Depolymerization of Nylon‑6 over a Supported Ruthenium Catalyst to ε‑Caprolactam

Plastic recycling is a crucial process to establish a circular economy in the plastic industry and combat the environmental threat caused by plastic waste. Catalytic hydro-depolymerization, which typically uses molecular hydrogen and metal catalysts, is a promising approach to recycling plastic waste. Recent studies have mainly focused on polyolefin plastics, while heteroatom-containing plastics are also consumed in large quantities and are less studied. In this work, we present a novel heterogeneous catalytic process where nylon-6 is efficiently hydro-depolymerized to produce its monomer (ε-caprolactam). Using ruthenium supported on zirconia with activated hydrogen as an active and selective catalyst, we received 94% yield of ε-caprolactam, with only 3.4% hexamethylenimine as a byproduct, for the breakdown of nylon-6 at 350 °C and 30 bar H₂. The depolymerization of nylon-6 to ε-caprolactam is shown to be sensitive to Ru particle size, with the disruption of the semicrystalline structure of nylon-6 being a prerequisite for C-N bond cleavage. A low metal loading of 2.3 wt % Ru, with a particle size of 9.5 ± 2.3 nm, was the most efficient catalyst. XPS and H₂-TPR revealed that the smaller Ru particles were easier to reduce, and this is suggested to be the reason for the higher activity. Moreover, we propose that Ru-supported catalysts, in conjunction with activated hydrogen, exert a synergistic effect, facilitating both the alteration of crystallinity and the cleavage of C-N bonds, thereby enhancing the depolymerization rate of nylon-6. The robustness of the catalyst system is studied using plasticizers and additives. Interestingly, the addition of water could suppress byproduct formation, resulting in 100% selectivity. Additionally, the reusability of the catalyst is demonstrated.