Turning plastic waste into fuel

Plastics are valued for their durability, but that quality also makes them difficult to break down. Tiny pieces of debris known as microplastics persist in soil, water and air and threaten ecosystems and human health. Traditional recycling reprocesses plastics to make new products, but each time this is done, the material becomes lower in quality due to contamination and degradation of the polymers in plastics. Moreover, recycling alone cannot keep pace with the growing volume of global plastic waste.

Now, a University of Delaware-led research team has developed a new type of catalyst that enhances conversion of plastic waste into liquid fuels more quickly and with fewer undesired byproducts than current methods. Featured on the cover of the Sept. 18 issue of Chem Catalysis, the pilot-stage work helps pave the way toward energy-efficient methods for plastic upcycling, reducing plastic pollution and promoting sustainable fuel production.

“Instead of letting plastics pile up as waste, upcycling treats them like solid fuels that can be transformed into useful liquid fuels and chemicals, offering a faster, more efficient and environmentally friendly solution,” said senior author Dongxia Liu, the Robert K. Grasseli Professor of Chemical and Biomolecular Engineering at UD’s College of Engineering.

One promising upcycling approach is hydrogenolysis, which uses hydrogen gas and a catalyst to convert the polymers in plastics into liquid fuels for transportation and industrial use. However, conventional catalysts have limited efficiency because bulky polymer molecules have a hard time interacting with the active sites of the catalyst where the reaction takes place. To address this, the researchers transformed MXenes (pronounced max-eens), a type of nanomaterial, into mesoporous MXenes, a form with larger, more open pores that had not previously been used for plastic upcycling. 

“MXenes form two-dimensional layers, like the pages of a book. These stacked layers in the closed book make it difficult for molten plastic to move through easily, limiting contact with the catalyst,” explained first author Ali Kamali, a doctoral candidate in the Department of Chemical and Biomolecular Engineering. “To improve the design, we inserted silica pillars to open up the space between MXene layers, allowing the polymers and intermediate compounds that form during the reaction to flow more easily.”

They tested their mesoporous MXene-supported ruthenium catalyst with low-density polyethylene (LDPE), a plastic often used in shopping bags and plastic films. In a small pressurized reactor, the team combined LDPE with the catalyst and hydrogen gas and heated the mixture, melting the plastic into a thick syrup. 

Their catalyst achieved reaction rates nearly two times faster than those previously reported for LDPE hydrogenolysis. The catalyst also displayed high selectivity, allowing for targeted production of liquid fuels while minimizing undesired byproducts like the greenhouse gas methane. The researchers attribute this selectivity to stabilization of ruthenium nanoparticles in the mesoporous space between MXene layers.

“We were able to produce a material that not only speeds the conversion but also improves the quality of the fuel products. This advance highlights the potential of nanostructured mesoporous catalysts to enhance plastic upcycling,” Liu said. 

Looking ahead, the research team plans to further refine the catalyst and to develop a broader library of MXene-based catalysts for use with different types of plastics. Ultimately, they hope to collaborate with industry partners to turn plastic waste from a problem into a resource, converting it into fuels and chemicals that not only help the environment but also bring economic value to local communities.

Other coauthors of the paper from UD’s Department of Chemical and Biomolecular Engineering were postdoctoral researcher Song Luo; graduate students Akash Warty, Antara Bhowmick and Brandon C. Vance; and Dionisios G. Vlachos, the Unidel Dan Rich Chair in Energy Professor of Chemical and Biomolecular Engineering. The research team also included investigators from the University of Maryland College Park, U.S. Army Combat Capabilities Development Command Army Research Laboratory, National Institute of Standards and Technology and Oak Ridge National Laboratory. The work was supported as part of the Center for Plastics Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under award number DE-SC0021166. 

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