PLA is Polylactic acid, a product of α-hydroxypropionic acid condensation, a type of thermoplastic aliphatic polyester, and a polymer material generated by polymerization of lactic acid extracted from renewable resources such as corn, sugarcane and other plants. It has good biocompatibility and biodegradability and is a new type of environmentally friendly plastic. After use, polylactic acid can be decomposed into carbon dioxide and water by microorganisms without polluting the environment. Its raw materials come from plants and are a renewable resource. Compared with petroleum-based plastics, it is more environmentally friendly. In addition, PLA has excellent melt processing properties and can be formed by various plastic processing equipments.
Polyester is the most important type of plastic, among which polyethylene terephthalate (PET) and PLA are very common. At present, PET can be recycled by physical, biological and chemical methods. Compared with traditional chemical degradation methods (such as high-temperature alcoholysis/hydrolysis), the hydrogenolysis method that has gradually developed in recent years has the advantages of high atomic economy, less pollution and low energy consumption, and has always attracted much attention. At present, there are few reports on the use of homogeneous catalytic systems for PET hydrogenolysis. In addition to homogeneous catalysts, heterogeneous catalysts can also be used for PET hydrogenolysis, but they still need to be carried out at a high temperature of >260 ℃. Therefore, it is urgent to develop a new method for polyester degradation that is mild, efficient and practical.
Some researchers believe that the difficulties in polyester degradation mainly lie in the following two aspects: 1) First, the ultra-large molecular weight characteristics of polyester lead to its low solubility and high reaction viscosity in the reaction system, which results in the inability of the reaction sites in the polyester to fully contact with the catalytic active species; 2) The activation of the ester C-O bond in the polyester is difficult. Based on the above considerations, the team first proposed a new alcoholysis/hydrogenation relay strategy to solve the above problems: first, CH3OH is used to depolymerize the macromolecular PET into small molecular oligomers to increase its solubility, and then the oligomers are hydrogenolyzed under mild conditions by designing a metal catalyst with a suitable structure to obtain the target product. However, traditional hydrogenation catalysts react with protic solvents to undergo re-aromatization to obtain [Cat.]-I-CH3OH, which cannot continue to activate hydrogen due to coordination saturation. Therefore, the biggest challenge facing this new relay strategy is how to overcome the poisoning effect of protic solvents on hydrogenation catalysts. The authors believe that if a new hydrogenation catalyst [Cat.]-II with quinaldine as the skeleton is designed, it may be possible to improve the stability of the catalyst in protic solvents, mainly because: 1) [Cat.]-II has a larger delocalization range and higher stability compared with [Cat.]-I; 2) The C=C bond in [Cat.]-II may be easily hydrogenated first to further obtain the active species [Cat.]-III. Since [Cat.]-III lacks the driving force for rearomatization, it can exist stably in protic solvents and smoothly participate in H2 activation.
In order to verify the feasibility of the reaction design idea, the authors first tested the viscosity of the reaction system under different conditions. The results showed that when CH3OH and tBuOK were added to the toluene solution of PET, the viscosity of the system dropped sharply, and the alcoholysis products DMT and HMT were observed. It can be seen that methanol can indeed depolymerize large molecular PET into small molecular oligomers, thereby improving its solubility. Next, according to the previous design, a series of new tridentate pincer ruthenium catalysts were synthesized using the easily available raw material 8-aminoquinaldine as the starting material, and their structures were characterized by single crystal, nuclear magnetic resonance, etc. After obtaining the catalyst, the authors began to investigate the stability of dearomatization intermediates such as [Ru]-1' in methanol. Experiments have found that only when more than 10 equivalents of methanol are added to the reaction will [Ru]-1' undergo aromatization with methanol to obtain the coordination-saturated intermediate [Ru]-1-alkoxo. The authors identified the structure of the above-mentioned new active species by nuclear magnetic resonance, high resolution, single crystal, etc., that is, the C=C bond of allylamine in [Ru]-1' was hydrogenated to obtain [Ru]-1'-H2.
After fully understanding the performance of the Ru catalyst, the authors began to explore its catalytic activity in polyester hydrogenolysis. Using commercially available PET particles as substrates, after a series of condition optimization, the optimal reaction system was obtained: [Ru]-1 as catalyst, tBuOK as base, methanol and toluene as solvents (v/v = 9:1), reacting at 80 °C for 72 h under 5 bar H2, the yield can reach up to 88%. It is worth mentioning that when the reaction pressure is further reduced to 1 bar, the reaction can still proceed smoothly. After obtaining the optimal conditions, the applicable scope of the substrate was further investigated. The experimental results show that this method has excellent universality. It can not only degrade various PET products, but also be compatible with polyester materials such as PLA. Experiments show that only 0.05 mol% catalyst is required for the PET hydrogenolysis reaction to proceed smoothly, and the TON can reach up to 1520.
