Producing aromatic enriched oil from mixed plastics using activated

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Producing aromatic enriched oil from mixed plastics using activated biochar as catalyst Kai Sun, Qunxing Huang, Mujahid Ali, Yong Chi, and Jianhua Yan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03710 • Publication Date (Web): 20 Feb 2018 Downloaded from http://pubs.acs.org on February 24, 2018

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Energy & Fuels

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Producing aromatic enriched oil from mixed plastics using activated biochar as catalyst

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Kai Sun, Qunxing Huang*, Mujahid Ali, Yong Chi, Jianhua Yan

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State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, 310027,

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People's Republic of China

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KEYWORDS: Plastics, Oil, Aromatics, Activated biochar, Catalyst

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ABSTRACT: Producing aromatic enriched oil from mixed plastics through catalytic pyrolysis

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has been experimentally studied. The effect of biochar catalysts has been investigated and the

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possible dominating catalytic mechanisms of biochars activated with different chemical agents

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have been discussed. Results indicated that when waste plastics were pyrolyzed with raw

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biochar, the alkene fraction in the oil product increased to 54.9%. When biochar was activated by

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ZnCl2, KOH and H3PO4, the oil product showed high selectively towards aromatics, and the

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proportions of aromatics were up to 47.6%, 44.7%, and 66.0%, respectively. Benzene, 1,1'-

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(1,3-propanediyl) bis- was the main composition in aromatics, the proportion of which could be

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up to 25.1% when KOH activated biochar was used. The enrichment part of aromatics was

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mainly bicyclic aromatics and C15-C16 compositions, the maximum proportions of which could

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reach 92.5% and 28.1% by KOH activated biochar. High surface functional group (e.g. C=O) 1 ACS Paragon Plus Environment

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content and low metal content of KOH activated biochar promoted hydrogen transfer reaction of

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alkenes to alkanes and aromatics. While aromatization process promoted by Lewis acid sites and

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Brønsted acid sites on ZnCl2 and H3PO4 activated biochar respectively significantly increased

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aromatic yield.

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1. Introduction

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As the production and consumption of plastic products grow rapidly, the waste plastics are

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leading to increasingly serious pollution of environment and waste of energy due to their low

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recovery rate and non-biodegradability1-4. Compared with the complicated and costly mechanical

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recycling method 1-3 or polluted and energy-wasting incineration method 5, converting waste

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plastics into valuable chemical products 6, 7 through catalytic pyrolysis is a more promising

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technology suitable for the volume reduction and resource recovery of dirty and mixed waste

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plastics 8, 9.

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Commonly used catalysts for plastics pyrolysis process include conventional zeolites (e.g.

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HZSM-5, HY), meso-structure catalysts (e.g. MCM-41), FCC catalysts, noble metal catalysts

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and some other catalysts. Conventional zeolites usually show high selectivity to aromatic

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compounds and have a low rate of deactivation. C Santellan et al. 6 found that adding HUSY and

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HZSM-5 during pyrolysis of real waste plastics could increase the yields of styrene (17.5%) and

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ethylbenzene (15%). However, conventional zeolites have single pore size distribution, resulting 2 ACS Paragon Plus Environment

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in poor adaptability to feedstocks. Besides, zeolites like HZSM-5 would decrease the oil yield

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significantly 10. Spend FCC catalysts are low-cost and could increase oil yield. GDL Puente et al.

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11

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of waste polyethylene catalyzed by commercial FCC catalysts. But FCC catalysts are less active

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than zeolites 12 and show low selectivity to aromatics 13. Noble metal catalysts such as Pt and Ga

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are usually loaded on chars or zeolites, which show high selectivity to aromatics. Y Uemichi et

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al. 14-16 obtained a maximum yield of aromatics up to 46.2 wt.% and 30 wt.% in pyrolysis oil of

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PE and PP by using 3 wt.% Pt-loaded activated carbon catalyst. However, high cost, easy-coking

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and deactivation make it difficult to scale up the process.

