Aromatic Hydrocarbon Production and Catalyst Regeneration in

Oct 10, 2017 - ZSM-5 zeolites were selected as catalysts to promote the aromatization during pyrolysis of oily sludge. The total aromatic yield and pr...
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Aromatic Hydrocarbon Production and Catalyst Regeneration in Pyrolysis of Oily Sludge Using ZSM‑5 Zeolites as Catalysts Jun Wang, Bing-Cheng Lin, Qun-Xing Huang,* Zeng-Yi Ma, Yong Chi, and Jian-Hua Yan Institute for Thermal Power Engineering, Zhejiang University, 38 Zheda Road, Hangzhou, Zhejiang 310027, People’s Republic of China ABSTRACT: ZSM-5 zeolites were selected as catalysts to promote the aromatization during pyrolysis of oily sludge. The total aromatic yield and product distribution were investigated to evaluate the efficiency of the catalysts and the influence of operating condition. The fresh and used catalysts were characterized by means of scanning electron microscopy, inductively coupled plasma optical emission spectrometry, X-ray diffraction, and ammonia temperature-programmed desorption. Results show that the highest catalytic activity (93.37%) was achieved with ZSM-5-O [the silicon/aluminum ratio (SAR) is 19] at a retention time of 40 s. The total aromatic yield increases with a longer retention time and higher catalyst dosage. An increase in the SAR value of ZSM-5 zeolites will reduce its acid density and aromatization activity. The coke deposition leading to deactivation for zeolites was observed, and calcination was used to regenerate the original catalytic performance. After regeneration, crystallinity was not affected but the acidity was reduced because of dealumination. The regenerated zeolites showed comparable catalytic activity for aromatization, and the tricyclic aromatic hydrocarbon content increased with reaction−regeneration cycles.

1. INTRODUCTION Oily sludge is the solid hazardous waste generated from petroleum exploitation, transportation, storage, and refinery. It is in the form of stable emulsion and contains a high proportion of petroleum hydrocarbons (PHCs). Consequently, the recovery of the PHCs has attracted wide interest, and many recover techniques, including solvent extraction, centrifugation treatment, pyrolysis, microwave irradiation, and ultrasonic treatment, have been developed.1 Among these technologies, pyrolysis is the most promising method to recover valuable liquid oils2 using catalysts or to obtain special hydrocarbon components by designed pyrolysis conditions.3,4 Aromatic compounds are important and widely used industrial raw materials for the synthesis of polymers.5 Especially, C8−C17 aromatic hydrocarbons play an important role in fuel oil used for aviation turbines.6−8 Currently, aromatics are mainly produced from petroleum and coal. For example, lignin has been widely used for aromatic production through pyrolysis for its rich source5 and high yields.9,10 As a result of the depletion of fossil fuels, many researchers have investigated the possibility of producing aromatic hydrocarbons from waste materials, such as wood sawdust,11 bio-oils,12 waste cardboard,13 and forest products.14 As a result of the high content of the oil fraction, oily sludge has high potential for aromatic hydrocarbon production. Previous studies15−18 have found many valuable aromatic species in the pyrolysis products of oily sludge. Nazem et al.17 obtained mono- and polyaromatic compounds through direct hydrothermal liquefaction of oily sludge from an Iranian oil refinery. Evans et al.16 recovered C5−C11 hydrocarbons from oily sludge, and the analysis results showed the predominance of polar aromatic compounds, some of which belonged to the polycyclic aromatic hydrocarbon (PAH) family. Qin et al.18 studied the pyrolysis treatment of oily sludge from the steel industry in a fluidized bed reactor. Although the aromatic hydrocarbon content in pyrolysis oil products was higher than © XXXX American Chemical Society

