Selective Conversion of Carbon Dioxide into Liquid Hydrocarbons and Long-Chain α-Olefins over Fe-Amorphous AlO<i><sub>x</sub></i> Bifunctional Catalysts
Muhammad Kashif Iqbal Khan, Paresh Butolia, Heuntae Jo, Muhammad Irshad, Daseul Han, Kyung‐Wan Nam, Jaehoon Kim
Abstract
Considerable progress has been made in the conversion of carbon dioxide (CO2), which is highly thermodynamically stable, into liquid hydrocarbons using metal oxide/zeolite composite catalysts. Nevertheless, producing liquid hydrocarbons with a single catalyst without utilizing additional C–C coupling agents remains a formidable challenge. Herein, we report a bifunctional iron aluminum oxide (FeAlOx) catalyst that directly converts CO2 into C5+ hydrocarbons with an overall selectivity of 77.0% and CO2 conversion of 20.2% at a H2/CO2 ratio of 1:1. Notably, the selectivity for linear α-olefins (LAOs) was 52.4%, accounting for 78.4% of the total C4+ olefins. At a high H2/CO2 ratio of 3:1, the yield of C5+ hydrocarbons was 19.7%. The concept of crystalline-/amorphous-structured active sites in the single FeAlOx catalyst was proposed. The reducible magnetite (Fe3O4) phase, which contains surface oxygen vacancies, facilitated the reverse-water–gas-shift (RWGS) reaction to form CO via CO2 hydrogenation, and subsequent C–C coupling over Hägg iron carbide afforded lower olefins (C2–C4=). Long-chain LAOs were then formed on the surface of amorphous aluminum oxide (AlOx) via the readsorption of C2–C4=. In addition, the amorphous AlOx phase enhanced CO2 and H2 adsorption, which facilitated the formation of carbonate, bicarbonate, and formate species via the RWGS reaction and the subsequent formation of long-chain hydrocarbons via the Fischer–Tropsch reaction. The bifunctional FeAlOx catalyst showed excellent stability for up to 450 h on-stream, demonstrating its potential as a practical-scale catalyst for the conversion of CO2 into value-added liquid fuels and chemicals.