Advances on the transition-metal based catalysts for aquathermolysis upgrading of heavy crude oil

Utilization of the abundant heavy crude oil resources has been accepted as an effective approach to tackle the shortage of light oil supply in the coming decades. However, the high viscosity and density of heavy crude oil severely hurdle the practical exploitation, transportation and refinement. To reduce the viscosity of heavy crude oil, numerous technologies, including physical upgradation, biological mining and chemical recovery, have been proposed. Catalytic aquathermolysis, which represents a very important branch of chemical recovery, transforms the heavy components (resins and asphaltenes) and heteroatom-containing compounds of heavy crude oil to light hydrocarbons, thereby leading to apparent viscosity reduction. The participation of transition metal-based catalysts is one of the most critical factors that govern the viscosity reduction efficiency of aquathermolysis. In this work, we reviewed the progress of transition metal-based catalysts in aquathermolysis upgrading of heavy crude oil. Specifically, homogeneous transition metal-based catalysts, including water-soluble inorganic salts, organic coordination compounds and ionic liquids, can enable uniform contact with oil phases, thereby achieving enhanced viscosity reduction efficiencies. However, separating these catalysts from of the recovered oil is rather difficult, which is not only nonbeneficial to the cost but might be harmful to the downstream refinery industry. In contrast, heterogeneous catalysts, involving transition metals and their oxides, sulfides, carbides, phosphides as well as solid acids, can be feasibly separated and even reutilized after aquathermolysis. To implement desired recovery efficiency, many strategies have been developed to improve the catalytic reactivity of the transition metal centers and to enable better dispersity of the catalysts in oil phase. Upon these progresses, the perspectives of transition metal-based catalysts are also proposed.