Direct production of lower olefins from synthesis gas using supported iron catalysts

Abstract

Lower olefins (ethylene, propylene and butylenes) are important commodity chemicals used for the manufacture of, amongst others, plastics, solvents and lubricants to cosmetics and drugs. C2 to C4 olefins are conventionally produced by steam cracking of naphtha. In view of economic, strategic, and environmental reasons there is a growing necessity to produce these key chemical building blocks from non-oil derived sources. Many processes have been devised to obtain lower olefins from synthesis gas, a mixture of CO and H2 that can be obtained from different carbon-containing feedstocks. Methanol-to-olefins (MTO) and Dimethyl ether-to-olefins (DMTO) are syngas-based processes that involve the synthesis of an intermediate. A direct route for the conversion of syngas into lower olefins is the so-called Fischer-Tropsch-to olefins (FTO) process. Promoted iron catalysts are the choice for FTO as they show low methane selectivity at the high reaction temperatures necessary to drive product distribution to shorter hydrocarbon chains and exhibit high olefin selectivity. The use of iron-based catalysts is advantageous for the conversion of CO-rich syngas such as the gas obtained from gasification of coal or biomass. The use of Fe-based catalysts eliminates the need of an upstream water gas shift process to adjust the H2/CO ratio. Iron is more resistant to poisoning by contaminants present in coal or biomass-based syngas in comparison to catalysts used for the conversion of syngas into methanol or DME. To achieve active and selective catalysts for the FTO reaction it is necessary to add small amounts of other elements to modify the properties of iron (carbide). In general, promoters are added to reduce methane selectivity and to increase the selectivity towards C2-C4 olefins. The complexity of promoted bulk iron catalysts is not only related to the different iron phases present in the working catalysts but also to the presence of modifying elements and their intricate interactions with the active phase. Iron catalysts experience physical changes during reaction. Large iron oxide crystals transform into iron carbide when they are put in contact with syngas. Internal forces originated by density differences of the iron phases and by nucleation of carbon filaments result in fragmentation of iron-containing particles. This might lead to problems during operation such as fouling of separation equipment or plugging of the catalyst bed. The stability of iron-based catalysts can be improved by dispersing iron nanoparticles on a support or carrier material. The complexity of iron catalysts increases further when introducing a support material as a result of the interactions between the active phase and the carrier. Additionally, structure sensitivity starts to play a role when iron (carbide) particle size is decreased from micrometers to the nanometer range and the spatial distribution of the particles on the support might influence catalytic properties. Other factors that determine the performance of FTO catalysts are reaction conditions and pretreatment processes. The present work is mainly focused on the study of the properties of iron-based catalysts that have an influence on activity, selectivity and stability during the conversion of synthesis gas into lower olefins.