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Catalytic conversion of coal pyrolysis vapors to light aromatics over hierarchical Y-type zeolites
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Catalytic conversion of coal pyrolysis vapors to light aromatics over hierarchical Y-type zeolites Q7,1
Peng Lv, Lunjing Yan, Yan Liu, Meijun Wang, Weiren Bao**, Fan Li* State Key Laboratory Breeding Base of Coal Science and Technology Co-founded By Shanxi Province and the Ministry of Science and Technology, Taiyuan University of Technology, Taiyuan, 030024, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 August 2019 Received in revised form 17 December 2019 Accepted 18 December 2019 Available online xxx
A series of hierarchical Y-type zeolites were prepared by a post-treatment method. All the samples were characterized using nitrogen adsorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and temperature-programmed desorption of ammonia (NH3-TPD). The results show that hierarchical Y-type zeolites with different porosities can be obtained, and the mesopore size can be controlled by changing the treatment conditions. The acidity of catalysts was also adjusted in this process. The catalysts were evaluated with respect to catalytic conversion of coal pyrolysis vapors to light aromatics online by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). Moreover, several model compounds were selected to evaluate the formation pathway of light aromatics during the upgrading of coal pyrolysis vapors over Y-type zeolite. It was found that pore-structure-modified Y-type zeolite has good catalytic performance for the upgrading of coal pyrolysis vapors. After catalytic cracking by EDY zeolites, the total amount of light aromatics such as benzene, toluene, ethyl-benzene, xylene, and naphthalene (BTEXN) in coal pyrolysis vapors increased from 5600 ng/mg (raw coal pyrolysis) to 18,800 ng/mg. The results of model compound catalytic pyrolysis show that Y-type zeolite is beneficial for the catalytic cracking of polycyclic aromatic hydrocarbons, the breaking of phenolic hydroxyl, and the decomposition of heterocyclic compounds, thus promoting the formation of BTEXN. Hierarchical catalysts with wide pore size and large mesopore volume contribute to the diffusion of bulky reactants and their contact with active sites in channels, further promoting the generation of light aromatics. © 2019 Published by Elsevier Ltd on behalf of Energy Institute.
Keywords: Coal pyrolysis Catalytic upgrading Light aromatics Hierarchical zeolite
1. Introduction Coal is the most abundant and widely distributed fossil energy on the earth [1], accounting for a relatively large proportion of fossil fuels consumed to meet China's primary energy demand [2,3]. Coal pyrolysis is the main stage of coal processing and utilization [4]. In addition to char and gas, coal pyrolysis also produces tar, a black and viscous mixture of hydrocarbons [5]. Coal tar contains a large proportion of heavy components. It is difficult to utilize heavy components as resources due to difficulty in separation [6]. The light components in tar such as benzene, toluene, ethyl-benzene, xylene, and naphthalene (BTEXN) are important chemical raw materials and widely used in the production of pesticides, plastics, and synthetic fibers [7]. Catalytic conversion of coal pyrolysis
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* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (W. Bao), [email protected] (F. Li).
vapors to light aromatics is a promising way for high value-added utilization of coal tar [8]. Carbon-based catalysts [9e11], metal catalysts [12e14], and zeolite catalysts [15e24] can promote the cracking of heavy components and generation of light aromatics in coal tar, thus improving the quality of tar. Among them, zeolite catalysts, especially Y-type zeolite and ZSM-5 zeolite, are favored because of their unique pore structure, suitable acid sites, and large surface area [18e24]. Ren et al. [20] used ZSM-5 with different SiO2/Al2O3 ratios to catalyze the conversion of lignite pyrolysis vapors to BTEXN and found that ZSM-5 with a high aluminum content is favorable for the production of light aromatics. Liu et al. [21] studied the catalytic upgrading of coal tar by metal-modified ZSM-5 and found that metal-modified ZSM-5 can promote the production of light aromatics and removal of oxygen-containing compounds. Sulfationacidified HZSM-5 catalyst can further improve the yield of light aromatics due to generation of the new strong acidic sites [22]. Yan et al. [23] used different types of zeolites for the catalytic conversion of gaseous tar. The results indicate that Y-type zeolite could
https://doi.org/10.1016/j.joei.2019.12.005 1743-9671/© 2019 Published by Elsevier Ltd on behalf of Energy Institute.
Please cite this article as: P. Lv et al., Catalytic conversion of coal pyrolysis vapors to light aromatics over hierarchical Y-type zeolites, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.12.005
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significantly increase the yield of BTEXN, while Al/SBA-15 had no obvious effect on the conversion of coal tar. Kong et al. [24] evaluated the catalytic conversion of gaseous tar using three Y-type zeolites with different acidities and found that Y-type zeolite had good catalytic performance for the production of light aromatics and catalytic cracking of heavy aromatics. Nevertheless, Y-type zeolite is typical microporous material with an average pore size of only 0.74 nm, severely limiting the mass transfer of reactants and products in the channel [26]. In addition, narrow pores easily deactivate the catalyst due to the blockage of pores by carbon deposition. Traditional mesoporous zeolites are limited in the field of catalytic cracking because of their weak catalytic activity and poor hydrothermal stability [27]. Hierarchical catalysts, where the advantages of microporous zeolites and mesoporous materials are combined, have great potential in catalytic reactions involving bulky molecules [28,29]. Gackowski et al. [30] obtained hierarchical Y-type zeolites by treating with an ammonia solution, and the samples exhibited promising catalytic properties in the liquid-phase isomerization of a-pinene. Verboekend et al. [31] also prepared hierarchical Y-type zeolites. Compared with conventional Y-type zeolite, pore-structuremodified Y-type zeolite has excellent adsorption and catalytic properties. Although hierarchical Y-type zeolites have been used in many catalytic reactions, studies on the catalytic conversion of coal pyrolysis vapors over hierarchical Y-type zeolites are limited. Especially, the effect of porous structure of Y-type zeolites on the formation of light aromatics has been rarely evaluated. In this
Table 1 Proximate and ultimate analyses of coal samples. Sample
Proximate analysis wt/%
Ultimate analysis wt/%, daf
Mad
Aad
Vad
C
H
O*
N
S
FCM
2.3
7.6
36.4
82.1
5.2
8.1
1.4
3.2
study, hierarchical Y-type zeolites with different porosities were prepared by sequential dealumination and desilication and evaluated in the upgrading of coal pyrolysis vapors by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). The effect of pore-structure-modified Y-type zeolites on the composition and distribution of gaseous tar from coal pyrolysis was evaluated. The results obtained in this study provide a new viewpoint for the design of catalysts used in the conversion of coal tar to light aromatics. 2. Experimental 2.1. Material A bituminous coal, named as FCM, obtained from Ningxia Hui Autonomous Region, China was used in this study. The proximate and ultimate analyses of coal samples are shown in Table 1. Commercial NaY zeolite was purchased from Catalyst Plant of Nankai University (Tianjin, China). Note: ad, air-dried basis; daf, dry and ash-free basis; *by difference. 2.2. Synthesis of hierarchical Y zeolites Hierarchical Y-type zeolites were synthesized by sequential dealumination and desilication [31]. In a specific procedure, NaY zeolites were stirred and refluxed in 0.1 M H4EDTA at 100 C for 6 h; subsequent washing, filtration, and drying afforded EY. This was then submitted to NHþ 4 ion exchange in 1.0 M NH4NO3 for three times at 80 C, followed by washing, drying, and calcination to obtain EDY0. In addition, the EY zeolites were treated with 0.1 and 0.3 M NaOH at 65 C for 0.5 h. After washing, filtration, and drying, the obtained products were stirred and refluxed in 0.1 M Na2EDTA at 100 C for 6 h, followed by washing, filtration, and drying and successively exchanged with 1.0 M NH4NO3 for three times. The final products were obtained in the H-form through calcination at 550 C for 4 h. The samples were denoted as EDY01 and EDY03,
Fig. 1. Isothermal N2 adsorption/desorption curve and aperture distribution of Y-type zeolites.
