Photocatalytic Hydrolysis-A Sustainable Option for the Chemical Upcycling of Polylactic Acid

Abstract

Plastic waste is a critical global issue, yet current strategies to avoid committing plastic waste to landfills include incineration, gasification, or pyrolysis high carbon emitting and energy consuming approaches. However, plastic waste can become a resource instead of a problem if high value products, such as fine chemicals and liquid fuel molecules, can be liberated from controlled its decomposition. This letter presents proof of concept on a low-cost, low energy approach to controlled decomposition of plastic, photocatalytic hydrolysis. This approach integrates photolysis and hydrolysis, both slow natural decomposition processes, with a photocatalytic process. The photocatalyst, α-Fe₂O₃, is embedded into a polylactic acid (PLA) plastic matrix. The photocatalyst/plastic composite is then immersed in water and subjected to low-energy (25 W) UV light for 90 h. The monomer lactide is produced as the major product. α-Fe₂O₃ (6.9 wt %) was found to accelerate the PLA degradation pathway, achieving 32% solid transformation into liquid phase products, in comparison to PLA on its own, which was found to not decompose, using the same conditions. This highlights a low energy route toward plastic waste upgrade and valorization that is less carbon intensive than pyrolysis and faster than natural degradation. By directly comparing a 25 W (0.025 kWh) UV bulb with a 13 kWh furnace, the photocatalytic reaction would directly consume 520× less energy than a conventional thermochemical pathway. Furthermore, this technology can be extended and applied to other plastics, and other photocatalysts can be used.

Figure 1

Schematic of the main steps involved in the photocatalytic hydrolysis of α-Fe₂O₃/PLA.

Figure 2

(a) Thermogravimetric analysis of raw PLA and Fe composite; (b) derivative weight loss profile; (c) FTIR spectra of PLA and α-Fe₂O₃/PLA composites before and after UV light exposure.

Figure 3

(a) Photocatalytic reaction selectivity after 90 h for both raw PLA and PLA composites. (b) Proposed reaction schematic with catalyst assignment (blue arrows denote reaction pathways; black arrow denotes γ-Fe₂O₃ selectivity, and red arrows denote α-Fe₂O₃ reaction selectivity).

Figure 4

GCMS chromatogram of the product of photocatalytic hydrolysis of PLA, with the main peaks showing the presence of lactide.