that in feed oil, it was still rather low (6.68−23.54%). To improve the yield of aromatic hydrocarbons through pyrolysis, a catalyst is essential. Zeolites, especially ZSM-5 and HZSM-5, have been wellknown as excellent catalysts in oil refinery, petrochemistry, and pollution control.19−22 In comparison to other amorphous silica−alumina catalysts, ZSM-5 zeolites possess higher acidity, resulting in enhanced selectivity.23 More importantly, ZSM-5 has high resistance to deactivation as a result of its threedimensional and well-connected micropore structure.24 Many studies have investigated aromatic hydrocarbon production from various raw materials over ZSM-5 zeolites, which exhibited remarkable capability.25 Mihalcik et al.26 and Li et al.25 found significant improvement in aromatic hydrocarbon production over ZSM-5 zeolites. The research of Liu et al.27 revealed that the aromatics could be generated directly from lignite by catalytic fast pyrolysis over metal-loaded HZSM-5 and the aromatic yield was remarkably increased. Although ZSM-5 zeolite has shown excellent performance in aromatization, it will be deactivated by coke deposition. Previous studies found that even the coke could be removed by calcination at high temperatures (>450 °C) under an oxidizing environment without any obvious framework damage,28,29 some active catalytic aluminate would be burned away,30−32 and it is very difficult to recover its original catalytic performance. This work is aimed to produce liquid products with a high aromatic content during pyrolysis treatment of oily sludge over ZSM-5 with different silicon/aluminum ratios (SARs). The effects of the retention time, SAR, and catalyst dosage on the product yield and quality were experimentally investigated. Received: June 28, 2017 Revised: October 9, 2017 Published: October 10, 2017 A

DOI: 10.1021/acs.energyfuels.7b01855 Energy Fuels XXXX, XXX, XXX−XXX

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10 min, then ramped to 270 °C in 10 min, and then kept at 270 °C for 10 min. The ammonia temperature-programmed desorption (NH3-TPD) was carried out on a Finesorb-3010 (Finetech, Zhejiang, China) to characterize the acid distribution on the ZSM-5 zeolite. The specimen was first heated to 500 °C in a stream of helium (He, 60 mL/min) and preheated at that atmosphere for 1 h. Then, the sample was cooled to 120 °C and subjected to ammonia (50% NH3/He, 60 mL/min) for 1 h to saturation. Then, the sample was heated to 700 °C at 10 °C/min after blowing a sweep of argon (Ar, 60 mL/min) for 1 h. The peaks were recorded by a mass spectrometer (MS), and the area demonstrates the acid amount. The total amount of Si and Al in the zeolites was quantified by inductively coupled plasma optical emission spectrometry (ICP−OES, iCAP6300, Thermo Fisher Scientific, Waltham, MA, U.S.A.). Prior to the analysis, the samples were digested in a mixed solution of hydrochloric acid (HCl), nitric acid (HNO3), and hydrofluoric acid (HF). A mixture of 0.1 g of sample and 10 mL of solution was heated in a polytetrafluoroethylene (PTFE) beaker on a heating plate until clear and transparent. After cooling, the solution was diluted for measurement. Thermogravimetric (TG) analysis was performed using a TA Instruments−Waters (New Castle, DE, U.S.A.) TGA Q50 device to characterize carbon deposition on the zeolite catalysts. Samples were heated from 50 to 900 °C at a heating rate of 10 °C/min with an air flow rate of 60 mL/min.

Moreover, the deactivated zeolites were regenerated and reused in the pyrolysis process. The finding of this paper can provide essential and useful insight into the improvement of the aromatic yield from oily sludge.

2. MATERIALS AND METHODS 2.1. Materials. The raw oily sludge was collected from the bottom of a 50 000 m3 crude oil storage tank at Sinochem Xingzhong Oil Staging Co., Ltd. The crude oil was imported from Middle East countries. The water content was determined by the ASTM D95-05 procedure, and the total hydrocarbons were derived according to Soxhlet extraction. The saturate, aromatic, resin, and asphaltene (SARA) contents of the oil components were analyzed according to the ASTM D2007-02 procedure. Commercial spherical ZSM-5 zeolites with a 3−5 mm diameter were purchased from Tianjin Yuanli Chemical Co., Ltd. The ZSM-5 zeolite with a lower SAR (19) was named as ZSM-5-O, and zeolites with a higher SAR (267) were referred to as ZSM-5-H. 2.2. Experimental Setups. The research was carried out on a twostage fixed bed reactor.33 The oily sludge (1.0 g) sample was placed in the first stage with the heating rate of 5 °C/min from room temperature to 500 °C. The catalyst was loaded at the second stage, which was kept at a constant temperature of 500 °C. The rapid cooling of pyrolytic vapors was achieved in a gas washing bottle filled with dichloromethane. The gas washing bottle was cooled by circulating water to guarantee the collection of all of the condensable organics. During the experiments, the inert atmosphere for the pyrolysis procedure was kept by nitrogen flow. Prior to the pyrolysis, the ZSM-5 zeolite should be pretreated at 550 °C for 4 h in the flow of air. The used ZSM-5-O zeolite was regenerated by calcination in the flow of oxygen (200 mL/min) at 500 °C for 60 min. Then, the calcined catalyst was reused in the catalytic pyrolysis of oily sludge under the same conditions of fresh zeolite. The used and regenerated ZSM-5-O zeolites were denoted as used ZSM-5-O and ZSM-5-Rx (x refers to the reaction−regeneration cycle), respectively. The regenerated ZSM-5-H catalysts were named as ZSM-H-Rx accordingly. The operating conditions of the pyrolysis experiments are listed in Table 1.