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found that oil with gasoline component content as high as 80% could be produced by pyrolysis

In recent years, carbonaceous materials like activated biochar are widely used in catalytic

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fields, such as hydrogen production 17, 18, methane reforming 19, 20, and tar reforming 21, etc., due

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to their low production cost, well developed and controllable pore structures, and convenient

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surface modification 22, etc. Some researches23, 24 have studied the catalytic effect of activated

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biochar on the pyrolysis of plastics in the comparative study with molecular sieves and silica gel,

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and found that activated biochar could decrease sulfur content and increase aromatic yield in oil.

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However, to the best of our knowledge, few researchers have studied the differences among

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different carbonaceous materials, or analyzed the possible catalytic mechanisms.

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In this study, raw biochar and different chemical activated biochars were used as catalysts for

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the pyrolysis of mixed waste plastics to produce aromatic enriched oil. Chemical activation could 3 ACS Paragon Plus Environment

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improve the specific surface area of catalysts and support chemical agent on the surface

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simultaneously. Thus, a large number of active sites can be introduced on char surface in single

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modification process, which make chemical activated biochar a promising catalyst. The effects

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of these catalysts on oil yield and aromatic compositions were comparatively investigated and

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the possible dominating catalytic mechanisms of different activation were also discussed.

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2. Materials and experimental methods

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2.1 Raw Materials

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In this work, commonly used plastics, such as polyethylene (PE), polypropylene (PP), and

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polystyrene (PS) were employed. Samples were purchased from Guanbu Electromechanical

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Technology Co., China with particle size less than 0.18µm. Before use, plastics were dried at 105

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°C for 24 h and mixed with 59 wt.% of PE, 22 wt.% of PP, and 19 wt.% of PS according to their

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typical relative fraction in municipal solid waste 25.

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2.2 Experimental preparation of the catalysts

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In this paper, both inactivated raw biochar (BC) and activated biochar (ACs) were used as

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catalysts for plastics pyrolysis. Raw biochar was prepared by pyrolyzing wood chips (Lishui,

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Zhejiang, China) from room temperature to 500 °C at a heating rate of 10 °C/min and kept at 500

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°C for 40 min. Activated biochars were prepared respectively through KOH, ZnCl2 and H3PO4

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activation methods (abbreviated as AC-KOH, AC-ZnCl2, and AC-H3PO4, respectively).

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To obtain AC-ZnCl2 catalyst, 20 g of wood chips were impregnated with 300 ml aqueous

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solution of ZnCl2 at an impregnation ratio (Defined as the weight ratio of activating agent to

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feedstock) of 1:1 for 24 h. Then the mixtures were dried, calcinated under N2 atmosphere at 600

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°C for 90min, washed with 0.1 M HCl solution and then distilled water until the pH reached

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neutral. For KOH activation, 20 g of raw biochar were impregnated with 300 ml aqueous

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solution of KOH at an impregnation ratio of 4:1 for 24 h. Then the mixtures were dried,

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calcinated under N2 atmosphere at 800 °C for 90 min, washed with 0.1 M HCl solution and then

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distilled water until the pH reached neutral. While for H3PO4 activation, 20 g of wood chips were

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impregnated with 300 ml aqueous solution of H3PO4 an impregnation ratio of 2:1 for 24 h. Then

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the mixtures were dried, calcinated under N2 atmosphere at 500 °C for 90 min, washed with 0.1

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M NaOH solution and then distilled water until the pH reached neutral. Detailed preparation

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parameters of ACs are summarized in Table 2. After preparation, all the catalysts were milled

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and sieved to a particle size of less than 0.4 mm.

