Importance of balanced distribution of metallic sites and acidic sites in Co/Ni-based catalysts for selective conversion of polycarbonate plastic into valuable chemicals

Recycling waste polycarbonate (PC) plastic via hydrogenolysis and hydrogenation to specific chemicals, such as bisphenol A, 4,4′-isopropylidenedicyclohexanol, and propane-2,2-diyldicyclohexane, is challenging, as balancing the activity of Ni-based and Co-based catalysts for hydrogenation and deoxygenation is required. In this study, various Ni-based and Co-based catalysts were synthesized via alloying metallic sites and tailoring acidity/basicity of the catalytic systems for achieving selective conversion of PC plastic to valuable chemicals. The results indicated that alloying Ni and Co with Sn formed Ni-Sn and Co-Sn alloys with larger crystallize size, created abundant Lewis acid sites and generated electron-rich Ni species, which suppressed the ring-saturation in bisphenol A and achieved selective conversion of PC to bisphenol A (yields: 93.3% over Ni-Sn-Al and 97.4% over Co-Sn-Al) in 1,4-dioxane. However, introducing alkaline MgO as the carrier in Ni-Mg and Co-Mg catalysts enhanced the distribution of hydrogenation sites but reduced the availability of acidic sites, thereby lowering the removal rate of C-OH bond in 4,4′-isopropylidenedicyclohexanol. H-ZSM-5, an acidic co-catalyst with abundant Brønsted and Lewis acid sites, was further introduced to work in tandem with metallic sites in Ni-Al/H-ZSM-5. The strong Lewis acidic sites in H-ZSM-5, with high affinity for both the benzene ring and C-OH bond, significantly enhanced the catalytic activity and stability of Ni-Al/H-ZSM-5 in n -heptane, showcasing superior activity for producing propane-2,2-diyldicyclohexane with yields of 92.0 and 92.9% from PC powder and DVD disk over Ni-Al/H-ZSM-5.