The deactivation mechanism of Zn/HZSM-5 zeolites in ethylene aromatization reaction
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摘要: 采用浸渍法制备了Zn负载量（质量分数）分别为1%、2%、3%的Zn/HZSM-5分子筛催化剂，通过XRD、N2吸附-脱附、NH3-TPD、Py-FTIR、XPS、TG-DTA等技术，系统考察Zn/HZSM-5分子筛在乙烯芳构化反应的失活机制。结果表明，积炭是催化剂失活的主要原因，HZSM-5中Zn的添加在较大程度上抑制了催化剂的积炭行为；低Zn含量时催化剂失活缓慢，但Zn含量较高时，由于催化剂比表面积和孔体积极剧下降，催化剂失活加剧。反应过程中，分子筛上Zn物种存在迁移和流失行为，迁移行为体现为催化剂表面Zn的富集和相对比例的变化；Zn流失速率在不同反应阶段保持恒定，但受到Zn含量的影响，Zn含量越高、流失速率越大。外表面ZnO是分子筛催化剂Zn流失的主要物种，且随Zn负载量升高变化趋势愈加明显，其含量与积炭速率存在一定关联。
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关键词:
- Zn/HZSM-5分子筛 /
- 乙烯芳构化 /
- Zn负载量 /
- Zn物种 /
- 失活
Abstract: Zn/HZSM-5 zeolites, with zinc contents of 1%, 2%, and 3%, were prepared by impregnation method and characterized by XRD, N2 adsorption, NH3-TPD, Py-FTIR, XPS, and TG-DTA techniques to investigate the deactivation mechanism in ethylene aromatization reaction. It shows that coking is the main reason for catalysts deactivation, which is considerably depressed with the presence of Zn in the HZSM-5 catalysts. The deactivation is slow for the catalysts with low Zn loading. However, high content of Zn in the catalysts brings in problems such as decrease of surface area and microporous volume, and hence accelerates the deactivation. In the reaction, zinc species lose from the catalysts, accompanied with the migration and redistribution of Zn from bulk to surface of zeolite. The losing rate was constant with the time on stream, but influenced by the zinc content of the catalyst. ZnO on the external surface of the catalyst are the main species leaching from zeolite. Zn leaching is accelerated with the increase of zinc content, and has correlation with coking rate to some extent.-
Key words:
- Zn/HZSM-5 zeolite /
- ethylene aromatization /
- zinc content /
- Zn species /
- deactivation
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图 1 HZSM-5和不同Zn负载量Zn/HZSM-5分子筛的XRD谱图
Figure 1 XRD patterns of HZSM-5 and Zn/HZSM-5 zeolites with different zinc contents
a: HZSM-5; b: 1%-Zn/HZSM-5; c: 2%-Zn/HZSM-5; d: 3%-Zn/HZSM-5
图 2 HZSM-5和不同Zn含量Zn/HZSM-5的N2吸附-脱附曲线
Figure 2 N2 adsorption-desorption of HZSM-5 and Zn/HZSM-5 zeolites with different contents a: HZSM-5; b: 1%-Zn/HZSM-5; c: 2%-Zn/HZSM-5; d: 3%-Zn/HZSM-5
(inset: pore size distribution of the samples)
图 3 HZSM-5和不同Zn含量Zn/HZSM-5分子筛的NH3-TPD曲线
Figure 3 NH3-TPD of HZSM-5 and Zn/HZSM-5 zeolites with different contents
a: HZSM-5; b: 1%-Zn/HZSM-5; c: 2%-Zn/HZSM-5; d: 3%-Zn/HZSM-5
图 4 HZSM-5和不同Zn含量Zn/HZSM-5分子筛的Py-FTIR谱图
Figure 4 Py-FTIR spectra of pyridine adsorption on HZSM-5 and Zn/HZSM-5 zeolites with different contents
a: HZSM-5; b: 1%-Zn/HZSM-5; c: 2%-Zn/HZSM-5; d: 3%-Zn/HZSM-5
图 5 不同Zn负载量Zn/HZSM-5分子筛的Zn 2p3/2 XPS谱图
Figure 5 XPS spectra of Zn 2p3/2 on Zn/HZSM-5 zeolites with different zinc contents
a: 1%-Zn/HZSM-5; b: 2%-Zn/HZSM-5; c: 3%-Zn/HZSM-5
图 6 HZSM-5和Zn/HZSM-5催化剂上乙烯转化率随时间的变化
Figure 6 Effect of time on stream on the ethylene conversion over HZSM-5 and Zn/HZSM-5 zeolites
a: HZSM-5; b: 1%-Zn/HZSM-5; c: 2%-Zn/HZSM-5; d: 3%-Zn/HZSM-5
图 7 不同反应阶段HZSM-5和Zn/HZSM-5催化剂的积炭速率
Figure 7 Coking rate ot HZSM-5 and Zn/HZSM-5 zeolites obtained at different reaction stages
a: HZSM-5; b: 1%-Zn/HZSM-5; c: 2%-Zn/HZSM-5; d: 3%-Zn/HZSM-5
图 8 不同反应阶段Zn/HZSM-5催化剂上(a)体相和(b)表面Zn含量