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ACs were washed with chemical agents to remove active matters. 5 g AC-KOH and AC-ZnCl2

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were respectively added into 40 ml 5 M HCl solution. 5g AC-H3PO4 was added into 40 ml 5 M

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NaOH solution. Then the mixtures were stirred for 24 h, washed with distilled water until the pH


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reached neutral, and dried at 105 °C. Washed samples were abbreviated as WAC-KOH,

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WAC-ZnCl2, and WAC-H3PO4, respectively.

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2.3 Chars characterization

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Ultimate analysis of raw and activated biochar was carried out by a vario MAX cube

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elemental analyzer (Elementar, German). Proximate analysis was analyzed by a 5E-IRSII

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industrial analyzer (Kaiyuan Instrument Co., Ltd, Changsha, China). Surface morphology and

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element distribution were characterized by a SIRON field emission scanning electron

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microscope equipped with EDAX-EDS (FEI, Holland). Specific surface area was measured by

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an AUTOSORB-IQ2-MP automatic specific surface and micropore size analyzer

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(Quantachrome, USA) using multipoint Brunauer-Emmett-Teller (BET) standard method.

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Microporous and mesoporous parameters were calculated by HK and BJH methods, respectively.

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Surface functional groups were recorded by a Nicolet 5700 Fourier transform infrared

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spectroscopy (Thermo Fisher scientific, USA), and a VG ESCALAB MKII X-ray photoelectron

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spectrophotometer (VG, UK). High-resolution spectra were curve-fitted by multiple Gaussian

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function.

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2.4 Experimental setup


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2 g catalyst and 8 g mixed plastic samples were mixed and heated from room temperature to

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500 °C at a heating rate of 20 °C/min and remained for 1.0 h using 0.3 L/min nitrogen flow as

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carrier gas. Pyrolysis oils were considered water-free and mainly collected by condensing device

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and two gas-washing bottles containing dichloromethane. The whole reaction pipe except the

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quartz boat was weighed before and after the test, then it was washed three times with 400 ml

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dichloromethane. All solutions were dried at 40 °C for 24 h and weighted respectively. Oil yield

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was calculated by adding up the mass of the oil both in pipe and collectors. Weight change of the

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char was calculated from the weight increment of the quartz boat after the reaction.

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Condensable oil product was recovered in dichloromethane and then analyzed by Agilent

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6890-5973 gas chromatography-mass spectrometry (GC-MS). For each experimental run, the

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oven stayed at 80 °C for 2 min, then was heated up to 250 °C at 20°C/min and kept for 10min.

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All tests were repeated and the results presented in tables were the average values.

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3. Results and discussion

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3.1 Characterization of catalysts

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Results of ultimate analysis and proximate analysis are shown in Table 2. Raw BC had a

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relatively high carbon and hydrogen content. The increase ratio of C/H in ACs indicated a high

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carbonization degree. High oxygen content, mostly caused by chemical activation process, may

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provide a large number of binding sites on the char surface. 7 ACS Paragon Plus Environment

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SEM and EDS analysis results are presented in Fig. 1. EDS spectra shows the elemental

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distribution on char surface. Minerals in BC came from the feedstock, while minerals in ACs

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both came from the feedstocks and the activating agents. Activating agents not only reshaped the

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physical morphology, but were also loaded on the char surface in specific form, endowing the

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ACs with catalytic capabilities. After activated by AC-ZnCl2 and AC-H3PO4, cylindrical

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channels along the direction of the wood fiber were formed. While after activated by AC-KOH,

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more intensive pore distribution and thinner pore walls can be observed, indicating that KOH

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could promote the formation of pores more efficiently. Acid or alkali washing didn’t change the

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surface morphology of ACs apparently. However, the content of active matters Zn, K, and P was

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respectively reduced by 53.6%, 92.1%, and 66.5% after washing process.

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Surface area and pore properties of the catalysts are shown in Table 3. Among all the ACs,

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AC-KOH had the highest specific surface area and minimum average pore size. The micropore

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ratio was as high as 82.6%, indicating that micropores played a dominant role in the pore

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structure of AC-KOH. AC-ZnCl2 was another typical microporous AC which had similar

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micropore ratio but much lower specific surface area compared with AC-KOH. AC-H3PO4 was a

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kind of mesoporous AC with a high mesopore ratio of 50.8%.

