The escalating global plastic waste crisis necessitates sustainable solutions, particularly for polyester, a significant portion of plastic production. Chemical recycling offers an attractive alternative to incineration and landfill, enabling the recovery of valuable chemicals. While various methods exist for polyester recycling (hydrolysis, alcoholysis, aminolysis, hydrogenolysis), challenges persist in achieving high reactivity and selectivity toward desired products. Diols, such as ethylene glycol and 1,2-propanediol, are valuable industrial chemicals, typically produced through energy-intensive processes. The inherent structure of polyesters, derived from the condensation of diols and dicarboxylic acids, suggests the potential for diol production via polyester hydrogenolysis. This approach requires selective hydrogenation of the carbonyl group to a CH2-OH group. Previous attempts have faced limitations due to the formation of byproducts. Atomically dispersed metal catalysts have emerged as promising candidates for their high atom utilization efficiency and tunable properties. Bimetallic single-atom catalysts often exhibit enhanced performance compared to their monometallic counterparts due to synergistic effects optimizing electron distribution and providing multiple active sites. This study focuses on developing a highly efficient catalyst system for achieving selective diol production from various polyester wastes.

Existing literature extensively covers various approaches to chemical recycling of plastic wastes, including thermocatalytic hydrocracking of polyethylene to alkanes and diverse methods for polyester recycling. However, challenges remain in achieving both high reactivity during polyester degradation and high selectivity for target products like diols. Studies have explored the hydrogenolysis of polylactic acid (PLA) to 1,2-propanediol, but achieving exclusive diol formation often proves difficult due to carboxylic acid and other byproduct formation. The use of atomically dispersed metal catalysts, particularly bimetallic single-atom catalysts, has shown promise in enhancing catalytic efficiency and selectivity in various reactions, motivating the design and synthesis of a novel catalyst for this application. The literature highlights the benefits of single-atom and dual-atom catalysts in improving atom utilization efficiency and promoting desirable reaction pathways.

Anatase TiO2 nanoparticle-supported Ru and Mo dual-atom catalysts (RuxMoy/TiO2) were synthesized. RuCl3 and ammonium molybdate were dissolved in water with TiO2 nanoparticles, followed by the addition of NaBH4 to form bimetallic nanoparticles. Subsequent air treatment at 200 °C yielded the final catalysts. A series of catalysts with varying Ru:Mo molar ratios were prepared, with Ru4Mo1/TiO2 (Ru:Mo = 4:1, 3.76 wt% Ru loading) exhibiting the highest activity. Comparative catalysts, including TiO2-supported Ru (Ru6/TiO2) and Mo-doped TiO2 (Mo2/TiO2), were also synthesized. Catalytic activity was evaluated using PLA depolymerization in water under H2. Reaction conditions (temperature, pressure) were optimized to maximize 1,2-propanediol yield. Catalyst stability was assessed through regeneration cycles involving O2 treatment at 200 °C. The optimized catalyst was tested on various polyesters (PGA, PBS, PCL, PBA, PBAT) and ethyl acetate. Characterization techniques employed included high-resolution transmission electron microscopy (HR-TEM), aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC HAADF-STEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), X-ray absorption fine structure (XAFS) spectroscopy, X-ray absorption near edge structure (XANES) spectroscopy, soft X-ray absorption spectroscopy (XAS), electron paramagnetic resonance (EPR) spectroscopy, operando Raman spectroscopy, and operando Fourier-transform infrared spectroscopy coupled with surface-enhanced infrared absorption spectroscopy (FTIR-SEIRAS). Density functional theory (DFT) calculations were performed using VASP to elucidate the reaction mechanism. Product yields and selectivities were determined using 1H and 13C NMR spectroscopy and GC-MS.

The Ru4Mo1/TiO2 catalyst exhibited exceptional performance in converting various polyesters to their corresponding diols with 100% selectivity under mild conditions. AC HAADF-STEM and EDS mapping confirmed the presence of atomically dispersed Ru and Mo atoms and O-bridged Ru-O-Mo dual-atom sites. The Ru single-atom sites effectively activated H2 for the hydrogenation of carboxylic acids, while the Ru-O-Mo sites prevented the hydrodeoxygenation of alcohols by increasing reaction energy barriers. Operando Raman spectroscopy revealed the formation of Ru-H bonds at 25°C, indicating the high hydrogen activation ability of the catalyst. Operando FTIR-SEIRAS studies provided evidence of the transformation of the carbonyl group to a -C-OH group. DFT calculations showed that the LA molecule preferentially adsorbs onto the Mo site, facilitating electron transfer from Mo to the carbonyl oxygen. This electron transfer pathway activates the C=O bond, lowering the reaction energy barrier. The catalyst showed high stability and recyclability, maintaining its activity after 10 regeneration cycles. The catalyst successfully recycled various polyesters (PLA, PGA, PBS, PCL, PBA) into their corresponding diols with high yields and selectivities. The study definitively demonstrates the synergistic effects of the Ru single-atom and Ru-O-Mo dual-atom sites in achieving efficient and selective polyester recycling.

The findings address the research question of developing a highly efficient catalyst for polyester recycling by demonstrating the superior performance of the Ru and Mo dual-atom catalyst. The 100% selectivity for diol formation highlights the effectiveness of the catalyst in preventing undesired side reactions. The catalyst's high activity under mild conditions, coupled with its regenerability, makes it a promising candidate for industrial applications. The mechanistic studies provide valuable insights into the roles of the Ru and Mo sites, demonstrating a synergistic effect that enhances catalytic activity and selectivity. This work contributes significantly to the field of chemical recycling by offering a robust and efficient method for converting polyester waste into valuable chemicals.

This study successfully developed a highly efficient and stable Ru and Mo dual-atom catalyst for the selective conversion of various polyester wastes into their corresponding diols. The unique structure of the catalyst, featuring both Ru single-atom and Ru-O-Mo dual-atom sites, enables efficient hydrogen activation and precise control over reaction pathways. The catalyst's high activity, selectivity, and regenerability demonstrate its potential for large-scale applications in chemical recycling. Future research could focus on exploring other bimetallic combinations and support materials to further optimize catalyst performance and expand its applicability to a wider range of polyester types.

The study primarily focused on a limited set of polyesters, and further investigation is needed to evaluate the catalyst's performance with other types of polyesters. The study's scale was limited to laboratory experiments, and additional work is required to assess the catalyst's scalability and cost-effectiveness for industrial implementation. While the catalyst demonstrated excellent stability over multiple cycles, long-term durability under continuous operation needs further investigation.