Effect of Particle Size on the Recoverability of Heterogeneous Catalysts Comprised of Magnetic Nanoparticles for Applications in Liquid-Phase Catalysis.

Plastic is a widely used material with limited mechanical and thermal recycling capabilities. In 2019, the United States consumed over 57 million metric tons of plastic, with less than 7% recovered for reuse, and only 54% of the plastics sent to mechanical recycling facilities were recovered for domestic recycling.² A promising alternative to other recycling methods is chemical recycling, which converts plastic polymers back into their parent monomers or to value-added chemicals. Among the challenges with using heterogeneous catalysts in chemical recycling is the mass transport, stability, and recovery of catalysts in batch reactors. One approach to effective catalyst recovery is the use of magnetic separation to rapidly separate catalysts and apply the necessary regeneration procedures. The focus of this research is to examine the magnetic separation of heterogeneous catalysts used in the chemical recycling of polyolefins. The goal is to determine the optimal nanoparticle size and loading to maximize recycling efficiency and catalyst recoverability using low-gradient magnetic separation with earth-abundant catalytic materials.

Catalysts comprised of Co, Ni, and Fe and their respective oxides have shown promise as materials for polyethylene deconstruction to alkanes using hydrogenolysis and are also magnetic catalysts, which can be effectively separated using low-gradient magnetic separation. These are commonly used in medical and water treatment applications due to their large magnetic saturation and superparamagnetic properties.³ The most employed magnetic nanoparticles for different applications tend to be Fe₃O₄, MnFe₂O₄, and CoFe₂O₄ because they are relatively easy to synthesize and have high saturation magnetization.³ The simple and environmentally friendly method for synthesizing Fe₃O₄ is aqueous coprecipitation from basic aqueous solutions of ferric and ferrous salts.³ For large nanoparticles, a reduction of the separation time was observed due to cooperative magnetophoresis when increasing the particle concentration. While for smaller nanoparticles, a significant increase in the separation time was observed⁴.

In this study, we synthesized a series of catalysts comprised of Co, Ni, and Fe nanoparticles supported on silica with varying weight loadings and nanoparticle sizes. We evaluated the rate of separation of the nanoparticles suspended in organic media that mimic the final product distributions from polyethylene hydrogenolysis (e.g. n-dodecane). A magnetic field was applied to the dispersions of nanoparticles, and the particle motion was measured using a camera with ImageJ analysis to evaluate the separation time of the MNPs and assess any induced convective flow and conglomerations. Ultra-violet visible spectroscopy was used to characterize the turbidity of the material as a function of time. Ultimately, this work aims to determine the most effective sizes and loadings of nanoparticle catalysts for efficient low-gradient magnetic separation and recovery in batch reactors.

References

1. Borkar, S. S.; Helmer, R.; Panicker, S.; Shetty, M. Investigation into the Reaction Pathways and Catalyst Deactivation for Polyethylene Hydrogenolysis over Silica-Supported Cobalt Catalysts. ACS Sustainable Chemistry & Engineering 2023, 11 (27), 10142–10157. DOI:10.1021/acssuschemeng.3c02202.
2. Hendrickson, T. P.; Bose, B.; Vora, N.; Huntington, T.; Nordahl, S. L.; Helms, B. A.; Scown, C. D. Paths to Circularity for Plastics in the United States. One Earth 2024, 7 (3), 520–531. DOI:10.1016/j.oneear.2024.02.005.
3. Rossi, L. M.; Costa, N. J. S.; Silva, F. P.; Wojcieszak, R. Magnetic Nanomaterials in Catalysis: Advanced Catalysts for Magnetic Separation and Beyond. https://doi.org/10.1039/c4gc00164h (accessed 2025-10-04).
4. Witte, K.; Müller, K.; Grüttner, C.; Westphal, F.; Johansson, C. Particle Size- and Concentration-Dependent Separation of Magnetic Nanoparticles. Journal of Magnetism and Magnetic Materials 2017, 427, 320–324. DOI:10.1016/j.jmmm.2016.11