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High resolution scans of C1s and O1s XPS spectra are presented in Fig. 2, respectively.

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Compared with BC, ZnCl2 activation led to an increase in carbon content and a decrease in

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oxygen content on the char surface, while KOH and H3PO4 treatments were opposite. For surface 8 ACS Paragon Plus Environment

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carbon atoms, activation treatment decreased the content of graphitized carbon, and increased the

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content of carbon atoms bonded to oxygen. C=O was the most abundant oxygen-containing

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functional group in all catalysts. Meanwhile, –OH and –COOH were also abundant in AC-KOH

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and AC-H3PO4.

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3.2 Product yields and composition distribution of oils

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Product yields of the gas, oil and weight change of catalysts after pyrolysis are shown in Fig.

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3. Significant differences can be observed in oil yields as well as the weight changes of different

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catalysts. Compared with the direct thermal pyrolysis, the catalysts have increased gas yields and

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reduced oil yields in various degree. High specific surface area could increase heat transmission

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area between the catalyst and the volatiles during thermal pyrolysis 24, meanwhile provide more

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spaces for active sites like AAEM species which could promote the secondary pyrolysis of the

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pyrolysis oil 26, 27, resulting in the production of smaller molecules.

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Alkanes, alkenes and aromatics were major components in pyrolysis oils. Addition of different

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catalysts changed the reaction mechanisms, thus changed the distribution of oil products to

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varying degrees. As showed in Fig. 4, BC showed a pronounced catalytic effect in promoting

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alkene yield (54.9%) and decreasing aromatic yield (16.9%). While chemically activated

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biochars showed obvious catalytic effects in increasing aromatic yield. Catalytic pyrolysis oil of

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AC-ZnCl2 had a high yield of aromatics (47.6%) at the expense of alkenes (24.1%). Catalytic 9 ACS Paragon Plus Environment

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effect of AC-KOH led to a significant decrease in alkene production, while the yield of alkanes

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(43.1%) and aromatics (44.7%) both increased remarkably. Catalytic effect of AC-H3PO4 was

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similar to AC-ZnCl2, which presented the highest selectivity of aromatics (66.0%) and the lowest

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selectivity of alkenes (9.8%). Among all catalysts, AC-H3PO4 possessed the largest potential in

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the enrichment of aromatics.

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The final residues after catalytic pyrolysis was higher than the initial mass of the catalysts,

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except when BC was used. Weight change of BC was mainly weight loss caused by further

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interaction with plastics, which will be discussed detailedly in section 3.4. Weight increases of

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ACs were more obvious compared with BC. The enrichment effects of ACs towards aromatics

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were AC-H3PO4 > AC-ZnCl2 > AC-KOH. Aromatics in oils provided abundant feedstocks for

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dehydrogenation and condensation process, which led to the formation of coke.

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Carbon atoms number against the concentration of the oil compositions is shown in Fig. 5 (a).

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The main compositions were hydrocarbons with carbon atoms between 6 and 26. For most

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catalytic reactions, C15-C16 hydrocarbons were the main compositions in pyrolysis oils.

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Compared with direct thermal pyrolysis oil, catalysis of BC resulted in a more dispersed carbon

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atoms number distribution. This could be due to the poor selectivity caused by the wide variety

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of minerals and surface functional groups in BC. However, catalysis by ACs improved the

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concentration of C15-C16 compositions (mainly branched bicyclic aromatics) and decreased the

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content of C10 compositions (mainly branched monocyclic aromatics), resulting in a narrower 10 ACS Paragon Plus Environment

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production distribution. According to Fig. 5 (b), the selectivities towards C15-C16 aromatics were

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increased 20.8%, 20.0%, and 29.0% by AC-ZnCl2, AC-KOH, and AC-H3PO4, respectively.