Table 2 Pore structure parameter and composition of Y-type zeolites. Samples
SBET (m2/g)
Vtotal (cm3/g)
Vmicro (cm3/g)
Vmeso (cm3/g)
SiO2/Al2O3
HY EDY0 EDY01 EDY03
609 587 585 684
0.28 0.31 0.44 0.54
0.21 0.16 0.13 0.20
0.06 0.12 0.27 0.32
5.34 9.01 8.12 6.56
Please cite this article as: P. Lv et al., Catalytic conversion of coal pyrolysis vapors to light aromatics over hierarchical Y-type zeolites, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.12.005
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Fig. 2. SEM images of Y-type zeolites.
Fig. 3. TEM images of Y-type zeolites.
Please cite this article as: P. Lv et al., Catalytic conversion of coal pyrolysis vapors to light aromatics over hierarchical Y-type zeolites, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.12.005
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respectively. In addition, HY zeolite was obtained from parent NaY zeolite after three consecutive NHþ 4 ion exchange and calcination. 2.3. Characterization of zeolites The N2 adsorption characterization of the catalysts was carried out using an ASAP2460 N2 adsorption analyzer (Micromeritics, USA). Spectro Arcos inductively coupled plasma atomic emission spectrometry (Spectro, Germany) was used to test the chemical composition of the zeolite. An H-7650 scanning electron microscope (Hitachi, Japan) was used to observe the surface morphology of catalysts. Transmission electron microscopy (TEM) was performed using a Tecnai G20 transmission electron microscope (FEI, Holland). The acidity of catalysts was measured by temperatureprogrammed desorption of ammonia (NH3-TPD) using an AUTOCHEM 2920 automated chemisorption analyzer (Micrometrics, USA).
2.4. Catalytic experiment
Fig. 5. Effect of Y-type zeolites on the composition of coal pyrolysis vapors.
Pyrolysis experiments of coal and model compounds were performed using a CDS Analytical Pyroprobe 5250 (CDS, USA). In a specific experiment, the coal sample (1 mg) or model compounds (0.1 mg) and catalyst (0.6 mg) were loaded in a sample tube, separated by a thin layer of quartz wool, and supported by using a quartz rod at the bottom of the sample. The samples were heated to 700 C at a heating rate of 10 C/ms and held at that temperature for 15 s. The pyrolysis products were injected to a Focus GC/DSQ II GC/ MS instrument (Thermo Fisher, USA) through a transfer-line for online separation and detection. The chromatographic peaks of BTEXN were identified using the corresponding chemical standards (AccuStandard Inc., USA) and quantified using an external standard method.
Fig. 6. Effect of Y-type zeolites on the total yield of BTEXN in coal pyrolysis vapors.
Fig. 4. NH3-TPD profiles of Y-type zeolites.
Table 3 Acidity of zeolites obtained from the NH3-TPD results. Samples
Amount (mmol/g) Weak acid
Strong acid
Total acid
HY EDY0 EDY01 EDY03
2.36 0.65 0.73 0.92
1.58 1.44 1.58 1.88
3.94 2.09 2.31 2.80
Fig. 7. Effect of Y-type zeolite on the yield of phenolic compounds in coal pyrolysis vapors.
Please cite this article as: P. Lv et al., Catalytic conversion of coal pyrolysis vapors to light aromatics over hierarchical Y-type zeolites, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.12.005
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3. Results and discussion 3.1. Textural property and composition analysis The isothermal N2 adsorption and desorption curves and aperture diameter distribution of Y-type zeolites are shown in Fig. 1. The
Fig. 8. Effect of Y-type zeolite on the yield of heteroatom compounds in coal pyrolysis vapors.
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isothermal adsorption curve of parent Y-type zeolite rapidly increased when the P/P0 is close to 0. With the increase in relative pressure, the adsorption rate became gentle and similar to the Itype isothermal curve, indicating that parent Y-type zeolite is a typical microporous material. A small number of mesopores with a
Fig. 10. Possible path of BTEXN formation from pyrene catalytic pyrolysis over Y-type zeolites.
Fig. 9. Total ion chromatogram of pyrene before (a) and after (b) catalytic pyrolysis over Y-type zeolites.
Please cite this article as: P. Lv et al., Catalytic conversion of coal pyrolysis vapors to light aromatics over hierarchical Y-type zeolites, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.12.005
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pore size of 2e3 nm was found in the pore size distribution of Ytype zeolite after dealumination treatment. After NaOH treatment, an apparent hysteresis loop appeared when the relative pressure was greater than 0.42, indicating the formation of mesopores. The pore size distribution shows that the diameter of mesopores increased with increasing concentration of alkali solution, from 2 to 3 nm up to ~10 nm as the concentration of NaOH increased to 0.3 M. Table 2 also shows that the mesopore volume (Vmeso) of HY is only 0.06 cm3/g. The Vmeso of zeolite clearly increased after sequential dealumination and desilication. When the concentration of NaOH is 0.3 M, the Vmeso reached 0.32 cm3/g. At the same time, it was also found that the SiO2/Al2O3 ratio of Y-type zeolite after dealumination increased from 5.34 to 9.01. After NaOH treatment, the SiO2/Al2O3 ratio of zeolite decreased due to the removal of silicon atom and constantly decreased with the increase in alkali solution concentration.
