1. Introduction

The catalytic reaction of hydrogenation of CO₂ to olefins is thermodynamically favorable. Nevertheless, there are only a few very efficient catalytic systems exhibiting high selectivity to olefins. The main reason is that the catalyst has to be able to simultaneously catalyze the reverse water-gas shift (RWGS) and modified Fischer-Tropsch to olefins (FTO) reactions working under the same operating conditions, and its final behavior depends on several combined factors, such as the type of metals, the selection of supports, the co-catalyst and the method of catalyst preparation and the selection of operating conditions ^[1][2][3]. For catalyst design, the selection of metals is limited to Fe, Co, Pd, Ni and Ru, which are the only ones that show high activity in the CO₂ hydrogenation upon moderate reaction conditions. Among them, only Fe-based catalysts show relatively high selectivity towards desired products while maintaining low selectivity towards undesired methane ^[1][2][3]. In addition, Fe-based catalysts are attractive to the industry due to their low cost, which is related to the abundance of iron in nature, and flexible product distribution ^[4].

2. Reaction Mechanism 

There are a large number of theories and hypotheses presented in the literature on the possible mechanism of CO₂ hydrogenation to olefins. This is due to the structure sensitivity of CO₂ activation ^[5] and the dependence of CO₂ hydrogenation on CO₂ coverage and hydrogen adsorption mode ^[6]. Therefore, the reaction mechanism depends on the catalyst formulation, catalyst morphology, catalyst pre-treatment, reaction conditions employed and proper hydrogen dosing according to the reactor configuration ^[7][8].

Depending on the catalyst used, the formation of olefins from CO₂ can occur via various alternative routes (Figure 1): (1), CO-mediated modified Fischer–Tropsch to olefins (FTO) route; (2), methanol to olefins (MTO) route, and (3), direct CO₂ hydrogenation to olefins using promoted and multifunctional catalysts (Equation (1)). The latter reaction pathway is kinetically more difficult and is still under development.

In general, it is observed that the FTO route (1) is more efficient in terms of hydrocarbon yield than the MTO route (2).

Figure 1. General scheme of the three routes of olefins formation from CO₂: (1) modified Fischer–Tropsch to olefins (FTO) route; (2) methanol to olefins (MTO) route; (3) direct olefin formation over multifunctional, promoted catalysts.

The production of olefins from CO₂ through the modified Fischer–Tropsch to olefins (FTO) synthesis pathway occurs by redox and associative reaction mechanisms:

(1) Redox mechanism: CO is formed by adsorption and activation of CO₂ on metals or metal oxides, and H₂ does not participate in the formation of intermediate products (Equation (2));

(2) Associative mechanism: multi-site adsorption of reagents, chain initiation, chain growth and termination and desorption of the final product (Equation (3)). In this mechanism, the association of hydrogen with CO₂ leads to the formation of different intermediate surface species, such as formate, carbonate or carboxylic species, which decompose into final products ^[9].

The RWGS is an endothermic reaction, so it is favored at higher temperatures, whereas FTO is an exothermic process [10]. Therefore, thermodynamic data indicate that low temperature favors the FTO reaction, while a high temperature is necessary to activate CO₂ for the RWGS reaction ^[1][2].

In general, the modified FT route of CO₂ hydrogenation could occur via redox (dissociation of CO₂ to CO and O) or via an associative mechanism, in which hydrogen reacts with CO₂ to form the intermediate HOCO. This intermediate then decomposes into CO and OH, the latter being hydrogenated to H₂O. Unfortunately, in the FTO reaction, product selectivity depends on chain growth, which can be predicted using the chain growth probability (α) of the Anderson–Schulz–Flory (ASF) distribution model ^[11], which can be calculated by the following equation (4):

where W_n is the mass fraction of hydrocarbons with carbon number n in the chain and α is the chain growth probability, which is a function of the rates of chain growth and chain termination ^[11].