Effect of zinc content on the structure of Zn species and catalytic properties over Zn/ZSM-5
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摘要: HZSM-5(Si/Al=30)为载体，用等体积浸渍法合成了系列不同Zn负载量的双功能Zn/ZSM-5催化剂，考察了Zn负载量在乙烯芳构化过程中的催化性能。用X射线粉末衍射（XRD）、N2吸附-脱附和吡啶吸附红外光谱（Py-FTIR）方法考察了催化剂的结构和酸性，用电感耦合等离子发射光谱（ICP）、紫外可见光谱（UV-vis DRS）、X射线吸收精细结构（XAFS）技术解析了Zn物种的结构及其流失行为。结果表明，Zn含量对其在HZSM-5上的存在状态及催化乙烯芳构化反应性能均有明显的影响，具有较多活性六配位ZnOH+物种的1.5%-Zn(IM)/Z5催化剂表现出较高的芳烃选择性和催化剂稳定性，且表现出较低的Zn流失速率。Abstract: Studying the status and distribution of Zn species on Zn/ZSM-5 zeolite catalysts were of great significance for determining the active centers and establishing structure-activity relationships in the ethylene aromatization process. The effect of zinc contents of Zn/ZSM-5 zeolites prepared by incipient-wetness impregnation method on catalytic performances in ethylene aromatization were investigated. The structures and acidic properties of the catalyst were studied through X-ray powder diffraction (XRD), N2 adsorption/desorption, and infrared spectra for pyridine adsorption (Py-FTIR). Besides, inductively coupled plasma-atomic emission spectrum (ICP), diffuse reflectance ultraviolet-visible spectrum (UV-vis DRS), extended X-ray absorption fine structure (EXAFS) and linear combination fitting (LCF) analysis on X-ray Absorption near edge spectra (XANES) had finely analyzed the structure and transition of Zn species and the losing rate of Zn species on HZSM-5 molecular sieve catalyst during ethylene aromatization process. The results showed that the introduction of Zn was advantage to improve the selectivity of aromatics hydrocarbon, and Zn contents of the catalyst had obvious influence on the structures, acidic properties, and the status of Zn species, as well as the catalytic performance of Zn/ZSM-5 catalysts. At low zinc loading, 1.5%-Zn(IM)/Z5 catalyst with more active 6-fold coordinated ZnOH+ species (55%) showed the highest selectivity to aromatics and catalyst stability. With the increase of zinc amount, the excessive Zn contents not only covered the acid sites and blocked the pore channel, but also changed the local coordination structure and state of Zn species. It was confirmed that the oxidizability of Zn species and the coordination number around Zn sites decreased, accompanied by a weakening of the interaction between Zn and zeolite, leading to the formation of large amounts of 4-fold coordinated ZnO clusters and ZnO crystallites. In the 4%-Zn(IM)/Z5 catalyst, Zn species composed of multi characteristics from ZnOH+, ZnO clusters inside the pores, and ZnO crystals on the external surface with relative contributions of 23.5%, 56.1%, and 20.4%, respectively. It meant that ZnO clusters and ZnO crystallites became the main component at the high Zn content. Furthermore, ZnO species located on the outer surface of Zn/ZSM-5 catalysts were easily reduced by H2 and then transported as zinc vapor to the outer surface, which eventually lead to the loss of Zn species from the catalyst and the decline of the catalytic performance of Zn/ZSM-5 catalyst. The relative proportion of ZnOH+ decreased with that of ZnO clusters and ZnO crystallites correspondingly increased considerably with the increase of Zn loading on ZSM-5, accompanied with the elevated rate for Zn losing, and shortened catalyst life. Therefore, a positively correlated between the content of ZnOH+ obtained through the UV-vis DRS and LCF analysis on XANES and the rate of aromatics formation was established, further confirming the catalytic nature of ZnOH+ as the active center, which played an important role in the aromatization reaction that enhancing the formation of aromatic hydrocarbons. Meanwhile, ZnO on the outer surface of Zn/ZSM-5 catalysts was the main species that losing from catalyst, and influenced the catalytic properties on a certain degree.