type zeolite shows homogeneous dark zones, indicating that they have a faultless structure. After sequential dealumination and desilication, the light gray areas were evenly distributed on the dark zones of TEM image of zeolites, indicating that mesopores were clearly generated after sequential dealumination and desilication. 3.3. Acidic properties To evaluate the effect of pore structure modification on zeolite acidity, NH3-TPD characterization of zeolite samples was carried
3.2. Morphology analysis The SEM images of parent and pore-structure-modified zeolites are shown in Fig. 2. The average particle size of parent Y-type zeolite is about 0.6e0.8 mm. The particle has a smooth surface and clear edges. After sequential dealumination and desilication, the surface of particles became rough and uneven with obvious erosion and cracking. This indicates that the dealumination and desilication destroyed the morphology of zeolite. TEM was applied to further observe the morphological changes in the samples. Fig. 3 shows that the morphology of EDY zeolite is clearly different from that of parent Y-type zeolite. The parent Y-
Fig. 12. Possible path of BTEXN formation from 3,4-dimethylphenol catalytic pyrolysis over Y-type zeolites.
Fig. 11. Total ion chromatogram of 3,4-dimethylphenol before (a) and after (b) catalytic pyrolysis over Y-type zeolites.
Please cite this article as: P. Lv et al., Catalytic conversion of coal pyrolysis vapors to light aromatics over hierarchical Y-type zeolites, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.12.005
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out. As shown in Fig. 4, the results show that all the curves of samples exhibit three distinct desorption peaks, corresponding to the physical adsorption of ammonia below 100 C, weak acid sites in the low-temperature zone of 100e250 C, and strong acid sites in the high-temperature zone of 250e600 C [32]. Table 3 shows the acidity distribution of zeolites calculated from the NH3 adsorption amount. The results show that HY zeolite has high weak and strong acid sites, 2.36 and 1.58 mmol/g, respectively. After dealumination treatment, the weak and strong acid sites of Y-type zeolite decreased to 0.65 and 1.44 mmol/g, respectively. Both the weak and strong acid sites of EDY zeolite increased after alkali treatment, from 0.65 to 1.44 mmol/g for EDY0 and up to 0.92 and 1.88 mmol/g for EDY03 with the increase in NaOH concentration. The acidity of Y-type zeolite is related to the amount and distribution of
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aluminum atoms in the zeolite. With the increase in alkali concentration, more and more silicon atoms were removed from the zeolite, and the SiO2/Al2O3 ratio of zeolite gradually decreased. Therefore, the amount of acid sites of EDY samples increased with the increase in the concentration of alkali. Similar results were reported by García et al. [32]. The acidic sites of Y-type zeolite increased after desilication by alkali treatment because of the retention of aluminum-rich species. 3.4. Catalytic performance of zeolites The composition of coal pyrolysis vapors before and after catalysis over Y-type zeolites is shown in Fig. 5. The results show that the relative content of aliphatic substances, phenolic
Fig. 13. Total ion chromatogram of dibenzothiophene before (a) and after (b) catalytic pyrolysis over Y-type zeolites.
Fig. 14. Possible path of BTEXN formation from dibenzothiophene catalytic pyrolysis over Y-type zeolites.
Please cite this article as: P. Lv et al., Catalytic conversion of coal pyrolysis vapors to light aromatics over hierarchical Y-type zeolites, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.12.005
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compounds, and heterocyclic compounds decreased, and the relative content of monocyclic aromatics and bicyclic aromatics increased after the catalytic conversion of coal pyrolysis vapors. The yield distribution of BTEXN before and after catalytic conversion is shown in Fig. 6. The HY zeolite significantly increased the amount of light aromatics in pyrolysis tar. After catalytic upgrading by HY, the total amount of BTEXN in pyrolysis products increased from 5600 ng/mg to 11,200 ng/mg. Bulky aromatic hydrocarbons underwent catalytic cracking to form free radicals under the action of acid centers of zeolites, and the radical fragments with small molecular weights were stabilized by the hydrogen-rich components in vapors to form light aromatics [33]. Several different catalysts were used in the catalytic upgrading of coal tar by Majka et al. [15]; the results show that Y-type zeolite has higher activity for the conversion of macromolecular tar to light aromatics. After sequential dealumination and desilication, the catalytic ability of Y-
type zeolite was improved in different levels. After catalysis using EDY03 zeolite, the amount of light aromatics increased up to 18,800 ng/mg. Wang et al. [34] found that hierarchical ZSM-5 obtained by HF treatment can promote the formation of light aromatics. Mesoporous ZSM-5 zeolite prepared by desilication can increase the yield of aromatics and decrease the coke formation also reported by Li et al. [35]. Fig. 7 shows the yield of several typical phenolic compounds in coal pyrolysis vapors before and after catalytic reaction. The amounts of phenolic substances such as phenol, cresol, xylenol, tricresol, ethyl-phenol, and methyl-ethyl-phenol decreased to varying degrees after the pyrolysis of volatiles was catalyzed by Y-type zeolite. The yield of several typical heterocyclic compounds in coal pyrolysis vapors before and after catalytic reaction is shown in Fig. 8. The results show that the yield of thiophene and furan derivatives significantly decreased after catalytic cracking. This indicates that Y-type zeolite can promote the conversion of phenolic substances and heterocyclic compounds in coal tar, thus improving the quality of tar.
3.5. Formation pathway of coal pyrolysis vapors to light aromatics
Fig. 15. Peak areas of BTEXN of different model compounds before and after catalytic pyrolysis over Y-type zeolites.
To determine the formation pathway of light aromatics during the catalytic conversion of coal pyrolysis vapors, three model compounds, pyrene, 3,4-dimethylphenol, and dibenzothiophene representing polycyclic aromatics, phenolic compounds, and heterocyclic compounds in coal tar, were selected. The products of pyrene before and after catalytic pyrolysis over Y-type zeolite are shown in Fig. 9. After catalytic pyrolysis, the yield of light aromatics such as BTEXN significantly increased, the total peak area of BTEXN is 9.1 times, as shown in Fig. 15, higher than that of direct pyrolysis. In addition to light aromatic hydrocarbons, 4,5-dihydropyrene, phenalene, and naphthalene derivatives were also found in the products of pyrene-catalyzed pyrolysis over Y-type zeolite. Shimada et al. [36] and Pujro et al. [37] carried out catalytic cracking experiments of polycyclic aromatics and inferred that during the catalytic upgrading of coal tar, polycyclic aromatic hydrocarbons first undergo hydrogenation reaction to saturate one of the aromatic rings, and then the saturated cyclic hydrocarbons undergo
Fig. 16. Schematic diagram of catalytic conversion of coal pyrolysis vapors to light aromatics.
