Effect of different acid-leached USY zeolites on in-situ catalytic upgrading of lignite tar

doi:10.1016/j.fuel.2020.117089

Fuel

Volume 266, 15 April 2020, 117089

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Highlights

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The acid-leached USY zeolites can significantly upgrade in-situ lignite tar.
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Leached USY with HCl and HNO₃ exhibit superior catalytic performance.
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The yields of naphthalene and methylnaphthalene increase by 7.6 and 6.7 times.
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Light tar contents are improved by about 47.7% compared with non-upgrading.

Abstract

In this work, USY zeolites obtained by hydrothermal pretreatment were leached by nitric acid (HNO₃), hydrochloric acid (HCl) and oxalic acid (H₂C₂O₄), respectively, and used as catalysts to upgrade in-situ lignite tar into light aromatics. The results showed that the acid leaching with HNO₃ and HCl removed extra-framework Al of USY, resulting in the decrease of total acid sites but the increase of ratio Brønsted/Lewis acid sites and mesopores volume, which improved catalytic performance and facilitated mass transfer in the reaction. The H₂C₂O₄ leaching dissolved framework Al of USY, leading to a dramatic reduction of acid sites. USY-HCl and USY-HNO₃ catalysts exhibited a good performance in the improvement of light aromatics and light tar (boiling point < 360 °C). The promotion of light aromatics, especially the naphthalene homologs, was obvious. The total yields of naphthalene and methylnaphthalene were increased by 7.6 and 6.7 times compared with non-upgrading tar. Light tar contents were also increased by about 47.7% in comparison with non-upgrading tar. This work provides a novel method to upgrade the in-situ lignite tar and generate more light aromatics by optimizing the physicochemical properties of USY zeolites.

Graphical abstract

Introduction

Lignite with the lowest degree of coalification and high volatiles can be easily converted to tar and gases through pyrolysis technology [1], [2], [3]. However, lignite tar usually contains less light aromatics and high heavy components, which causes various operation problems and limits the subsequent processing [4]. Light aromatics, including benzene, toluene, xylene, naphthalene (BTXN), are very important raw materials in chemical industry. For instance, naphthalene is widely used to produce plasticizers, synthetic fibers and drugs [5]. Therefore, in order to improve the economic value of lignite tar, selective conversion of lignite pyrolysis volatiles into light aromatics through the conversion of heavy components over catalysts is a hot topic [6]. Many materials including char, metal oxides and zeolites are used as catalysts to crack pyrolysis volatiles into light components [7], [8], [9]. Especially, zeolites with regular crystal structure and adjustable acid sites are widely employed to promote the generation of light aromatics in coal tar [10], [11].

Zeolite HY with accessible microspores (0.76 nm) exhibits good catalytic cracking, hydrocracking and isomerization performances [12]. However, its strong acidity and narrow channels of HY zeolite compared with polycyclic aromatic hydrocarbons (PAHs) easily result in the excessive cracking of coal tar and coke formation [13]. Therefore, various methods are taken to modify zeolite Y for pyrolysis volatiles upgrading and the formation of light aromatics. Li et al. prepared two hierarchical Y zeolites by alkaline treatment with NaOH and NaOH&TBPH, and found the latter showed a better catalytic performance in the cracking of 1,3,5-TIPB [14], which is attributed to its appropriate acidity and mesoporosity, especially moderate Brønsted acid sites and higher B/L ratio. Liu et al. employed four zeolites including NaY, HY, hydrothermal treatment Y (HTY) and hydrothermal treatment-acid leaching Y (HTY-AL) to upgrade coal pyrolysis volatiles [15]. The results presented that HTY and HTY-AL benefited the generation of light aromatics, with the yield of BTEXN increased from 0.78 wt% (raw coal pyrolysis) to 3.63 wt% and 5.31 wt%. The outstanding performance of HTY and HTY-AL correlated with pore structure and appropriate acidity. HCl acid leaching could remove the extra-framework Al of USY zeolite and improve the catalytic activity in the upgrading of heavy oil [16], [17].

Our previous work found that hydrothermal and HNO₃ acid treatment obviously affected HY zeolite structure and upgrading of pyrolysis volatiles, but the influences of different acid leaching are not clear. Therefore, this work is aimed to investigate the effect of different acid-leaching (HNO₃, HCl and H₂C₂O₄) on the structure of the hydrothermal treated USY and its catalytic performances in in-situ catalytic cracking of the heavy components in lignite pyrolysis volatiles.

Section snippets

Catalysts preparation

Ultra-stable Y zeolite, marked as USY, was prepared by hydrothermal treatment of raw HY (SiO₂/Al₂O₃ = 5.4, Nankai University Catalyst Co., Ltd) at 650 °C for 4 h, which was described in Ref. [18]. Different acid solution (HNO₃, HCl and H₂C₂O₄) with 0.8 mol/L was used to leach USY at 40 °C for 4 h (the ratio of acid solution volume to USY zeolite mass is 10 mL/1 g). Thereafter, the solid product was filtrated and washed with deionized water until neutral. The resultant filter cake was dried at

XRD analysis

The effect of different acid leaching on the structure of USY was examined by XRD analysis. As presented in Fig. 1a, compared with USY, the samples by HNO₃ and HCl exhibit typical characteristic patterns of zeolite Y, indicating that the FAU framework is retained [23], [24]. However, the diffraction peaks of USY-HNO₃ and USY-HCl shift slightly to higher angles, which means that HNO₃ and HCl acid leaching results in lattice shrinkage and structural stabilization because the Si-O (0.166 nm) is

Conclusions

The USY was leached by HNO₃, HCl and H₂C₂O₄ acid and used as catalysts for in-situ upgrading of lignite tar. Different acid leaching exerted a different effect on the structure, textural properties and the catalytic performances. The acid leaching with HNO₃ and HCl resulted in the decrease of total acid sites but the increase of Brønsted acid sites and mesopores volume. The acid leaching with H₂C₂O₄ led to a dramatic reduction of total acid sites and no obvious improvement of textural

CRediT authorship contribution statement

Baoyong Wei: Conceptualization, Methodology, Writing - original draft. Lijun Jin: Supervision, Funding acquisition. Dechao Wang: Project administration. Yankun Xiong: Investigation. Haoquan Hu: Formal analysis. Zongqing Bai: Resources.

Declaration of interest statement

The authors declare that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

Acknowledgments

This research was performed with the support of the National Natural Science Foundation of China (No. 21878044, U1510101, U1710105), the Fundamental Research Funds for the Central Universities (DUT18LK10) and the Found of State Key Laboratory of Coal Conversion (Grant No. J19-20-301).

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Full Length ArticleEffect of different acid-leached USY zeolites on in-situ catalytic upgrading of lignite tar

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Catalysts preparation

XRD analysis

Conclusions

CRediT authorship contribution statement

Declaration of interest statement

Acknowledgments

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Full Length Article
Effect of different acid-leached USY zeolites on in-situ catalytic upgrading of lignite tar