Elucidating the effect of nanocube support morphology on the hydrogenolysis of polypropylene over Ni/CeO2 catalysts†

Abstract

The catalytic hydrogenolysis process offers the selective production of high-value liquid alkanes from waste polymers. Herein, through normalisation of Ni structure, Ni mass and density, and CeO₂ crystallite size, the importance of CeO₂ nanocube morphology in the hydrogenolysis of polypropylene (M_w = 12 000 g mol⁻¹; M_n = 5000 g mol⁻¹) over Ni/CeO₂ catalysts was determined. High liquid productivities (65.9–70.9 g_liquid g_Ni⁻¹ h⁻¹) and low methane yields (10%) were achieved over two different Ni/CeO₂ catalysts after 16 h reaction due to the high activity and internal scission selectivity of the supported ultrafine Ni particles (<1.3 nm). However, the Ni/CeO₂ nanocube catalyst exhibited higher C–C scission rates (838.1 mmol g_Ni⁻¹ h⁻¹) than a standard benchmark mixed shape Ni/CeO₂ catalyst (480.3 mmol g_Ni⁻¹ h⁻¹) and represents a 75% increase in depolymerisation activity. This led to shorter hydrocarbon chains achieved by the nanocube catalyst (M_w = 2786 g mol⁻¹; M_n = 1442 g mol⁻¹) when compared to the mixed shape catalyst (M_w = 4599 g mol⁻¹; M_n = 2530 g mol⁻¹). The enhanced C–C scission rate of the nanocube catalyst was determined to arise from a combination of improved H-storage and favourable basic properties, with higher weak basic site density key to facilitate a greater degree of hydrocarbon chain adsorption.