3. RESULTS AND DISCUSSION The water, oil, and solid fractions of the oily sludge, the ultimate analysis, and the SARA results are listed in Table 2. Table 2. Ultimate Analysis and SARA Results of the Oily Sludge oily sludge ultimate analysis (%)

Table 1. Operating Conditions for Oil Products catalyst

residence time (s)

mass ratio (zeolite/sludge)

oil product

none ZSM-5-O ZSM-5-O ZSM-5-O ZSM-5-O ZSM-5-H ZSM-5-H ZSM-5-H ZSM-5-H ZSM-O-R1 ZSM-O-R2 ZSM-H-R1 ZSM-H-R2

none 10 20 40 40 10 20 40 40 40 40 40 40

none 20 20 20 10 20 20 20 10 20 20 20 20

oil-N oil-10s oil-20s oil-40s oil-d10 oil-H-10s oil-H-20s oil-H-40s oil-H-d10 oil-R1 oil-R2 oil-H-R1 oil-H-R2

water, solid, and oil fractions (wt %)

SARA fraction of oil (wt %)

a

Cada Had Oad Nad Sad water by distillation solid particles oil by solvent extraction saturates aromatics resins asphaltenes

64.44 8.39 10.47 0.36 1.7 32.22 1.55 66.23 33.05 35.58 18.53 12.84

ad = air-dried basis.

The raw sludge sample possessed a low solid fraction (1.55%) and a relative high C/H ratio (7.68). Over 66% of the oil is saturates and aromatics, which is very suitable for aromatic product recovery. Figure 1 shows the original sludge, recovered pyrolysis oil, and catalyst before and after usage. The original oily sludge sample is black and viscous, while the pyrolysis oil products are yellow to brown. Furthermore, the pyrolysis oils have better fluidity. Obvious coke deposition can be observed on used ZSM-5 according to Figure 1, demonstrating the deactivation. No significant difference can be seen between fresh zeolites and regenerated zeolites in color. The SARs of the fresh zeolites and regenerated zeolites, determined by ICP−OES, were 267.3 (ZSM-5-H), 19.00 (ZSM-5-O), 21.20 (ZSM-5-R1), 22.48 (ZSM-5-R2), and 25.56 (ZSM-5-R3).

2.3. Analysis Methods. The microstructure of ZSM-5 zeolites was studied by scanning electron microscopy (SEM, SIRON, FEI Co., Netherlands). X-ray diffraction (XRD, DMAX-RA, Rigaku, Tokyo, Japan) was used to characterize the crystallographic structures of the samples. The data were collected in the 2θ range of 5−50° with Cu Kα radiation (λ = 0.154 06 nm). The quantitative analyses of oil products from the catalytic pyrolysis were conducted on a 7890B gas chromatograph with a 5977A mass selective detector (GC−MSD, Agilent Technologies, Santa Clara, CA, U.S.A.). The instrument was equipped with an extractor electron impact (EI) ion source. The oven temperature was held at 50 °C for B

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Figure 1. (a) Raw oily sludge, (b) pyrolysis oil products, (c) fresh ZSM-5 zeolites, (d) used zeolites with coke, and (e) regenerated zeolites.

