Recyclable Hydrazine-Passivated NiBx/Ni Heterostructured Catalyst for Enhanced Hydrogenation of Polystyrene Pyrolysis Oil

Nickel boride (Ni2B) is known to be an excellent catalyst for hydrogenation of olefin structures under mild reaction conditions. Its susceptibility to oxidation in air environments, however, limits its practical applications. Here, a hydrazine-passivated nickel boride/nickel (NiBx/Ni) heterostructured catalyst is developed to address the oxidation problem while improving the conversion efficiency of styrene to ethylbenzene in a real polystyrene (PS) pyrolysis oil. The calculated turnover frequency for styrene to ethylbenzene conversion is 24.9 mmol/h·g, with 99.9% selectivity, representing a 38.3% improvement compared with the control Ni2B catalyst (18.0 mmol/h·g). This outstanding performance is attributed to the increased charge density on both Ni and B, induced by a strong hydrazine reductant, which surpasses other amine-structured ligands, such as ethanolamine and ethylamine. Additionally, the enhanced magnetic properties of NiBx/Ni, resulting from the Ni(111) nanocrystalline structure along the easy axis of magnetization, enable facile recovery after a neodymium magnet. The catalyst demonstrated excellent stability, retaining catalytic efficacy over five consecutive cycles with only a 4% loss in activity. This study highlights the potential of the NiBx/Ni catalyst for low-cost and efficient hydrogenation in industrial applications.