A research team led by Prof. ZENG Jie from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS) has made a significant breakthrough in the field of plastic upcycling. The researchers introduced a novel dehydroaromatisation and hydrogenolysis tandem strategy for converting high-density polyethylene (HDPE) plastics into valuable cyclic hydrocarbons without the need for solvents or hydrogen. The study was published in Nature Nanotechnology. 

Polyethylene, one of the most commonly used plastics, poses challenges in terms of natural degradation due to its stable chemical structure. Recycling technologies for waste polyethylene plastics not only mitigate pollution but also offer economic benefits.  

Inspired by two processes in the petroleum industry, namely catalytic reforming of short-chain gasoline fractions and hydrocracking of heavy oils, the researchers sought to treat waste HDPE plastics as a "solid petroleum" raw material through environmentally friendly catalytic conversion, thereby producing downstream petroleum-based chemical products. They focused on catalytic reforming of short-chain gasoline fractions to obtain higher-value cyclic hydrocarbons, which generates hydrogen, and the hydrocracking of heavy oils to produce short-chain hydrocarbons, which consumes hydrogen. 

Building upon these processes, the researchers devised a "hydrogen-breathing" strategy for degrading HDPE plastics. They developed a molecular sieve-loaded metallic ruthenium catalyst (Ru/HZSM-5) that facilitates the dehydrogenation of the plastic into cyclic hydrocarbons, "breathe out" hydrogen in the process. Simultaneously, the plastic "breathe in" the released hydrogen and undergoes cracking, transforming into short-chain hydrocarbons.  

Then, the researchers explored the upcycling reaction pathways of high-density polyethylene plastics. They conducted catalytic experiments on the recycling of HDPE plastics with different molecular sieve loadings of ruthenium metal, and examined the effect of the molecular sieve pores on the reaction.

The results showed that the HZSM-5 molecular sieve has a moderate pore size, which not only avoids the formation of thick cyclic aromatic hydrocarbons and carbon deposits, but also ensures the smooth desorption of cyclic hydrocarbons, thus guaranteeing the continuity and stability of the catalytic reaction. Ru/HZSM-5 catalysts have a very good cyclic stability, and are also suitable for different types of polyethylene plastics. 

This study provides an innovative solution for the conversion of HDPE plastics into valuable cyclic hydrocarbons, and it contributes to the ongoing efforts to address plastic waste and to promote a sustainable future.