Cobalt-Catalyzed Fischer–Tropsch Synthesis: Chemical Nature of the Oxide Support as a Performance Descriptor

Abstract

The effect of the chemical nature of the oxide support on the performance of cobalt Fischer–Tropsch catalysts is investigated. A series of supports is synthesized via monolayer coverage of porous γ-Al2O3 with various oxides representative of a wide range of Lewis acid–base character, as quantified by UV–vis spectroscopy coupled to alizarin adsorption. Incorporation of cobalt (20 wt %) results in model catalysts with identical porosities and similar Co particle sizes (>10 nm), allowing the study of support effects without overlap from diffusional or particle size factors. Under realistic reaction conditions, the initial TOF scales with the acidity of the oxide support, whereas the cobalt time yield and selectivity to industrially relevant C13+ hydrocarbons show a volcano dependence, with a maximum at an intermediate acid–base character. As inferred from in situ CO-FTIR, “selective” blockage of a few cobalt sites, though crucial for CO hydrogenation, by atoms from basic oxides and “unselective” site blockage via decoration of Co nanoparticles (strong metal–support interaction) with acidic, reducible oxides cause a decrease in reaction rate for supports with pronounced alkaline and acidic character, respectively. The extent of secondary isomerization reactions of α-olefin products, of relevance for chain reinsertion processes and product selectivity, also correlates with the support acidity. For a TiO x /Al2O3 as support, a remarkable C13+ productivity exceeding 0.09 molC gCo –1 h–1 is achieved, owing to the combination of optimal activity and selectivity. These results provide a unifying view of the support effects over a considerably broad study space and delineate a blueprint toward advanced Fischer–Tropsch catalysts.

Fischer–Tropsch synthesis over various Fe/Al2O3–Cr2O3 catalysts

“…They reported that zeolite catalysts which owned higher quantity of the acidic centers with medium strength showed also higher isomerization activity in Fischer-Tropsch synthesis. Prieto et al [31] claimed that the overall reaction rate of the F-T synthesis carried out on cobalt catalysts reaches a maximum for oxide supports characterized by an intermediate acid-base character. Such activity behaviour authors related with the nature of the support.…”

Influence of Carbon Deposits on the Cobalt-Catalyzed Fischer–Tropsch Reaction: Evidence of a Two-Site Reaction Model

“…Prieto et al reported that the methane selectivity is at a maximum as a function of the acid–basic character of the support, and that a highly basic dopant such as SmO x can suppress methane formation. 13 …”

Fischer–Tropsch Synthesis Over Zr-Promoted Co/γ-Al2O3 Catalysts

“…Contrariwise, the CoZrAl ME catalyst, which presented a homogeneous distribution, showed a lower activity than the CoAl catalyst. Unfortunately, these results cannot be explained by differences in support acidity, factor that is believed to play a crucial role in the FTS activity [46,47]. According to Johnson and Bell [33], promotion with Zr originates from sites with enhanced activity at the Co-ZrO 2 interface.…”

“…Prieto et al [46] found that the TOF and the selectivity to long chain hydrocarbons increases with increasing acidity of the oxide support. However, excessive acidity leads to a partial coverage of the cobalt particles by partially reduced oxide support species.…”

“…Recent investigations have found that the support Lewis acidity plays an important role in cobalt-catalyzed FTS [46,47]. Prieto et al [46] found that the TOF and the selectivity to long chain hydrocarbons increases with increasing acidity of the oxide support.…”