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Our newspaper, Hefei, June 28, by wire (reporter Ding Yiming, correspondent Wang Min)Recently, Professor Zeng Jie's research group at the University of Science and Technology of China has made breakthrough progress in the field of plastic cycle upgrading.Researchers have designed a "hydrogen breathing" strategy to convert high-density polyethylene plastics into high value-added cyclic hydrocarbons without adding additional hydrogen or solvents, which provides a new method for the "artificial carbon cycle" of waste plastics.The research results were recently published in the international academic journal Nature Nanotechnology.
Polyethylene is one of the five general plastics with high stability and difficulty in natural degradation.Disposal of waste polyethylene plastics by incineration or landfill will cause air, soil and water pollution.Considering that polyethylene and petroleum have similar chemical structure and composition, Zeng Jie and others boldly speculated that whether polyethylene can be directly regarded as a "solid petroleum" and used for reference to petrochemical technology to synthesize downstream chemical products of petroleum?
In this study, researchers first focused on an important process of petroleum processing - hydrocracking, which can split long chain heavy oil to obtain short chain oil products, such as gasoline, kerosene and diesel.With reference to this method, researchers conducted hydrocracking experiments with polyethylene as raw material and successfully converted it into chain hydrocarbon products of gasoline fractions, further confirming that polyethylene is equivalent to "solid oil".
"As the literal meaning of 'hydrocracking', this process needs to consume a lot of hydrogen, which is very expensive. In addition, the existing hydrogen production process will also cause carbon emissions." Zeng Jie said, further, they started from improving the process, trying to achieve the recycling and upgrading of waste polyethylene plastics without the use of hydrogen.
Catalytic reforming is another important means in petroleum processing, which can convert light gasoline fractions into high octane gasoline rich in aromatics, or chemical raw materials such as benzene, toluene and xylene, and generate hydrogen.
Hydrogen is generated during catalytic reforming and consumed during hydrocracking.If these two processes are connected in series and applied to polyethylene degradation, it is equivalent to letting the plastic "breathe" out hydrogen while dehydrogenating into a ring hydrocarbon, while letting the plastic "suck" in hydrogen and crack into short chains.
Zeng Jie introduced that "this strategy uses polyethylene's own hydrogen atom to replace the added hydrogen, which not only reduces costs, but also saves energy and emissions, so as to realize the" self production and self sale "of hydrogen".Most importantly, in this process, the stable carbon skeleton in polyethylene can also be broken, making the molecular chain segment shorter, and the product obtained is cyclic hydrocarbon, which has higher value than chain hydrocarbon.
In order to realize the dynamic "hydrogen breathing" strategy, the key is to find a suitable catalyst.Zeng Jie proposed that "acidic sites can promote the cyclization of olefins into cyclic hydrocarbons".The researchers introduced molecular sieves with acidic sites on the original ruthenium catalyst as support.They found that this new catalyst can successfully dehydrogenate and cyclize polyethylene, release hydrogen, and successfully initiate the subsequent hydrocracking process.
Under the action of ruthenium catalyst supported on molecular sieve, waste polyethylene plastics were gradually degraded.Finally, after 24 hours of catalytic reaction, the conversion rate of HDPE plastic reached 69.6%, and the main degradation product was liquid cyclic hydrocarbon.Cyclic hydrocarbon is one of the chemicals with high added value. It can be used as raw materials for synthetic drugs, dyes, resins and fibers, and has a wide range of applications.
"In the future, we will develop a catalyst without precious metal ruthenium to further reduce the cost of the catalyst, and at the same time introduce a large amount of cheap and easily available co reactants in nature to continuously improve the product value," said Zeng Jie.