Please cite this article as: P. Lv et al., Catalytic conversion of coal pyrolysis vapors to light aromatics over hierarchical Y-type zeolites, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.12.005
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ring-opening reaction under the action of active center of solid-acid catalysts. Finally light aromatics are formed [24]. Fig. 10 shows the possible routes for catalytic pyrolysis of pyrene over Y-type zeolite. The products obtained from the pyrolysis of 3,4-dimethylphenol with and without Y-type zeolite are shown in Fig. 11. After catalytic pyrolysis, the yield of light aromatic hydrocarbons significantly increased, and phenol and other phenolic compounds were also produced. The possible reaction pathway of 3,4-xylenol to light aromatics over Y-type zeolite is shown in Fig. 12. Solid-acid catalyst can reduce the content of phenols in tar and promote the conversion of phenols to light aromatics, similar results are also reported in other studies [38e40]. Heuchel et al. [40] elucidated the catalytic conversion mechanism of phenolic compounds over a zeolite catalyst in detail and found that phenolic compounds could form aromatics by dehydrogenation under the effect of active center of catalysts. At the same time, other benzene series and phenolic compounds are produced by dealkylation and transalkylation of methyl side chain [41]. Fig. 13 shows the product distribution of dibenzothiophene before and after catalytic pyrolysis over Y-type zeolite. The total peak area of BTEXN after catalytic pyrolysis is 17.2 times as large as that without catalyst. In addition to light aromatic hydrocarbons, biphenyl, fluorene, benzothiophene derivatives, and phenylmercaptan were also found in the catalytic pyrolysis products. This indicates two possible catalytic pathways for the conversion of dibenzothiophene over Y-type zeolite, as shown in Fig. 14. One route can be described as follows: A benzene ring of dibenzothiophene first undergoes hydrogenation and ring-opening reaction to produce benzothiophene derivatives. Benzothiophene derivatives continue to undergo hydrogenation and ring-opening reaction of thiophene ring to finally produce phenylmercaptan and benzene series. Another route can be described as follows: The breaking of sulfur-carbon bond of dibenzothiophene under the action of catalyst and free radical fragments is stabilized by hydrogen-rich free radicals to form biphenyl derivatives and fluorene, and the biphenyl derivatives and fluorene continue to undergo cracking to produce light aromatic hydrocarbons. The catalytic mechanism of sulfur-containing aromatic heterocyclic compounds to light aromatics is similar to that of hydrodesulfurization of dibenzothiophene reported previously, because Y-zeolite has been applied to the removal of organic sulfur because of its unique pore structure and excellent H-transfer ability [42,43]. Therefore, we can propose a possible pathway for the catalytic conversion of coal pyrolysis vapors to light aromatics over hierarchical Y-type zeolites, as shown in Fig. 16. Y-type zeolite is a promising catalyst for the conversion of coal pyrolysis vapors to light aromatics because it can promote the catalytic cracking of polycyclic aromatics, phenols, and heterocyclic compounds. The large pore size and pore volume of hierarchical Y-type zeolites further facilitate the mass transfer of reactants in the channel and contact with the active center, thus promoting the formation of light aromatics. In addition, the acid sites of zeolite after pore structure modification have been adjusted, which is also an important factor in improving the catalytic activity. 4. Conclusion In this study, the catalytic conversion of coal pyrolysis vapors to light aromatics over hierarchical Y-type zeolites was evaluated. Ytype zeolites with different porosities were prepared by sequential dealumination and desilication. The mesopore size of catalysts continually increased with increasing severity of the alkali treatment, from around 2.5 nm to up to ~10 nm. Hierarchical Y-type zeolites exhibited excellent catalytic performance for the catalytic upgrading of coal pyrolysis vapors and significantly improved the yield of light aromatics. Compared with raw coal pyrolysis, the total
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amount of BTEXN in FCM coal gaseous tar increased from 5600 ng/ mg to 18,800 ng/mg after catalytic upgrading over EDY03 zeolites. Catalytic pyrolysis experiments of model compounds show that Ytype zeolite can promote the cracking of polycyclic aromatics and heterocyclic compounds and the breaking of phenolic hydroxyl groups, thus promoting the formation of light aromatics. The wide pore diameter distribution and large pore volume of porestructure-modified Y-type zeolites are favorable for bulky reactants in the pyrolysis tar for transport in channels and access to the active site, thus promoting the generation of light aromatics. In addition, the distribution and amount of acidity of the zeolites were adjusted during pore structure modification, which also has a substantial role in the catalytic upgrading of coal pyrolysis vapors. Uncited references
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[25]. Acknowledgement The authors gratefully acknowledge the financial support received from the National Key R&D Program of China (2018YFB0605000) and National Natural Science Foundation of Q3 China (21706173). References [1] P. Rathsack, Analysis of pyrolysis liquids obtained from the slow pyrolysis of a German brown coal by comprehensive gas chromatography mass spectrometry, Fuel 191 (2017) 312e321. [2] Z.Y. Niu, G.J. Liu, H. Yin, et al., Effect of pyridine extraction on the pyrolysis of a perhydrous coal based on in-situ FTIR analysis, J. Energy Inst. 92 (3) (2019) 428e437. [3] Y. Zhang, Z.W. Yuan, M. Margni, et al., Intensive carbon dioxide emission of coal chemical industry in China, Appl. Energy 236 (2019) 540e550. [4] Y.Y. Zhu, W. Wen, Y.M. Li, et al., Pyrolysis study of Huainan coal with different particle sizes using TG analysis and online Py-PI-TOF MS, In Press, J. Energy Inst. (2019). Q4 [5] Y. Zhao, J.T. Liu, W.S. Liang, et al., Influence of the valence state change of iron oxidation for pyrolysis by using density functional theory, Appl. Surf. Sci. 478 (2019) 313e318. [6] M.Y. Wang, L.J. Jin, Y. Li, et al., In situ catalytic upgrading of coal pyrolysis tar over carbon-based catalysts coupled with CO2 reforming of methane, Energy Fuel. 31 (9) (2017), 9356e8362. [7] Y.Y. He, R.F. Zhao, L.J. Yan, et al., The effect of low molecular weight compounds in coal on the formation of light aromatics during coal pyrolysis, J. Anal. Appl. Pyrolysis 123 (2017) 49e55. [8] G.L. Li, L.J. Yan, R.F. Zhao, et al., Improving aromatic hydrocarbons yield from coal pyrolysis volatile products over HZSM-5 and Mo-modified HZSM-5, Fuel 130 (2014) 154e159. [9] D.Q. Fu, X.H. Li, W.Y. Li, et al., Catalytic upgrading of coal pyrolysis products over bio-char, Fuel Process. Technol. 