Fischer-Tropsch Synthesis Steps into the Solar Era: Lower Olefins from Syngas

In this issue of Chem, Ma, Wen, and co-workers have demonstrated a photo-driven Fischer-Tropsch to olefins (FTO) process over an oxygen-decorated Fe5C2 catalyst and achieved a very high olefin/paraffin ratio of 10.9 with high CO conversion. Such high selectivity toward lower olefins originates from the spontaneous oxygen decoration on the Fe5C2 surface under photo-irradiation. In this issue of Chem, Ma, Wen, and co-workers have demonstrated a photo-driven Fischer-Tropsch to olefins (FTO) process over an oxygen-decorated Fe5C2 catalyst and achieved a very high olefin/paraffin ratio of 10.9 with high CO conversion. Such high selectivity toward lower olefins originates from the spontaneous oxygen decoration on the Fe5C2 surface under photo-irradiation. Fischer-Tropsch synthesis (FTS) of syngas (CO + H2) to highly valuable fuels or chemicals is one of the most important processes from both academic and industrial points of view. It is also a promising alternative technology for the production of lower olefins from a non-petroleum (i.e., coal, natural gas, or biomass) route, namely, the Fischer-Tropsch to olefins (FTO) process.1Torres Galvis H.M. de Jong K.P. Catalysts for production of lower olefins from synthesis gas: a review.ACS Catal. 2013; 3: 2130-2149Crossref Scopus (688) Google Scholar FTS has been under extensive study for more than 50 years, and great progress has been made recently.2Jiao F. Li J. Pan X. Xiao J. Li H. Ma H. Wei M. Pan Y. Zhou Z. Li M. et al.Selective conversion of syngas to light olefins.Science. 2016; 351: 1065-1068Crossref PubMed Scopus (884) Google Scholar, 3Zhong L. Yu F. An Y. Zhao Y. Sun Y. Li Z. Lin T. Lin Y. Qi X. Dai Y. et al.Cobalt carbide nanoprisms for direct production of lower olefins from syngas.Nature. 2016; 538: 84-87Crossref PubMed Scopus (544) Google Scholar, 4Zhao B. Zhai P. Wang P. Li J. Li T. Peng M. Zhao M. Hu G. Yang Y. Li Y.-W. et al.Direct transformation of syngas to aromatics over Na-Zn-Fe5C2 and hierarchical HZSM-5 tandem catalysts.Chem. 2017; 3: 323-333Abstract Full Text Full Text PDF Scopus (185) Google Scholar It has been reported that Mn-modified Co carbide catalysts or Na- and Zn-promoted Fe5C2 catalysts are highly selective for olefin production.3Zhong L. Yu F. An Y. Zhao Y. Sun Y. Li Z. Lin T. Lin Y. Qi X. Dai Y. et al.Cobalt carbide nanoprisms for direct production of lower olefins from syngas.Nature. 2016; 538: 84-87Crossref PubMed Scopus (544) Google Scholar, 5Zhai P. Xu C. Gao R. Liu X. Li M. Li W. Fu X. Jia C. Xie J. Zhao M. et al.Highly tunable selectivity for syngas-derived alkenes over zinc and sodium-modulated Fe5 C2 catalyst.Angew. Chem. Int. Ed. 2016; 55: 9902-9907Crossref PubMed Scopus (268) Google Scholar However, because of the energy demand for activating CO and H2 molecules, the existing FTS processes are typically carried out in a harsh reaction (e.g., 230°C–450°C and 2–5 MPa). Such FTS therefore has a significant carbon footprint and is unsustainable in the long term. In order to further increase FTS efficiency and enhance the product selectivity of FTS catalysts (e.g., the ratio of olefin to paraffin and selectivity of CH4 and CO2), the development of more mild-condition-activated catalysts and a greener and more environmentally friendly pathway is highly desirable. Solar-driven catalysis offers enormous potential for transforming abundant solar energy into fuels and valuable chemical feedstocks via processes such as water splitting, CO2 reduction, and N2 fixation, providing a possible alternative, green, and sustainable route for FTS to hydrocarbons.6Meng X. Liu L. Ouyang S. Xu H. Wang D. Zhao N. Ye J. Nanometals for solar-to-chemical energy conversion: from semiconductor-based photocatalysis to plasmon-mediated photocatalysis and photo-thermocatalysis.Adv. Mater. 