(154ap) Mechanochemical Degradation of Polyethylene into Waxy Residue, Olefin Containing Products and Gaseous C1-C6 Products

Plastics have become widespread in their use with the majority of plastic being single use, thus, leading to approximately 300 million tons of plastic waste are generated annually. The recycling industry and infrastructure, however, is not equipped to handle this magnitude of plastic leading to only ~10-15% being recycled and the remaining waste being landfilled, incinerated, or polluted to the environment (i.e., oceans).[1, 2] In order to combat the plastic waste crisis, chemical recycling to convert waste into value-added products, such as hydrocarbon fuels and petrochemicals has been explored. Current methods include pyrolysis to convert plastic waste into hydrocarbon fuels by using high temperatures (500-1000 °C) in the absence of oxygen to thermally crack the polymer chains into lower molecular weight fractions.[3] The formation of these lower MW is implanted in refinery processes to convert to chemicals as well as may be used as transportation fuel. Although pyrolysis has resulted in production of olefins, the many drawbacks, including catalyst coking, high energy requirements, and a relatively large carbon footprint, limit the implementation of this technology. [4-8] To overcome the drawbacks and challenges inherently present in pyrolysis methods, this work targets sustainable solutions to combat plastic waste using a greener method of conducting chemistry known as mechanochemistry. Mechanochemistry is the process of conducting a solid-state chemical reaction in an environment under extreme mechanical stress, and here, a planetary ball mill is used for these reactions with the major advantages including operation without the use of solvents, reduced reaction times, and changes in product selectivity, and being less energy intensive.[9]

In this work, polyethylene (MW = 4000 da) was degraded using mechanochemical reactions catalyzed with aluminosilicate materials of varying properties such as Si/Al and topology (i.e., FAU and MFI zeolites). Mechanochemical reactions were conducted in an oxygenated environment at ambient temperature for up to 12 h using a planetary ball mill (Retsch PM 400) with stainless-steel jars containing grinding balls, polyethylene, and the aluminosilicate catalyst. Post reaction, a series of steps were taken to extract the formed olefin-containing residue from the catalyst and stainless-steel grinding balls including sonication, centrifugation, and rotary evaporating. Following the extraction of the residue, thermogravimetric analysis (TGA) was measured on the resulting product and revealed the product degraded faster and at a lower temperature than the reference polyethylene. ¹H, ¹³C, and Heteronuclear Single Quantum Coherence (HSQC) Nuclear Magnetic Resonance (NMR) spectra were collected and revealed the presence of olefinic peaks in the range of ~5-5.3 ppm (¹H) and ~170-200 pm (¹³C) with HSCQ allowing for a correlation between proton and carbon NMR spectra to determine the bond correlations. Additionally, gaseous products were extracted and injected into a gas chromatography-mass spectrometer (GC-MS) containing a specialized plot column allowing for full separation of the gaseous mixture into its individual components in order to identify each product through both a flame ionization detector and mass spectrometer. Current efforts have been able to both qualitatively and reproducibly identify gaseous products as C1-C6 hydrocarbons with future efforts moving towards the quantification of each species as well as identifying products that form during the reaction as a waxy residue. These exciting results are directing future efforts to optimize the reaction conditions to enhance the formation of valuable chemicals directly from polyolefin plastic feedstock, move towards the quantification of gaseous products, and investigate the recyclability of the catalyst. Overall, this low temperature, energy efficient mechanochemical process demonstrates that polymers can be successfully degraded to lower MW residues containing olefinic functional groups. This reveals the ability and effectiveness of mechanochemistry in degrading polymers into smaller constituent building blocks, including olefins and alkanes, thus paving way for greener and energy efficient solutions to breakdown polymers into monomers.

1. Parker, L. A Whopping 91 Percent of Plastic Isn’t Recycled. National Geographic Society, 2019.

2. Programme, U.N.E., Single-use plastics: A roadmap for sustainability. 2018, United Nations Environment Programme.

3. Jiang, J., et al., From plastic waste to wealth using chemical recycling: A review. Journal of Environmental Chemical Engineering, 2022. 10(1).

4. Li, Q., et al., Progress in catalytic pyrolysis of municipal solid waste. Energy Conversion and Management, 2020. 226.

5. Xue, Y., P. Johnston, and X. Bai, Effect of catalyst contact mode and gas atmosphere during catalytic pyrolysis of waste plastics. Energy Conversion and Management, 2017. 142: p. 441-451.

6. Wang, X., et al., Soot formation during polyurethane (PU) plastic pyrolysis: The effects of temperature and volatile residence time. Energy Conversion and Management, 2018. 164: p. 353-362.

7. Santos, B.P.S., et al., Petrochemical feedstock from pyrolysis of waste polyethylene and polypropylene using different catalysts. Fuel, 2018. 215: p. 515-521.

8. Huang, J., et al., Chemical recycling of plastic waste for sustainable material management: A prospective review on catalysts and processes. Renewable and Sustainable Energy Reviews, 2022. 154.

9. Howard, J.L., Q. Cao, and D.L. Browne, Mechanochemistry as an emerging tool for molecular synthesis: what can it offer? Chemical Science, 2018. 9: p. 3080-3094.