pyrolysis intermediates of biomass, through a Diels-Alder reaction leading to the synergistic formation of aromatic hydrocarbons during the catalytic co-pyrolysis. The synergistic aromatics formation efficiency is a function of the properties of the feeding materials and catalysts. Kim et al. [28] examined the effects of different co-feeding plastics, i.e., PE and PP, on the catalytic pyrolysis of cellulose. They reported that the co-conversion efficiency of different plastics with biomass for aromatic production is correlated with the diffusion efficiency of the plastic molecules and the pore size of the catalyst. For example, the diffusion of plastic molecules with a larger kinetic diameter was severely hindered over small pore ZSM-5, leading to a lower rate of aromatic formation. Similar to their results, it is expected that the aromatic formation efficiency varies significantly according to the composition of WPC (WPC1 versus WPC2) and the catalyst used (ZSM-5 versus HDMZSM-5). In the TMR-GC/MS chromatograms, the catalytic pyrolysis of both WPCs over HDMBeta with the largest pore diameter showed the lower peak intensities for the typical pyrolyzates of lignin, such as vinylguaiacol, syringol, eugenol, vinylsyringol, and propenylsiringol, followed by HDMZSM-5 and ZSM-5. This suggests that the diffusion of large molecular pyrolyzates of the WPCs to the catalyst is limited more severely using a catalyst with a smaller pore size, as shown in Fig. 5 and Table 4. The higher elimination efficiency of large molecular lignin pyrolyzates over HDMZSM-5 than that over ZSM-5 suggests that the desilication of ZSM-5 is effective in increasing the mass transfer rate of large molecular intermediates produced during the catalytic pyrolysis of WPCs. Fig. 6 shows the MS peak intensities for benzene, toluene, ethylbenzene, and xylene (BTEXs), other mono aromatics (OMAHs), naphthalenes, and other poly aromatics (OPAHs) obtained from catalytic pyrolysis at 500 °C. Although HDMBeta showed the highest elimination efficiency of the large molecular products, it produced the smallest amount of aromatics during the catalytic pyrolysis of both WPCs among the catalysts used in this study (Fig. 6). The relative MS peak intensities for linear and branched hydrocarbon having a similar carbon range to gasoline (C5-C11) on the catalytic pyrogram of WPCs over HDMBeta were much higher than those over HDMZSM-5 and ZSM-5. This suggests that HDMBeta can provide sufficient mass transfer and high cracking efficiency because of its largest pore size (Table 3) and larger number of acid sites than ZSM-5 (Fig. 2). On the other hand, the properties of HDMBeta are insufficient to produce aromatic hydrocarbons compared to HDMZSM-5 and ZSM-5 because of its lower acid strength (Fig. 2) and insufficient shape selectivity for aromatization. Meanwhile, HDMBeta produced a larger amount of aromatic hydrocarbons (WPC1: 437 × 106 counts, WPC2: 403 × 106 counts) than Beta (WPC1: 426 × 106 counts, WPC2: 365 × 106 counts), because of its higher mesoporosity (Table 4) and acidity (Fig. 2). HDMZSM-5 produced the much larger amount of aromatic hydrocarbons (WPC1: 835 × 106 counts, WPC2:734 × 106 counts) than ZSM-5 (WPC1: 540 × 106 counts, WPC2: 488 × 106 counts) because of its higher mesoporosity (Table 4) and acidity (Fig. 2) [32]. The co-presence of mesopores and micropores in HDMZSM-5 (Table 4) can provide an enhanced mass transfer rate through mesopore-opening and enhanced shape selectivity for aromatic formation through the microporous MFI structure, which has a similar size to the benzene ring. The increased acidity of HDMZSM-5 and HDMBeta by the desilication treatment of ZSM-5 and Beta is also an important factor in increasing the production of aromatic hydrocarbons during the catalytic pyrolysis of WPCs [35]. Between the two WPCs, WPC1 produced larger quantities of aromatics (835 × 106 counts) than WPC2 (734 × 106 counts) during their catalytic pyrolysis over all the catalysts at 500 °C (Fig. 6). This suggests that the formation of aromatics from the catalytic pyrolysis of WPC2 is

Fig. 6. The MS peak intensities for aromatic hydrocarbons obtained from the non-catalytic and catalytic pyrolysis of (a) WPC1 and (b) WPC2 at 500 °C.

