Mechanistic Insights into Polypropylene Hydrogenolysis Using Ni/Al2O3 Catalysts

Catalytic hydrogenolysis is emerging as an attractive strategy for converting polyolefins into high-value hydrocarbon liquids. A key challenge in catalytic hydrogenolysis is the high methane yield. Recently, Ni-based catalysts have shown promise as a cost-effective alternative to noble metals in polyolefin hydrogenolysis. In this study, three alumina-supported Ni catalysts (12-13 wt % Ni) were prepared using acidic, neutral, and basic gamma-Al2O3 via impregnation. The resulting Ni/A-Al2O3, Ni/N-Al2O3, and Ni/B-Al2O3 catalysts were used to investigate reaction pathways in n-hexadecane and isotactic polypropylene hydrogenolysis. Experiments conducted in a batch autoclave at 300 degrees C with 30 bar of H2 showed that Ni/B-Al2O3 exhibited the highest reactivity, 5 h for n-hexadecane and 30 h for polypropylene, respectively. Using n-hexadecane as a model compound for hydrogenolysis, we attributed the origin of methane selectivity to terminal C-C bond scission, occurring through both single-step and cascade mechanisms. Detailed product analysis (GC-FID, GPC, and NMR) and comprehensive catalyst characterization revealed the origins of varied activity and product distribution in the hydrogenolysis of n-hexadecane and polypropylene. The increased ratio of tetrahedrally coordinated Ni2+ to metallic Ni0, attributed to stronger metal-support interactions, along with stronger surface basicity, promotes terminal C-C scission, leading to enhanced hydrogenolysis reactivity.