Bifunctional Catalysis to convert Synthesis Gas to Chemicals and Fuels

Abstract

The world population is increasing and the demand for chemicals and fuels is higher than ever. The use of fossil feedstocks to serve these needs adds to the concentration of greenhouse gases in the atmosphere and drives unwanted climate change. Hence, new pathways for the production of chemicals and fuels are investigated and implemented. Synthesis gas (a mixture of H2 and CO) is an alternative to crude oil as it can be obtained from renewable sources such as municipal waste or biomass or from CO2 combined with H2 obtained by electrolysis of water. Iron-carbide based Fischer-Tropsch (FT) catalysts allow to convert synthesis gas to hydrocarbon products. The promotion of these FT catalysts with sodium and sulfur reduces the formation of undesired methane and increases the olefin content in the products as well as the activity in synthesis gas conversion. These types of catalysts are called Fischer-Tropsch to Olefins (FTO) catalysts. By the addition of a suitable zeolite to the FTO catalysts (forming a bifunctional catalyst) the formed olefins can (partially) be converted into aromatics. However, in close proximity between the FTO catalysts and zeolites alkaline promoters, such as sodium, can migrate from the FTO catalyst to the zeolite causing decreased activity of both catalysts. Colloidal iron nano particles attached to zeolites show promising application as model catalysts. The method of introducing sodium and sulfur promoters to these bifunctional catalysts has a large influence on the activity and stability in synthesis gas conversion to form olefins and aromatics. Bulk iron catalysts (promoted with alkaline metals) are widely applied in the high temperature Fischer-Tropsch synthesis to produce olefin rich hydrocarbons. Operating a potassium promoted bulk iron catalyst at medium temperature (250°C-300°C) causes a shift towards heavier products compared to high temperature applications. Placing a zeolite catalyst bed downstream of the bulk iron catalyst allows to convert higher olefins to aromatics and crack long hydrocarbons into C5-C11 products. Additionally, isomerization of hydrocarbons is promoted. The resulting hydrocarbons mixture complies with requirements for gasoline fuel, such as a high octane number. The aim of the research was to gain insight into bifunctional catalysts consisting of iron based FT catalysts and zeolites to convert synthesis gas to chemicals, namely olefins and aromatics, as well as gasoline. Chapters 3 and 4 describe a study of the processes of aromatization and deactivation by promoter migration for sodium- and sulfur-containing FTO catalysts. Colloidal iron nano particles attached to a zeolite catalyst were applied as model catalysts in chapter 5 to investigate the influence of promotion on the growth of the iron nano particles. Chapter 6 describes the use of bulk iron catalysts in combination with zeolites at medium temperatures (250°C-300°C) to understand aromatization, isomerization and cracking during the direct conversion of synthesis gas to gasoline.