Synthesis of bimetallic Mn–Co stearate catalyst for the conversion of plastic waste to surfactant via chemical upcycling and its application

In this work, polyethylene (PE) and polypropylene (PP) wastes were converted into anionic carboxylate surfactants by catalytic oxidation using a newly synthesized bimetallic manganese-cobalt (Mn-Co) stearate catalyst with a Mn:Co molar ratio of 4.5:1. The synthesized catalyst was characterized using Fourier Transform Infrared (FTIR), confirming the successful formation of Mn-Co stearate by the deprotonation of the acid and formation of metal–carboxylate bonds. X-ray diffraction analysis (XRD) of the catalyst shows a peak shift at a lower 2θ due to metal-carboxylic binding. Also, the energy dispersive X-ray (EDX) elemental mapping of the Mn-Co stearate catalyst confirms the catalyst's composition and homogeneity. While the BET analysis shows low adsorption of the catalyst, the contact angle measurement of the recovered surfactants indicates a steep decrease with a corresponding increase in concentration for all samples, reflecting good wetting properties. Notably, the high-density polyethylene (HDPE)-derived surfactant demonstrates lower contact angle measurements, with the angle reduced from 117.5° to 35° at a low concentration of 0.5 g/L, comparable to commercial sodium dodecyl sulfate (SDS). Characterization of Interfacial tension (IFT) and foaming behavior provides a comprehensive assessment of the surfactant's amphiphilic efficiency and its suitability in stabilizing complex multiphase systems. Additionally, the Mn-Co stearate-derived surfactant solutions tested in deionized water and saltwater indicate high chemical stability and ionic tolerance as manifested by the absence of precipitate. Finally, this work confirms the effectiveness and viability of Mn-Co stearate as a wax-oxidizing catalyst and acknowledges PE and PP wastes as promising feedstocks for producing high-value surfactants, capable of supporting circular economy-driven plastic waste valorization.