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These increasing proportions of aromatics may be both from the decomposition of aromatic

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polymers and condensation of monocyclic aromatics, or from the aromatization of alkanes and

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alkenes, which will be discussed in the following section.

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3.3 Aromatics in oils

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The compositions of aromatics classified by the number of aromatic rings are shown in Fig. 6.

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The aromatics in direct thermal pyrolysis oil were mainly bicyclic aromatics (63.3%) and

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monocyclic aromatics (36.7%). The increasing proportion of monocyclic aromatics (44.0%) and

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the appearance of a relatively high proportion of tricyclic aromatics (12.6%) in catalytic

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pyrolysis oil proved that the presence of BC affected the distribution of aromatics, though the

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catalytic selectivity was not apparent. ACs all showed obvious selectivities towards bicyclic

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aromatics, especially AC-KOH (92.5%). The occurrence of tricyclic aromatics and even

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tetracyclic aromatics indicated the occurrence of condensation reaction during the catalytic

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processes, which was different from direct thermal pyrolysis process.

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In the direct thermal pyrolysis oil, monocyclic aromatics were probably both derived from the

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secondary reaction (e.g. alkylation reaction, transalkylation reaction, and dehydrogenation of the

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side chains) of the PS pyrolysis products (such as A1 and A4), and the dehydrocyclization and 11 ACS Paragon Plus Environment

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aromatization of long-chain alkanes and alkenes (such as A2 and A3). Pyrolysis of PE and PP

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produced a large number of n-alkenes, providing sufficient reactants for both alkylation reaction

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of aromatics and dehydrocyclization process. For BC, AC-ZnCl2, and AC-KOH catalytic

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pyrolysis oils, monocyclic aromatics were composed of alkyl and alkenyl benzenes, which were

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similar to thermal pyrolysis oils. Main monocyclic aromatics in AC-KOH group were alkyl

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benzenes which had a higher saturation degree than the other groups. While high content of

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cyclized productions in AC-H3PO4 group showed that the dehydrocyclization reaction was

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obviously enhanced.

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Bicyclic aromatics were the main compositions of the aromatic components. Most of the

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bicyclic aromatics were multi-phenyl alicyclic hydrocarbons like B11, and a few were fused-ring

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aromatics or biphenyl type aromatics such as B1 and B4. B11 (Benzene,

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1,1'-(1,3-propanediyl)bis-, 64.9%) was the main bicyclic aromatics in direct thermal pyrolysis

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oil, followed by B15 (16.3%), B10 (10.3%), and B5 (8.5%). They were mostly derived from the

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secondary reactions (e.g. alkylation, transalkylation, dehydrogenation, and condensation) of

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monocyclic aromatics. Catalysis of ACs enriched the species of the bicyclic aromatics, especially

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for AC-H3PO4. A high proportion of B8 and the emergence of B1, B12, and B14, etc. in

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AC-H3PO4 group clearly showed the enhancement of dehydrogenation, cyclization, and

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aromatization during secondary reactions.


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In tricyclic aromatics, C1 and C2 were fused-ring aromatics with higher condensation degree

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compared with C3 and C4. Thus, higher proportions of C1 and C2 in AC-H3PO4 group suggested

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AC-H3PO4 had more pronounced effects on condensation reaction. Presence of tetracyclic

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aromatics (3.5%) in oil compositions of AC-H3PO4 group also supported this view.

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In conclusion, catalysis of all ACs enriched the bicyclic aromatics. AC-H3PO4 significantly

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increased the proportions of aromatics (especially bicyclic ones), and its catalytic effect on

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dehydrogenation and condensation was the most obvious. As showed in Fig. 7, AC-KOH,

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AC-ZnCl2, and AC-H3PO4 all showed remarkable selectivities to Benzene,

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1,1'-(1,3-propanediyl)bis- (B11), the proportion of which in could be up to 25.1%, 20.3%, and

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16.2%, respectively. This was beneficial to the production and extraction of aromatics from

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plastic pyrolysis oil.

