Enhancement of Aromatic Products from Catalytic Fast Pyrolysis of

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Enhancement of aromatic products from catalytic fast pyrolysis of lignite over hierarchical HZSM-5 by piperidine-assisted desilication Xue-Yu Ren, Jing-Pei Cao, Xiao-Yan Zhao, Zhen Yang, Sheng-Nan Liu, and Xian-Yong Wei ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03185 • Publication Date (Web): 03 Dec 2017 Downloaded from http://pubs.acs.org on December 21, 2017

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ACS Sustainable Chemistry & Engineering

Enhancement of aromatic products from catalytic fast pyrolysis of lignite over hierarchical HZSM-5 by piperidine-assisted desilication Xue-Yu Ren, Jing-Pei Cao*, Xiao-Yan Zhao**, Zhen Yang, Sheng-Nan Liu, Xian-Yong Wei

Key Laboratory of Coal Processing and Efficient Utilization (Ministry of Education), China

University of Mining & Technology, No. 1 Daxue Road, Xuzhou, Jiangsu, 221116, China

*

To whom correspondence should be addressed.

Email address: [email protected]; [email protected] (Prof. J. P. Cao)

[email protected] (A./Prof. X. Y. Zhao)

Phone: +86 516 83591059

*

Corresponding author. Tel./fax: +86 516 83591059. E-mail address: [email protected]; [email protected] (J. P. Cao) ** Corresponding author. Tel./fax: +86 516 83591059. E-mail address: [email protected] (X. Y. Zhao)

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KEYWORDS: CFP, PAD, Hierarchical HZSM-5, 5Zr/Co-AT0.2-PI0.3, Aromatics, Pyrolysis vapors ABSTRACT HZSM-5 was post-treated by piperidine-assisted desilication (PAD) and metallic Co and/or Zr modification for introducing the meso/microporous system and metal active sites to enhance the activity for catalytic fast pyrolysis (CFP) of lignite for aromatic products. CFP was conducted over parent and hierarchical HZSM-5 in a drop tube reactor at 600 oC and a gas resident time of 1.5 s. The results showed that assisted desilication with piperidine (PI) concentration of 0.3 mol/L (AT0.2-PI0.3), retained the morphology of HZSM-5 and avoided severe alkaline corrosion. It was due to the shield of the zeolite crystals from extensive dissolving of NaOH by organic amines. It not only decreased the deactivation rate of the catalyst, but also enhanced the mass transfer in the catalyst. The selectivity of light aromatics (LAs) such as benzene, toluene, ethylbenzene, xylene and naphthalene (BTEXN) remarkably increased to 24.9% over AT0.2-PI0.3 in comparison to the HZSM-5. In addition, introducing bimetallic Zr-Co facilitated the hydrogen transfer of pyrolysis fragments at the metal sites and speeded up the cracking reaction and deoxygenation step of the cascade reactions. 5Zr/Co-AT0.2-PI0.3 with Zr-loading of 5% exhibited an excellent activity for upgrading of pyrolysis vapors, and its LAs selectivity further increased to 30.5%. Meanwhile, the organic oxygen species (OOSs) and macromolecular compounds (C14+ and C18+) contents were decreased gradually. This work provides a potential approach for directional production of LAs from lignite. INTRODUCTION The focus of the catalytic fast pyrolysis (CFP) of lignite at moderate temperature is to convert heavy species in pyrolysis vapor into valuable chemicals, mainly composed of light aromatics (LAs), such as benzene (B), toluene (T), ethylbenzene (E), xylene (X) and naphthalene (N).1-3


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HZSM-5 was considered to be the most frequently used catalyst in CFP.4,5 Its moderate pore openings, internal pore space and strong acid sites are in favor of aromatic products.6,7 However, the diffusion restrictions and fast deactivation rates are the main drawbacks of HZSM-5.6,8-10 Generally, the relatively small pore constrains the molecular diffusion, transportation and access to the active sites located inside zeolite channels.6,11 Coke will also be easily formed due to low mass transport, which accelerates the deactivation of the catalyst. However, hierarchical networks and a portion of acidity can participate in the catalytic cracking of heavy species. Especially, the mesoporous system facilitates the capture of the macromolecular compounds (MCs), and thus alleviates the coke formation.12,13 The post-synthesis treatment, such as desilication with alkali solutions (i.e., CH3ONa, NaOH, NaHCO3, NH4OH, TPAOH), was regarded as a simple approach to create mesoporosity within zeolite crystals.14-18 Wang et al.17 investigated the one-step catalytic aromatization of glycerol over different hierarchical HZSM-5 prepared by desilication with different alkali liquors. They found that hierarchical structure with smaller intramesopores (3-5 nm) had better shape-selective performance for BTX aromatics and slower carbon deposition rate. Tarach et al.18 studied the effects of acidity and accessibility of desilicated HZSM-5 in terms of their effectiveness as catalyst for acid-catalyzed cracking processes, and found that the cracking of macromolecular (1,3,5-triisopropylbenzene (TIPB) versus n-decane) over the hierarchical zeolites proceeded more effectively than over their microporous counterparts. NaOH&tetrabutylammonium hydroxide (TBAOH) treatment gave stronger sites than NaOH and improved the TIPB cracking activity. However, the rate of silicon dissolution is relatively fast when using NaOH alone, resulting in an uncontrollable formation of mesopores and the loss of a certain amount of acid sites. Moreover, the desilicated HZSM-5 is a potential support for CFP because of the improved diffusion and access to



