Homogeneous Catalysis in Aquathermolysis for Heavy Oil Upgrading: A Critical Review of Advances, Challenges, and Perspectives

As global energy demand rises and conventional crude oil supplies dwindle, unconventional sources, such as heavy and extra-heavy oils, are gaining prominence. However, the intrinsic characteristics of heavy oils, including their high viscosity and low mobility, combined with the complex nature of reservoir formations, present significant challenges for efficient recovery. Traditional in situ recovery methods face limitations like low sweep efficiency, substantial capital costs, the risk of formation damage, and environmental impacts, necessitating more sustainable and cost-effective solutions. Aquathermolysis, a promising thermal upgrading technique for heavy oils, has garnered considerable attention due to its ability to reduce viscosity and improve oil recovery. This Review examines the role of homogeneous catalysis in optimizing the aquathermolysis process, focusing on water soluble, oil soluble, and amphiphilic catalysts. Water-soluble catalysts promote carbon–heteroatom cleavage (e.g., breaking C–S, C–N, and C–O bonds) through interactions with hydrophilic components of heavy oil. Oil-soluble catalysts integrate directly with the hydrocarbon matrix, facilitating the cracking of large molecules and hydrogenation of unsaturated bonds. Amphiphilic catalysts, combining the properties of both, enhance catalytic activity at water–oil interfaces. Each catalyst type offers unique advantages in viscosity reduction, desulfurization, and overall heavy oil upgrading. This Review provides a comprehensive analysis of these catalysts, discussing their mechanisms, benefits, limitations, and applications. By elucidating their contributions to process efficiency and product quality, this Review highlights their potential to address the challenges associated with heavy oil recovery, paving the way for more sustainable and effective energy solutions.