Figure 3. Components of the liquid products derived from catalytic pyrolysis with ZSM-5-O and ZSM-5-H at different residence times.

found that oils were mainly composed of aromatic hydrocarbons, while there was no aromatics in the raw materials. This indicated the phenomenal aromatic selectivity of ZSM zeolites. Twaiq et al.35 found that the yield of total aromatics decreased with a higher SAR. The total aromatic yield decreased by 18% with the SAR rising from 50 to 400. They proposed that the lower acidity resulting from a high SAR decreased the secondary cracking reactions and led to aromatic reduction. The aromatic hydrocarbons contained in the catalytic pyrolysis oils were mainly monocyclic aromatic hydrocarbons (MAHs) and bicyclic aromatic hydrocarbons (BAHs), with a small amount of tricyclic aromatic hydrocarbons (TAHs). The MAHs accounted for 29.72% in oil-10s, 15.64% in oil-20s, and11.17% in oil-40s. Meanwhile, the MAH contents of oil-H10s, oil-H-20s, and oil-H-40s were 36.93, 22.54, and 11.42%, respectively. It is reported that the olefin cracking and aromatization reaction mainly happened on the Brønsted acid sites.36 Thus, the aromatization occurred more completely over ZSM-5-O, which possessed more acid sites. The branches on the MAHs were converted into benzene rings with deeper aromatization, resulting in BAHs. Ultimately, MAHs generated with ZSM-5-O decreased as a consequence of higher acidity. In this paper, the reduction in the total aromatic yield over ZSM5-H was mainly caused by reduced alkenylbenzene. This indicated that the aromatization reaction was not sufficient on ZSM-5-H as a result of the lower acidity of zeolites, resulting from a higher SAR. 3.2. Influence of the Residence Time and Dosage of ZSM-5 Zeolites. It is suggested that a longer retention time

Figure 2. GC−MS data of pyrolysis oil without a catalyst.

3.1. Influence of the SARs on the Aromatic Yield and Distribution. Figure 2 shows the gas chromatography−mass spectrometry (GC−MS) data of the oil product (named oil-N) from non-catalytic pyrolysis of oily sludge. As observed, the main products of oil-N were saturates with a fraction over 60% and aromatics accounting for 27.65% by weight. The rest are organic compounds containing O, N, and S. The effects of SAR on the component distribution and total aromatic yield under different retention times (10, 20, and 40 s) were given in Figure 3. The catalyst/sludge mass ratio was 20. The oil products derived with ZSM-5-O and ZSM-5-H were designated as oil-t and oil-H-t (t refers to the residence time), respectively. The associated GC−MS data of these oil products were plotted in Figure 4. As seen, the total aromatic yield over zeolites increased significantly compared to that of oil-N. The maximum yields over ZSM-5-O and ZSM-5-H were 93.37 and 87.33% at 40 s, respectively. This indicated the extraordinary catalytic performance of ZSM-5 zeolites with various SARs. A similar phenomenon was reported by other researchers using the same catalysts. Cai et al.20 and Wang et al.34 carried out experiments with ZSM-5 zeolites for bio-oil generation and C

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Figure 4. GC−MS data of liquid products for ZSM-5 zeolites with SARs of 19 and 267 at 10, 20, and 40 s.

continuously aromatized. Thus, BAHs account for the largest portion (over 60% for ZSM-5-O and 50% for ZSM-5-H) of the total aromatic yield. The total aromatic yield and product distribution changed significantly with a lower zeolite dosage of 10:1. The residence time for this set of experiments is 40 s to obtain a higher aromatic yield. Oil-d10 and Oil-H-d10 were used to represent

can benefit the total aromatic yield and BAH selectivity. For example, the MAH yields of ZSM-5-O and ZSM-5-H decreased by 62.4 and 69.1%, while the BAH yield increased to 207 and 214%, with the residence time rising from 10 to 40 s. Significant increases in naphthalene and methylnaphthalene are noticed. It can be deduced that the gaseous products from the first stage formed MAHs in zeolites, and then the branched olefins were D

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Figure 5. Components of the liquid products derived from catalytic pyrolysis with a catalyst dosage of 10:1.

Figure 7. XRD patterns of the ZSM-5 zeolites (ZSM-5-O, used ZSM5-O, ZSM-5-R1, ZSM-5-R2, ZSM-5-R3, and ZSM-5-H).

Figure 6. GC−MS data of liquid products for zeolites with SARs of 19 and 267 with a zeolite dosage of 10:1.

