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Polyethylene Upcycling to Diacids Using Acid-Only Activation
Summary
Researchers developed an acid-only activation process using chlorosulfuric acid and sulfuric acid mixtures to sulfonate polyethylene waste, enabling subsequent depolymerization with H2O2 and Fe(III) catalyst to achieve molar yields of C2-C4 diacids exceeding 40%, eliminating the need for costly organic solvents in plastic upcycling.
There is a pressing need to develop new plastics recycling technologies. Chemical upcycling converts low-value waste plastics into higher-value products. Here, polyethylene (PE) upcycling is accomplished using acid-only activation of PE (PEAA), thus eliminating costly and toxic organic solvents. Acid mixtures of chlorosulfuric acid (CSA) and sulfuric acid (SA) were used to effectively sulfonate PE, allowing subsequent facile depolymerization. Molecular deconstruction of sulfonated PE using H2O2 in the presence of Fe(III) catalyst achieved molar yields of C2-C4 diacids exceeding 40\% (based on estimates of acids generated from sulfonates). The sulfonation step using PEAA is hypothesized to follow a shrinking core mechanism (SCM) in which PE particles quickly reach a saturated state in the outer sulfonated layer as CSA diffuses radially inward. This proposed mechanism is supported by the fact that, regardless of the extent of sulfonation, similar molar yields of products and ratios of liberated carbon to sulfur are observed. Modeling and experimentation show that deviations from the SCM occur by lowering the Damköhler number (Da) or increasing CSA solubility in PE. In both cases CSA diffusion is enhanced, which favors kinetically limited sulfonation.