Selective Oxidative Upcycling of Polyethylene Pyrolyzed Wax to High-Performance Surfactants Using Synthesized Co and Mn–Co Stearate Catalysts

In this work, post-consumer low-density polyethylene (LDPE) and high-density polyethylene (HDPE) were converted into surfactant-grade materials using a two-step process involving controlled pyrolysis and catalytic oxidation. Low-viscosity hydrocarbon waxes (approximately 30% yield) were recovered through pyrolysis at 500°C under nitrogen flow. Subsequently, they were oxidized at 150°C for 8 h using monometallic cobalt stearate Co(C₁₈H₃₅O₂)₂ and bimetallic manganese-cobalt stearate Mn₅Co(C₁₈H₃₅O₂)₁₂ catalysts synthesized via coprecipitation. Fourier transform infrared spectroscopy (FT-IR), ¹³C nuclear magnetic resonance (NMR), and transmission electron microscopy (TEM) were used to identify the functional, structural, and morphological characteristics of the wax, oxidized wax, and carboxylic acid extract, respectively. Gas chromatography-mass spectrometry (GC–MS) was used to analyze the chemical composition of the wax, while the molecular structure of the carboxylic acid extracts was determined using ¹H NMR. Moreover, the formation of aldehyde functionality was confirmed by FT-IR, supporting the proposed radical-mediated oxidation mechanism. Following neutralization with potassium hydroxide (KOH), carboxylate surfactants were obtained and evaluated for their critical micelle concentration (CMC), surface tension at the CMC (γ_cmc), maximum surface excess (Γ_max), minimum area per molecule (A_min), foam stability, and solubility in deionized water and seawater, respectively.