Evaluation of the catalytic performance of the transition metal oxide nanoparticles (M = Mo, Ni, Co, Fe, Cu, Ce, Mn) for the hydrothermal upgrading of Ashalcha heavy crude

Heavy oil upgrading remains an important challenge because of the high viscosity, high content of resins and asphaltenes, and elevated sulfur concentration of heavy crude feedstocks, which limit their production, transportation, and refining. In this work, the catalytic performance of commercially available transition metal oxide nanoparticles, namely NiO, Co 3 O 4 , CuO, Fe 2 O 3 , MoO 3 , CeO 2 , and MnO 2 , was comparatively evaluated for the hydrothermal upgrading of high-sulfur Ashalcha heavy crude oil under identical reaction conditions. Upgrading experiments were conducted in a high-pressure high-temperature batch reactor at 300 °C for 24 h using an oil/water mass ratio of 70:30 and a catalyst loading of 0.3 wt%, under nitrogen with an initial pressure of 2 bar. The products were characterized by viscosity measurements, SARA analysis, elemental analysis, gas chromatography of evolved gases and fractions of saturates , as well as X-ray diffraction and scanning electron microscopy-energy dispersive X-ray spectroscopy of spent catalysts. All tested metal oxide nanoparticles improved the quality of the heavy oil to different extents, but NiO exhibited the best overall performance. In the presence of NiO, the oil viscosity decreased from 3336 to 1014 mPa·s, the resin content decreased from 19.97 to 11.95 wt%, the asphaltene content decreased from 6.66 to 5.25 wt%, the H/C atomic ratio increased to 1.89, and sulfur removal reached 8.95%. In addition, NiO provided the highest yield of low-molecular-weight alkanes in the saturate fraction (46%) and the highest selectivity toward gaseous products associated with bond cleavage and hydrodesulfurization. Post-reaction X-ray diffraction and scanning electron microscopy-energy dispersive X-ray spectroscopy analyses showed that Ni-containing sulfide phases formed after upgrading, indicating that sulfidation plays an important role in the catalytic behavior of NiO. The main contribution of this work is a systematic side-by-side comparison of several transition metal oxide nanoparticles under the same hydrothermal upgrading conditions, combined with post-reaction catalyst characterization. The results demonstrate that NiO provides the most favorable balance of viscosity reduction, compositional improvement, and desulfurization for Ashalcha heavy crude oil under the investigated conditions.