Recently, the research team of Professor Zeng Jie from the National Research Center for Microscale Matter Science of the University of Science and Technology of China in Hefei and the Department of Chemical Physics designed and constructed a Cu-Fe carbide interfacial catalyst to achieve high-selectivity preparation of long-chain olefin by hydrogenation of carbon dioxide under atmospheric pressure. The relevant results were published in the journal Nature Communications under the title "Ambient pressure hydrogenation of CO2 into long-chain olefins". The co-first authors of the paper are Li Zhongling, a doctoral student, and Wu Wenlong, a special associate researcher.

Long-chain alkenes (C4+=) have a wide range of applications in the field of fine chemicals, such as synthetic detergents, high-octane gasoline, lubricants, pesticides, plasticizers, etc. At present, the main way to synthesize long chain olefin is olefin polymerization which relies on the petrochemical industry. If the use of renewable energy to electrolyze water to produce hydrogen, and then react with greenhouse gas carbon dioxide directly to produce long-chain olefin, there will be huge environmental benefits. Due to the small scale and dispersed layout of water electrolytic equipment, in order to directly docking electrolytic water to produce hydrogen, it is necessary to make carbon dioxide hydrogenation reaction under normal pressure. However, at present, the hydrogenation of carbon dioxide to produce long chain alkenes is mostly carried out under high pressure reaction conditions. Moreover, according to Le Chatelier's principle, atmospheric pressure is not conducive to the formation of long-chain alkenes. Therefore, the realization of atmospheric pressure hydrogenation of carbon dioxide to produce long-chain olefin remains a great challenge.

The hydrogenation of carbon dioxide to olefin is mainly through methanol intermediates and carbon monoxide intermediates. Since low pressure is not conducive to either methanol synthesis or methanol hydrocarbon production, the research team chose the carbon monoxide intermediate path. The challenge of this pathway is to design suitable active sites for Fischer-Tropsch synthesis (FTS) at atmospheric pressure. Drawing on the design idea of modified Fischer-Tropsch catalyst used for alcohol synthesis, the researchers introduced a copper site with non-dissociative adsorption capacity of carbon monoxide on the basis of the iron-based catalyst, and prepared a copper-iron catalyst with copper-iron carbide interface working at atmospheric pressure. The catalyst contains a variety of phases such as copper metal, ferric oxide and iron carbide.

FIG. 1. (a) Performance comparison with conventional iron-based catalysts. (b) Carbon number distribution of hydrocarbon products. (c) Performance comparison at different airspeeds. (d) Performance comparison at different H2/CO2 ratios.

The performance of the catalyst was evaluated and it was found that at 320oC, 1 bar (H2:CO2 = 3:1) and 2400 mL h-1 gcat-1, the selectivity of the catalyst for long-chain olefin reached 66.9%, the conversion of carbon dioxide was 27.3%, and the selectivity of carbon monoxide was 43.7%. Compared with traditional iron-based catalysts, the catalyst is less selective to carbon monoxide and methane, and more selective to long-chain olefins (FIG. 1a,b). In order to evaluate the suitability of the catalyst, we changed the reaction space speed and the H2/CO2 ratio. When the reaction space speed increased, the carbon dioxide conversion rate decreased but the selectivity of long-chain olefin did not decrease significantly; when the H2/CO2 ratio increased, the selectivity of long-chain olefin only slightly decreased, indicating that the catalyst could be applied to a wide range of reaction conditions (FIG. 1c,d). Although the catalyst is operated under atmospheric pressure, its selectivity for long-chain olefins is basically equivalent to the optimal value reported in the literature under high-pressure reaction conditions (Figure 2). The researchers found that the long-chain olefin selectivity of the catalyst decreased after a long reaction time, but it could be restored after a simple regeneration treatment.

Figure 2. (a) Selectivity comparison of catalysts for carbon dioxide hydrogenation to long-chain olefin.

The coupling mechanism of carbon-carbon bonds was investigated by combining in situ characterization and theoretical calculations. The carbon monoxide desorption experiment showed that there were two active sites of carbon monoxide non-dissociative adsorption and desorption in the activated catalyst, indicating the possible mechanism of carbon monoxide insertion (FIG. 3a). Synchrotron radiation vacuum ultraviolet photoionization time-of-flight mass spectrometry detected traces of acetaldehyde, an oxygen-containing product, which also partially confirmed the existence of carbon monoxide insertion mechanism (FIG. 3b). The results of theoretical calculations show that the reaction energy barrier of carbon monoxide insertion at the Cu-Fe carbide interface is much lower than that of iron carbide alone, and the reaction energy barrier of CH2 coupling is also lower than that of iron carbide alone (FIG. 3c,d). Therefore, the researchers believe that in addition to the carbon-carbon coupling through the carbide path on iron carbide as with traditional iron based catalysts, carbon chain growth can also be performed at the copper-iron carbide interface through carbon monoxide insertion process, which can utilize a large amount of carbon monoxide on the catalyst surface that is not dissociated due to low pressure. The synergistic effect of the carbide path and the carbon monoxide insertion path results in good long-chain olefin selectivity.

Figure 3. (a) desorption spectra of carbon monoxide at programmed temperature. (b) photoionization spectra. (c) Carbon monoxide insertion mechanism barrier. (d) Carbide energy barrier.

The results of this study reveal the carbon-carbon coupling mechanism in the process of carbon dioxide hydrogenation reaction, and also provide ideas for the design of long chain olefin catalyst from carbon dioxide hydrogenation. The research has been supported by the National Key Research and Development Plan, the National Science and Technology Research Plan, the National Science Fund for Outstanding Young People, and the key project of the Anhui Province Joint Fund.

Paper link:

https://www.nature.com/articles/s41467-022-29971-5

(Hefei National Research Center for Microscale Matter Science, School of Chemistry and Materials Science, Department of Scientific Research)
Source: HKUST News Network