Recent advances in catalytic hydrogenolysis of polyester

The annual<?A3B2 tlb=1.15h?> release of vast quantities of plastic waste into the environment has led to significant contamination of soil, water and air. This is because the natural degradation of plastic waste typically takes several centuries, and microplastics generated during the natural degradation of plastics persist longer. Plastic waste management has therefore become one of the major challenges facing the global community. In recent years, the chemical recycling of plastic waste into useful chemicals has attracted global attention. Polyethylene<?A3B2 tlb=1.15h?> terephthalate (PET) is one of the most widely produced plastics, accounting for approximately 20% of the total number of plastics produced. Therefore, developing efficient methods for the recycling of PET is highly important. Hydrolysis and alcoholysis of PET into its monomers have been carried out as industrial processes. However, these methods require harsh conditions and involve high costs, and the degradation process results in a significant amount of waste, which is harmful to the environment. Therefore, the development of an efficient process for converting polyester waste into value-added chemicals with lower emissions is highly desirable. Owing to the production of a variety of high-value chemicals with high atomic economy, the catalytic hydrogenolysis of polyesters has received increasing attention. To promote the environmentally friendly recycling of polyester waste into high-value chemicals, this review summarizes the progress made in the field of polyester hydrogenolysis over the last decade. The ester group is the main functional group of a polyester, and its selective catalytic hydrogenation is the main topic of polyester hydrogenolysis. An ester group contains three types of carbon‒oxygen (C‒O) bonds: a C=O double bond, a Cacyl−O bond and a Calkoxy−O bond. As reported, the selective activation of different C−O bonds could lead to different products. Therefore, this review focused on the selectivity of C-O bond activation over homogeneous and heterogeneous catalysts, including Cacyl−O bond, Calkoxy−O bond or both Cacy−O and Calkoxy−O bond cleavage. First, the catalytic hydrogenative cleavage of both Cacyl−O and Calkoxy−O bonds to produce arene or alkane compounds, including benzene, toluene, p-xylene, and cyclohexane, over heterogeneous catalysts (Ru/Nb2O5, Co/TiO2, CuFeCr, CuNa/SiO2, Cu/ZnZrOx, or Ru-ReOx/SiO2) occurs under H2 (3−40 bar) or alcohols as the hydrogen sources at 190−240°C. Mechanistic studies indicate that both the metal centre and the surface functional groups of the support play important roles in hydrogenation, C-O bond cleavage and decarbonylation. Second, the catalytic hydrogenative cleavage of the Cacyl−O bond to produce diols, for example, 1,4-benzenedimethanol, ethylene glycol, propane-1,2-diol and butane-1,4-diol, which is mainly catalysed by homogeneous Ru or Mn complexes or heterogeneous og-CuZn or RuMo/TiO2 catalysts under H2 (5–100 bar) at 80–200°C. Mechanistic studies have indicated that the combined involvement of the metal centre and the ligand is key for H–H bond splitting and ester group reduction. Interestingly, the participation of the solvent methanol in the hydrogenolysis process could cause the reaction to occur under very mild conditions (5 bar H2 at 80°C). Third, catalytic hydrogenative cleavage of the Calkoxy−O bond produces terephthalic acid and ethylene (ethane). Unlike Cacyl−O bond cleavage, the catalyst is used to reduce the C=C double bond of the intermediate olefin rather than the C=O double bond of the ester group. Therefore, the hydrogenolysis of polyester via the Calkoxy−O bond does not require high-pressure H2 but rather a high reaction temperature (180–265°C). Finally, the next steps and challenges in the field of polyester hydrogenolysis are discussed, with emphasis on the development of high-performance and low-cost catalysts, milder reaction conditions, easy scalability and compatibility with complex mixed polyester reaction systems.