E

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Figure 8. NH3-TPD profiles of fresh and regenerated zeolites.

Figure 11. Main components of pyrolysis oil produced with regenerated zeolites as catalysts (retention time, 40 s; dosage, 20).

the oil products. The main compound distribution is illustrated in Figure 5, and the gas chromatograms are shown in Figure 6. The BAH yield was reduced in comparison to the liquid products under a catalyst/feedstock ratio of 20:1. The BAHs decreased over 9 and 5% for ZSM-5-O and ZSM-5-H, respectively. Moreover, the total aromatic yield reduced as well. The results reveal the incompletion of the aromatization procedure, which resulted in intermediates. 3.3. Zeolite Characterization. Figure 7 shows the XRD patterns of fresh ZSM-5 zeolites (ZSM-5-O and ZSM-5-H), used ZSM-5-O, and regenerated zeolites (ZSM-5-R1, ZSM-5R2, and ZSM-5-R3). All of the XRD patterns exhibited the same diffraction peaks in the ranges of 2θ = 7−10°, 22−25°, and 30°, matching well with the standard pattern of the ZSM-5 zeolite.37 Hence, the regenerated zeolites still reserved the initial structure of ZSM-5. The relative crystallinity values of regenerated zeolites (ZSM-5-R1, 98.2%; ZSM-5-R2, 94.9%; and ZSM-5-R3, 93.0%) are relative to the standard ZSM-5 zeolite reference. Therefore, there is no obvious framework change or damage caused by the regeneration process. Other researches focusing on regeneration confirmed this conclusion.28 Figure 8 displays the NH3-TPD profiles of fresh ZSM-5 zeolites (ZSM-5-O and ZSM-5-H) and regenerated zeolites (ZSM-5-R1, ZSM-5-R2, and ZSM-5-R3). Obviously, there is only one prominent peak, which is ascribed to the Brønsted acid site, in every profile. They are classified as Na-ZSM-5 zeolites, which contained a large quantity of Na+, and show

Figure 9. SEM micrographs of fresh, used, and regenerated ZSM-5 zeolites with a SAR of 19.

Figure 10. TG and DTG curves for used ZSM-5-O and used ZSM-5H. F

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Figure 12. GC−MS data of liquid products over regenerated zeolites.

only one peak belonging to medium acid. The peak position of ZSM-5-R1 (341 °C) changed slightly compared to that of ZSM-5-O (342 °C). Meanwhile, the area of ZSM-5-O decreased after being calcined, demonstrating lower acidity. Moreover, the peak area and peak center (325 °C) of ZSM-5R2 decreased significantly compared to that of ZSM-5-R1. However, the curve of ZSM-5-R3 changed slightly in the peak center (322 °C) and area. Thus, the regeneration treatment would cause a decrease in acid intensity and density. As shown in Figure 8, the peak of ZSM-5-H centered at 289 °C and the peak area was far lower than that of ZSM-5-O. This confirms that zeolites with a higher SAR possess a lower acid intensity.36 The SAR of our catalyst samples rose from 19.00 to 25.56 after the regeneration treatment and increased with more reaction− regeneration cycles. This suggested that the regeneration resulted in dealumination. Luo et al.29 and Campbell et al.30 also discovered the lower densities of acid sites in regenerated ZSM-5 zeolites, especially the reduction in the concentration of a strong acid site. They assumed that this was caused by dealumination. 3.4. Regeneration of Used Catalysts. The surface morphology of original ZSM-5-O zeolite, used zeolite, and regenerated zeolites were investigated using SEM. The results are shown in Figure 9. A significant carbon deposition can be observed on the used ZSM-5 zeolite. The network structure of original zeolite was destroyed, and the surface became more

