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 mg_LDPE mg_Ru⁻¹ h⁻¹ in low-density polyethylene (LDPE) conversion, a fivefold increase compared to traditional nanoparticles (NPs) at 10.63 mg_LDPE mg_Ru⁻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 mg_Ru⁻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.