mesoporos/micropores. However, it is rarely reported that how to suppress the loss of acid sites during desilication. It has been confirmed that the addition of protective agents, such as TPA+, TBA+ and piperidine (PI), has indeed played a major role in curbing the serious desilication.19-21 In addition, the metal or metal ions exchanged or loaded on the zeolites can improve the selectivity of pyrolysis tar.22,23 Cheng et al.24 prepared a series of bimetallic Co-Zn modified HZSM-5 for upgrading pyrolysis bio-oil and found that bimetallic Co-Zn/HZSM-5 was more effective for decarboxylation and decarbonylation than monometallic Co/HZSM-5 or Zn/HZSM-5. This was due to the effect of synergies between Co and Zn on HZSM-5 support. Li et al.25 studied the zeolites modified by Fe, Zr, and Co and found that 4Zr/HZSM-5 promoted the formation of more B and its derivatives, whereas 4Fe/HZSM-5 produced a higher yield of N and its derivatives during CFP of biomass. Therefore, the efficient and inexpensive metal-modified bifunctional HZSM-5 might be designed for CFP of vapors, according to the selectivity of products by different metal ions. In this work, we focus on the protective effect of the NaOH&PI mixture with different concentration on zeolite and the reactivity of hierarchical zeolites during the CFP of Baiyinhua lignite (BYHL). The presence of PI as strong organic base during desilication does offer a greater mesoporosity development and higher acid strength than NaOH alone. Furthermore, the metallic Co and/or Zr was introduced into AT0.2-PI0.3 to upgrade pyrolysis vapor, and the optimal Zr loading for improving the BTEXN selectivity was investigated in detail. EXPERIMENTAL SECTION BYHL sample and catalyst preparation BYHL as the feed sample was collected from Inner Mongolia, China, and sieved to a particles size of 0.5-1.0 mm and subsequently dried at 107 oC for 24 h before use.26-27 The basic properties of


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BYHL27 were described in detail in the Supporting Information (SI-1). A commercial HZSM-5 (SiO2/Al2O3=27) was purchased from Nankai University Catalyst Corp, China. Prior to treatment, it was calcined in a tubular furnace at 550 oC for 5 h to remove impurities and absorbed water.28 The hierarchical HZSM-5 was prepared by piperidine-assisted desilication (PAD) method using various PI concentrations (0.1, 0.3, 0.5 and 1.0 mol/L) and an aqueous solution of 0.2 M NaOH.29 Then all the Na-form zeolites were converted to H-form. The sample treated by NaOH&PI was regarded as AT0.2-PIx, where x represents the molar concentration of PI. Meanwhile, the catalyst, which was treated with NaOH alone or uncalcined, was prepared and denoted as AT0.2 or AT0.2-Tem, respectively. The Co and/or Zr modified zeolites were synthesized by wet impregnation method. The support, denoted as 3Co-AT0.2-PI0.3, was prepared through the mixture of Co(NO3)2·6H2O solution with AT0.2-PI0.3. Then the Zr(NO3)4·5H2O as second introduced metal with various Zr loading (1, 3, 5, 8, 10 and 15 wt.%) was impregnated in 3Co-AT0.2-PI0.3. The synthesis of yZr/Co-AT0.2-PI0.3, where y represents the Zr loading, was based on 3 wt.% of Co content.30 Meanwhile, 5Zr-AT0.2-PI0.3 was prepared and preferentially selected for comparison. A detailed description of catalyst preparation can be found in the Supporting Information (SI-2). Finally, the post-treated zeolites were characterized by several techniques (Supporting Information, SI-3), such as ICP-AES, N2 adsorption-desorption instrument, SEM, XRD, NH3-TPD and FTIR. Catalytic pyrolysis of lignite CFP of BYHL was carried out in a drop tube reactor (360 mm length, 22 mm i.d.).26 Based on the previous study, the CFP temperature and a gas resident time were set at 600 oC and 1.5 s, respectively.26 The reactor was first flushed with Ar and heated up to 600 oC at a heating rate of 15 o

C/min. However, the metal loaded catalyst need to be reduced for 1 h before feeding.28 The

experimental process was amply described in the Supporting Information (SI-4). Each experiment



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was operated at least twice to obtain reliable data and an average value. The product yield and selectivity of BTEXN in tar were calculated according to the following equations: Product Yield (wt.%) = Selectivity (C-%)=

