Plastic is an indispensable material globally due to its versatility, affordability, durability, and lightweight properties. Since the 1950s, plastic production has soared, providing essential applications in packaging, construction, transportation, and electronics. However, the non-degradability of plastics has resulted in a substantial increase in waste, with most ending up in landfills or being incinerated, raising significant environmental concerns. This study addresses these challenges by investigating the catalytic depolymerization of polyethylene (PE) and elucidating the role of water in modifying reaction pathways. Ru-based catalysts, supported on SiO2, SBA-15, γ-Al2O3, TiO2, and Zeolite-Y, were synthesized and evaluated. In the absence of water, catalyst performance varied, with conversions ranging from moderate levels to an impressive 80.6% for the Ru/Zeolite-Y catalyst. Remarkably, the addition of water increased the conversion of the 5% Ru/Zeolite-Y catalyst to 96.9%, while simultaneously enhancing the yield of valuable gasoline- and diesel-range products. Conversely, water reduced conversions with other catalysts, indicating that its promotional effect is catalyst-specific. Furthermore, the promotional effect of water was evaluated using other zeolites (HZSM-5, HBEA, and HMOR), and conversion enhancement by water was observed exclusively on hydrophilic catalysts. Mechanistic investigations, including Pyridine-DRIFTS analysis, revealed that Brønsted acid sites were present only on Ru/Zeolite-Y, suggesting a key role in water-induced reactivity enhancement. Experiments employing D2O confirmed that water functions as a proton donor, actively participating in the depolymerization reaction. Additional tests using n-dodecane as a surrogate for PE demonstrated that water significantly alters isomer selectivity, particularly at lower metal loadings. These findings support a bifunctional depolymerization mechanism wherein water facilitates both hydrogenolysis and hydrocracking by promoting interactions at metal and acid sites. The study highlights the critical interplay between water, catalyst composition, and reaction conditions, offering valuable insights for optimizing plastic waste valorization strategies.
