Cs-Promotion in Fe-FTS Catalysts for Selective Olefin Production

Olefins are the essential raw materials for production of fuel, pharmaceuticals, and consumer goods. However, they are typically produced by hydrocracking fossil fuels, which have limited reserves and are a significant contributor to global warming and climate change. Alternative olefin production methods that are sustainable and renewable are necessary. Through high-temperature and moderate-pressure catalysis, Fischer-Tropsch-to-Olefin (FTO) can convert syngas (CO + H₂) from renewable sources like landfill gas into olefins. In iron- (or cobalt- or ruthenium-) based catalysts, hydrogens tend to overreact and break carbon-carbon bonds, resulting in more CO₂ and CH₄, and fewer olefins. It has been demonstrated that post-transition metal elements such as indium can alter the surface of FTS catalysts to stabilize reactions that generate more olefins and fewer greenhouse gases. To reduce the cost of FTO, inexpensive catalysts with a high level of activity and enhanced surface chemistry control are required. This study aims to determine whether cesium—an alkali metal—can reliably increase the selectivity of iron-FTS catalysts toward more olefins and fewer greenhouse gases at high temperatures and moderate pressure.

Cesium-promoted iron catalysts were synthesized by wet co-impregnation method and pretreated under a reductive environment (H₂/N₂). Catalyst performance testing was carried out in a fixed-bed reactor at 400 °C, 5 atm, and H₂/CO = 2. The Fe₅Cs/SiO₂ exhibited stable catalytic activity (40 hrs) and, in comparison to the monometallic Fe catalyst, the CO reactivity increased 4 times and the selectivity of CH₄ decreased by 40%. Cs enhanced olefin selectivity by 60% while reducing CO₂ emissions by 55%. There was a volcano-like relationship between the molar concentration of Cs and the activity of CO and the selectivity for olefins.

This finding suggests that Cs alters the surface chemistry in the Fe-based FTO reaction, where CO activation, C-C coupling, and hydrogenation can be modulated to increase olefin production. Cs-promoted Fe-based catalysts are low-cost alternatives that could help in the high conversion of syngas produced from the reforming of biogas and landfill gas into olefins, potentially creating a sustainable alternative to crude oil cracking.