Selective production of light α-olefins and long-chain α-olefins from CO2/H2 and CO/H2 over iron-based catalysts: Effects of Na2S and H2O

Abstract

Conversion of CO₂/H₂ and CO/H₂ into α-olefins via Fischer-Tropsch synthesis (FTS) reaction has stimulated great interest. Herein, iron-based catalysts were developed to produce α-olefins with a tailor-made manner. As-synthesized 1.5Na-Fe1Zn1 exhibited high activity for CO₂ conversion, high selectivity toward C₂-C₅ light olefins along with stable 1000 h run. Separate catalytic cracking of C₅₊ products over ZSM-5 nanosheet promoted light olefins selectivity up to 64.6 %, benefiting from the elimination of hydrogenation reaction. Interestingly, Na₂S showed different effects in CO₂-FTS and CO-FTS over iron-based catalysts. In detail, Na₂S significantly suppressed catalytic activity and increased selectivity toward alkanes/CO in CO₂-FTS reaction. However, it can surprisingly improve catalyst stability in CO-FTS reaction, leading to high activity at about 96 % CO conversion and high selectivity to long-chain C₄₊ olefins at about 65.4 %. In addition, it was demonstrated that H₂O accumulation hindered H₂/CO adsorption and dissociation, subsequent hydrogenation into CH_x, and their carbon–carbon coupling, which resulted in catalyst deactivation and selectivity shifting toward CH₄ and CO in atmospheric pressure CO₂-FTS reaction.