Moles of carbon in a specifial product ×100% Moles of carbon in feed

Moles of carbon in BTEXN ×100% Moles of carbon in the pyrolysis tar

(1) (2)

Moreover, as for the pyrolysis products including pyrolysis tar and water, char, gas and coke, they were analyzed by Karl-Fischer titrator, CHNS elemental determinator, GC-MS, GC and TG analyzer11 (Supporting Information, SI-5). RESULTS AND DISCUSSION Structural and textural characterization As listed in Table 1, the SiO2/Al2O3 ratio decreased from 27 to 13 over AT0.2, while the SiO2/Al2O3 ratio of AT0.2-PI0.3 decreased to 20, indicating that adding PI greatly controlled desilication. However, adding a higher PI concentration leads to the remarkable decrease in SiO2/Al2O3 ratio. It could be interpreted by the desilication mechanism that the Si-O-Al bond is relatively more difficult to fission than the Si-O-Si bond.17 Moreover, the hydrophobicity of PI makes it tend to interact with the hydrophobic Si-OH groups in zeolite crystals via electrostatic interactions other than with the water molecules.20,31 The development of mesopore system after desilication leads to the enhancement of extra surface area (Sext) over AT0.2, compared with the HZSM-5. The alkaline leaching typically leads to a slight drop in the micropore volume (Vmicro), implying that the formation of mesopores inevitably causes the disappearance of a certain micropores.12 However, the assisted desilication caused the decrease in specific surface area (SBET) and Sext when the PI concentration lower than 0.3 mol/L, once beyond 0.3 mol/L, the SBET and Sext began to increase. The pore size of HZSM-5 mainly centers in the range of less than 2 nm (Figure 1b). Obviously, desilication with NaOH alone caused


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the disappearance of micropores (Figure 1c).14 Moreover, the pore size distribution is relatively dispersed when 0.3 mol/L PI was added (Figure 1d). It is feasible for this hypothesis that adding PI protected the zeolite framework against the alkali corrosion.21 The textural properties of AT0.2-Tem are similar to that of AT0.2-PI0.1, indicating that the template plays an important role in protecting the structure of zeolite from damage during alkali treatment. However, it is possible for the template to occupy the channel without calcination. Therefore, after calcined, the desilication with adding PI achieved better channel protection and mesoporous development. In addition, Figures S1 and S2a (SI-6) displayed that the post-treated zeolites exhibited both types I and IV isotherms with an obvious hysteresis loop, revealing that the formation of a hierarchical porous system with microporosity and mesoporosity.14,32 The SBET and total pore volume decreased when adding Co and/or Zr to the AT0.2-PI0.3, which was possibly related to pore blocking by metal species dispersed in the internal pores and channels or the presence of metal oxide aggregates on the external surface.30,33 Figure S2b (SI-6) showed that the pore diameter increased slightly with increasing Zr loading, although the size range remained relatively constant. Therefore, considering the above factors, 5Zr/Co-AT0.2-PI0.3 was regarded as potentially beneficial for the gas-solid interaction during CFP process. As shown in Figure 2, HZSM-5 has a quite smooth surface with typical hexagonal shape and sheet-like structure.17 Upon alkali-treatment, lots of gullies appear due to alkali corrosion. Both the overall sizes and morphology of outer surfaces of the zeolite crystals show different features depending on the PI concentration and severity of alkaline treatment (Figures 2b-f).14,21 AT0.2 has a rougher surface and deeper destruction that its sheet-like structure was almost destroyed completely and even disappeared due to the high surface energy and unstable chemical properties. Besides, its hexagonal shape was also damaged deeply. With the co-existence of PI, the morphology of



post-treated zeolites was retained. In particular, the AT0.2-PI0.3 avoided severe alkaline corrosion (Figure 2d), indicating that the addition of organic amines shielded the zeolite crystals from extensive dissolving by NaOH attacking.21 However, the morphology was corroded seriously with raising the PI concentration (Figures 2e-f). This provides some evidences that the protective effect of PI could prevent deep destruction of HZSM-5 skeleton from severe desilication.21 The SEM micrographs in Figures 2g-i indicated that the AT0.2-PI0.3 support appeared in agglomeration due to the interconnection of particles. Besides, its rough surface could easily adsorb and accumulate metal particles to the outer surfaces of zeolite particles.34 In Figure 3, it can be confirmed that the post-treated zeolites still retained the characteristic reflections of MFI topology, although some reduction of χ can be observed due to the removal of framework T-atoms (Table1).6,23,32 The desilication performed in NaOH alone greatly caused the decrease in peak intensities and led to severe destruction of zeolite framework (Figure 3a). Besides, the treatment in the mixture of NaOH&PI also destructed the crystal structures of HZSM-5 with raising PI concentration (Figure 3a). PI as a protective agent in NaOH solution plays a certain inhibitory effect on the destruction of HZSM-5 framework structure. Similar results were reported in some literatures.19-21 The framework of zeolite almost unchanged after loaded Co and/or Zr (Figure 3b). However, the intensities of 3Co-AT0.2-PI0.3, 5Zr/Co-AT0.2-PI0.3 and 5Zr-AT0.2-PI0.3 decreased significantly, especially for 3Co-AT0.2-PI0.3, which its χ value severely reduced to 33 (Table 1). This might be due to the deposition of Co and/or Zr metal oxides in the inside pore and channels of zeolite.23,32 In addition, the crystallite peaks of metal oxides cannot be found in the spectra, revealing that the metallic components were well dispersed on the internal and external surfaces of the zeolite.23,25 It was noted that the χ value of 5Zr/Co-AT0.2-PI0.3 was between 3Co-AT0.2-PI0.3 and 5Zr-AT0.2-PI0.3, implying that the formation of Zr-Co alloy replaced the