2016; 28: 6781-6803Crossref PubMed Scopus (352) Google Scholar Until now, group VIII metals, especially for Ni-, Co-, and Fe-based heterostructures, have shown potential catalysis mainly because of their tunable electronic structure. In recent years, using group VIII elements (such as Ni) as catalysts, researchers have developed a large number of photo-driven CO or CO2 methanation processes over Ni or NiO catalysts.7Sastre F. Puga A.V. Liu L. Corma A. García H. Complete photocatalytic reduction of CO2 to methane by H2 under solar light irradiation.J. Am. Chem. Soc. 2014; 136: 6798-6801Crossref PubMed Scopus (197) Google Scholar, 8Zhao Y. Zhao B. Liu J. Chen G. Gao R. Yao S. Li M. Zhang Q. Gu L. Xie J. et al.Oxide-modified nickel photocatalysts for the production of hydrocarbons in visible light.Angew. Chem. Int. Ed. 2016; 55: 4215-4219Crossref PubMed Scopus (178) Google Scholar Compared with the traditional fossil-based thermal catalysis, these photo-driven reactions toward CO or CO2 utilization are more sustainable without an obvious carbon footprint, achieving the conversion from solar to chemical energy. In previous work, Zhang, Ma, and co-workers8Zhao Y. Zhao B. Liu J. Chen G. Gao R. Yao S. Li M. Zhang Q. Gu L. Xie J. et al.Oxide-modified nickel photocatalysts for the production of hydrocarbons in visible light.Angew. Chem. Int. Ed. 2016; 55: 4215-4219Crossref PubMed Scopus (178) Google Scholar, 9Zhao Y. Li Z. Li M. Liu J. Liu X. Waterhouse G.I.N. Wang Y. Zhao J. Gao W. Zhang Z. et al.Reductive transformation of layered-double-hydroxide nanosheets to Fe-based heterostructures for efficient visible-light photocatalytic hydrogenation of CO.Adv. Mater. 2018; 30: e1803127Crossref Scopus (78) Google Scholar combined experimental and theoretical methods to report hydrocarbon synthesis over Ni/NiO heterostructure photocatalysts. The existence of Ni oxide decorated on the surface of Ni nanoparticles led to Ni nanoparticles that were apt for CO hydrogenation under light irradiation, which alters the geometric and electronic structure of Ni nanoparticles, favors the C–C coupling reaction on its surface, confers the catalyst with unexpected reaction power toward higher hydrocarbons at moderate reaction conditions, and shows a green and sustainable approach for the photocatalytic production of highly valuable chemical fuels. However, the synthesis of olefins under light irradiation is remains a challenge. In this issue of Chem,10Gao W. Gao R. Zhao Y. Peng M. Song C. Li M. Li S. Liu J. Li W. Deng Y. et al.Photo-driven syngas conversion to lower olefins over oxygen-decorated Fe5C2 catalyst.Chem. 2018; 4https://doi.org/10.1016/j.chempr.2018.09.017Abstract Full Text Full Text PDF PubMed Scopus (58) Google Scholar Ma, Wen, and co-workers report an efficient photo-driven FTO process over O-decorated Fe5C2 catalysts and show that the selectivity of lower olefins is highly dependent on the partial O modulation of the Fe5C2 surface (Figures 1A and 1B ) and solar irradiation. They synthesized the Fe5C2 catalysts by a facile chemie douce route. Under photo-irradiation (atmospheric pressure and CO/H2 molar ratio = 1/2), the results of the CO hydrogenation reaction over Fe5C2 showed that the Fe5C2 catalyst had an outstanding selectivity of lower olefins. The CO conversion was as high as 49.5%, and the olefin-to-paraffin (o/p) ratio in the C2−4 hydrocarbons even reached 10.9, indicating that most of the C2–4 hydrocarbons were olefins. Furthermore, the lower CO2 selectivity of 18.9% ensured a high carbon resource utilization efficiency. Consequently, the authors carefully monitored the temperature of the catalyst bed at various conditions. Under whole-range photo-irradiation, the temperature rapidly reached 200°C within 60 s and rocketed up to 490°C within 28 min. Meanwhile, the UV/visible spectra proved that Fe5C2 indeed had a strong absorbance at 300−2,500 nm, covering full-spectrum photo-irradiation, implying a high utilization efficiency of solar energy over the Fe5C2 catalyst. The remarkable increased temperature of Fe5C2 can provide energy to overcome the reaction barrier and initiate the reaction process. The recycling test showed that the catalyst under photo-irradiation was very stable, and the catalyst performance was almost unchanged as well, indicating that the nature of the catalyst does not change during recycling. To gain an in-depth understanding of the structure of the Fe5C2 catalyst during reaction conditions, the authors performed in situ X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) experiments during the photo-driven CO hydrogenation reaction (Figures 1C and 1D), and the results suggested that the bulk phase of Fe5C2 remains mostly unchanged during the photo-driven FTO process. This differs from that of the traditional thermal reaction with the fully covered O layer on the Fe5C2 surface,4Zhao B. Zhai P. Wang P. Li J. Li T. Peng M. Zhao M. Hu G. Yang Y. Li Y.