176 (2018) 240e248. [10] L.J. Jin, X.Y. Bai, Y. Li, et al., In-situ catalytic upgrading of coal pyrolysis tar on carbon-based catalyst in a fixed-bed reactor, Fuel Process. Technol. 147 (2016) 41e46. [11] J.Z. Han, X.D. Wang, J.R. Yue, et al., Catalytic upgrading of coal pyrolysis tar over char-based catalysts, Fuel Process. Technol. 122 (2014) 98e106. [12] Q.X. Lu, N. Zhang, Q. Yang, et al., Influence of calcium promoter on catalytic pyrolysis characteristics of iron-loaded brown coal in a fixed bed reactor, In Press, J. Energy Inst. (2019). Q5 [13] L. He, H.L. Hui, S.G. Li, et al., Production of light aromatic hydrocarbons by catalytic cracking of coal pyrolysis vapors over natural iron ores, Fuel 216 (2018) 227e232. [14] L.J. Yan, Y.H. Bai, X.J. Kong, et al., Effects of alkali and alkaline earth metals on the formation of light aromatic hydrocarbons during coal pyrolysis, J. Anal. Appl. Pyrolysis 122 (2016) 169e174. [15] M. Majka, G. Tomaszewicz, A. Mianowski, Experimental study on the coal tar hydrocracking process over different catalysts, J. Energy Inst. 91 (6) (2018) 1164e1176. €gler, et al., Influence of catalysts on the pyrolysis of [16] M. Seitz, W. Heschel, T. Na lignites, Fuel 134 (2014) 669e676. [17] Q. Li, X.Y. Feng, X. Wang, et al., Pyrolysis of Yulin coal over ZSM-22 supported catalysts for upgrading coal tar in fixed bed reactor, J. Anal. Appl. Pyrolysis 126 (2017) 390e396. [18] X.Y. Ren, J.P. Cao, X.Y. Zhao, et al., Catalytic upgrading of pyrolysis vapors from lignite over mono/bimetal-loaded mesoporous HZSM-5, Fuel 218 (2018)
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Catalytic conversion of coal pyrolysis vapors to light aromatics over hierarchical Y-type zeolites Q7,1
Peng Lv, Lunjing Yan, Yan Liu, Meijun Wang, Weiren Bao**, Fan Li* State Key Laboratory Breeding Base of Coal Science and Technology Co-founded By Shanxi Province and the Ministry of Science and Technology, Taiyuan University of Technology, Taiyuan, 030024, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 August 2019 Received in revised form 17 December 2019 Accepted 18 December 2019 Available online xxx
A series of hierarchical Y-type zeolites were prepared by a post-treatment method. All the samples were characterized using nitrogen adsorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and temperature-programmed desorption of ammonia (NH3-TPD). The results show that hierarchical Y-type zeolites with different porosities can be obtained, and the mesopore size can be controlled by changing the treatment conditions. The acidity of catalysts was also adjusted in this process. The catalysts were evaluated with respect to catalytic conversion of coal pyrolysis vapors to light aromatics online by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). Moreover, several model compounds were selected to evaluate the formation pathway of light aromatics during the upgrading of coal pyrolysis vapors over Y-type zeolite. It was found that pore-structure-modified Y-type zeolite has good catalytic performance for the upgrading of coal pyrolysis vapors. After catalytic cracking by EDY zeolites, the total amount of light aromatics such as benzene, toluene, ethyl-benzene, xylene, and naphthalene (BTEXN) in coal pyrolysis vapors increased from 5600 ng/mg (raw coal pyrolysis) to 18,800 ng/mg. The results of model compound catalytic pyrolysis show that Y-type zeolite is beneficial for the catalytic cracking of polycyclic aromatic hydrocarbons, the breaking of phenolic hydroxyl, and the decomposition of heterocyclic compounds, thus promoting the formation of BTEXN. Hierarchical catalysts with wide pore size and large mesopore volume contribute to the diffusion of bulky reactants and their contact with active sites in channels, further promoting the generation of light aromatics. © 2019 Published by Elsevier Ltd on behalf of Energy Institute.
Keywords: Coal pyrolysis Catalytic upgrading Light aromatics Hierarchical zeolite
1. Introduction Coal is the most abundant and widely distributed fossil energy on the earth [1], accounting for a relatively large proportion of fossil fuels consumed to meet China's primary energy demand [2,3]. Coal pyrolysis is the main stage of coal processing and utilization [4]. In addition to char and gas, coal pyrolysis also produces tar, a black and viscous mixture of hydrocarbons [5]. Coal tar contains a large proportion of heavy components. It is difficult to utilize heavy components as resources due to difficulty in separation [6]. The light components in tar such as benzene, toluene, ethyl-benzene, xylene, and naphthalene (BTEXN) are important chemical raw materials and widely used in the production of pesticides, plastics, and synthetic fibers [7]. Catalytic conversion of coal pyrolysis
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* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (W. Bao), [email protected] (F. Li).
vapors to light aromatics is a promising way for high value-added utilization of coal tar [8]. Carbon-based catalysts [9e11], metal catalysts [12e14], and zeolite catalysts [15e24] can promote the cracking of heavy components and generation of light aromatics in coal tar, thus improving the quality of tar. Among them, zeolite catalysts, especially Y-type zeolite and ZSM-5 zeolite, are favored because of their unique pore structure, suitable acid sites, and large surface area [18e24]. Ren et al. [20] used ZSM-5 with different SiO2/Al2O3 ratios to catalyze the conversion of lignite pyrolysis vapors to BTEXN and found that ZSM-5 with a high aluminum content is favorable for the production of light aromatics. Liu et al. [21] studied the catalytic upgrading of coal tar by metal-modified ZSM-5 and found that metal-modified ZSM-5 can promote the production of light aromatics and removal of oxygen-containing compounds. Sulfationacidified HZSM-5 catalyst can further improve the yield of light aromatics due to generation of the new strong acidic sites [22]. Yan et al. [23] used different types of zeolites for the catalytic conversion of gaseous tar. The results indicate that Y-type zeolite could
https://doi.org/10.1016/j.joei.2019.12.005 1743-9671/© 2019 Published by Elsevier Ltd on behalf of Energy Institute.
