Tsinghua Niu Zhiqiang’s team has made important progress in the field of catalytic recycling of waste plastics

Tsinghua News Network, May 11 Recently, Niu Zhiqiang’s research group of the Department of Chemical Engineering has made important progress in the field of catalytic recycling of waste plastics. The catalytic cycle of waste plastics can effectively reduce their environmental harm, reduce carbon emissions throughout the life cycle of plastics, and improve the comprehensive utilization rate of carbon resources. Based on the strategy of biomimetic catalysis, Niu Zhiqiang’s research group of the Department of Chemical Engineering of Tsinghua University discovered a class of dual-core zinc catalysts, which transformed intermolecular nucleophilic reactions into intramolecular nucleophilic reactions with the help of the active center of “bimetallic site”, which not only promoted the depolymerization of polyethylene terephthalate (PET) plastics under environmental conditions, but also realized the efficient chemical cycle of PET under mild conditions. This work provides new ideas and new methods to solve the global plastic pollution problem.

Figure 1.Dual-core catalyst design concept
The heavy use of plastics has caused serious environmental and energy problems, especially the huge annual production of polyethylene terephthalate (PET), which accounts for nearly 12% of the world’s total solid waste. At present, the chemical cycle of PET needs to be carried out under harsh conditions such as high temperature, high pressure or high concentration of acid and alkali, and it is challenging to develop energy-saving and environmentally friendly PET degradation pathways. PET hydrolases enable the depolymerization of PET under mild conditions, mainly due to the proximity localization effect catalyzed by the enzyme: the active center binds to both PET and nucleophiles, bringing them closer to each other and taking the correct spatial orientation, greatly increasing the local effective concentration of the substrate. This reaction mechanism is also fully reflected in metallolytic enzymes. For example, organophosphorus-degrading enzyme (OpdA) from Radiobacterium has an active center composed of two metal ions (such as di-Zn, di-Co, etc.). It uses the same proximity localization effect to accelerate organophosphorus hydrolysis (Figure 1A). As of now, no activity has been documented to indicate that OpdA or other dinuclear metallolytic enzymes have the activity to degrade PET, possibly because the polymer molecules do not have access to the active center of OpdA. Niu’s research group conjectures that if the non-catalytic protein backbone of OpdA is removed, the binuclear site may be fully exposed to catalytic PET degradation (Figure 1b).

Figure 2.Depolymerization of PET under environmental conditions
Based on the above conjecture, Niu Zhiqiang’s group found that a dual-core zinc catalyst can effectively achieve the depolymerization of PET under mild conditions or even environmental conditions (Figure 2a). Compared with PET hydrolase, the catalyst has the advantages of good stability, low cost and wide range of plastic applications, which breaks the limitation of low activity of PET hydrolase for high crystallinity PET (Figure 2b). The catalyst enables continuous depolymerization of PET in a real seawater environment (pH 7.9) (Figure 2c), providing the possibility of environmental remediation.

Figure 3. PET chemical cycle and substrate universality
The structural stability of the two-core catalyst allows it to be used over a wide operating temperature (30−340oC) and pH (8−14) range. Under relatively mild conditions (pH 13 and 90oC), the specific activity of 577 gPETh-1gcat-1, the spatiotemporal yield of 70 g TPAL-1h-1 terephthalate monomer, and a TON of not less than 25,000 can be achieved (Figure 3a, b). Compared to the traditional PET alkaline hydrolysis technique, the specific activity of the catalyst is increased by an order of magnitude on the basis of a 10-fold reduction in alkaline concentration (Figure 3a). In addition, the catalyst has good resistance to various additives and pigments in waste plastics and shows universal applicability to different kinds of polyester plastics (Figure 3c).
The results were published in the international academic journal Nature Sustainability under the title “Depolymerization of polyesters by a binuclear catalyst for plastic recycling”.
The first authors of the paper are Shengbo Zhang, a postdoctoral fellow in the Department of Chemical Engineering of Tsinghua University, Qikun Hu, a 2021 doctoral student, and Yuxiao Zhang, a postdoctoral fellow. The corresponding author is Associate Professor Niu Zhiqiang, Department of Chemical Engineering, Tsinghua University. The DFT calculation work was completed under the guidance of Professor Liu Ping of Brookhaven National Laboratory. The secondary polymerization of recovered monomers and the characterization of polymer melt properties were completed by Zhu Xingsong, senior engineer of Sinopec. The research work is supported by the National Key Research and Development Program of China, the National Natural Science Foundation of China, the Frontier Interdisciplinary Project of Tsinghua University Independent Research Program, and the China Postdoctoral Science Foundation.
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