hindered more severely than WPC1 because of its higher PP content. On the other hand, the quantities of aromatics obtained from the catalytic pyrolysis of both WPCs over all the catalysts were increased by increasing the catalytic pyrolysis temperature from 500 to 600 °C, as shown in Fig. 7 and Fig. S3. In addition, WPC2 produced a larger quantity of aromatic hydrocarbons (901 × 106 counts) than WPC1 (889 × 106 counts) during the catalytic pyrolysis of WPCs over all catalysts at 600 °C. The molecular size of the reaction intermediates decreased with increasing reaction temperature due to the increased cracking efficiency caused by higher temperature [36]. In addition, the pore entrance size of the catalysts can be increased at elevated temperatures [37]. In the case of WPC2, which has a larger PP content than WPC1, the molecular size of the reaction intermediates, which cannot diffuse easily to the pores of the catalyst at 500 °C, can be diffused effectively to the pore entrance enlarged at 600 °C. This can lead to the additional formation of aromatic hydrocarbons during the catalytic pyrolysis of WPC2 over all the catalysts. Therefore, the reaction temperature is an important parameter for the catalytic pyrolysis of WPCs and can be adjusted to control the overall aromatics formation efficiency over the catalysts. Fig. 8 shows the effects of the catalyst to sample ratio on the catalytic pyrolysis of WPC1 over HDMZSM-5. The amount of aromatic hydrocarbons produced increased with increasing catalyst to sample ratio due to the higher acidity provided by the larger amount of catalyst. Fig. 9 shows the MS intensities for aromatic hydrocarbons obtained from the sequential catalytic pyrolysis of WPC 1 over HDMZSM-5 and ZSM-5 at 600 °C. HDMZSM-5 produced a larger quantity of aromatic hydrocarbons than ZSM-5 during the seven sequential catalytic 735

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Fig. 9. The absolute peak area of aromatic hydrocarbons obtained during the sequential ex-situ catalytic pyrolysis of WPC1 over HDMZSM-5 and ZSM-5 at 600 °C.

commercial micropore zeolites and applied to the catalytic pyrolysis of two WPCs having different PE/PP contents. WPC2 having the higher PP content than WPC1 produced the smaller amount of aromatic hydrocarbons during the catalytic pyrolysis at 500 °C because of the limited diffusion of PP to the catalyst pore. The formation efficiency of aromatic hydrocarbons from WPC2 was considerably increased by increasing the catalytic pyrolysis reaction temperature from 500 to 600 °C due to the increased secondary cracking of reactant molecules and the enlargement of catalyst pore size at elevated temperatures. Among the tested catalysts here, i.e., Beta, ZSM-5, HDMBeta, HDMZSM-5, HDMBeta exhibited the lowest decomposition temperatures of plastics (PE and PP) in WPCs and the highest elimination efficiency of large molecular pyrolyzates of wood, PE, and PP in WPCs due to its largest pore size. However, in terms of aromatic formation efficiency, HDMZSM-5 showed the best activity due to its strongest acidity, the presence of mesoporosity, and appropriate shape selectivity for aromatics formation. HDMZSM-5 also demonstrated the longer catalyst lifetime for the aromatic formation during the catalytic pyrolysis of WPC compared to microporous ZSM-5 due to the presence of mesoporosity. For the actual commercialization of this process, a more detailed study on the deactivation and regeneration of the catalyst will be needed using a larger scale reactor. The use of CaCO3 in WPC as a catalyst will also be considered in a future study to improve the process efficiency.

Fig. 7. The MS peak intensities for aromatic hydrocarbons obtained from the non-catalytic and catalytic pyrolysis of (a) WPC1 and (b) WPC2 at 600 °C.

Declaration of Competing Interest None. Acknowledgement This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2018R1A2B2001121). Fig. 8. The effect of catalyst to sample ratio (4:1–1:4) on the formation of aromatic hydrocarbons via the catalytic pyrolysis of WPC1 over HDMZSM-5 at 600 °C.

Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.enconman.2019.05.034.

pyrolysis reactions of WPC1. This suggests that HDMZSM-5 has a longer lifetime than ZSM-5 during the continuous catalytic pyrolysis process of WPC.

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4. Conclusions HDMBeta and HDMZSM-5 were prepared by the desilication of 736

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