Catalytic aquathermolysis for partial upgrading of heavy oil with Fe-based catalysts: Effect of competitive oxidation–sulfidation

Over the past two decades, Fe-based catalysts have attracted significant attention owing to their potential applications. While numerous studies have explored the role of Fe-based catalysts in enhancing oil recovery, their phase transformations and mechanisms of activity during aquathermolysis remain inadequately understood. This study used α-Fe2O3, FeS, and Fe3O4 to investigate Fe-based materials’ structural evolution and catalytic performance in heavy-oil aquathermolysis. Among the tested catalysts, α-Fe2O3 showed the best aquathermolysis product distribution and quality, including optimal coke formation and effective sulfur- and nitrogen-based impurities removal. An isotopic technique using D2O instead of water for aquathermolysis was used to quantify the hydrogen-transfer capabilities of the catalysts to estimate the degree of hydrogen donation in each system. Systematic morphological and crystallographic analyses indicated the development of a core–shell structure during aquathermolysis, where the proportion of amorphous Fe-oxysulfide shells strongly correlated with the catalyst’s heavy-oil upgrading efficiency. Remarkably, α-Fe2O3 demonstrated sustainability over five catalytic cycles, producing less than 1 wt% of coke. Furthermore, a detailed phase transformation scheme for α-Fe2O3 describes its dynamic behavior in heavy oil under aquathermolysis conditions. This study offers insight into the design of Fe-based catalysts that can facilitate aquathermolysis, enabling oil recovery from heavy oil resources.