Please cite this article as: P. Lv et al., Catalytic conversion of coal pyrolysis vapors to light aromatics over hierarchical Y-type zeolites, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.12.005
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significantly increase the yield of BTEXN, while Al/SBA-15 had no obvious effect on the conversion of coal tar. Kong et al. [24] evaluated the catalytic conversion of gaseous tar using three Y-type zeolites with different acidities and found that Y-type zeolite had good catalytic performance for the production of light aromatics and catalytic cracking of heavy aromatics. Nevertheless, Y-type zeolite is typical microporous material with an average pore size of only 0.74 nm, severely limiting the mass transfer of reactants and products in the channel [26]. In addition, narrow pores easily deactivate the catalyst due to the blockage of pores by carbon deposition. Traditional mesoporous zeolites are limited in the field of catalytic cracking because of their weak catalytic activity and poor hydrothermal stability [27]. Hierarchical catalysts, where the advantages of microporous zeolites and mesoporous materials are combined, have great potential in catalytic reactions involving bulky molecules [28,29]. Gackowski et al. [30] obtained hierarchical Y-type zeolites by treating with an ammonia solution, and the samples exhibited promising catalytic properties in the liquid-phase isomerization of a-pinene. Verboekend et al. [31] also prepared hierarchical Y-type zeolites. Compared with conventional Y-type zeolite, pore-structuremodified Y-type zeolite has excellent adsorption and catalytic properties. Although hierarchical Y-type zeolites have been used in many catalytic reactions, studies on the catalytic conversion of coal pyrolysis vapors over hierarchical Y-type zeolites are limited. Especially, the effect of porous structure of Y-type zeolites on the formation of light aromatics has been rarely evaluated. In this
Table 1 Proximate and ultimate analyses of coal samples. Sample
Proximate analysis wt/%
Ultimate analysis wt/%, daf
Mad
Aad
Vad
C
H
O*
N
S
FCM
2.3
7.6
36.4
82.1
5.2
8.1
1.4
3.2
study, hierarchical Y-type zeolites with different porosities were prepared by sequential dealumination and desilication and evaluated in the upgrading of coal pyrolysis vapors by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). The effect of pore-structure-modified Y-type zeolites on the composition and distribution of gaseous tar from coal pyrolysis was evaluated. The results obtained in this study provide a new viewpoint for the design of catalysts used in the conversion of coal tar to light aromatics. 2. Experimental 2.1. Material A bituminous coal, named as FCM, obtained from Ningxia Hui Autonomous Region, China was used in this study. The proximate and ultimate analyses of coal samples are shown in Table 1. Commercial NaY zeolite was purchased from Catalyst Plant of Nankai University (Tianjin, China). Note: ad, air-dried basis; daf, dry and ash-free basis; *by difference. 2.2. Synthesis of hierarchical Y zeolites Hierarchical Y-type zeolites were synthesized by sequential dealumination and desilication [31]. In a specific procedure, NaY zeolites were stirred and refluxed in 0.1 M H4EDTA at 100 C for 6 h; subsequent washing, filtration, and drying afforded EY. This was then submitted to NHþ 4 ion exchange in 1.0 M NH4NO3 for three times at 80 C, followed by washing, drying, and calcination to obtain EDY0. In addition, the EY zeolites were treated with 0.1 and 0.3 M NaOH at 65 C for 0.5 h. After washing, filtration, and drying, the obtained products were stirred and refluxed in 0.1 M Na2EDTA at 100 C for 6 h, followed by washing, filtration, and drying and successively exchanged with 1.0 M NH4NO3 for three times. The final products were obtained in the H-form through calcination at 550 C for 4 h. The samples were denoted as EDY01 and EDY03,
Fig. 1. Isothermal N2 adsorption/desorption curve and aperture distribution of Y-type zeolites.
Table 2 Pore structure parameter and composition of Y-type zeolites. Samples
SBET (m2/g)
Vtotal (cm3/g)
Vmicro (cm3/g)
Vmeso (cm3/g)
SiO2/Al2O3
HY EDY0 EDY01 EDY03
609 587 585 684
0.28 0.31 0.44 0.54
0.21 0.16 0.13 0.20
0.06 0.12 0.27 0.32
5.34 9.01 8.12 6.56
Please cite this article as: P. Lv et al., Catalytic conversion of coal pyrolysis vapors to light aromatics over hierarchical Y-type zeolites, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.12.005
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Fig. 2. SEM images of Y-type zeolites.
Fig. 3. TEM images of Y-type zeolites.
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respectively. In addition, HY zeolite was obtained from parent NaY zeolite after three consecutive NHþ 4 ion exchange and calcination. 2.3. Characterization of zeolites The N2 adsorption characterization of the catalysts was carried out using an ASAP2460 N2 adsorption analyzer (Micromeritics, USA). Spectro Arcos inductively coupled plasma atomic emission spectrometry (Spectro, Germany) was used to test the chemical composition of the zeolite. An H-7650 scanning electron microscope (Hitachi, Japan) was used to observe the surface morphology of catalysts. Transmission electron microscopy (TEM) was performed using a Tecnai G20 transmission electron microscope (FEI, Holland). The acidity of catalysts was measured by temperatureprogrammed desorption of ammonia (NH3-TPD) using an AUTOCHEM 2920 automated chemisorption analyzer (Micrometrics, USA).
2.4. Catalytic experiment
Fig. 5. Effect of Y-type zeolites on the composition of coal pyrolysis vapors.
Pyrolysis experiments of coal and model compounds were performed using a CDS Analytical Pyroprobe 5250 (CDS, USA). In a specific experiment, the coal sample (1 mg) or model compounds (0.1 mg) and catalyst (0.6 mg) were loaded in a sample tube, separated by a thin layer of quartz wool, and supported by using a quartz rod at the bottom of the sample. The samples were heated to 700 C at a heating rate of 10 C/ms and held at that temperature for 15 s. The pyrolysis products were injected to a Focus GC/DSQ II GC/ MS instrument (Thermo Fisher, USA) through a transfer-line for online separation and detection. The chromatographic peaks of BTEXN were identified using the corresponding chemical standards (AccuStandard Inc., USA) and quantified using an external standard method.
Fig. 6. Effect of Y-type zeolites on the total yield of BTEXN in coal pyrolysis vapors.
Fig. 4. NH3-TPD profiles of Y-type zeolites.
Table 3 Acidity of zeolites obtained from the NH3-TPD results. Samples
Amount (mmol/g) Weak acid
Strong acid
Total acid
HY EDY0 EDY01 EDY03
2.36 0.65 0.73 0.92
1.58 1.44 1.58 1.88
3.94 2.09 2.31 2.80
Fig. 7. Effect of Y-type zeolite on the yield of phenolic compounds in coal pyrolysis vapors.
