As the first of the three major synthetic materials, plastics are discarded after use and have a serious impact on the ecological environment. According to statistics, by 2016, human beings have produced 8.3 billion tons of plastics, of which 6.3 billion tons have become waste plastics, except for a very small part (<10%) has been recycled, a small part (~20%) has been incinerated, and most of them have been abandoned in the natural environment. One possible solution is the use of biodegradable plastics, such as the recent widespread use of biodegradable plastic polylactic acid (PLA) in supermarkets in Beijing as a shopping bag, hoping to reduce the environmental impact of plastic products. However, the degradation process of polylactic acid plastics in the actual environment is very slow, and even if the discarded polylactic acid eventually degrades into CO2 and H2O, this 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 to recycle carbon resources and is the direction of scientists from all over the world.
Figure 1. Schematic diagram of polylactic acid catalyzed amination to alanine
Recently, the Martin/Meng Wang research group at Peking University reported for the first time in the world a new process of catalyzing the conversion of polylactic acid plastics to alanine (Figure 1). Using the Ru/TiO2 catalyst, ammonia can be easily heated to achieve the efficient preparation of alanine from polylactic acid plastics (77% yield, reaction temperature 140 degrees) without additional hydrogen. Studies have shown that PLA is first ammonialyzed in ammonia to form lactamide, followed by lactamide hydrolysis to form ammonium lactate, and ammonium lactate is further aminated on the surface of the catalyst to form alanine (Figure 2a). Isotopic tracing experiments show that activation of ammonium lactate α-H is an important step in the reaction, and the reaction follows the dehydrogenation-amination-rehydration route (Figure 2d). The metal catalyst plays a key role in the activation and subsequent amination of α-H. The yield of alanine can be further increased by separation-cycling, which is 94% selectivable overall and more than 95% pure. The efficiency of this PLA-to-alanine process was evaluated using a commercial PLA straw (containing approximately 83% PLA), a 5.0 g PLA straw that is catalytically converted to obtain 3.0 g of pure alanine, and polylactic acid conversion can be achieved by simply adding ammonia to the whole process without the need for additional hydrogen.
Figure 2. Polylactic acid catalyzed the reaction process of amination to alanine
This new approach to converting PLA into high-value-added chemicals, guided by the idea of "carbon circulation," has greater advantages than natural degradation pathways and will inspire other types of waste plastics to recycle. The research was recently published in the Journal of the American Chemical Society under the title "Catalytic Amination of Polylactic Acid to Alanine".
The research has been funded by the National Natural Science Foundation of China, the Key Research and Development Program of the Ministry of Science and Technology, and the Beijing National Research Center for Molecular Sciences. Professor Martin and Associate Researcher Wang Meng of Peking University are the corresponding authors of the work, and the first authors are Tian Shuheng and Jiao Yuchen, doctoral students in the School of Chemistry and Molecular Engineering of Peking University.
Source: Peking University
Original link:
https://pubs.acs.org/doi/10.1021/jacs.1c08159