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interaction between metal and proton of zeolite.32,33 Based on the literature,35,36 the FTIR spectra in Figure S3 (SI-6) clearly showed that the typical characteristic peaks of HZSM-5 are not destroyed during alkali treatment and metal modification. However, the band at 3000 cm-1 in the catalyst treated by the NaOH&PI mixture solution are assigned as the C-H stretching vibrations of the PI molecules.21 These organic species-related bands, absent in the parent HZSM-5, should be due to the PI molecules incorporated into HZSM-5 channels or on the crystal external surface. The PI molecules could protect the surrounding framework silicon from extensive crystal dissolution, leading to a controllable desilication.17,21 Acidity and accessibility of sites Figure S4 (SI-6) displayed typical two desorption peaks of the zeolites centering at around 200 and 400 oC, corresponding to the weak and strong acid sites, respectively.37 The acid quantities of the zeolites (Table 2) showed that the high temperature peak intensity became weaker after desilicating with NaOH only, whereas the low temperature peak became broader. It indicated that alkali treatment mainly caused the decrease in the amounts of Brønsted acid sites because the NaOH solution might damage not only Si, but also the framework Al.38 When adding the PI, the high temperature peak was greatly retained, and its mid-strong acid value increased gradually with raising the PI concentration. The total acid amount of AT0.2-PI1.0 is up to 0.88 mmol/g, which is higher than HZSM-5 (0.75 mmol/g), due to the decrease of SiO2/Al2O3 ratio. AT0.2-PI0.3 shows relatively low total acid amount (0.69 mmol/g), suggesting that excessive PI does not protect the zeolite skeleton, but enhances alkaline corrosion, which exposing the framework aluminum while removing the Si.21 Adding Co significantly affects the acidic properties of the AT0.2-PI0.3. The strong acid peak around 600 oC should be attributed to the tendency of Co to form a tetrahedral CoO4−2 arrangement, especially in mesoporous structure, which offered more acidity in the zeolite.39 When



metal-Zr was further introduced, the total acidity significantly increased because the attached Zr had a chance to internally incorporate to the desilicated zeolite framework and the vacant sites were already equipped by Co ions. Therefore, some of metallic Zr was just attached on the external surface of AT0.2-PI0.3, which should partly be responsible for the increase in the strong acidity of 5Zr/Co-AT0.2-PI0.3. However, the total acidity of 3Co-AT0.2-PI0.3 is low compared with 5Zr-AT0.2-PI0.3, which can be interpreted that the amount of incorporated Zr was much higher than Co. Mechanism of desilication and metal modification A detailed analysis of alkali treatment process is showed in Figure 4 to provide insight into the mesopore formation mechanism and acidity changes and its effect on the catalytic performance. Alkali treatment of zeolite is a silicon-selective process,6,11,17 leading to the formation of secondary mesopore and the bared Al species due to their regular distribution throughout the zeolite structure. Typically, the Si-O-Al bond is not easy to hydrolyze in alkaline solutions, because of the negative charge of the AlO4- tetrahedron with the four-coordinated Al is protected from the attack of OH-, whereas each four-coordinated Al can also protect the four adjacent Si atoms from attack. Thus, the Si-O-Si bond without adjacent AlO4- tetrahedron is more easily broken to form terminal Si-OH.12 The HZSM-5 with lower SiO2/Al2O3 ratio owns higher Al content and acid strength/ amount, thus it is inevitable to dealumination accompanied by loss of four-coordinated Al during alkali treatment. In addition, desilication significantly affects the zeolite crystallinity and leads to a decrease in the particle size, as shown in the Figures 1 and 3. However, the introduction of PI affected the formation of mesopore (Figure 4(b)). Evidently, a balance between the PI protection and dissolution of zeolite crystal can be realized at a suitable PI concentration, introducing a large amount of mesopores into zeolites while preserving the maximum Vmicro.21 Alkali corrosion acts on the outer


