The effect of hydrothermal pretreatment on the catalytic performance of Zn/HZSM-5 catalysts for ethylene aromatization reaction
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摘要: 针对用于低碳烯烃芳构化的Zn/HZSM-5催化剂存在易于结焦失活的问题,采用高温水热的方法对催化剂进行预处理,通过XRD、N2物理吸附-脱附、NH3-TPD、Py-FTIR、XPS和TG等技术对样品进行表征,并以乙烯芳构化为探针反应考察了高温水热预处理对催化剂反应性能和稳定性的影响。结果表明,Zn/HZSM-5催化剂经高温水热预处理48 h后表现出优异的催化性能,虽然乙烯转化率略微降低,但是催化剂寿命显著延长,由72 h延长至216 h,同时芳烃选择性保持在60%以上;水热处理促进了ZnO物种与B酸中心的相互作用及ZnOH+物种的生成,在抑制氢转移反应的同时显著促进了催化剂的脱氢性能,提高了氢气选择性;此外,水热处理后催化剂容碳量明显增加、积炭速率降低,表现出优异的抗结焦积炭特性。Abstract: To address the issue of coking and deactivation of Zn/HZSM-5 catalysts used for light olefins aromatization, a high-temperature hydrothermal method was employed for catalyst pretreatment. The catalysts were characterized using XRD, N2 physical adsorption-desorption, NH3-TPD, Py-FTIR, XPS and TG techniques. The effect of high-temperature hydrothermal pretreatment on the catalytic performance and stability of the catalyst was investigated using ethylene aromatization as a probe reaction. The results showed that the Zn/HZSM-5 catalyst exhibited excellent catalytic performance after 48 h of high-temperature hydrothermal pretreatment. Although the conversion of ethylene slightly decreased, the catalyst lifetime was significantly extended, increasing from 72 to 216 h, while the aromatics selectivity remained above 60%. It was suggested that the hydrothermal treatment enhanced the interaction between ZnO species and Brønsted acid sites, promoting the generation of ZnOH+ species. This not only suppressed the hydrogen transfer reaction but also significantly enhanced the dehydrogenation performance of the catalyst, improving the selectivity towards hydrogen. Additionally, the catalyst exhibited increased carbon capacity and reduced carbon deposition rate after hydrothermal treatment, demonstrating excellent anti-coking properties.
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Key words:
- Zn/HZSM-5 /
- hydrothermal pretreatment /
- ethylene aromatization /
- Zn species
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表 1 新鲜和水热预处理不同时间的Zn/HZSM-5催化剂上乙烯芳构化反应产物分布和积炭速率
Table 1 Products distribution and carbon deposition rate of ethylene aromatization reaction over fresh and hydrothermal pretreated Zn/HZSM-5 catalystsa
Catalyst Product selectivity/% H2 selec./% C4-HTIb Carbon deposition
rate/(%·h−1)CH4 ${\rm{C}}_{2}^{0}- {\rm{C}}_{4}^{0}$ ${\rm{C}}_{3}^{=} -{\rm{C}}_{4}^{=} $ C5+ arom. Zn/HZSM-5 5.05 26.31 1.42 1.2 66.01 35.98 0.91 0.23 Hyd-6h 4.78 27.97 2.46 1.42 63.17 39.16 0.87 0.19 Hyd-12h 4.46 26.88 4.10 2.18 62.30 42.70 0.86 0.17 Hyd-48h 3.69 24.88 6.56 4.37 60.47 43.08 0.82 0.21 a: Reaction conditions: 470 ℃, 0.1 MPa, ethylene WHSV 1.8 h−1, TOS=24 h; b: C4-HTI = ${\rm{C}}_{4}^{0} $/(${\rm{C}}_{4}^{=}+{\rm{C}}_{4}^{0} $). 表 2 Zn/HZSM-5和Hyd-xh催化剂的组成及结构性质
Table 2 Composition and textural properties of Zn/HZSM-5 and Hyd-xh
Sample Si/AlFa Zn contentb/% SBET/(m2·g−1) SE/(m2·g−1) Smicro/(m2·g−1) vtotal/(m3·g−1) vmicro/(m3·g−1) Zn/HZSM-5 33 1.1 315 102 213 0.31 0.09 Hyd-6h 35 1.2 325 98 227 0.33 0.10 Hyd-12h 35 1.4 341 105 236 0.33 0.11 Hyd-48h 39 1.5 348 100 248 0.35 0.11 a: Si/AlF were obtained from 29Si MAS NMR spectra; b: Obtained from XPS. 表 3 Zn/HZSM-5和Hyd-xh催化剂的酸性
Table 3 Acidic properties of Zn/HZSM-5 and Hyd-xh catalysts
Sample Acidity by Py-FTIR at 150 ℃/(μmol·g−1) Acidity by NH3-TPD/(mmol·g−1) Brönsted Lewis L/B weak medium strong total Zn/HZSM-5 97 445 4.58 0.10 0.19 0.17 0.46 Hyd-6h 61 471 7.72 0.10 0.16 0.16 0.42 Hyd-12h 69 420 6.09 0.09 0.16 0.14 0.40 Hyd-48h 64 364 5.69 0.09 0.17 0.12 0.37 -
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