Insights into the role of H radicals in a plasma-enabled hydrogenation of heavy oil model compound

Plasma hydrogenation technology represents a sustainable and environment-friendly solution to upgrade heavy oil, but the underlying mechanisms of plasma hydrogenation for heavy oil or its model compounds remain insufficiently understood. Here we demonstrate the utilization of hydrogen-rich plasma for the hydrogenation of heavy oil model compound ethylbenzene under ambient conditions without catalysts, highlighting the crucial role of free radicals in this process. Simulation results show that H2 plasma yields a higher density (1.98 x 1014 vs 1.06 x 1014 cm-3) and slower consumption rate (1.0 x 1016 vs 2.5 x 1018 cm-3 s- 1) of H radicals, and a higher average H number (1.8-4.3 vs 0.5-1.7) added to the aromatic ring, leading to a more efficient hydrogenation of aromatic rings compared to CH4 plasma. Moreover, an increase in H radical density may result in the cleavage of hydrogenated products due to elevated high-energy electron levels, hence potentially undermining the overall effectiveness. Experimental results further reveal that radical recombination is the dominant reaction mechanism for H-involved reactions, since almost no hydrogenated products were observed after CH4 plasma treatment. These novel insights into plasma-enabled aromatic ring hydrogenation provide a new and green approach for future upgrading of heavy oils.