Polyethylene terephthalate (PET) is one of the most widely used general-purpose plastics in the world. PET is a milky white or light yellow, highly crystalline polymer with a smooth and shiny surface. The raw material of plastic bottles belongs to food-grade PET, which is used to package beverages, food and medicines, etc.
Converting waste plastics into recyclable materials is essential to solving environmental pollution and promoting social sustainable development. Among the many PET recycling methods, chemical catalysis is considered to be the most attractive and industrially applicable option, and metal solid catalysts have shown excellent performance in the degradation of PET and other polyesters. However, many recycling methods require harsh reaction conditions (such as alkaline/acidic treatment, high temperature and high pressure), which limits their application potential in industrial production. Therefore, the development of a simple and environmentally friendly PET catalytic degradation system is crucial for future sustainable development.
A research team proposed a new method for the selective degradation of PET plastic waste based on iron salt photocatalysts. The method can convert PET plastic waste into terephthalic acid (TPA) with a yield of up to 99%. The catalytic system exhibits high catalytic activity, excellent turnover number (TON) and conversion rate (TOF) values, and uses oxygen or air as an environmentally friendly oxidant. In addition, the solvent recovery process in this method does not affect the yield of TPA, and can achieve gram-level reaction amplification. This study provides new ideas and methods for the efficient conversion of PET waste.
The research team first constructed a photocatalytic system using cheap FeCl₃ and NH₄Cl. Under 365 nm LED irradiation, after 24 hours of reaction, commercially available PET powder was completely converted, and the final product was terephthalic acid with a yield of up to 97%. At the same time, the loading amount of photocatalyst iron salt can be reduced to 0.05%, and the reaction can be carried out in an air environment without a significant decrease in yield. This method shows good economy and efficiency, and provides a new way for the sustainable recycling of PET.
Subsequently, the research team conducted aerobic degradation research on PET bottles commonly seen in daily life. All PET waste was effectively converted to TPA with high selectivity under air conditions. The hexafluoroisopropanol (HFIP) solvent was successfully recovered through a simple distillation process, and no negative impact on the reaction was observed. This achievement effectively solves the problem of solvent recovery in a sustainable system and provides a new idea for the environmentally friendly treatment of PET.
The research team then used mixed fragments from four PET bottles for gram-scale reactions. By using HFIP recovered by distillation, 1.82 g of mixed bottle fragments were successfully reacted and TPA was obtained with an isolated yield of 91%. It is worth noting that despite the relatively slow reaction rate, 1.152 g of PET fragments still showed significant conversion after 24 hours of reaction under 400 nm LED irradiation, with an isolated yield of TPA of 28%. The research team then scaled up the reaction to 5 grams and 10 grams. In the reaction using 5.76 g of mixed PET fragments, TPA was successfully obtained with an isolated yield of 93% after 24 hours; and 11.52 g of PET fragments were also smoothly and effectively converted within the same 24 hours, with an isolated yield of TPA of 88%. The success of these scaled-up reactions highlights the potential of this system for future practical applications.
Possible mechanisms have also been speculated. After UV irradiation, the excited state [FeCl4]- undergoes a ligand-to-metal charge transfer (LMCT) process to produce a chlorine radical. The chlorine radical obtains the hydrogen atom at the α-position of the substrate oxygen through hydrogen atom transfer (HAT) to form a carbon radical intermediate A; the HAT process can also be achieved by oxidation of active oxidizing species present in the system. The carbon radical is then captured by oxygen to produce a peroxy radical species B, which then oxidizes Fe(II) to regenerate Fe(III) and generate a hydroperoxide species C. Alkoxy radical D is generated by Fe(II) reduction of C, which then undergoes β-scission to generate an aldehyde species E and a carbon radical F. F is further oxidized to generate E, which then undergoes further oxidation and decarboxylation to generate the final benzoic acid product. In addition, intermediate C can also be dehydrated to generate an anhydride intermediate G, which is easily hydrolyzed with the solvent HFIP to form a diester intermediate and generate a molecule of benzoic acid. Hydrolysis of the diester intermediate regenerates HFIP and generates acid H, which is then decarboxylated to generate intermediate F.
