Preparation of layered K-Fe-Zn-Ti catalyst and its performance in the hydrogenation of carbon dioxide to light olefins
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摘要: 采用高温固相法制备了系列Zn改性的层状K-Fe-Zn-Ti催化剂，用于CO2加氢经费托合成直接制烯烃反应。采用SEM、TEM、XRD、H2-TPR、CO2-TPD、XPS、N2吸附-脱附和TG等手段对反应前后的催化剂进行了表征，对K-Fe-Zn-Ti催化剂的组成-结构-性能关系进行了关联研究。结果表明，所制备的催化剂均出现K2.3Fe2.3Ti5.7O16物相，为典型的层状金属氧化物（Layered Metal Oxides，LMO）结构；Zn改性后生成了ZnFe2O4物相，降低了催化剂样品结晶度，增强了表面碱性，促进了CO2表面吸附。在CO2加氢反应中，K-Fe-Zn-Ti系列催化剂均具有较高的烯烃选择性（O/P>6.5），Zn改性促进了C5+的生成，显著提高了C4+线性α-烯烃（linear α-olefins，LAOs）的选择性，C4+烃中LAOs含量由Zn改性前的54.6%提高至75.2%。在所考察的范围内，随Zn/Fe比的增加，烯/烷比（C2-4=/C2-40，O/P）先增加后降低，但对重烃含量以及LAOs选择性影响不明显。K-Fe-Zn-Ti催化剂具有较好的稳定性，经100 h在线反应后，仍保持LMO结构。
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关键词:
- 层状K-Fe-Zn-Ti催化剂 /
- CO2加氢 /
- 低碳烯烃 /
- 线性α-烯烃
Abstract: A series of layered K-Fe-Zn-Ti catalysts with different Zn/Fe molar ratios were prepared by high-temperature solid state reaction and characterized by SEM, TEM, XRD, H2-TPR, CO2-TPD, XPS, N2 sorption and TG measurements; the performance of K-Fe-Zn-Ti catalysts in the hydrogenation of CO2 to light olefins was investigated. The results indicate that the K-Fe-Zn-Ti catalysts have the typical layered structure with K2.3Fe2.3Ti5.7O16 as the main phase. ZnFe2O4 appears on the Zn promoted K-Fe-Zn-Ti catalysts, which may reduce the crystallinity, enhance the surface basicity, and promote the adsorption of CO2. The K-Fe-Zn-Ti catalysts exhibit high selectivity to olefins in CO2 hydrogenation; the ratio of olefins to paraffins in the products (O/P) is higher than 6.5. The addition of Zn can enhance the formation of C5+ hydrocarbons and especially C4+ linear alpha-olefins (LAOs); the content of LAOs in C4+ hydrocarbons over Zn promoted K-Fe-Zn-Ti reaches 75.2%, in comparison with the value of 54.6% over the Zn-free K-Fe-Ti catalyst. In particular, the 0.8K-1.8Fe-0.6Zn-1.3Ti catalyst displays the highest O/P value (7.8), although the effect of Zn content in the Zn-promoted K-Fe-Zn-Ti catalysts on the yield of heavy hydrocarbons and selectivity to alpha-olefins is less significant. Moreover, the K-Fe-Zn-Ti catalysts display high stability in CO2 hydrogenation and the LMO structure remains almost intact after a long term reaction test of 100 h.-
Key words:
- layered K-Fe-Zn-Ti catalyst /
- CO2 hydrogenation /
- light olefins /
- linear α-olefins
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图 2 反应后催化剂样品的XRD谱图
