Kinetic Analysis of CO₂ Hydrogenation to Long-Chain Hydrocarbons on a Supported Iron Catalyst

Abstract

Hydrogenation of CO2 to long-chain hydrocarbons via combined reverse water gas shift (RWGS) and Fischer–Tropsch (FT) gained much attention in the last years as a way to produce sustainable hydrocarbons for the chemical industry or fuel applications. Despite the large amount of interest in the reaction, so far only a few studies have been conducted regarding the kinetics. In this study we carefully investigated the kinetics of an alumina supported iron catalyst at 280–320 °C, 10–20 bar, 900–120 000 mLN h–1 g–1, and a H2/CO2 molar inlet ratio of 2–4. Special attention was focused toward the thermodynamic constraints under reaction conditions. Based on elementary reaction steps according to recent mechanistic investigations, we derived new Langmuir–Hinshelwood–Hougen–Watson type kinetic expressions which allow an excellent reproduction of the experimental data and outperform existent models. Possible model combinations were discriminated against each other, and the best fit was obtained for the assumption of H-assisted CO2 and H-assisted CO dissociation mechanisms for RWGS and FT, respectively. Model uncertainties that are introduced by the RWGS being close to equilibrium are discussed in detail and are possibly a reason for strongly varying results for activation energies between different studies. The detrimental effect of water vapor on the reaction progression is analyzed numerically and can be attributed to two parameters: kinetic inhibition via strong adsorption of oxygen containing species and thermodynamic constraints by shifting the equilibrium CO partial pressures to lower values.

Influence of Recycle Operation on the Catalytic Hydrogenation of CO2 to Long-Chain Hydrocarbons

“…The long-term catalytic activity, as well as particle size variation tests, were performed in a lab-scale setup that has been previously presented. 9 The long-term tests were conducted with 2 g catalyst (100− 200 μm), diluted with 2 g of SiC (ESK-SIC GmbH, 200− 300 μm), and the particle size variation tests were conducted with 1 g catalyst, diluted with 12 g of SiC (200−300 μm). Different from the original publication, we used a 1/2′′ stainless steel tube with an inner diameter of 10.9 mm for the particle size variation tests to avoid wall effects.…”

“…From a qualitative point of view, there are only small differences in comparison to the product distribution for the once-through mode that we had published previously. 9 Most notably, short-chain oxygenates could now be reliably measured. Besides that, the product seems to display a higher ethane/ethene ratio under recycle conditions.…”

CO2 Conversion by Fischer‐Tropsch Synthesis Using Na‐Modified Fe Catalysts

“…This strengthens the CO bonding to the iron surface and weakens the H 2 adsorption, resulting in a pronounced chain growth and a higher alkene-to-alkane ratio [18]. When CO 2 is used directly in FTS, a K-promoted Fe/Al 2 O 3 catalyst has been shown to have a considerable C5+ selectivity of 63 % [23], while the addition of Na to a bare Fe catalyst increases both the C5+ and alkene selectivities [21,22,24].…”

Development and Validation of a Detailed Microkinetic Model for the CO2 Hydrogenation Reaction toward Hydrocarbons over an Fe–K/Al2O3 Catalyst

“…14 mechanism-based kinetic model was recently developed by Brubach et al, but it includes only C 4 species as products. 15 The purpose of this work is to develop a microkinetic model that is based on a hypothesis about the reaction mechanism with the aim of understanding the formation of the different groups of products observed.…”

“…However, this model is based on a semiempirical hypothesis and does not comprehensively explain the reaction mechanism. A first mechanism-based kinetic model was recently developed by Brübach et al, but it includes only C 4 species as products …”