The target molecules in this work are ethylene, propylene, and butenes, collectively known as light olefins. These compounds sit at the very top of the petrochemical value chain, serving as feedstocks for polyethylene, polypropylene, and a vast array of downstream products. Conventionally, they are produced by steam cracking of fossil-derived naphtha at temperatures approaching 900 degrees Celsius, an energy-intensive process with a large carbon footprint. Catalytic pyrolysis of plastic waste offers an alternative route: instead of burning fossil fuels to make new plastics, the plastics already in circulation can be broken down and fed back into the supply chain. The catch has always been selectivity, because the harsh chemistry that cleaves polyethylene chains also tends to destroy the very olefins the process is meant to produce.

ZSM-5, a zeolite with the MFI framework topology, has long been a workhorse catalyst for plastic cracking. Its microporous channels and tunable acidity make it excellent at cutting long hydrocarbon chains into smaller fragments. But the commercial form of ZSM-5 carries an abundance of strong Brønsted acid sites, the proton-donating centers embedded in the zeolite framework. Those sites do more than crack chains. They also promote aromatization, a cascade of secondary reactions in which the desired light olefins are consumed to form benzene, toluene, xylenes, and ultimately coke. In other words, the standard catalyst destroys its own product. The researchers set out to tame this acidity without sacrificing the cracking activity that makes ZSM-5 valuable.

Previous attempts to solve this problem with a single metal additive had fallen short. Zinc modification is a well-known strategy in zeolite chemistry, but on its own it cannot deliver satisfactory olefin yields, and neither can zirconium alone. The insight behind the new study is that the two metals work synergistically. The team prepared a series of Zn and Zr co-modified ZSM-5 catalysts with varying molar ratios using incipient wetness impregnation, a straightforward method in which a metal salt solution is added to the zeolite in just enough volume to fill its pores, then dried and calcined. The resulting catalysts were tested for LDPE cracking in a two-stage fixed-bed reactor, a configuration that separates the thermal breakdown of the plastic from the catalytic upgrading of the volatile intermediates.

Characterization told a subtle story about what each metal contributes. X-ray diffraction confirmed that metal loading did not destroy the MFI topological framework, meaning the zeolite’s crystalline pore structure remained intact. Yet the single zinc-modified sample showed a marked decrease in crystallinity and a loss of long-range framework order. The explanation lies in how zinc interacts with the zeolite. Zinc species tend to react with the framework bridging hydroxyl groups, the structural features responsible for Brønsted acidity, to form ZnOH+ species. These unconstrained zinc hydroxyl species act as strong dehydrogenation active sites, and when present in abundance they trigger severe secondary reactions that once again consume the light olefin intermediates the process is designed to yield.

This is where zirconium changes the picture. In the bimetallic samples, zirconium species couple with zinc species through an electronic interaction, and this partnership restrains the consumption of the bridging hydroxyl groups by zinc. The result is a rebalanced acid-site distribution: the ratio of Brønsted to Lewis acid sites, often abbreviated B/L, shifts into a range more favorable for light-olefin production. Brønsted sites perform the cracking chemistry that fragments the polyethylene, while an appropriate population of Lewis sites supports dehydrogenation and other steps without letting the reaction cascade run away into aromatization. By moderating how much zinc can bind to the framework hydroxyls, zirconium prevents the overproduction of the troublesome ZnOH+ species while preserving the beneficial acidity profile.

The performance numbers are striking. Among all the prepared samples, the catalyst designated Zn0.25Zr0.75/ZSM-5, with a zinc-to-zirconium molar ratio favoring zirconium, delivered the best results. At a reaction temperature of 500 degrees Celsius, it achieved a gas yield of 77.40 weight percent and a light olefin yield of 62.54 weight percent from waste LDPE. Ethylene production in particular exceeded that of the monometallic modified samples, an important benchmark given that ethylene is the most commercially valuable of the light olefins and the most prone to secondary consumption. For comparison, unmodified ZSM-5 under similar conditions channels a large share of the carbon into aromatics and coke rather than gaseous olefins, which is precisely the outcome the metal modification strategy is designed to avoid.

Just as important as the initial yield is durability, because industrial catalysts must survive hundreds of hours of operation and repeated regeneration. Coke deposition is the inevitable byproduct of zeolite-catalyzed plastic cracking, and catalysts are routinely restored by burning off the carbon in air. The researchers subjected their best catalyst to regeneration-cycle tests, and the results were encouraging: after ten regeneration cycles, the light olefin yield was still maintained at 59.94 weight percent, only modestly below the fresh-catalyst value. This cycling stability suggests that the zinc-zirconium synergy is not a fragile transient effect but a robust feature of the catalyst’s structure that survives the thermal stress of regeneration.

The study’s analytical toolkit underscores how modern catalyst development proceeds at multiple scales simultaneously. The team employed X-ray photoelectron spectroscopy to probe the electronic states of the metal species, magic-angle spinning nuclear magnetic resonance to track changes in the framework aluminum environment, pyridine adsorption infrared spectroscopy to quantify Brønsted and Lewis acid sites, and ammonia temperature-programmed desorption to measure acid strength distributions. Scanning electron microscopy with energy-dispersive X-ray spectroscopy confirmed metal dispersion, while Brunauer-Emmett-Teller measurements tracked surface area and porosity. Gas chromatography with flame ionization detection and gas chromatography-mass spectrometry provided detailed product analysis. Together these techniques allowed the authors to connect the electronic interaction between zinc and zirconium directly to the acid-site balance and, ultimately, to the olefin yield.

The broader implications reach into the economics of plastic recycling and the decarbonization of the chemical industry. If waste polyethylene can be converted to light olefins at moderate temperatures with high selectivity and a catalyst that survives repeated regeneration, the process becomes a far more attractive complement or alternative to steam cracking. The work also offers a design principle that extends beyond this particular system: rather than adding a single promoter and hoping for the best, catalyst designers can pair metals whose interactions tune each other’s binding to the zeolite framework, achieving an acidity balance no single modifier can deliver. As research groups worldwide race to close the loop on polyolefin plastics, this zinc-zirconium partnership in the channels of ZSM-5 stands out as a compelling example of how atomic-level catalyst engineering can translate an environmental liability into a stream of valuable industrial feedstock.

Subject of Research: Zn-Zr co-modified ZSM-5 zeolite catalysis for converting waste LDPE plastic into light olefins

Article Title: Synergistic Effect of Zn and Zr Co-modified ZSM-5 Zeolite for Catalytic Cracking of Waste LDPE to High-Yield Light Olefins

Article References: Wang, W., Shao, Z., Wang, C., Shi, H., & Geng, P. (2026). Synergistic Effect of Zn and Zr Co-modified ZSM-5 Zeolite for Catalytic Cracking of Waste LDPE to High-Yield Light Olefins. Catalysis Letters, 156(10), Article 290. https://doi.org/10.1007/s10562-026-05530-1

Image Credits: AI Generated

DOI: 10.1007/s10562-026-05530-1

Keywords: catalytic pyrolysis, LDPE, ZSM-5 zeolite, light olefins, zinc modification, zirconium modification, plastic recycling, Brønsted acid sites, Lewis acid sites, ethylene, catalyst regeneration, chemical recycling