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sphere of HZSM-5 and creates sheet-like pores. Diffusion of OH- into the inner micropores further resulted in alkali corrosion around them. Hollow mesopores were then formed. Different from the apparent desilication caused by NaOH alone, the hydrophobicity of PI makes it tend to interact with the hydrophobic Si-OH groups in zeolite crystals via electrostatic interactions to prevent HZSM-5 crystals from further desilication.17 When the active metal Co and Zr was introduced as the acceptors of electrons, zeolite structure with defective atoms was internally incorporated with them, leading to the formation of metal active site.14 Catalytic performance in CFP Effect of the PI concentration in alkali treatment As shown in Figure 5, the coke yield derived from AT0.2 decreased by 0.1 wt.% as against that of HZSM-5. The coke yield was gradually decreased from 3.2 to 1.9 wt.% over AT0.2-PI0.3. Water formation was enhanced and its yield reached the maximum about 11.2 wt.% over AT0.2-PI0.3. Accordingly, the tar yield was the lowest among the catalysts. It might be due to the cracking, dehydration, decarbonylation and decarboxylation reactions catalyzed by the acid sites in AT0.2-PI0.3.36 As for the desilicated HZSM-5, the gas yield reached 18.6 wt.% over AT0.2-PI0.3, indicating that AT0.2-PI0.3 favored for the catalytic reactions to form water and gaseous products, which attributed to preserving the large amount of acid sites and recreating hierarchical pore.21 In addition, AT0.2-Tem as reference also showed a good catalytic performance, which can give a good explanation for this hypothesis. The pyrolysis vapor derived from pyrolysis reaction could undergo secondary cracking reactions to generate small-molecule gaseous products (Table 3).25,38 The main components of the gas are H2, CO and CO2. Small amounts of light alkenes/alkanes were also detected. The desilicated zeolites could promote decarbonylation and decarboxylation to generate CO and CO2, which is the preferable method of oxygen removal during CFP process.40 The



CO2/CO mole ratio gradually increased until it reached 0.62 over AT0.2-PI0.3. It can be concluded that the desilicated zeolite favors the decarboxylation reaction compared with decarbonylation reaction.26 In addition, the carbon balance of special products was calculated based on Eq. (1) and displayed in Figure S5 (SI-6). The gaseous carbon (Gas-C) yield consisted of CO, CO2 and light alkenes/alkanes, while the carbon in tar (Tar-C) was obtained by difference. AT0.2 enhanced the formation of Tar-C, but no obvious change can be found in Tar-C yield when PI was added. It can be inferred that insufficient acid sites on AT0.2 caused a reduction in catalytic activity for CFP, and finally led to a high Tar-C yield.21,40 A number of the MCs was deposited on the catalyst to form the carbon deposition (Dep-C).41 Besides, adding PI led to the significant increase in Gas-C yield at the expense of the tar, because adding the PI retains a certain amount of acid sits in zeolites. Figure 6 displayed the selectivity of BTEXN calculated by Eq. (2). The selectivity of BTEXN increased over desilicated HZSM-5 in comparison with parent HZSM-5. The total BTEXN amount increased to 24.9% over AT0.2-PI0.3, whereas B and T, as the main components of LAs, and its selectivity was up to 11.0% and 6.7%, respectively. However, the selectivity of B and T began to decrease gradually at PI concentration above 0.3 mol/L. As for AT0.2-Tem, the selectivity of BTEXN equated to AT0.2-PI0.1, indicating that the presence of a protective agent protected the active sites of HZSM-5 from -OH attack.22 The acid sites provide the adsorption site for the hydrogen transfer reaction of the pyrolysis vapor, while the hierarchical porous provide the diffusion channels for cracking of small-molecule compounds.12,40 In addition, the selectivity of m, p-X reduced somewhat, whereas the selectivity of Ns increased with the raising PI concentration. The selectivity of Ns over AT0.2-PI0.3 was 5.8%, which is about 1.3 times higher than that over HZSM-5, indicating that AT0.2-PI0.3 has high catalytic activity for the formation of LAs. Compared with the previous


