The synthesis and evaluation of a novel low-pH co-impregnated Co-Mo bimetallic catalyst system for Fischer-Tropsch Synthesis

Abstract

As the global struggle with depleting crude oil resources and ever rising costs continues, gas-to-liquids conversion of vast geographically stranded gas resources via the Fischer-Tropsch synthesis (FTS), offers a most viable alternative-to-crude oil energy avenue for the production of ultra-clean transportation fuels and petrochemicals feedstock. Since the extent of future success of this technology is braided with intelligent catalyst design, the focus of this thesis is to synthesise, characterise and test novel bimetallic Co-Mo formulations, with the vision to harness the high FT activity of cobalt, and endow it with the carbon and sulfur resilience of molybdenum-based species. Catalysts were prepared by sub-isoelectric co-impregnation using cobalt nitrate and ammonium molybdate on g-alumina support. Five different Co and/or Mo formulations are analysed, each comprising of 20% (wt/wt) total metal loading. Comprehensive physicochemical characterisation is performed, including determination of BET area, pore volume, pore-size distribution, H2/O2/CO chemisorption, NH3/CO2 temperature programmed desorption to assess acidity/basicity character, total carbon analysis, solid-state kinetics via thermogravimetric analysis, temperature programmed reduction/oxidation and XRD analysis to elucidate the prevalent catalyst oxidation states, etc. Mo addition increased the catalyst acidity, carbon stability, while bimetallic mixtures exhibited synergism for the acid site strength and concentration, total dispersion, and the solid-state kinetics from TG analysis. Catalyst testing and reaction kinetics study is predominantly performed in a differential fixed bed reactor, at 13 different synthesis gas (H2+CO) compositions, and 4 temperatures. The ASF parameters and their further derived attributes, product selectivities, olefin-to-paraffin ratio modelling, Arrhenius parameters, and other performance indices are discussed. Mechanistic methanation modelling revealed that the reaction was best characterised by a twin-RDS Eley-Rideal-Langmuir-Hinshelwood (ERLH) model. Higher hydrocarbon modelling is also achieved via empirical power law treatment, and a single comprehensive model is proposed, capable of predicting product formation based on H2 and CO feed partial pressures, and the carbon chain length. Preliminary work in a 2 L high pressure stirred tank slurry reactor is also performed, with the objective of laying the foundation for future work in the area.