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3.4 Discussion of reaction mechanism

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It is believed that direct thermal pyrolysis of polyolefins followed the free radical

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mechanism 28, 29, while the dominating mechanism of catalytic pyrolysis depends on the catalyst

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types, which are summarized in Fig. 8.

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BC was the only non-activated carbonaceous material, the preparation temperature of which

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was the same as the pyrolysis temperature of plastics. Since the mass loss of BC was around 3%

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after catalytic reaction (Fig. 3), it can be inferred that the presence of plastics promoted the 13 ACS Paragon Plus Environment

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further pyrolysis of BC. Hydrogen transfer reaction from hydrogen-rich materials (polyolefin

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chains), to hydrogen-deficient materials (BC) occurred during their interaction 30. The

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dehydrogenation process was enhanced by the abstraction of hydrogen from hydrogen enriched

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hydrocarbons like alkanes onto the carbon surface 31, thus the production of alkenes increased

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substantially. The occurrence of unsaturated compositions like cycloalkanes, dienes and even

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cycloalkene, as showed in Fig. 9, had obviously proved the existence of hydrogen transfer and

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cycloaddition. What’s more, the high content of Ca species in inherent minerals may contribute

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to the reduction of aromatic yield 32.

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When WAC-ZnCl2 and WAC-H3PO4 were used as catalysts, the content of aromatics

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decreased remarkably and alkanes became the main components in the oil product. The reason is

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that the concentration of Zn2+ and H3PO4 decreased after washing, and the catalytic effect of acid

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sites on Diels-Alder reaction, hydrogen transfer reaction and dehydrocyclization reaction to

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produce aromatics were reduced. It indicated that the effect of active matters (Zn2+ and H3PO4)

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was dominating the catalytic pyrolysis reaction when AC-ZnCl2 and AC-H3PO4 were used as

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catalysts. On the contrary, when WAC-KOH was used as catalyst, the change of production

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composition was relatively small, which suggested that the catalytic effect of AC-KOH was less

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sensitive to the concentration of the introduced active sites, and biochar properties may play a

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more important role in catalytic reaction.


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Zn species on AC-ZnCl2 could form Lewis acid sites 33, 34, as showed in Fig. 8. The

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presence of Zn species increased the consumption of alkenes, and led to the formation of

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aromatics through Diels-Alder reaction 34, hydrogen transfer reaction 33 and direct

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dehydrocyclization 35. Meanwhile, Lewis acid sites could effectively catalyze the alkylation of

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aromatics, leading to the further consumption of alkenes and formation of a large number of

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aromatics with side chains.

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AC-KOH was a relatively pure AC. Mineral content on carbon surface was very low due to

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alkali treatment and the loss of potassium in the process of pre-washing. It was reported that pure

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carbon support was also effective in promoting hydrogen transfer reaction 31, 36 and cyclization of

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straight-chain intermediate 37. The dehydrogenation step in hydrogen transfer was enhanced by

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the abundant surface functional groups on carbon surface such as carbonyl groups 38, 39, while the

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step of hydrogen release was relatively limited due to the lack of metal sites 36, leaving more

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hydrogen atoms on carbon surface available for the hydrogenation of hydrogen acceptors such as

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alkenes. Therefore, hydrogen transfer reaction of alkenes was enhanced greatly, and the

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proportions of alkanes and aromatics increased significantly. This may also be the main catalytic

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mechanisms of WACs. The slight decrease in aromatic content was probably caused by the

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restrained hydrogen transfer reaction due to the lack of metal site.