compact. Obviously, the coke deposited on the surface was removed, and the surface structure was recovered after regeneration. No significant change is discovered on the surface of recycled zeolites according to the micrographs. Researchers have found that the main reason leading to deactivation for zeolites was surface coke.29,31,32 The coke can be oxidized at a high temperature with oxygen, air, or water stream environment. The used zeolites were calcined in oxygen stream to remove the coke. Figure 10 displays the TG− differential thermogravimetric (DTG) curves of used ZSM-5-O and used ZSM-5-H under air flow. Two evident peaks can be observed in the DTG curve. The first peak (88 °C for used ZSM-5-O and 96 °C for used ZSM-5-H) was ascribed to the water evaporation. The other (542 °C for used ZSM-5-O and 583 °C for used ZSM-5-H) could be attributed to the combustion of coke. It is reported that peaks at a high temperature (500−600 °C) usually correspond with polyaromatic coke.38 The performance of regenerated zeolites was evaluated under the same experimental conditions as fresh zeolites. The total aromatic yield and component distribution are taken into consideration. The main components of pyrolysis oils over regenerated zeolites are listed in Figure 11, and the gas chromatograms were shown in Figure 12. There is no significant loss in the total aromatic yield, while the losses for MAHs and BAHs are noticeable. Meanwhile, the TAH yield G

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increases by about 30% (8.84, 9.2, and 9.07% for oil-R1, oil-R2, oil-H-R1, and oil-H-R2, respectively) compared to oil products over fresh zeolites. This indicates that the aromatic hydrocarbons in oils generated with regenerated zeolites owned more benzene rings. It can be supposed that the hydrocarbons tended to accumulate and polymerize in the zeolites, forming highmolecular-weight compounds. The decrease of the total aromatic yield is reasonable, revealing the stable activity of regenerated zeolites in aromatization. The previous characterization displayed the decrease of acid density in the regenerated zeolites, which was triggered by dealumination during the calcination. Studies found that the regeneration could increase the catalyst lifetime.30 Some researchers proposed that the main factors were either the dealumination or the residual coke.31,32 Zhang et al.32 discovered a modified texture of regenerated ZSM-5 zeolites caused by the reaction−regeneration cycle. The total concentration of acid sites and the ratio of strong acid sites/ weak acid sites decreased simultaneously. Thus, the reaction became gentle and led to an obvious reduction in coke and coking rate. This consequently affected the catalytic performance. A slightly decreased total aromatic yield demonstrated the stable activity, and the increased TAH yield revealed higher selectivity. It is supposed that the performance of recycled zeolites was attributed to the dealumination caused by regeneration. The dealumination resulted in a modified texture and lower acidity, leading to higher selectivity and a lower total aromatic yield of zeolites, whereas too much reaction− regeneration cycles would lead to a low total aromatic yield and high macromolecular aromatics.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-0571-87952834. Fax: +86-571-87952438. Email: [email protected]. ORCID

Qun-Xing Huang: 0000-0003-1557-3955 Yong Chi: 0000-0001-6360-6198 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 51576172) and the Innovative Research Groups of the National Natural Science Foundation of China (Grant 51621005).

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REFERENCES

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4. CONCLUSION This work is aimed at aromatic production from pyrolysis of oily sludge in a fixed bed reactor over ZSM-5 zeolites. Relative compositions of pyrolysis oils (MAHs, BAHs, and TAHs) were presented to show the effects of catalyst choice and operating conditions. Zeolites exhibited strong aromatization, and the highest activity of 93.37% was achieved over ZSM-5-O (retention time = 40 s). The NH3-TPD profiles showed lower acid intensity and weaker acid sites of ZSM-5-H zeolites, resulting in a reduced aromatic hydrocarbon yield and more alkenylbenzenes. A shorter retention time led to the formation of MAHs, while the total aromatic yield decreased. It suggested that the aromatization was enhanced with a longer residence time, and the branched olefins were converted into benzene rings, i.e., converting MAHs into BAHs. The total aromatic yield decreased with a lower zeolite dosage, resulting from the uncompleted reaction. To regenerate the used zeolites with coke deposition, calcination was conducted under an oxygen atmosphere. No framework damage or change has been observed after calcination according to XRD patterns, which is coincident with former studies. Nonetheless, dealumination happened during regeneration and resulted in lower acidity, which has been found on the regeneration procedure of ZSM-5 zeolites in previous works. The oil products with regenerated zeolites possessed more TAHs and a slightly decreased total aromatic yield, demonstrating the promotion of macromolecular aromatic compounds by the reaction−regeneration cycle. H

DOI: 10.1021/acs.energyfuels.7b01855 Energy Fuels XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.energyfuels.7b01855 Energy Fuels XXXX, XXX, XXX−XXX