Please cite this article as: P. Lv et al., Catalytic conversion of coal pyrolysis vapors to light aromatics over hierarchical Y-type zeolites, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.12.005
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3. Results and discussion 3.1. Textural property and composition analysis The isothermal N2 adsorption and desorption curves and aperture diameter distribution of Y-type zeolites are shown in Fig. 1. The
Fig. 8. Effect of Y-type zeolite on the yield of heteroatom compounds in coal pyrolysis vapors.
5
isothermal adsorption curve of parent Y-type zeolite rapidly increased when the P/P0 is close to 0. With the increase in relative pressure, the adsorption rate became gentle and similar to the Itype isothermal curve, indicating that parent Y-type zeolite is a typical microporous material. A small number of mesopores with a
Fig. 10. Possible path of BTEXN formation from pyrene catalytic pyrolysis over Y-type zeolites.
Fig. 9. Total ion chromatogram of pyrene before (a) and after (b) catalytic pyrolysis over Y-type zeolites.
Please cite this article as: P. Lv et al., Catalytic conversion of coal pyrolysis vapors to light aromatics over hierarchical Y-type zeolites, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.12.005
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pore size of 2e3 nm was found in the pore size distribution of Ytype zeolite after dealumination treatment. After NaOH treatment, an apparent hysteresis loop appeared when the relative pressure was greater than 0.42, indicating the formation of mesopores. The pore size distribution shows that the diameter of mesopores increased with increasing concentration of alkali solution, from 2 to 3 nm up to ~10 nm as the concentration of NaOH increased to 0.3 M. Table 2 also shows that the mesopore volume (Vmeso) of HY is only 0.06 cm3/g. The Vmeso of zeolite clearly increased after sequential dealumination and desilication. When the concentration of NaOH is 0.3 M, the Vmeso reached 0.32 cm3/g. At the same time, it was also found that the SiO2/Al2O3 ratio of Y-type zeolite after dealumination increased from 5.34 to 9.01. After NaOH treatment, the SiO2/Al2O3 ratio of zeolite decreased due to the removal of silicon atom and constantly decreased with the increase in alkali solution concentration.
type zeolite shows homogeneous dark zones, indicating that they have a faultless structure. After sequential dealumination and desilication, the light gray areas were evenly distributed on the dark zones of TEM image of zeolites, indicating that mesopores were clearly generated after sequential dealumination and desilication. 3.3. Acidic properties To evaluate the effect of pore structure modification on zeolite acidity, NH3-TPD characterization of zeolite samples was carried
3.2. Morphology analysis The SEM images of parent and pore-structure-modified zeolites are shown in Fig. 2. The average particle size of parent Y-type zeolite is about 0.6e0.8 mm. The particle has a smooth surface and clear edges. After sequential dealumination and desilication, the surface of particles became rough and uneven with obvious erosion and cracking. This indicates that the dealumination and desilication destroyed the morphology of zeolite. TEM was applied to further observe the morphological changes in the samples. Fig. 3 shows that the morphology of EDY zeolite is clearly different from that of parent Y-type zeolite. The parent Y-
Fig. 12. Possible path of BTEXN formation from 3,4-dimethylphenol catalytic pyrolysis over Y-type zeolites.
Fig. 11. Total ion chromatogram of 3,4-dimethylphenol before (a) and after (b) catalytic pyrolysis over Y-type zeolites.
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out. As shown in Fig. 4, the results show that all the curves of samples exhibit three distinct desorption peaks, corresponding to the physical adsorption of ammonia below 100 C, weak acid sites in the low-temperature zone of 100e250 C, and strong acid sites in the high-temperature zone of 250e600 C [32]. Table 3 shows the acidity distribution of zeolites calculated from the NH3 adsorption amount. The results show that HY zeolite has high weak and strong acid sites, 2.36 and 1.58 mmol/g, respectively. After dealumination treatment, the weak and strong acid sites of Y-type zeolite decreased to 0.65 and 1.44 mmol/g, respectively. Both the weak and strong acid sites of EDY zeolite increased after alkali treatment, from 0.65 to 1.44 mmol/g for EDY0 and up to 0.92 and 1.88 mmol/g for EDY03 with the increase in NaOH concentration. The acidity of Y-type zeolite is related to the amount and distribution of
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aluminum atoms in the zeolite. With the increase in alkali concentration, more and more silicon atoms were removed from the zeolite, and the SiO2/Al2O3 ratio of zeolite gradually decreased. Therefore, the amount of acid sites of EDY samples increased with the increase in the concentration of alkali. Similar results were reported by García et al. [32]. The acidic sites of Y-type zeolite increased after desilication by alkali treatment because of the retention of aluminum-rich species. 3.4. Catalytic performance of zeolites The composition of coal pyrolysis vapors before and after catalysis over Y-type zeolites is shown in Fig. 5. The results show that the relative content of aliphatic substances, phenolic
Fig. 13. Total ion chromatogram of dibenzothiophene before (a) and after (b) catalytic pyrolysis over Y-type zeolites.
Fig. 14. Possible path of BTEXN formation from dibenzothiophene catalytic pyrolysis over Y-type zeolites.
Please cite this article as: P. Lv et al., Catalytic conversion of coal pyrolysis vapors to light aromatics over hierarchical Y-type zeolites, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.12.005
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compounds, and heterocyclic compounds decreased, and the relative content of monocyclic aromatics and bicyclic aromatics increased after the catalytic conversion of coal pyrolysis vapors. The yield distribution of BTEXN before and after catalytic conversion is shown in Fig. 6. The HY zeolite significantly increased the amount of light aromatics in pyrolysis tar. After catalytic upgrading by HY, the total amount of BTEXN in pyrolysis products increased from 5600 ng/mg to 11,200 ng/mg. Bulky aromatic hydrocarbons underwent catalytic cracking to form free radicals under the action of acid centers of zeolites, and the radical fragments with small molecular weights were stabilized by the hydrogen-rich components in vapors to form light aromatics [33]. Several different catalysts were used in the catalytic upgrading of coal tar by Majka et al. [15]; the results show that Y-type zeolite has higher activity for the conversion of macromolecular tar to light aromatics. After sequential dealumination and desilication, the catalytic ability of Y-
type zeolite was improved in different levels. After catalysis using EDY03 zeolite, the amount of light aromatics increased up to 18,800 ng/mg. Wang et al. [34] found that hierarchical ZSM-5 obtained by HF treatment can promote the formation of light aromatics. Mesoporous ZSM-5 zeolite prepared by desilication can increase the yield of aromatics and decrease the coke formation also reported by Li et al. [35]. Fig. 7 shows the yield of several typical phenolic compounds in coal pyrolysis vapors before and after catalytic reaction. The amounts of phenolic substances such as phenol, cresol, xylenol, tricresol, ethyl-phenol, and methyl-ethyl-phenol decreased to varying degrees after the pyrolysis of volatiles was catalyzed by Y-type zeolite. The yield of several typical heterocyclic compounds in coal pyrolysis vapors before and after catalytic reaction is shown in Fig. 8. The results show that the yield of thiophene and furan derivatives significantly decreased after catalytic cracking. This indicates that Y-type zeolite can promote the conversion of phenolic substances and heterocyclic compounds in coal tar, thus improving the quality of tar.
