Metal Hydroxide-Catalyzed Heavy Oil Upgrading in Supercritical Water: Deuterium Tracing Study

Conversion of heavy oil is urgent as most of the explored fossil oil reserves and renewable raw biodiesel can be categorized as heavy oil. Supercritical water upgrading (SCWU) is a green and promising technology for heavy oil conversion, but the hydrogen-donating capacity of pure water is inadequate. This study adopts transition-metal hydroxide (M–OH) to enhance the in situ hydrogen-donating capacity of water for the first time. Both M–OH and the corresponding metal oxide nanoparticle (MOn) of one post-transition metal (Al) and five transition metals (Cr, Cu, Fe, Ni, Zn) are tested. The SCWU conditions are selected as 425 °C, 60 min, water-to-oil ratio of 4:1, catalyst-to-oil ratio of 1:10, and water density of 308 kg m–3. Deuterium oxide (D2O) instead of H2O is used as the reaction medium for a deuterium tracing study. Results indicate that the different metal-based MOn nanoparticles display various effects on heavy hydrocarbon cracking, gasification, coke suppression, and D addition reactions. The overall performance is found to be on the order of NiO > Cr2O3 > Fe2O3 > CuO > Al2O3 > ZnO. Compared to MOn, M–OH can generally improve the oil yield and H/C ratio and reduce the coke yield simultaneously. The mechanism analysis suggests that the M–OH decomposition behavior under hydrothermal conditions is the primary cause by which MOn and lattice H2O are generated in the oil-rich phase. The lattice H2O can not only take part in SCWU reactions but also suppress coke formation. In conclusion, M–OH is a promising catalyst as it combines the good dispersibility and catalytic activity of MOn nanoparticles and the low cost of water-soluble inorganic metal salts (M-IAc) but does not result in any corrosive species, such as inorganic acid from M-IAc.