Figure 8 Residual Zn content on bulk phase ((a), taking that on fresh catalyst as 100%) and surface phase (b) of the spent Zn/HZSM-5 zeolites obtained at different reaction stages
a: 1%-Zn/HZSM-5; b: 2%-Zn/HZSM-5; c: 3%-Zn/HZSM-5
图 9 不同反应阶段Zn/HZSM-5催化剂上(a)ZnOH+和(b)ZnO相对含量
Figure 9 Distribution of ZnOH+ (a) and ZnO (b) on surface of the spent Zn/HZSM-5 zeolites obtained at different reaction stages
a: 1%-Zn/HZSM-5; b: 2%-Zn/HZSM-5; c: 3%-Zn/HZSM-5
表 1 HZSM-5和不同Zn负载量Zn/HZSM-5分子筛的结构性质
Table 1 Textural properties of HZSM-5 and Zn/HZSM-5 zeolites with different zinc contents
Catalyst Crys.a/% Zn w/% Specific surface area A/(m2·g-1) Pore volume v/(cm3·g-1) ICP XPS ABETb external total c micropore d HZSM-5 100 - - 388 131 0.387 0.112 1%-Zn/HZSM-5 93 1.01 1.38 364 117 0.362 0.110 2%-Zn/HZSM-5 97 1.99 1.97 340 112 0.385 0.102 3%-Zn/HZSM-5 90 3.03 2.41 314 105 0.370 0.093 a：relative crystallinity, estimated by comparing the total XRD peak area of a zeolite sample in the range of 2 θ from 22° to 25°, with that of the parent HZSM-5 having the strongest diffraction intensity; b：surface area calculated by the BET-plot; c：total volume absorbed at p/p0=0.99; d: micropore volume determined by t-plot 
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表 2 HZSM-5和不同Zn负载量Zn/HZSM-5分子筛的酸性
Table 2 Acidic properties of HZSM-5 and Zn/HZSM-5 zeolites with different zinc contents
Catalyst Acid strength /(mmol·g-1)a Acid type /(μmol·g-1)b L/B weak medium strong total BAS LAS HZSM-5 0.27 - 0.36 0.63 267 79 0.30 1%-Zn/HZSM-5 0.12 0.22 0.30 0.64 191 296 1.55 2%-Zn/HZSM-5 0.13 0.25 0.27 0.65 107 270 2.52 3%-Zn/HZSM-5 0.13 0.27 0.25 0.65 110 415 3.77 a：density of the acid sites, assorted according to the acidic strength, determined by NH3-TPD, weak-NH3 desorbed at 120-200 ℃; medium-NH3 desorbed at 200-300 ℃; strong-NH3 desorbed at 300-550 ℃; b：density of the acid sites, determined by Py-FTIR
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表 3 HZSM-5和Zn/HZSM-5催化剂上乙烯芳构化反应产物分布
Table 3 Products distribution of ethylene aromatization reaction over HZSM-5 and Zn/HZSM-5 zeolitesa
Catalyst C2H4 conversion x/% Product selectivity sC mol/% H2 selectivity b smol/% C1-4- C3-5= C5+ non-aromatic aromatics BTX HZSM-5 98.55 47.27 6.73 2.99 44.40 37.69 11.21 1%-Zn/HZSM-5 98.86 27.51 3.56 2.55 67.50 61.58 43.99 2%-Zn/HZSM-5 99.45 31.47 4.36 2.64 63.28 57.58 34.75 3%-Zn/HZSM-5 98.80 33.49 5.26 2.78 59.64 52.74 30.60 a：reaction conditions: 470 ℃, 0.1 MPa, ethylene WHSV of 2.7 h1; the data were obtained at TOS of 24 h; b：molar percent of H2 in the products
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表 4 不同反应阶段HZSM-5和Zn/HZSM-5催化剂上的积炭量
Table 4 Coke content on the spent HZSM-5 and Zn/HZSM-5 zeolites obtained at different reaction stagesa
Catalyst Coke content w/% 24 h 48 h deactivated b HZSM-5 10.96 24.61 28.25 (60 h) 1%-Zn/HZSM-5 7.59 10.86 16.07 (72 h) 2%-Zn/HZSM-5 7.69 12.53 16.82 (72 h) 3%-Zn/HZSM-5 9.49 14.24 15.01 (54 h) a: calculated from the TG-DTA analyses; b: the deactivated catalysts were obtained when the ethylene conversion decreased to 30%, with HZSM-5 lifetime of 60 h, 1%-Zn/HZSM-5 and 2%-Zn/HZSM-5 lifetime of 72 h, and 3%-Zn/HZSM-5 lifetime of 54 h
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