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H3PO4 treatment also introduced large number of acid sites and increased the acidity of catalyst surface 40, 41. AC-H3PO4 was abundant in phosphorus and oxygen-containing functional 15 ACS Paragon Plus Environment

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groups, especially acid ones like -P=OOH 42, -COOH and -OH. These functional groups could

271

act as Brønsted acid sites, which had obvious catalytic effect on hydrogen transfer reaction,

272

Diels-Alder reaction, and Friedel-Crafts alkylation 43. Thus, AC-H3PO4 significantly enhanced

273

the consumption of alkenes and alkanes and improved the yield of branched aromatics.

274

Intermediate products of apparent dehydrocyclization and dehydro-condensation in aromatic

275

compositions (mentioned in section 3.3) were a strong proof of dehydrogenation reaction.

276

Page 16 of 38

Production distribution will be affected by not only the pore structure but also catalytic active

277

sites. In this paper, AC-KOH had the highest BET surface area and micropore ratio. As KOH

278

activation didn’t introduce much external active sites on AC, the product distribution was mainly

279

affected by the physical properties like pore structure. While for AC-ZnCl2 and AC-H3PO4, the

280

acid sites introduced played more important roles in changing production distribution. Strong

281

acid sites on the surface of AC-ZnCl2 and AC-H3PO4 promoted the end-chain scission of

282

polymers and favored the yield of gaseous products 44. High density of strong acid sites not only

283

led to coke formation, but also increased gas yield. So, the production distributions of AC-ZnCl2

284

and AC-H3PO4 catalyzed reactions were affected more by the introduced active sites.

285

4. Conclusions


Page 17 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Carbonaceous material, including biochar and chemically activated biochars, had obvious

287

catalytic effects on pyrolysis oils of mixed plastics. From the analysis and comparison among

288

pyrolysis oils and different catalytic pyrolysis oils, the main conclusions were as follows:

289

(1) The catalysis of raw and activated biochars resulted in an increase in the proportion of

290

pyrolysis gas and a slight decrease in the proportion of pyrolysis oil.

291

(2) The catalytic effect of BC greatly increased the proportion of alkenes in the oil (up to

292

54.9%). The selectivity of aromatics was greatly improved by chemically activated biochars at

293

the cost of alkenes. AC-H3PO4 showed the strongest enrichment effect towards aromatics,

294

which could reach 66.0%.

295

(3) C15-C16 compositions (bicyclic aromatics) contributed most to the increase of aromatic

296

fraction. The enrichment effect of AC-H3PO4 was the most obvious, which increased C15-C16

297

compositions by 29%. Benzene, 1,1'- (1,3-propanediyl) bis- was the main composition in oils,

298

the proportion of which in AC-KOH, AC-ZnCl2, and AC-H3PO4 groups could be up to 25.1%,

299

20.3%, and 16.2%.

300

(4) The presence of plastics promoted the further pyrolysis of BC. AC-KOH’s catalytic effect

301

was mainly characterized by promoting hydrogen transfer process, which increased the yield

302

of alkanes and aromatics at the cost of alkenes. ZnCl2 and H3PO4 treatment could form

303

Lewis/Brønsted acid sites on the char surface, which favored dehydrogenation process, 17 ACS Paragon Plus Environment

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Page 18 of 38

304

hydrogen transfer reaction, Diels-Alder reaction, etc., and promoted the conversion of alkenes

305

to aromatics.


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Associated content

307

Supporting Information

308

Main compositions in direct thermal and catalytic pyrolysis oils from GC-MS results (based on

309

area %). (Table S1)

310

Author information

311

Corresponding Author

312

*Telephone: +86-571-87952834. Fax: +86-571-87952438. E-mail: [email protected]

313

Notes

314

The authors declare no competing financial interest.

315

Acknowledgments

316

This work was supported by National Natural Science Foundation of China (Grant No.

317

51621005), the National Key Research and Development Program of China (2016YFE0202000),

318

the Environmental Protection Special Funds for Public Welfare (201509013) and the

319

Fundamental Research Funds for the Central Universities.