3.5. Formation pathway of coal pyrolysis vapors to light aromatics
Fig. 15. Peak areas of BTEXN of different model compounds before and after catalytic pyrolysis over Y-type zeolites.
To determine the formation pathway of light aromatics during the catalytic conversion of coal pyrolysis vapors, three model compounds, pyrene, 3,4-dimethylphenol, and dibenzothiophene representing polycyclic aromatics, phenolic compounds, and heterocyclic compounds in coal tar, were selected. The products of pyrene before and after catalytic pyrolysis over Y-type zeolite are shown in Fig. 9. After catalytic pyrolysis, the yield of light aromatics such as BTEXN significantly increased, the total peak area of BTEXN is 9.1 times, as shown in Fig. 15, higher than that of direct pyrolysis. In addition to light aromatic hydrocarbons, 4,5-dihydropyrene, phenalene, and naphthalene derivatives were also found in the products of pyrene-catalyzed pyrolysis over Y-type zeolite. Shimada et al. [36] and Pujro et al. [37] carried out catalytic cracking experiments of polycyclic aromatics and inferred that during the catalytic upgrading of coal tar, polycyclic aromatic hydrocarbons first undergo hydrogenation reaction to saturate one of the aromatic rings, and then the saturated cyclic hydrocarbons undergo
Fig. 16. Schematic diagram of catalytic conversion of coal pyrolysis vapors to light aromatics.
Please cite this article as: P. Lv et al., Catalytic conversion of coal pyrolysis vapors to light aromatics over hierarchical Y-type zeolites, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.12.005
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ring-opening reaction under the action of active center of solid-acid catalysts. Finally light aromatics are formed [24]. Fig. 10 shows the possible routes for catalytic pyrolysis of pyrene over Y-type zeolite. The products obtained from the pyrolysis of 3,4-dimethylphenol with and without Y-type zeolite are shown in Fig. 11. After catalytic pyrolysis, the yield of light aromatic hydrocarbons significantly increased, and phenol and other phenolic compounds were also produced. The possible reaction pathway of 3,4-xylenol to light aromatics over Y-type zeolite is shown in Fig. 12. Solid-acid catalyst can reduce the content of phenols in tar and promote the conversion of phenols to light aromatics, similar results are also reported in other studies [38e40]. Heuchel et al. [40] elucidated the catalytic conversion mechanism of phenolic compounds over a zeolite catalyst in detail and found that phenolic compounds could form aromatics by dehydrogenation under the effect of active center of catalysts. At the same time, other benzene series and phenolic compounds are produced by dealkylation and transalkylation of methyl side chain [41]. Fig. 13 shows the product distribution of dibenzothiophene before and after catalytic pyrolysis over Y-type zeolite. The total peak area of BTEXN after catalytic pyrolysis is 17.2 times as large as that without catalyst. In addition to light aromatic hydrocarbons, biphenyl, fluorene, benzothiophene derivatives, and phenylmercaptan were also found in the catalytic pyrolysis products. This indicates two possible catalytic pathways for the conversion of dibenzothiophene over Y-type zeolite, as shown in Fig. 14. One route can be described as follows: A benzene ring of dibenzothiophene first undergoes hydrogenation and ring-opening reaction to produce benzothiophene derivatives. Benzothiophene derivatives continue to undergo hydrogenation and ring-opening reaction of thiophene ring to finally produce phenylmercaptan and benzene series. Another route can be described as follows: The breaking of sulfur-carbon bond of dibenzothiophene under the action of catalyst and free radical fragments is stabilized by hydrogen-rich free radicals to form biphenyl derivatives and fluorene, and the biphenyl derivatives and fluorene continue to undergo cracking to produce light aromatic hydrocarbons. The catalytic mechanism of sulfur-containing aromatic heterocyclic compounds to light aromatics is similar to that of hydrodesulfurization of dibenzothiophene reported previously, because Y-zeolite has been applied to the removal of organic sulfur because of its unique pore structure and excellent H-transfer ability [42,43]. Therefore, we can propose a possible pathway for the catalytic conversion of coal pyrolysis vapors to light aromatics over hierarchical Y-type zeolites, as shown in Fig. 16. Y-type zeolite is a promising catalyst for the conversion of coal pyrolysis vapors to light aromatics because it can promote the catalytic cracking of polycyclic aromatics, phenols, and heterocyclic compounds. The large pore size and pore volume of hierarchical Y-type zeolites further facilitate the mass transfer of reactants in the channel and contact with the active center, thus promoting the formation of light aromatics. In addition, the acid sites of zeolite after pore structure modification have been adjusted, which is also an important factor in improving the catalytic activity. 4. Conclusion In this study, the catalytic conversion of coal pyrolysis vapors to light aromatics over hierarchical Y-type zeolites was evaluated. Ytype zeolites with different porosities were prepared by sequential dealumination and desilication. The mesopore size of catalysts continually increased with increasing severity of the alkali treatment, from around 2.5 nm to up to ~10 nm. Hierarchical Y-type zeolites exhibited excellent catalytic performance for the catalytic upgrading of coal pyrolysis vapors and significantly improved the yield of light aromatics. Compared with raw coal pyrolysis, the total
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amount of BTEXN in FCM coal gaseous tar increased from 5600 ng/ mg to 18,800 ng/mg after catalytic upgrading over EDY03 zeolites. Catalytic pyrolysis experiments of model compounds show that Ytype zeolite can promote the cracking of polycyclic aromatics and heterocyclic compounds and the breaking of phenolic hydroxyl groups, thus promoting the formation of light aromatics. The wide pore diameter distribution and large pore volume of porestructure-modified Y-type zeolites are favorable for bulky reactants in the pyrolysis tar for transport in channels and access to the active site, thus promoting the generation of light aromatics. In addition, the distribution and amount of acidity of the zeolites were adjusted during pore structure modification, which also has a substantial role in the catalytic upgrading of coal pyrolysis vapors. Uncited references
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