Plastics are one of the most used materials in modern society and have changed our lives deeply. However, the increasing use of polymer plastics and the accumulation of plastic waste in the environment have posed a huge threat to humans. According to statistics, humans have produced 8.3 billion tons of plastic by 2016, of which 6.3 billion tons have become waste plastics, except for a very small part (<10%) that is recycled, most of it is discarded in the natural environment. One possible solution is to use biodegradable plastics. For example, recently supermarkets in Beijing have widely adopted biodegradable plastic, polylactic acid (PLA), as shopping bags, hoping to reduce the impact of plastic products on the environment. However, the degradation process of polylactic acid plastic in the natural environment is very slow, and even if the waste polylactic acid is finally degraded into CO2 and H2O, it is also a carbon emission process and a huge waste of carbon resources. Converting waste plastics, including biodegradable plastics, into high value-added chemicals is an important way of carbon resource recycling, and it is also the direction of the current efforts of scientists from all over the world.
Fig.1 Schematic diagram of catalytic amination of polylactic acid to alanine
Recently, for the first time in the world, Ding Ma/Meng Wang’s group at the College of Chemistry and Molecular Engineering, Peking University has reported a one-pot catalytic method to upcycle PLA into alanine (Fig. 1). By a simple ammonia solution treatment at 140 °C in the presenceof a Ru/TiO2 catalyst, PLA can be transformed into alanine with high activity and good stability. Importantly, no externally added H2 is required and the overall selectivity of alanine can reach 94% with 95% purity via a separation-recycling process.It is demonstrated that PLA was first depolymerized into lactamide and then hydrolyzed to ammonium lactate, which is aminated on the catalyst surface to form alanine (Fig. 2a). Isotopic tracing experiments show that the activation of ammonium lactate α-H is an important step in the reaction, and the reaction follows the dehydrogenation-amination-rehydrogenation route (Fig. 2d). The metal catalyst plays a key role in the activation of α-H and subsequent amination.
Fig.2 Catalytic transformation of PLA to alanine
To evaluate the real-world efficiency of the process from PLA to alanine, the commercial PLA straws (~83% PLA) was used. After five cycles for 5.0 g of used straws, 3.0 g purified alanine was obtained, demonstrating the effectiveness of this process.
Under the guidance of the "carbon circulation" idea, this new method of converting PLA into high value-added chemicals has greater advantages compared with natural degradation pathways and will inspire other types of waste plastic recycling. The related work was recently published in the Journal of the American Chemical Society ("Catalytic Amination of Polylactic Acid to Alanine", DOI: 10.1021/jacs.1c08159). Prof. Ding Ma andAssociate Prof. Meng Wang are the co-corresponding authors. Shuheng Tian and Yuchen Jiao are the co-first authors. This work received financial support from the National Natural Science Foundation of China, the National Key R&D Program of China and Beijing National Laboratory for Molecular Sciences.
Original link for the paper: https://doi.org/10.1021/jacs.1c08159
