Review article Aquathermolysis of heavy oil: A review and perspective on catalyst development Oki Muraza a,b,⇑ , Ahmad Galadima a a Center of Research Excellence in Nanotechnology, King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia b Chemical Engineering Department, King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia highlights  Aquathermolysis has many advantages over other heavy oil upgrading methods.  Progress made and issues on catalysts development were adequately reviewed.  Minerals and organic systems of transition metals were the early catalysts.  Nanometals, hydrophobic solid acids and carbides are potential catalysts.  Catalyst design, operational conditions and mechanisms need further studies. article info Article history: Received 9 February 2015 Received in revised form 24 April 2015 Accepted 28 April 2015 Available online 8 May 2015 Keywords: Aquathermolysis Heavy oil Solid acids Catalyst development Nanosized alloys abstract Aquathermolysis process is one of the key and economical viscosity reduction technologies being consid- ered for the successful exploitation of huge heavy oil deposits across the world. The paper substantially reviewed the progress made and the fundamental issues related to catalysts development. The most appropriate catalysts are those containing strong, active sites that can actively break the CAC, CAS, CAO and related bonds in resins and asphaltenes, with a net increase in the concentration of saturates and lighter aromatic hydrocarbons. Solid catalysts based on natural zeolites, heteropoly acids and mod- ified zirconia oxides have so far demonstrated excellent activities. Catalysts such as alloys of Mo, W, C and Ni, nano oxides of Cu and Fe, Ni-chelates, supported nano-Fe particles and Cu 2+ and Fe 3+ complexes of toluene sulfonic acid were also recently reported as good candidates for the aquathermolysis reactions. Hydrophobic zeolites prepared in the fluoride or organic silane routes, carbides of W and Mo are prospec- tive catalysts for the future due to their excellent performance in related reactions. In all cases, further studies are required to explore the optimal catalyst synthesis procedures, operation conditions and actual reaction mechanisms. Ó 2015 Elsevier Ltd. All rights reserved. Contents 1. Introduction ......................................................................................................... 220 2. Historical background ................................................................................................. 221 2.1. An overview of main mechanism................................................................................... 222 3. Solid catalysts in aquathermolysis ....................................................................................... 222 3.1. Solid acid catalysts: zirconia, alumina, natural zeolites ................................................................. 223 3.2. Hydrophilic and soluble catalysts .................................................................................. 224 3.3. Other potential catalysts .......................................................................................... 225 4. Stability of catalysts in aqueous medium (hot water) ........................................................................ 225 5. Cracking and hydrogen donor ........................................................................................... 226 http://dx.doi.org/10.1016/j.fuel.2015.04.065 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: Center of Research Excellence in Nanotechnology, King Fahd University of Petroleum & Minerals, Dhahran, 31261, Saudi Arabia. Tel.: +966 13 860 7612. E-mail address: omuraza@kfupm.edu.sa (O. Muraza). Fuel 157 (2015) 219–231 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel