Zhen Huang, Shen Wang, Yinghong Yue, Weiming Hua, Wei Shen, Hualong Xu
The Department of Chemistry, Laboratory of Advanced Materials, Fudan University,
Shanghai 200438, CHINA
Aromatics, especially light aromatics (BTX) play an important role in modern chemical industry, and are mainly produced from naphtha reforming and oil thermal cracking processes. Syngas can be converted into value-added hydrocarbons through a well-known Fischer-Tropsch synthesis (FTS) process, the products follow the Anderson-Schulz-Flory (ASF) distribution with little aromatics.1,2 Although FTS catalysts combined with zeolites can produce aromatics, the selectivity is insufficient and generally limited by the operating conditions.3,4 On the other hand, methanol can be produced from syngas, and then transferred into aromatics through methanol-to-aromatics (MTA) process catalysed by zeolites such as ZSM-5. However, the low selectivity to BTX and rapid deactivation of zeolites caused by coking have severely restricted the development of this process. Besides, the two-step process is not energy and cost efficient. The combination of these two steps could provide a strategy to highly efficient produce aromatics directly from syngas. Prior work has reported that aromatics could be obtained on a combined metal oxides and ZSM-5 catalyst with a moderate selectivity, implying that metal oxide can be adopted to activate CO and hydrogen molecules. But, development of a catalyst with improved activity and selectivity to light aromatics is highly desired for this process.
The OX-ZEO catalysts developed by Bao and co-workers using partially reduced metal oxides and zeolites exhibited an extraordinary activity and selectivity for direct conversion of syngas to olefins and aromatics.5,6 The surface oxygen vacancy has been considered as active sites for C-O bond activation, where *C species formed can be afterwards hydrogenated into hydrocarbons as intermediates for the formation of olefins and aromatics on zeolites. Y. Wang and P. Zhang reported their works on modified ZSM-5 to enhance light aromatics, Zn-modified ZSM-5 catalyst decorated by Silicalite-1 shell, the selectivity to p-xylene can be significantly improved over modified ZSM-5.7,8 Aromatics could be formed through the aromatization of intermediates diffusion from metal oxides, BTX mainly formed inside the micropores, C9+ Aromatics are considered to produce on the acid sites in the pore-mouth region or on external surfaces of ZSM-5 through the alkylation of aromatic intermediates. So, the acidity modification of ZSM-5 on external surface is the key routes to enhance BTX selectivity.
Here in this report, we integrated Ceria-Zirconia solid solutions with modified ZSM-5 as a bi-functional catalyst for direct conversion of CO-rich syngas to aromatics. Ceria has been widely applied in heterogeneous catalysis due to its facile redox property and oxygen storage/release capacity (OSC), which is related to the oxygen vacancies and lattice oxygen mobility in ceria crystals. The oxygen vacancy could be improved by introducing various metal ions into ceria lattice.9,10 In addition, ZSM-5 zeolite has been regarded as one of the most promising catalysts in the production of light aromatics due to the pore-size-initiated shape selectivity. Firstly, we synthesized a series of ceria-zirconia solid solutions (CZS) with different Ce content. The structure homogeneity of CZS in a wide range of Ce content can be attributed to the similar cation radius of Zr4+ (0.084 nm) to that of Ce4+ (0.097 nm). Surprisingly, aromatics can be obtained from CO-rich syngas (H2/CO =1) with a high selectivity over these oxides which is integrated with HZSM-5 zeolite (Si/Al=40) by powder-mixing. However, when the Ce content in CZS samples increases from 20% to 100%, the selectivity of aromatics gradually decreases from 75.2% to 59.0% with a slight fluctuation of CO conversion around 5% at 380 oC, which indicates that the relatively low Ce content can favour the formation of aromatics. Moreover, the BTX components account for nearly half of the aromatics, making light aromatics as the main products. The experiment results indicated that the CO conversion increased constantly as reaction temperature increasing from 380 oC to 450 oC, while the aromatic selectivity decreased significantly from 75% to 45%. After optimizing reaction condition, the CO conversion reaches to 30% with selectivity of aromatics above 50%, the fraction of BTX in aromatics is about 23%.
In order to enhance the selectivity of BTX in the fraction of aromatics, modified ZSM-5 catalysts decorated by SiO2 on external surface, combined with CZS have been also applied for enhancing BTX formation. The results indicated that CZS /ZSM-5-Si exhibited a high selectivity to aromatics, which is 55% with a CO conversion of 32% at 450 oC and 3.6 MPa, and the fraction of BTX reach about 41% among aromatics . After eliminating the strong acid sites on external surface of ZSM-5 by SiO2 decoration, the fractions of BTX in aromatics increase significantly from 25% to 41%. The light aromatic products (BTX) have increased due to the restrained production of heavier aromatics from alkylation of light aromatic intermediates.
Reference:
1. H. M. Torres Galvis and K. P. de Jong, ACS Catal., 2013, 3, 2130-2149.
2. C. D. Chang, W. H. Lang and A. J. Silvestri, J. Catal., 1979, 56, 268-273.
3. S. Sartipi, M. Makkee, F. Kapteijn and J. Gascon, Catal. Sci. Technol., 2014, 4, 893-907.
4. J. Plana-Pallejà, S. Abelló, C. Berrueco and D. Montané, Appl. Catal., A, 2016, 515, 126-135.
5. F. Jiao, J. Li, X. Pan, J. Xiao, H. Li, H. Ma, M. Wei, Y. Pan, Z. Zhou, M. Li, S. Miao, J. Li, Y. Zhu, D. Xiao, T. He, J. Yang, F. Qi, Q. Fu and X. Bao, Science, 2016, 351, 1065-1068.
6. J. Yang, X. Pan, F. Jiao, J. Li and X. Bao, Chem. Commun., 2017, 53, 11146-11149.
7. P. Zhang, L. Tan, G. Yang and N. Tsubaki, Chem. Sci., 2017, 8, 7941-7946.
8. K. Cheng, W. Zhou, J. Kang, S. He, S. Shi, Q. Zhang, Y. Pan, W. Wen, Y. Wang, Chem, 2017, 334-347?
9. Y. Wang, J. Posthuma de Boer, F. Kapteijn and M. Makkee, ChemCatChem, 2016, 8, 102-105.
10. Q. Li, Z. Huang, P. Guan, R. Su, Q. Cao, Y. Chao, W. Shen, J. Guo, H. Xu and R. Che, ACS Appl. Mater. Interfaces, 2017, 9, 16243-16251.
See more of this Group/Topical: General Submissions