Preparation of Co/Zr/Al2O3-Pt/ZSM-5 catalysts for syngas to liquid fuels
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摘要: 以Co基催化剂耦合沸石分子筛催化剂应用于合成气催化转化可以有效改善催化剂的产物选择性。本研究通过浸渍法制备得到Zr/Al2O3载体和Pt/ZSM-5催化剂，再通过超声分散法制备了Co/Al2O3、Co/Zr/Al2O3和Co/Zr/Al2O3-Pt/ZSM-5催化剂。通过系列表征技术对载体和催化剂理化性质进行分析，评价了催化剂费-托合成反应性能。结果表明，Zr的引入有助于提升Co/Zr/Al2O3上Co物种的还原性，改善催化活性，增加C12+重质烃的选择性。当Co/Zr/Al2O3与Pt/ZSM-5耦合后，由于贵金属Pt的助剂效应，进一步促进Co物种的还原，Co/Zr/Al2O3-Pt/ZSM-5催化剂的CTY值提高至8.3×10−5 mmol/(g·s)，同时具有较低的CH4、C2−C4产物选择性。此外，Pt/ZSM-5的酸性促进C12+产物的部分裂解，使产物分布向C5−C11液态烃偏移，C5−C11产物选择性达到45.9%。本研究为设计和制备高效的费-托合成催化剂提供了参考。Abstract: The Co catalysts exhibit high catalytic activity and low water gas shift activity, as well as excellent chain growth ability and low by-product selectivity for Fischer-Tropsch synthesis. The performance of the Co catalyst is influenced by several factors, including its structural composition and physical and chemical properties. The traditional cobalt Fischer-Tropsch catalyst follows the ASF distribution, resulting in a wide range of hydrocarbon products. This makes it difficult to achieve high selectivity for liquid hydrocarbons. Due to the presence of a large number of acidic sites on the surface of zeolite, it has excellent catalytic performance for hydrocracking. The integration of zeolite molecular sieves with Fischer-Tropsch catalysts to form a multi-component catalyst can significantly improve product selectivity, optimize liquid hydrocarbon yields and bypass conventional wax treatment steps. In this study, the catalysts Co/Al2O3, Co/Zr/Al2O3 and Co/Zr/Al2O3-Pt/ZSM-5 were prepared by ultrasonic dispersion method, and the effects of Zr promoter modification and multicomponent coupling catalyst on the activity and product selectivity of Fischer-Tropsch synthesis were investigated. In Co/Al2O3, Co/Zr/Al2O3 and Co/Zr/Al2O3-Pt/ZSM-5 catalysts, the Co species are uniformly dispersed on the support surface and have similar particle sizes. Pt and Zr were uniformly dispersed in the catalyst, Zr was mainly dispersed on the surface of Al2O3 supports, and Pt was mainly dispersed on the surface of ZSM-5 molecular sieve. The reduction of Co species was promoted by Zr-modified alumina. With the addition of Pt/ZSM-5 catalyst, the adsorbed hydrogen is more easily dissociated and converted into active hydrogen, further promoting the reduction of Co species. The catalytic performance of Fischer-Tropsch synthesis was evaluated. Compared with Co/Al2O3 catalyst, the selectivity of C12+ heavy hydrocarbon products on Co/Zr/Al2O3 catalyst increased from 28.2% to 38.1%, with a corresponding decrease in CH4, C2−C4 and C5−C11 products, indicating that Zr promoter promoted the generation of heavy hydrocarbon products. Coupled with Pt/ZSM-5 catalyst, the Co/Zr/Al2O3-Pt/ZSM-5 catalyst showed low CH4 selectivity (10.0%) and C2−C4 selectivity (15.0%), while the selectivity of C5−C11 liquid hydrocarbon products increased from 32.1% to 45.9%, the selectivity of heavy hydrocarbon products (C12+) decreased from 38.1% to 29.1%. Compared to Co/Zr/Al2O3, the TOF and CTY of Co/Zr/Al2O3-Pt/ZSM-5 catalysts are increased by 212.6% and 62.7%, respectively. The improvement in catalytic activity was mainly due to the addition of Zr promoter and Pt/ZSM-5 catalyst, which promoted the reduction of Co species. Under the synergistic effect of Zr promoter and Pt/ZSM-5 catalyst, Zr promoter promotes the formation of C12+ heavy hydrocarbons, while Pt/ZSM-5 catalyst promotes the hydrocracking of heavy hydrocarbons to C5−C11 liquid hydrocarbons, thereby improving the selectivity of Co/Zr/Al2O3-Pt/ZSM-5 catalyst for C5−C11 liquid hydrocarbons. In this study, a functional catalyst was constructed by coupling Fischer-Tropsch synthesis catalyst and hydrocracking catalyst at nanoscale, which achieved high selectivity of C5−C11 liquid hydrocarbon and low selectivity of CH4 and C2−C4 products, which provided a reference for the design and implementation of catalysts with high selectivity for liquid hydrocarbons.
