The increase in global demand for lower olefins (ethylene, propylene, and butylenes) coupled with the regional diversification of carbon raw materials bring about opportunities and challenges for emerging technologies. Crude oil has been the primary carbon feedstock for the past 50 years, but alternative carbon feedstocks, including coal, natural gas, and biomass, received considerable attention in recent years. These alternative resources could first be converted to synthesis gas, a mixture of CO and H2. Natural gas produces H2-rich synthesis gas (H2/CO ≥ 2) while coal and biomass produce CO-rich synthesis gas (H2/CO < 2). Synthesis gas could subsequently be used to produce fuels and chemicals, including lower olefins and oxygenates. Lower olefins and their derivatives are essential chemical building blocks for many industries, ranging from plastics to pharmaceuticals. Currently the commercial production of lower olefins from synthesis gas is taking place via methanol (methanol-to-olefins), but direct routes, namely Oxide-Zeolite (OX-ZEO) and Fischer-Tropsch to Olefins (FTO), offer potentially higher efficiencies in volume, energy, materials, and cost.
Although the Fischer Tropsch (FT) process has been developed for almost a century, its main application is liquid fuels instead of chemicals (lower olefins, oxygenates, and aromatics). The biggest challenge of the Fischer-Tropsch to Olefins (FTO) process is the product selectivity which is governed the Anderson-Schulz-Flory (ASF) distribution. The serendipitous discovery of adding Na and S promoters to supported Fe-based catalysts which led to unprecedented selectivity towards lower olefins offers a solution to this selectivity problem.
The goal of this work is to design and develop catalysts for the direct production of lower olefins from synthesis gas and a two-pronged approach was used to achieve this. On one front, experimental and theoretical methods were used to understand the Fe particle size and promoters (Na and S) effects of state-of-the-art Fe-based FTO catalysts in terms of activity, selectivity and stability. These insights were then used to rationally design model Fe-based FTO catalysts which were highly active, selective and stable for direct conversion of CO-rich syngas feed to lower olefins. On the other front, Co-based FTO catalysts were developed to meet the need of directly converting H2-rich syngas feed to lower olefins.