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work, this study achieved a breakthrough that enhanced the BTEXN content through the utilization of desilicated HZSM-5.26 The chemical compositions of tar were categorized into five species including monocyclic aromatics (C6+/MAHs), polycyclic aromatic hydrocarbons (PAHs), including bicyclic aromatics (C10+) and tricyclic aromatics (C14+) and tetracyclic aromatics (C18+), organic-oxygen species (OOSs), organic-nitrogen species (ONSs) and organic-sulfur species (OSSs). As shown in Figure 7, in the presence of HZSM-5, the total aromatics content is about 79.7% (based on peak area%), whereas the OOSs content was around 19.1%. AT0.2 led to the decrease in total aromatics content and the increase in OOSs content. The results confirmed that the activity for CFP is closely related to the acid sites and pore structure of the catalyst.6,42 Adding PI in desilication process improved the catalytic performance of desilicated HZSM-5 obviously. The total aromatics content over AT0.2-PI0.3 reached the highest value of 84.6%, especially the C6+ species content. At a higher PI concentration, the total aromatics content began to decrease gradually. These results indicate that the heavy species that underwent catalytic cracking were converted into aromatics and small-molecular gaseous products.6,38 The acid sites determine the crack degree of the aromatic side chains, while the channels of zeolite control the shape selectivity of the pyrolysis molecules.6 Based on the order of bond energy of Car-H > Cal-H > Car-Cal, the side chain in the aromatic ring is unstable.42 These pyrolysis fragments containing aromatic rings are absorbed by the acid site when passing through the catalyst layer. Their side-chains are activated by protonic acids, which trigger cracking reaction and the dealkylation of side chain.1 Moreover, the acid sites can promote the deoxygenation reaction. In particular, the -OH in phenols can be polarized after being absorbed by acid sites and spilt off from the aromatic ring, then the aromatic nucleus combined with H radical.6,43 Besides, controllable micro/mesoporous system in AT0.2-PI0.3 provides implantation sites for heavy species to



go through a secondary cracking reaction. Consequently, large amounts of aromatics can be produced over AT0.2-PI0.3 during CFP. Effect of Bimetallic Zr-Co loading As shown in Figure 8, the catalytic performance of Co and/or Zr modified zeolites were improved somewhat compared with the AT0.2-PI0.3 (Figure 5). The gas and water yield first increased and then decreased with the increase in Zr loading. The gas and water yield over 5Zr/Co-AT0.2-PI0.3 were up to 21.9 wt.% and 12.9 wt.% respectively, whereas the coke yield reduced to 2.3 wt.%. Introducing metallic Co and/or Zr caused the increase in the yields of H2, CO2 and CO significantly compared with AT0.2-PI0.3 support (Table 3). It revealed that the metal ions could promote the formation of gaseous products by cracking, reforming and other reactions.25,26 In addition, the removal of oxygen as CO and CO2 caused the decline in the oxygen content in tar and the increase in the hydrocarbons yield.25,39 Thus, bimetallic Zr-Co in zeolite is a potential active component for CFP. As for the mono-metallic Co or Zr modified zeolite which displayed in Figure S6 (SI-6), the Tar-C yield was only 12.7 wt.% or 12.8 wt.%, respectively. However, the Tar-C yield first decreased and then increased with an increase of Zr loading. Thus, the Zr loading with 5 wt.% is a potential catalyst for CFP of BYHL. The 5Zr/Co-AT0.2-PI0.3 enhanced reactions of decarboxylation and decarbonylation that produced CO2 and CO. Similar bimetallic effects were confirmed by Cheng et al.24 Figure 9 shows that the metal modified zeolites significantly promoted the formation of BTEXN compared with that without metal modification. The selectivity of total BTEXN, especially for B, increased with raising the Zr loading on 3Co-AT0.2-PI0.3. However, there is a limit to the activity exerted by Zr loading, and 5 wt.% might be the optimum loading. The highest selectivity of B was 18.2% over 5Zr/Co-AT0.2-PI0.3, while the released amount of BTEXN increased to 30.5%, although


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the total tar yield reduced. This indicated that the formation of LAs is favored at bimetallic Zr-Co modified zeolite, in which the acid sites and metal sites promoted the oligomerization and aromatization.39 Monometallic Co or Zr modified zeolites give relatively high T selectivity of 5.8% or 6.5%, respectively. The selectivity of Ns is up to 4.9% over 5Zr-AT0.2-PI0.3, whereas its value significantly reduced at higher Zr loading. This result can be ascribed to the agglomeration phenomena of metal particles that retard the reaction between aromatics and other OOSs to form alkylated-B or PAHs during CFP.30,44 However, PAHs are commonly regarded as indicators for coke formation which may lead to catalyst deactivation.41,45 Besides, it can be seen that introducing bimetallic Zr-Co has little influence on the selectivity of X, 1-methyl-N and 2-methyl-N. Thus, the hydrogen transfer reaction and aromatization of vapor in the metal sites of zeolite contributed to the formation of LAs.22,24 As displayed in Figure 10, when the metallic Co or Zr was impregnated on the AT0.2-PI0.3, the relative content of total aromatics significantly increased to 91.7% or 90.5%, respectively. Large number of reactive sites, including acid sites and metal sites, were provided by the catalyst to convert OOSs and macromolecules into MAHs and PAHs.44,46 The total aromatics content increased with the increase in Zr loading, and reached the maximum of 96.8% at Zr loading of 5 wt.%, especially for B species increased significantly to 65.1%, while PAHs decreased, especially for C18+ decreased to 0.3%. It can also be seen that the OOSs content reduced to 2.0% over 5Zr/Co-AT0.2-PI0.3, whereas most of the OOSs are simple phenols. It can be interpreted that the metal part on zeolite contributed to the formation of MAHs and simultaneously retarded the further polymerization reaction of MAHs and other OOSs which is a competitive reaction causing PAHs formation.23,44 In addition, the phenols are easily absorbed on the metallic surface to form surface phenoxy species, which would undergo several reactions, such as breaking of C-O and C-C, to



