Mechano-catalytic depolymerization of plastic waste

Speaker: Prof. Ina Vollmer, Inorganic Chemistry and Catalysis group, Utrecht University

Host: on invitation of Prof. Dierk Raabe

Abstract:

Only 12% of plastic waste is recycled, mainly because the predominantly applied technique of melting and re-extrusion produces a lower quality material [1]. Alternatively, chemical depolymerization can produce monomers to make high-quality plastics again. 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.

Figure 1. A) Milling container modified with gas in- and outlet to track gaseous product formation. B) Visualization of the mechano-chemical chain cleavage upon ball impact as well as density functional theory optimized structure of a radical interacting with the catalytic surface of the grinding sphere. C) Propene flow during milling of PP with untreated ZrO₂ spheres, sulfuric acid treated (S-ZrO₂) spheres, untreated spheres with sulfated ZrO₂ added as powder, and untreated spheres with sulfuric acid added as liquid. C) Cumulative yields obtained from model PP after 1 h of milling with untreated and S-ZrO₂ spheres using optimized conditions.

References

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).

short bio:

Ina Vollmer is an assistant professor at the Inorganic Chemistry and Catalysis group at Utrecht University. She is looking for pathways to recycle plastics to chemical building blocks using catalysis. Vollmer studied process engineering at Hamburg University of Applied Science, while also obtaining a journalistic degree with a scholarship from the journalistic trainee program of the Konrad-Adenauer foundation. She went on to study chemical engineering at Massachusetts Institute of Technology (graduation 2015). In her doctorate, she investigated the aromatization of methane over zeolite supported metal catalysts using operando spectroscopy with Prof. Freek Kapteijn and Prof. Jorge Gascon (graduated 2019). This included a research stay at King Abdullah University of Science and Technology in Saudi Arabia. After a postdoc on chemical recycling of plastic and natural degradation of plastics to nanoplastics at Utrecht University with Prof. Bert Weckhuysen, Vollmer became an assistant professor in 2021. She teaches sustainable chemistry, polymer chemistry and electron paramagnetic resonance spectroscopy. She is a faculty council member and in the board of the Institute of Sustainable and Circular Chemistry. With her interdisciplinary background in process engineering, chemical engineering, heterogeneous catalysis, spectroscopy, chemical recycling of plastics, polymer chemistry, and mechano-catalysis, Vollmer connects different approaches to come up with new ways to drive chemical conversions. Her idea to convert polyolefins at room temperature in the ball mill via direct catalysis received a Veni and XS grant from the Dutch Research Council (NWO) in 2021 and was patented in 2022.