Effects of the promotion with bismuth and lead on direct synthesis of light olefins from syngas over carbon nanotube supported iron catalysts

Light olefins are important platform molecules in chemical industry. Fischer-Tropsch synthesis provides an alternative technology for direct synthesis of light olefins from syngas, which can be generated from renewable feedstocks such as organic waste, used plastics and biomass. The Bi- and Pb-promoted iron catalysts supported by carbon nanotubes have been prepared for direct conversion of syngas to lower olefins. Compared to the unpromoted iron catalysts, a twice higher Fischer-Tropsch reaction rate and higher selectivity to lower olefins were obtained. A combination of characterization techniques reveals remarkable migration the promoting elements, which occurs during the catalyst activation. After the activation, the iron carbide nanoparticles are decorated with the promoting elements. The lower melting points and high mobility of these two metal promoters during the catalyst activation are crucial for the intimate contact between Fe and promoters. The promoting effects of bismuth and lead result in a better reducibility and easier carbidisation of iron nanoparticles. The higher yield of light olefins over the carbon nanotube supported catalysts in the presence of promoters is due to slowing down secondary hydrogenation of olefins and a decrease in the chain growth probability.