320 19 ACS Paragon Plus Environment

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reactor: Potassium carbonate versus calcium hydroxide as a catalyst. Fuel Processing

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Energy & Fuels

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Tables

437

Table 1. Preparation parameters of ACs.

Activation Activating

Impregnation Impregnation

method

ratio a

ACs agent

one-step

ZnCl2

activation

solution

two-step

KOH

activation

solution

one-step

H3PO4

activation

solution

AC-ZnCl2

AC-KOH

AC-H3PO4

438

a

Activation

Activation

temperature

time

(°C)

(min)

time (h)

1:1

24

600°C

90

4:1

24

800°C

90

2:1

24

500°C

90

Defined as the weight ratio of activating agent to feedstock.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

439

Page 28 of 38

Table 2. Physico-chemical properties of catalysts. Catalysts Properties BC

AC-ZnCl2

AC-KOH

AC-H3PO4

Cd

84.0

77.6

78.1

64.5

Hd

3.4

1.1

0.0

2.4

Nd

2.9

1.5

1.2

1.3

Std

0.1

0.2

0.1

0.04

Od (diff.)

4.8

8.7

9.1

20.5

Ultimate analysis (wt.%)

Proximate analysis (wt.%) Md

4.8

11.0

11.5

11.2

Ad

7.5

11.8

8.4

21.9

Vd

14.7

8.8

7.0

23.5

FCd

73.0

68.4

73.1

43.4

Yield b (wt. %)

28.5

45.8

14.4

59.3

440


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441

Energy & Fuels

Table 3. Surface areas and pore properties of different catalysts a. BET

Total

Average

surface

pore

pore

Micropore Sample

volume area 2

442 443 444

volume 3

diameter

Mesopore

Mesopore

volume

ratio c

(cm3/g)

(%)

Micropore (cm3/g)

b

ratio (%)

(m /g)

(cm /g)

(nm)

BC

79.2

0.0587

2.965

0.0323

55.1

0.0157

26.7

AC-ZnCl2

1082.2

0.5859

1.983

0.4834

82.5

0.0662

11.3

AC-KOH

2110.7

1.1290

1.945

0.9327

82.6

0.1086

9.6

AC-H3PO4

600.1

0.5193

3.249

0.2400

46.2

0.2642

50.8

a

All data in Table 3 were calculated with adsorption isotherm. Defined as the ratio of micropore volume to total pore volume. c Defined as the ratio of mesopore volume to total pore volume. b


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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445

Figures

446

Figure 1. SEM and EDS results of (a) BC, (b) AC-ZnCl2, (c) AC-KOH, (d) AC-H3PO4, (e)

447

WAC-ZnCl2, (f) WAC-KOH, (g) WAC-H3PO4

448 449 30 ACS Paragon Plus Environment

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450

Energy & Fuels

Figure 2. High resolution C1s and O1s XPS spectra of different catalysts.

451 452


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453

454 455

Page 32 of 38

Figure 3. Product yields of mixed plastics by direct thermal and catalytic pyrolysis.

(a)

456


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457

Figure 4. Content of alkanes, alkenes, aromatics, and other compounds in direct thermal

458

and catalytic pyrolysis oils.

459 460


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461

Figure 5. Concentration of the compositions in (a) pyrolysis oils, (b) aromatic compounds

462

as a function of carbon atoms number.

463 464 465

(a)

(b)


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Figure 6. Compositions of aromatics in pyrolysis oils classified by number of aromatic

467

rings.

468 469


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470

Figure 7. Hydrocarbons distribution in the gas chromatogram of the direct thermal and

471

catalytic pyrolysis oils.

472 473


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Energy & Fuels

Figure 8. Dominating reaction mechanisms of different catalytic processes.

475 37 ACS Paragon Plus Environment

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Figure 9. Cycloalkanes, dienes, and cycloalkene in BC catalytic pyrolysis oil.

477


Producing aromatic enriched oil from mixed plastics using activated

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