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Depolymerization of Nylon‑6 over a Supported Ruthenium Catalyst to ε‑Caprolactam
Prabin Dhakal1, Abdenour Achour1, Phuoc Hoang Ho1
1Chemical Engineering, Competence Centre for Catalysis, Chalmers University of Technology, Gothenburg 41296, Sweden.
ACS Environmental Au
|July 23, 2026
Summary
This study introduces a new catalytic method for recycling nylon-6 plastic into its monomer, ε-caprolactam, using ruthenium catalysts and hydrogen. The process achieves high yields and selectivity, offering a sustainable solution for plastic waste management.
Area of Science:
- Materials Science
- Chemical Engineering
- Environmental Science
Background:
- Plastic waste poses a significant environmental challenge, necessitating effective recycling strategies.
- Catalytic hydro-depolymerization is a promising technology for plastic recycling, particularly for polyolefins.
- Heteroatom-containing plastics like nylon-6 are widely used but less explored in catalytic recycling.
Purpose of the Study:
- To develop an efficient heterogeneous catalytic process for the hydro-depolymerization of nylon-6 into its monomer, ε-caprolactam.
- To investigate the influence of catalyst properties, such as ruthenium particle size and loading, on depolymerization efficiency.
- To explore the synergistic effects of ruthenium catalysts and activated hydrogen in nylon-6 breakdown and assess catalyst robustness and reusability.
Main Methods:
- Nylon-6 was subjected to hydro-depolymerization using ruthenium supported on zirconia as a heterogeneous catalyst.
- Activated hydrogen was employed under specific temperature (350 °C) and pressure (30 bar H₂) conditions.
- Catalyst characterization involved techniques such as X-ray photoelectron spectroscopy (XPS) and H₂-temperature-programmed reduction (H₂-TPR).
- The effect of water addition on selectivity and catalyst reusability were also evaluated.
Main Results:
- A high yield of 94% for ε-caprolactam was achieved with only 3.4% hexamethylenimine byproduct.
- Ruthenium particle size was found to be critical, with smaller particles (9.5 ± 2.3 nm) and low loading (2.3 wt %) showing optimal performance.
- XPS and H₂-TPR indicated that smaller Ru particles are more easily reduced, contributing to higher activity.
- The addition of water improved selectivity to 100% by suppressing byproduct formation.
- The catalyst demonstrated robustness in the presence of plasticizers and additives, and its reusability was confirmed.
Conclusions:
- A novel and efficient heterogeneous catalytic system using Ru/ZrO₂ and activated hydrogen was successfully developed for nylon-6 depolymerization.
- The process effectively converts nylon-6 into its monomer, ε-caprolactam, with high yield and selectivity, especially with water addition.
- Catalyst performance is strongly linked to Ru particle size and reducibility, highlighting the importance of catalyst design.
- This method offers a sustainable pathway for nylon-6 recycling, contributing to a circular economy in the plastic industry.
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