# Synthesis and Characterization of Iron-Based Catalysts for Carbon Dioxide Valorization
have reported the production of lower olefins via the modified FT synthesis, employing bifunctional catalysts of In\({}_{2}\)O\({}_{3}\)-ZnZrO\({}_{\text{x}}\) oxides supported on SAPO-34 zeolites, reaching 17% CO\({}_{2}\) conversion with 84% selectivity to C\({}_{2}\)-C\({}_{4}\) olefins [34]. Potassium-modified HY zeolites coupled with iron species have exhibited improved selectivity to C\({}_{5+}\) products (i.e., 18%), compared to the non-modified HY zeolite (i.e., 10%); adding cerium as a promoter further increased the selectivity to 32% [35]. Employing Fe-Zn-Zr species supported on HZSM-5, Hbeta and HY; among the samples tested, Fe-Zn-Zr/HZSM-5 catalysts showed the best performance with 21.0% CO\({}_{2}\) conversion and 50% selectivity to C\({}_{5+}\) hydrocarbons (including CO), at 5.0 MPa, 340 \({}^{\circ}\)C, 3000 mL gcat\({}^{-1}\) h\({}^{-1}\), and H\({}_{2}\)/CO\({}_{2}\)3 [36]. Another group has used a sodium-modified Fe\({}_{3}\)O\({}_{4}\) catalyst combining SAPO-11 and ZSM-5 supports, at H\({}_{2}\)/CO\({}_{2}\) = 3, 320 \({}^{\circ}\)C, 3.0 MPa, 0.5 gcat, to achieve 31% CO\({}_{2}\) conversion to gasoline hydrocarbon fraction (72% selectivity) and isoparaffins (38%) [37]. [PERSON] et al. have prepared a multifunctional Na-Fe\({}_{3}\)O\({}_{4}\)/HZSM-5 catalyst, observing 22.0% CO\({}_{2}\) conversion towards gasoline-range hydrocarbons--C\({}_{5}\)-C\({}_{11}\) (78% selectivity) and methane (4% selectivity) under the following reaction conditions: H\({}_{2}\)/CO\({}_{2}\) = 3, 320 \({}^{\circ}\)C, 3.0 MPa, 4000 mL h\({}^{-1}\) gcat\({}^{-1}\), in a dual-bed reactor [17]. Unsupported Na-Fe\({}_{3}\)O\({}_{4}\) converted 34% CO\({}_{2}\) towards C\({}_{5}\)-C\({}_{11}\) (38% selectivity), CO (14% selectivity), and methane (4% selectivity) under the same experimental conditions [17]. The same group has employed Na-promoted Fe\({}_{3}\)O\({}_{4}\) nanoparticles supported on HZSM-5 in order to convert CO\({}_{2}\) (25.6% conversion) mainly into C\({}_{4+}\) products, including isoparaffins (78% selectivity), under the following reaction conditions: H\({}_{2}\)/CO\({}_{2}\) = 2, 320 \({}^{\circ}\)C, 3.0 MPa, 4000 mL gcat\({}^{-1}\) h\({}^{-1}\)[38]. In a similar study, [PERSON] et al. synthesized an Na-modified ZnFeO\({}_{x}\)/HZSM-5 that converts CO\({}_{2}\) into aromatics with 41.2% conversion and 75.6% selectivity under the following reaction conditions: H\({}_{2}\)/CO\({}_{2}\)/N\({}_{2}\): 73/24/3, 320 \({}^{\circ}\)C, 3.0 MPa, 4000 mL gcat\({}^{-1}\) h\({}^{-1}\)[39]. Another study employed multifunctional catalysts combining K-Fe/C and ZSM-5, to reduce the acidity of the aluminosilicate, resulting in 35% CO\({}_{2}\) conversion into C\({}_{5+}\) hydrocarbons (70% selectivity) at H\({}_{2}\)/CO\({}_{2}\) = 2.5, 320 \({}^{\circ}\)C, 2.0 MPa, 1200 mL gcat\({}^{-1}\) h\({}^{-1}\)[40]. In this work, multi-functional iron-based catalysts, employing promoters such as alkalis (i.e., Na, K) or zinc (Zn) and dispersed on HZSM-5, have been synthesized in order to upgrade the CO\({}_{2}\) into C\({}_{2+}\), hydrocarbons. The aim is to study potassium- and sodium-doped state-of-the art catalysts one step further in two different amounts. The respective catalysts have been successfully synthesized as proved by the diffraction diagrams of the TEM, importing the alkali ions in the Fe\({}_{3}\)O\({}_{4}\) lattice. Synthesized samples have been physico-chemically characterized, and their performance has been evaluated through preliminary experimental CO\({}_{2}\) hydrogenation tests in order to correlate physicochemical properties with product distribution. The association of the obtained experimental data with the detected differences in TPD-NH\({}_{3}\) measurements, concerning the strength of the acid sites, shows that product distribution is highly affected by the presence of acid sites; Na addition suppresses formation of CO while the addition of K generates stronger acid sites that hinder the formation of C\({}_{2+}\), hydrocarbons, promoting C\({}_{1}\) product formation.