generate more B and T.43 The metal phases can actually induce hydrogen transfer reaction by using the small amount of H radical, and the H2 and CH4 can serve as the source of H radical which combining with fragment radicals in the cracking reaction to form LAs.42 Analysis of the coke Figure 11 showed the amount of coke deposition on the catalyst calculated by Eq. (S1) (Supporting Information, SI-5). HZSM-5 displayed a high coke yield (29.5 mg/mcat) due to the limited diffusion properties. The formation of coke begins with a series of precursors such as PAHs, OOSs, small olefins and so on, while coke precursors could easily diffuse into the micropores and lead to the blockage of pore, poisoning of active sites and reduction in number/strength of active sites.41 In addition, the MCs were difficult to diffuse into the micropores and would deposit on the surface of HZSM-5.47 As for the desilicated zeolites, the tendency of forming coke was in accordance with the results shown in Figure 5. The coke yield even decreased to 16.9 mg/mcat over AT0.2-PI0.3, revealing that the introduced mesopores and retained acid sites by mild PAD improved the accessibility of MCs to the internal channels of zeolite, relieving the coke formation on the external surface of zeolites.21 Subsequently, more small molecules could access the active sites at the micropores through mesopores and were converted to aromatics. In addition, for the metal modified zeolites (Figure 11b), the coke yields over 3Co-AT0.2-PI0.3 and 5Zr-AT0.2-PI0.3 are about 19.4 and 23.9 mg/mcat, respectively. However, the coke yield over 5Zr/Co-AT0.2-PI0.3 is around 20.3 mg/mcat, which is slightly higher than AT0.2-PI0.3, indicating that the synergistic effect of Zr-Co active sites at loading amount of 5 wt.% might inhibit coke formation.32,44 However, the coke yield increased at higher Zr loading probably be due to the agglomeration of Zr metal particles in 3Co-AT0.2-PI0.3. CONCLUSION


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Hierarchical HZSM-5 combining microporosity and mesoporosity together was readily post-treated by PAD and presented perfect catalytic performance in directional production of BTEXN from lignite. The appropriately desilicated HZSM-5 controllably promoted the generation of mesopores and minimized the loss of micropores and acid sites, because HZSM-5 was shielded from the attack by OH- with the addition of cyclic secondary amines. Although the recreated mesopores suppressed the formation of coke to some extent, the desilicated zeolites possessed a lower hydrogen transfer activity and inhibited the secondary reactions of pyrolysis fragments, expressing significantly higher OOSs content and MCs (C14+ and C18+) in tar than HZSM-5. In addition, introducing bimetallic Zr-Co facilitated the hydrogen transfer of pyrolysis fragments at the metal sites and speeded up the cracking reaction and deoxygenation step of the cascade reactions. 5Zr/Co-AT0.2-PI0.3 exhibited an excellent ability for upgrading pyrolysis vapors, which can give a high BTEXN selectivity of 30.5%. Meanwhile, the OOSs and MCs contents were decreased to 2.0% and 4.9%, respectively. The carbon deposition mechanism involved in the process and the anti-carbon deposition performance of the catalyst is a challenging subject that deserves further research. ASSOCLATED CONTENT Supporting Information Zeolite catalysts preparation, catalysts and products characterization techniques, BJH pore size distributions of metal-loaded zeolites, N2 adsorption-desorption isotherms, FTIR spectra and NH3-TPD profiles of zeolite catalyst, effect of the parent and post-treated zeolite on carbon distribution for CFP of BYHL (PDF). AUTHOR INFORMATION Corresponding Author



* Email: [email protected]; [email protected] (Prof. J.P. Cao) [email protected] (A./Prof. X.Y. Zhao) Author Contributions The manuscript was written through contributions of all authors. Notes The authors declare no competing financial interests. ACKNOWLEDGEMENTS This work was subsidized by the Fundamental Research Funds for the Central Universities (China University of Mining & Technology, Grant 2017XKZD10), and the Priority Academic Program Development of Jiangsu Higher Education Institutions. REFERENCES (1) Li G. L., Yan L. J., Zhao R. F., Li F. Improving aromatic hydrocarbons yield from coal pyrolysis volatile products over HZSM-5 and Mo-modified HZSM-5. Fuel 2014, 130, 154-159. (2) Chareonpanich M., Boonfueng T., Limtrakul J. Production of aromatic hydrocarbons from Mae-Moh lignite. Fuel Process. Technol. 2002, 79, 171-179. (3) Chareonpanich M., Tomita A. Selective production of BTX by hydrocracking of coal volatile matter over zeolite catalyst. Energy Fuels 1994, 8, 1522-1523. (4) Foster A. J., Jae J., Cheng Y. T., Huber G. W., Lobo R. F. Optimizing the aromatic yield and distribution from catalytic fast pyrolysis of biomass over ZSM-5. Appl. Catal., A 2012, 423, 154-161. (5) Zhou G. F., Jensen P. A., Le D. M., Knudsen N. O., Jensen A. D. Direct upgrading of fast pyrolysis lignin vapor over the HZSM-5 catalyst. Green. Chem. 2016, 18, 1965-1975. (6) Shao S. S., Zhang H. Y., Shen D. K., Xiao R. Enhancement of hydrocarbon production and


