Mechano-catalytic depolymerization of plastic waste

However, depolymerization via thermal and even catalytic pyrolysis of commodity polymers such as polypropylene (PP) offers only low selectivities and low-value product mixtures, due to the high temperature applied, which is required for thermal C–C bond cleavage [1]. The team of Vollmer researches several strategies to design better chemical recycling strategies using heterogenous catalysts. In the lecture, she will mainly focus on the investigation of polymer conversion in a mechano-chemical ball mill reactor (Figure 1A), which allows us to achieve conversion at room temperature instead of the more than 500 °C used in the state-of-the-art pyrolysis process. It was previously found that organic radicals can form even at −196 °C when PP is exposed to mechanical force [2,3] and she exploits this effect for plastic recycling to chemical building blocks. Her team combines mechano-chemical bond scission with heterogeneous catalysis by directly functionalizing the surface of ceramic grinding spheres to create catalytically active sites. This led to the discovery of a new catalytic mechanism, where the activated surface of the grinding spheres can interact with the organic radicals formed by the mechano-chemical action of colliding grinding spheres. This is fundamentally different from thermal conversion using typical heterogeneous catalysts, such as solid acids, where the polymer backbone C-C bonds are activate via the formation of carbocations. The catalytic grinding spheres also allows to overcome difficulties in contacting porous catalyst materials with bulky polymer molecules that are encountered in thermal catalysis [4]. The superiority of the catalytic spheres over catalysts is evidenced by the lack of activity of powder catalysts in the ball mill (Figure 1B). The contact between the polymer and the catalyst surface is ensured by the repeated ball-ball and ball-wall collision in the ball mill. In addition, high mixing by the ball mill action avoids clogging observed in conventional reactors because of the high viscosity of molten polymer.

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1. Vollmer, I., Jenks, M.F.J., Roelands, M.C.P., et al. Angew. Chem., Int. Ed. 59, 36 (2020).

2. Aydonat, S., Hergesell, A. H., Seitzinger, C. L., Lennarz, R., Chang, G., Sievers, C., Meisner, J., Vollmer, I., & Göstl, R. Polymer Journal (2024).

3. Sakaguchi, M., Sohma, J. J. Polym. Sci., Part B: Polym. Phys. 13, 6 (1975).

4. Rejman, S., Vollmer, I., Werny, M. J., Vogt, E. T. C., Meirer, F., & Weckhuysen, B. M. Chemical Science, 14(37), 10068–10080 (2023).