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Key words:
- Zn/ZSM-5 /
- incipient-wetness impregnation /
- Zn content /
- Zn species /
- ethylene aromatization
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图 1 HZSM-5和x%-Zn（IM）/Z5催化剂上乙烯转化率（a）和产物分布（b）随时间的变化
Figure 1 Ethylene conversion and product distribution of ethylene aromatization over the HZSM-5 and x%-Zn(IM)/Z5 catalysts
图 2 HZSM-5和x%-Zn（IM）/Z5催化剂的XRD谱图
Figure 2 XRD patterns of the HZSM-5 and x%-Zn(IM)/Z5 catalysts
图 3 HZSM-5和x%-Zn（IM）/Z5催化剂的Py-FTIR谱图
Figure 3 Py-FTIR patterns of the HZSM-5 and x%-Zn(IM)/Z5 catalysts
图 4 x%-Zn（IM）/Z5催化剂的UV-vis谱图（a）及其拟合结果（b）
Figure 4 UV-vis patterns of the x%-Zn(IM)/Z5 catalysts (a) and the deconvolved results (b)
图 5 金属Zn、ZnO和x%-Zn（IM）/Z5样品的XANES（a）谱图、EXAFS的谱图（b）和拟合谱图（c）
Figure 5 Zn K-edge XANES (a)、EXAFS spectra (b) and the fitting (c) of Zn foil, ZnO and x%-Zn(IM)/Z5 catalysts
图 6 x%-Zn（IM）/Z5样品XANES谱图的LCF分析（a）−（d）及其拟合结果（e）
Figure 6 LCF analysis of the XANES spectra collected for x%-Zn(IM)/Z5 (a)−(d) and the fitting result (e)
表 1 HZSM-5和x%-Zn（IM）/Z5催化剂的结构性质和酸性
Table 1 Textural properties and acidic properties of the HZSM-5 and x%-Zn(IM)/Z5 catalysts
Catalyst Surface area/
(m2·g−1)Pore volume/
(cm3·g−1)Acidity by
Py-FTIRBET total micro meso L/B HZSM-5 385 0.379 0.113 0.266 0.30 1.5%-Zn(IM)/Z5 364 0.366 0.110 0.256 2.23 2%-Zn(IM)/Z5 345 0.350 0.105 0.245 3.96 3%-Zn(IM)/Z5 348 0.360 0.106 0.254 4.64 4%-Zn(IM)/Z5 337 0.352 0.105 0.247 5.61 
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表 2 不同Zn负载量新鲜和失活x%-Zn（IM）/Z5催化剂上Zn含量
Table 2 Zn contents of the fresh an deactivation x%-Zn(IM)/Z5 catalysts
Catalyst Zn content w/% Reaction time/h Zn loss rate/(%·h−1) fresh catalysts deactivation catalysts 1.5%-Zn(IM)/Z5 1.54 1.40 78.00 0.12 2%-Zn(IM)/Z5 1.81 1.64 54.00 0.17 3%-Zn(IM)/Z5 2.78 2.51 56.00 0.17 4%-Zn(IM)/Z5 3.70 3.14 48.00 0.32
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表 3 ZnO和x%-Zn（IM）/Z5催化剂的XAFS拟合参数
Table 3 EXAFS fit parameters of ZnO and x%-Zn(IM)/Z5 catalysts
Sample E0/eV Zn K-edge EXAFS fit parameters a contribution CN R/Å ${{S}}_0^2 $ σ/Å2 R-factor Zn 9658.7 − − − − − − ZnO 9661.7 Zn-O 4.0(±0.6) 1.97(±0.01) 0.904 0.004(±0.002) 0.007 1.5%-Zn(IM)/Z5 9664.1 Zn-O 5.4(±0.3) 2.05(±0.01) 0.904 0.011(±0.002) 0.004 2%-Zn(IM)/Z5 9664.4 Zn-O 5.1(±0.7) 2.05(±0.01) 0.904 0.009(±0.003) 0.005 3%-Zn(IM)/Z5 9663.4 Zn-O 4.7(±0.7) 2.02(±0.02) 0.904 0.011(±0.003) 0.006 4%-Zn(IM)/Z5 9663.1 Zn-O 4.6(±0.4) 2.00(±0.01) 0.904 0.010(±0.002) 0.006 a: CN = coordination number, R =Interatomic distances, σ= Debye-Waller factor, and R-factor=∑i(datai -fiti)2/(datai)2.
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