Professor Zeng Jie’s group at the University of Science and Technology of China has made a breakthrough in the field of plastic recycling and upgrading. The researchers devised a “hydrogen breathing” strategy to convert high-density polyethylene plastics into high-value-added cyclic hydrocarbons without adding additional hydrogen or solvents, providing a new method for the “artificial carbon cycle” of waste plastics. The research results have been published in the international academic journal Nature Nanotechnology.
Catalytic reforming is an important tool in petroleum processing that converts light gasoline fractions into aromatic-rich, high-octane gasoline, or chemical feedstocks such as benzene, toluene and xylene, and produces hydrogen. Hydrogen is produced during catalytic reforming and consumed during hydrocracking. If these two processes are applied to polyethylene degradation in tandem, it is equivalent to letting the plastic “exhale” hydrogen while allowing the plastic to “inhale” hydrogen to crack into short chains by dehydrogenating the rings into cyclic hydrocarbons.
This strategy uses polyethylene’s own hydrogen atoms to replace additional hydrogen, which not only reduces costs, but also saves energy and reduces emissions. Most importantly, the process also breaks the stable carbon skeleton in polyethylene, making the molecular chain segments shorter, and the resulting product is a cyclic hydrocarbon, which has a higher value than chain hydrocarbons.
Acidic sites can promote the cyclization of olefins into cyclic hydrocarbons. The researchers introduced a molecular sieve with acidic sites as a carrier to the original ruthenium metal catalyst. They found that the new catalyst could smoothly catalyse dehydrocyclization of polyethylene and release hydrogen to trigger the subsequent hydrocracking process. Under the action of the molecular sieve-loaded ruthenium catalyst, the waste polyethylene plastic was gradually degraded. Finally, after 24 hours of catalytic reaction, the conversion rate of HDPE plastic reached 69.6%, where the main degradation product was liquid cyclic hydrocarbons. Cyclic hydrocarbons are one of the high value-added chemicals that can be used in a wide range of applications as raw materials for the synthesis of drugs, dyes, resins and fibers.
In the future, they hope to develop catalysts that do not contain the precious metal ruthenium to further reduce the cost of catalysts, while introducing co-reactants that are large in volume and cheap and easily available in nature to continuously improve the value of the products.