Selective hydrogenation of carbon monoxide (CO) is a common method for converting syngas into various organic chemicals like light olefins and wax. In particular, light olefins that are generally considered as the building block of modern plastics have been extensively studied. Three main pathways have been proposed for efficient syngas conversion to light olefins. The first one is the Fe-based catalytic pathway, which mainly uses Fe/CNF catalysts supported by S/Na synergistic effects and can achieve a high hydrocarbon selectivity of up to 53% at a pressure of 20 bar. Another zeolite/oxide catalytic pathway resembles a tandem catalysis coupling carbon-carbon reaction and methanol synthesis, which uses a range of catalysts like ZrOx-ZnO/SAPO-34. The other pathway involves a cobalt-based catalytic reaction using cobalt carbide nanoprisms as the main catalyst. This cobalt-based pathway is known for excellent light olefin production at low pressure and low-temperature conditions.
Although the available pathways have proved effective for converting syngas into light olefins, there is still a need to design other effective catalysts or pathways. To this end, adjusting of product selectivity for CO hydrogenation by adding additional element has attracted significant research attention for improving selective hydrogenation of CO. Specifically, Fischer-Tropsch to light olefins (FTO) catalysis is a promising approach for selective CO hydrogenation to produce ethylene, propylene, butylene, among other compounds.
On this account, Professor Zhenxin Liu, Mr. Gaopeng Jia, Ms. Chenxi Zhao, and Professor Yu Xing from Zhengzhou University of Light Industry proposed selective iron catalysts for direct Fischer-Tropsch synthesis to light olefins. Specifically, they developed a zeolite-free and potassium-promoted iron FTO catalysts modulated by composite oxides (Al – O – (Zn)) annealed at high temperatures. The effectiveness of the catalysts in terms of hydrocarbon selectivity performance was studied and discussed. Their research work is currently published in the journal, Industrial and Engineering Chemistry Research.
Results showed that FTO selectivity at lower carbon monoxide conversions was dominated by catalyst composition, while FTO selectivity at higher CO conversions was dominated by reaction temperatures. The catalysts contained a ZnAl2O4 component that reduced the carbon chain propagation by restraining the carbon-carbon coupling. Compared to pure-shape supports, the combination of ZnAl2O4 and high temperature annealed alumina may not only lower down the C5+ hydrocarbon selectivity more significantly, but also generate higher degree for iron carburization, which is positively correlated with the C2-C4 olefin/paraffin ratio. At a pressure of 20 bar, the catalyst achieved a maximum hydrocarbon selectivity of 64%. Furthermore, the contents of ethylene, propylene and butylene in C2, C3 and C4 hydrocarbons were 88%, 92% and 89%, respectively.
In summary, zeolite-free and potassium supported iron FTO catalyst modulated by spinel/alumina annealed at high temperatures is reported and its performances tested. The obtained results were impressive and comparable to those of the existing selective catalysts. For instance, the reported maximum selectivity of 64% is one of the best practicable hydrocarbon selectivity values for Fe-based FTO catalysts so far. The olefin fractions in ethylene, propylene and butylene hydrocarbons are also remarkable. In a statement to Advances in Engineering, Professor Yu Xing pointed out that the study would promote the application of selective iron catalysts for FTO reactions.
Liu, Z., Jia, G., Zhao, C., & Xing, Y. (2021). Selective Iron Catalysts for Direct Fischer–Tropsch Synthesis to Light Olefins. Industrial & Engineering Chemistry Research, 60(17), 6137-6146.