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Table and Figure Table 1. Chemical composition, crystallinity and textural properties of catalysts Catalyst

SiO2/Al2O3a

HZSM-5 AT0.2 AT0.2-PI0.1 AT0.2-PI0.3 AT0.2-PI0.5 AT0.2-PI1.0 AT0.2-Tem 3Co-AT0.2-PI0.3 1Zr/Co-AT0.2-PI0.3 3Zr/Co-AT0.2-PI0.3 5Zr/Co-AT0.2-PI0.3 8Zr/Co-AT0.2-PI0.3 10Zr/Co-AT0.2-PI0.3 15Zr/Co-AT0.2-PI0.3 5Zr-AT0.2-PI0.3

27 13 15 20 18 17 15 -

SBETb m2/g 378 393 366 365 380 376 361 322 292 331 311 303 295 277 356

Sextc m2/g 100 115 98 91 100 107 92 83 81 101 94 105 89 86 112

Vmesod cm3/g 0.11 0.12 0.13 0.11 0.12 0.13 0.13 0.12 0.12 0.12 0.11 0.12 0.10 0.13 0.13

Vmicroc cm3/g 0.13 0.13 0.12 0.13 0.13 0.12 0.12 0.11 0.09 0.11 0.10 0.10 0.10 0.08 0.11

Vtotale cm3/g 0.24 0.25 0.25 0.24 0.25 0.25 0.25 0.23 0.21 0.23 0.21 0.22 0.20 0.21 0.24

χf (%) 100 77 80 87 73 71 33 59 67

a

Determined by ICP-AES.

b

Determined by multipoint BET method.

c

Measured by the t-plot method.

d

By difference.

e

Calculated from absorbed volume of nitrogen for a relative pressure P/P0 of 0.99.

f

Relative crystallinity (χ) is estimated from the areas of the peak in the 2θ range from 22.3o to 25o.



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Table 2. Acid amount of the HZSM-5 and post-treated zeolites Catalyst

Total amount (mmol/g)

Amount (mmol/g-NH3) Mid-strong acid Weak acid sites sites

HZSM-5 AT0.2 AT0.2-PI0.1 AT0.2-PI0.3 AT0.2-PI0.5 AT0.2-PI1.0 3Co-AT0.2-PI0.3 5Zr/Co-AT0.2-PI0.3 5Zr-AT0.2-PI0.3

0.74 0.34 0.71 0.69 0.70 0.88 0.43 0.53 0.64

0.44 0.31 0.47 0.43 0.43 0.59 0.28 0.26 0.43

0.30 0.03 0.24 0.26 0.27 0.29 0.13 0.27 0.21


Strong acid sites 0.02 -

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Table 3. Gaseous product distribution derived from catalytic pyrolysis of BYHL Gaseous product distribution (mmol/g lignite, daf) Catalyst HZSM-5 AT0.2 AT0.2-PI0.1 AT0.2-PI0.3 AT0.2-PI0.5 AT0.2-PI1.0 AT0.2-Tem 3Co-AT0.2-PI0.3 1Zr/Co-AT0.2-PI0.3 3Zr/Co-AT0.2-PI0.3 5Zr/Co-AT0.2-PI0.3 8Zr/Co-AT0.2-PI0.3 10Zr/Co-AT0.2-PI0.3 15Zr/Co-AT0.2-PI0.3 5Zr-AT0.2-PI0.3

CO2/CO H2

CO2

CO

CH4

C 2 H6

C2H4

C3-C4

1.8 1.9 1.8 2.2 1.8 1.8 2.0 3.1 3.0 3.1 3.3 3.3 3.4 3.2 2.3

1.2 1.4 1.5 1.7 1.6 1.5 1.7 2.1 1.9 2.1 2.3 2.2 2.2 2.2 1.8

2.2 2.4 2.6 2.8 2.7 2.6 2.6 2.9 2.9 3.0 3.0 3.0 2.9 2.9 2.6

1.0 0.9 1.1 1.1 1.1 1.1 1.1 1.3 1.2 1.3 1.4 1.4 1.3 1.3 1.1

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.1

0.3 0.3 0.4 0.3 0.4 0.3 0.3 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.3

0.1

Enhancement of Aromatic Products from Catalytic Fast Pyrolysis of

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