Figure 2 XRD patterns of the spent AR-K-Fe-Zn-Ti catalysts after enduring the CO2 hydrogenation reaction
图 3 新鲜样品的SEM照片
Figure 3 SEM images of the fresh K-Fe-Zn-Ti catalysts
(a): 0.8K-2.4Fe-1.3Ti; (b): 0.8K-2.0Fe-0.4Zn-1.3Ti; (c): 0.8K-1.92Fe-0.48Zn-1.3Ti; (d): 0.8K-1.8Fe-0.6Zn-1.3Ti; (e): 0.8K-1.6Fe-0.8Zn-1.3Ti
图 4 0.8K-1.92Fe-0.48Zn-1.3Ti催化剂的TEM照片
Figure 4 TEM images of 0.8K-1.92Fe-0.48Zn-1.3Ti catalyst
(a): fresh sample; (b): used sample
图 6 AR-0.8K-1.92Fe-0.48Zn-1.3Ti的热重分析曲线
Figure 6 TGA curves of the AR-0.8K-1.92Fe-0.48Zn-1.3Ti catalyst
表 1 催化剂的织构性质
Table 1 Textural properties of the K-Fe-Zn-Ti catalysts
Catalyst Surface area A/(m2·g-1) Pore volume v/(cm3·g-1) Average pore size d/nm 0.8K-2.4Fe-1.3Ti 4.11 0.020 1.51 0.8K-2.0Fe-0.4Zn-1.3Ti 2.91 0.010 2.59 0.8K-1.92Fe-0.48Zn-1.3Ti 3.38 0.004 1.96 0.8K-1.8Fe-0.6Zn-1.3Ti 5.91 0.013 1.96 0.8K-1.6Fe-0.8Zn-1.3Ti 1.66 0.008 3.86 
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表 2 反应前后催化剂样品的表面组成
Table 2 Surface composition of the K-Fe-Zn-Ti catalysts determined by XPS
Catalyst Surface element content wmola/% Fe/Ti Fe/K Fe/Zn K Fe Ti Zn O C 0.8K-2.4Fe-1.3Ti 14.99 7.61 7.67 0 50.72 18.19 0.99 0.51 0 0.8K-2.0Fe-0.4Zn-1.3Ti 15.53 7.28 5.55 1.55 47.48 22.31 1.31 0.47 4.70 0.8K-1.8Fe-0.6Zn-1.3Ti 12.98 3.07 4.42 1.12 37.14 37.39 0.69 0.24 2.74 0.8K-1.6Fe-0.8Zn-1.3Ti 11.25 2.50 3.51 1.08 34.83 46.83 0.71 0.22 2.32 AR-0.8K-2.4Fe-1.3Ti 16.34 1.69 2.78 0 32.19 46.09 0.61 0.10 0 AR-0.8K-2.0Fe-0.4Zn-1.3Ti 12.02 1.39 3.32 0.91 28.51 53.97 0.42 0.12 1.53 AR-0.8K-1.8Fe-0.6Zn-1.3Ti 18.57 1.06 2.11 0.59 33.10 44.55 0.66 0.54 1.31 AR-0.8K-1.6Fe-0.8Zn-1.3Ti 14.28 0.42 0.98 0.26 22.52 61.52 0.43 0.03 1.62 a: calculated from peak area of XPS spectra
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表 3 催化剂的活性评价
Table 3 Performances of the K-Fe-Zn-Ti catalysts in CO2 hydrogenation
Catalyst CO2
conv. x/%Selectivity s/% Hydrocarbon distribution C/% O/P CO HC oxy CH4 C2-4= C2-40 C5+ α-olefins(C4+) 0.8K-2.4Fe-1.3Ti 35.2 70.1 22.4 7.5 23.3 59.3 8.1 9.3 54.6 7.3 0.8K-2.0Fe-0.4Zn-1.3Ti 13.2 56.4 38.8 4.8 21.0 51.1 7.8 20.1 72.6 6.6 0.8K-1.92Fe-0.48Zn-1.3Ti 14.6 53.6 41.6 4.9 19.2 52.2 7.3 21.3 75.2 7.1 0.8K-1.8Fe-0.6Zn-1.3Ti 16.1 75.4 21.8 2.8 20.4 51.6 6.6 21.4 73.2 7.8 0.8K-1.6Fe-0.8Zn-1.3Ti 13.0 49.1 46.0 4.8 19.9 51.2 7.9 21.1 70.9 6.5 reaction condition: 320 ℃, 2 MPa, H2/CO2 = 3.0, GHSV = 1000 h-1, TOS = 72 h
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