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

Mechanistic kinetic models have been developed for the CO2 hydrogenation reaction with the aim to provide insights into the mechanism followed for the formation of CO, linear alkanes and alkenes containing up to 20 carbon atoms, and alcohols and acids containing up to six carbon atoms over an Fe-K/Al2O3 catalyst in a continuous fixed-bed reactor. On the basis of a redox mechanism for the reverse water-gas shift reaction, an alkyl mechanism to explain the chain-growth mechanism and the formation of linear hydrocarbon chains, and a CO insertion mechanism to explain the formation of oxygenate products, the kinetic rates for the compounds considered are derived according to the Langmuir-Hinshelwood-Hougen-Watson method. Two models are proposed: the first one considers the existence of only one site for the FT step, where all of the products are formed; the second model is based on the hypothesis that two different active sites exist, one for the formation of hydrocarbons and the other for the formation of oxygenates. Mathematical optimization via least-squares methods allowed estimation of the kinetic parameters for the two models. The models were validated against the available experimental data. Globally, both models give good predictions of the experimental data, with mean average relative residuals <5%. However, the monosite model shows a better fit of the experimental data and has a lower statistical error. Nevertheless, it is not able to accurately predict the formation of oxygenates, giving a value of the chain-growth probability that is significantly far from the experimental value. An improved description of oxygenates is provided by the multisite model, but it has larger confidence intervals and suffers from a high number of kinetic parameters. This study globally provides a first investigation of the mechanism of CO2 hydrogenation, including the formation of hydrocarbons and oxygenates. It has been shown that a complex mechanism is involved that includes chain-growth via an alkyl mechanism combined with a CO insertion mechanism to form the oxygenate products.