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Key words:
- Co-based catalysts /
- ZSM-5 /
- ultrasonic dispersion method /
- Fischer-Tropsch synthesis
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图 1 TEM图像 （a） Al2O3，（b） 、（c） Zr/Al2O3及EDX元素分布
Figure 1 TEM images (a) Al2O3, (b), (C) Zr/Al2O3 and EDX mapping images
图 2 Pt/ZSM-5催化剂：（a） TEM图像、（b） EDX元素分布、（c）粒径统计、（d） XRD谱图、（e） Py-FTIR和（f） NH3-TPD
Figure 2 Pt/ZSM-5 catalyst: (a) TEM, (b) EDX mapping images, (c) particle size statistics, (d) XRD，(e) Py-FTIR and (f) NH3-TPD
图 7 催化剂的N2物理吸附-脱附曲线（a）和孔径分布（b）
Figure 7 N2 physical adsorption-desorption isotherms (a) and pore size distribution curves (b) of catalysts
图 10 催化剂的产物选择性（a）及产物分布（b）
Figure 10 Product selectivity (a) and product distribution (b) of catalyst
图 11 反应后Co/Zr/Al2O3-Pt/ZSM-5催化剂的（a） TEM像（b）−（f）EDX元素分布
Figure 11 Spent of Co/Zr/Al2O3-Pt/ZSM-5 catalyst: (a) TEM; (b)−(f) EDX mapping images
表 1 催化剂的结构参数
Table 1 Structural parameters of the catalysts
Catalyst Co3O4 d a/nm Co0 d b/nm Dispersionc/% TOFd/(10−5mmol·s−1) Co/Al2O3 12.5 9.4 10.2 46.9 Co/Zr/Al2O3 11.9 8.9 10.7 47.7 Co/Zr/Al2O3-Pt/ZSM-5 11.5 8.6 11.1 149.1 a: Obtained by particle size statistics; b: According to the formula calculation[32], d(Co0)=0.75×d(Co3O4); c: According to the formula calculation[32], Dispersion=96/d(Co0); d: According to the formula calculationon[33], TOF=rAc/nOF. 
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表 2 催化剂的孔结构参数
Table 2 Pore structure parameters of catalyst
Catalyst Surface area/(m2·g−1) Pore volume/(cm3·g−1) BJH pore size/nm Al2O3 242.4 1.06 17.5 Co/Al2O3 222.0 0.89 16.1 Co/Zr/Al2O3 184.4 0.70 16.3 Co/Zr/Al2O3-Pt/ZSM-5 236.0 0.42 16.1/37.1
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表 3 催化剂的费-托合成反应性能
Table 3 FTS performance of the catalysts
Catalyst CO
conv./%CTY/
(10−5mmol·g−1·s−1)CO2
sel./%Hydrocarbon selectivity/% α4−30 CH4 C2−C4 C5−C11 C12+ Co/Al2O3 18.7 4.8 0.7 12.8 18.7 40.3 28.2 0.798 Co/Zr/Al2O3 19.7 5.1 0.6 12.5 17.3 32.1 38.1 0.822 Co/Zr/Al2O3-Pt/ZSM-5 16.5 8.3 0.5 10.0 15.0 45.9 29.1 0.776
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