-W. et al.Direct transformation of syngas to aromatics over Na-Zn-Fe5C2 and hierarchical HZSM-5 tandem catalysts.Chem. 2017; 3: 323-333Abstract Full Text Full Text PDF Scopus (185) Google Scholar showing the superiority of the photo-driven reaction processes over the Fe5C2 catalyst. Up until this point, the above results could not explain why the unprompted iron carbide is apt for the production of olefins in the case of photo-irradiation. So, the authors studied the surface structures of the fresh and used catalysts by ex situ X-ray photoelectron spectroscopy (XPS) (Figures 1A and 1B); the detailed analysis suggested that the catalyst surface is mainly Fe5C2, and the slightly increased peaks at ∼709.3 and 529.8 eV revealed that the Fe5C2 surface is decorated by a small amount of O. Furthermore, they prepared three other kinds of catalysts (O2-passivated, air-treated, and H2-treated Fe5C2) as references to tune the surface structure of the Fe5C2 catalyst. The authors discovered that the O2-passivated Fe5C2 catalyst surface had been mostly transformed into iron oxide (Figure 1A), and the air- and H2-treated Fe5C2 catalysts were transformed into Fe2O3 and metallic Fe nanoparticles, respectively. However, the catalytic performance of the reference catalysts clearly decreased or was almost inactive for CO activation. On the basis of these results, they hypothesized that Fe5C2 partly decorated with surface O promotes the formation of olefins under photo-irradiation. Furthermore, to understand the relationship between the surface O atoms on Fe5C2 and the selectivity of FTO products, the authors carried out density functional theory (DFT) calculations on the Fe5C2 catalysts. By comparing the C2H6 formation mechanisms from CH2 coupling and hydrogenation on two Fe5C2 surfaces decorated with and without O atoms, they showed that C2H4 can be easily desorbed to atmosphere on Fe5C2(111)-4Oads because of its lower adsorption energy, whereas C2H4 tends to further hydrogenate to form C2H6 on Fe5C2(111). In addition, analysis of the electron structure properties showed that the electron can be transferred from the Fe atom to O and C2H4, and the excited DFT calculations further showed that the excited-state energies of C2H5 and C2H6 are obviously suppressed on the O-decorated Fe5C2(111) surface in comparison with the ground DFT results, indicating that the excited process can modify the local electronic structure and optical band gap of the surface. All of these calculations agree well with the experimental results, demonstrating that a small number of O atoms adsorbed on the Fe5C2 surface can indeed dramatically improve the o/p ratio. This work reports an extremely promising photo-driven FTO process over an O-decorated Fe5C2 catalyst with high olefin selectivity and low CO2 selectivity. Compared with traditional thermal-driven FTO processes, the photo-driven FTO based on the Fe5C2 catalyst is a sustainable route to harvesting the most abundant energy on Earth and to producing highly valuable fuels and chemicals. This heralds the coming of high-performance, energy-efficient, and green FTO processes with solar power as the energy source and cost-effective iron carbide as the catalyst, showing the potential of solar energy in the production of industrially important chemicals. Photo-Driven Syngas Conversion to Lower Olefins over Oxygen-Decorated Fe5C2 CatalystGao et al.ChemOctober 18, 2018In BriefWith the depletion of crude oil and especially the growing demand for lower olefins, the FTO process is a promising alternative for the petroleum route. In this work, a photo-driven FTO process was realized over Fe5C2 catalyst under photo-irradiation. The unique slight oxygen-modulated Fe5C2 structure formed in situ under the photo-irradiation condition, resulting in high selectivity to olefins, making it an excellent catalyst in photo-driven FTO reaction. Full-Text PDF Open Archive