Hydrogen/Solvent-Free Plastic Valorization via Loosely Coordinated Ruthenium Sites Steering Selective Aromatization

Abstract

The development of hydrogen-and solvent-free catalytic systems for plastic valorization requires precise control over metal speciation to enhance both catalytic efficiency and selectivity. Ru atomic clusters (ACs) on HZSM-5 zeolite show a turnover frequency of 52.71 mgLDPE•mgRu⁻¹•h⁻¹ in low-density polyethylene (LDPE) conversion, a fivefold increase compared to traditional nanoparticles (NPs) at 10.63 mgLDPE•mgRu⁻¹•h⁻¹. This improved catalytic performance is further evident in cyclohexane dehydro-aromatization, where atomic-scale Ru sites exhibit a 17.8-fold activity increase (544.34 vs. 30.48 μM•mgRu⁻¹•h⁻¹), linking subnanometric structures to reaction pathway selectivity. Mechanistic analysis highlights two key effects: (1) optimized interactions between isolated Ru sites and intermediates lower dehydrogenation barriers, and (2) reduced cyclization energy barriers compared to NPs. Process optimization leads to 87% LDPE conversion with 69% aromatic selectivity, maintaining 70% efficiency across various polyolefins (including high-density PE and ultra-high-molecular-weight PE) and polystyrene. A techno-economic analysis shows the potential of AC catalysts, predicting a net present value of 1.78 million over five years, compared to an annual loss of 8.17 million for NPs. This study demonstrates the critical role of metal dispersion and acid-metal balance, forming the basis for a sustainable plastic-to-aromatics process with strong economic viability.