Boosted production of aromatics by catalyzing upgrade pyrolysis vapors from lignite over Sn-Ga/HZSM‑5 catalysts
Introduction
Pyrolysis provides mild conversion of lignite volatiles to improve coal tar quality [1,2]. One economically viable option is to obtain some valuable chemical materials such as BTEXN (benzene, toluene, ethylbenzene, xylene and naphthalene) in catalytic reforming of lignite pyrolysis volatile process on account of its complexity [3,4], while, the traditional aromatic production depended on the catalytic reforming of the meager petroleum resources. Catalytic reforming of lignite pyrolysis volatile can efficiently manipulate pyrolysis products distribution to desired products. Several catalysts have been investigated during the pyrolysis process such as alkaline earth metals, metal oxides, metal chlorides and zeolites [[5], [6], [7]].
Among the catalysts, HZSM-5 (H5) exhibited moderate pore structure, strong acidity and high hydrothermal stability which were widely used as catalysts for cracking, deoxygenation, dehydrogenation, oligomerization, and aromatization [[8], [9], [10]]. However, H5 was limited by low reactant and product mass-transfer rates to form coke at continuous elevated temperatures. It was confirmed that hierarchical H5 was beneficial to reduce the diffusion resistances of reactants and products [11]. Besides, various kinds of modified zeolites including Co/H5, sulfation-acidified H5, Pt/H5, Mo/H5 and Ni/H5 were evaluated for upgrading coal tar due to the desired characteristics for catalytic reforming volatiles from lignite and biomass [3,4,[12], [13], [14]]. Consequently, the BTEXN yield increased to 7000 ng/mg from coal pyrolysis at 900 °C over Mo/H5 which was three times than that without H5. It indicated that Mo has been loaded on H5 as MoO3 [13]. Compared with non-catalytic experiment, Zn, Mo, Ni, Fe and Ga modified H5 significantly increased the light aromatics formation during the catalytic upgrading of coal pyrolysis volatiles process [15]. Among these transition metals, Ga and Sn exhibited superior aromatization performance and stability. For instance, the highest BTEX selectivity was obtained in n-heptane catalytic aromatization process owing to the increased strong Lewis acidic sites and mesopore volumes induced by the framework Ga species [16]. Mo and Ga modified ZSM-5 with moderate concentrations of tetrahedral aluminum exhibited excellent catalytic performance and durability for light alkanes including ethane and propane dehydroaromatization [17,18]. The underutilized pyrolysis gas including H2, CH4, C2H4, C3H6, C2H4 and CO2 could be activated to produce valuable olefins and aromatics simultaneously via the interaction of cationic Ga species with protonic sites by acceleration of the dehydrogenation rate-determining step [11,19,20]. Upgrading of furanics by zeolite-catalyzed aromatization typically resulted in a large loss of carbon due to coke deposition which could be mitigated by the addition of ethylene and the modified zeolite with Lewis acid Ga sites during the catalytic pyrolysis process [21]. Compared with H5, Ga-H5 led to a two-fold increase in the BTX yield during the ethanol conversion process, Ga contributed to H2 yield and suppressed hydrogen transfer reactions for light alkane formation [22]. Furthermore, polycyclic aromatic hydrocarbons result of carbon deposition could be alleviated by the addition of Sn over Pt-Mo/H5 during the methane conversion process. Meanwhile, it also exhibited higher benzene selectivity [23]. Sn species also were observed that preferentially healed the defects in H5 crystals to create new active sites to improve the catalytic performance in terms of selectivity and BTX yield [24]. Noteworthy, the catalytic activity and stability of H[Sn, Al]ZSM-5/0.3AT zeolite was improved by doping Sn species during glycerol aromatization [25].
In this work, the BTEXN formation over AT0.2H5 (HZSM-5 treated with 0.2 mol/L NaOH) with different Ga loadings was investigated to determine the loading threshold for controlling the aromatic selectivity. After identifying the optimum Ga loading for catalytic reforming of lignite pyrolysis volatile, modification of this catalyst with Sn was investigated. A series of characterization technics including XRD, BET, NH3-TPD and SEM were used to investigate the effect of physicochemical properties and catalytic activities in catalytic cracking/reforming processes.
Section snippets
Coal samples and modified H5
The Shengli lignite (SL) that came from Xilinhot, InnerMongolia, China was crushed and sieved to 0.5–1.0 mm. Then, the samples were dried at 105 °C for 10 h to remove moisture. The detailed proximate and ultimate analyses of SL were listed in Table S1.
H5 (SiO2/Al2O3 = 25) were purchased from Nankai University Catalyst Co., Ltd. It was calcined in air at 550 °C for 5 h to remove impurities before use. The hierarchical H5 was obtained by treating with 0.2 mol/L NaOH (AT0.2H5). Different loading
Results and discussion
As displayed in Figs. 1 and S1, all catalysts exhibited the typical MFI framework with sharp and intense diffraction peaks, suggesting that the alkali treatment process and the introduction of Sn and Ga did not change the crystalline structure of H5. The presence of Ga2O3 and SnO peaks observed from XRD analysis might ascribe to the dispersion of Ga(NO3)3 and SnCl4 during calcination over AT0.2H5. In addition, no obvious diffraction peaks corresponding to Ga species were observed in XRD
Conclusion
The synthesized bimetallic 2%Sn-4%Ga/AT0.2H5 exhibited high selectivity of aromatics during the catalytic reforming of SL pyrolysis volatile. The total BTEXN yield increased dramatically over 2%Sn-4%Ga/AT0.2H5 in comparison with pure H5 (from 22.3 to 25.1 mg/g), especially the benzene yield reached to 15.4 mg/g which attributed to the appropriate meso/microporosity, acidic distribution and active component. In addition, the BTEXN yield maintained over 22 mg/g during the first three experiments
CRediT authorship contribution statement
Zhen Yang: Conceptualization, Methodology, Writing - original draft. Jing-Pei Cao: Supervision, Funding acquisition, Writing - original draft. Tian-Long Liu: Visualization, Investigation. Chen Zhu: Writing - review & editing. Xiao-Yan Zhao: Data curation. Xiao-Bo Feng: Methodology. Yun-Peng Zhao: Writing - review & editing. Hong-Chun Bai: Formal analysis.
Declaration of Competing Interest
The authors declared that they have no conflicts of interest to this work.
Acknowledgements
This work was subsidized by the Natural Science Foundation of Jiangsu Province (Grant BK20200028), the National Natural Science Foundation of China (Grant 21978317), the Qing Lan Project, and the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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