Walnut shell catalyst converts farm plastic waste into olefins at 69% yield

Published in Sustainable Carbon Materials, 2026.

Every growing season, farmers lay down sheets of low-density polyethylene mulch film to hold moisture in the soil and keep weeds at bay. After harvest, the shredded, dirt-caked remnants are nearly impossible to recycle through conventional means. They pile up in fields, degrade into microplastics over decades, and contaminate the soil they were meant to protect.

A team of researchers has now demonstrated a chemical alternative: heating that waste plastic with a catalyst made from another agricultural by-product, walnut shells, to break the polymer chains into useful chemicals. The process, called catalytic pyrolysis, yielded promising results, but also revealed a fundamental engineering trade-off that will need to be solved before it can scale.

69% olefins at the sweet spot

Pyrolysis involves heating plastic in the absence of oxygen until its long polymer chains fracture into smaller molecules. Adding a catalyst can steer the breakdown toward specific, more valuable products. The team used biochar derived from walnut shells and activated with phosphoric acid as their catalyst, testing it at three different temperatures.

At 350 degrees Celsius, the results were striking. Olefins, the chemical building blocks used to make new plastics, fuels, and industrial materials, made up about 69% of the liquid products. Total pyrolysis oil yield reached roughly 72%, indicating highly efficient conversion of the waste film.

That temperature, however, came with a cost. The chemical reactions that maximized olefin production also generated oxygen-containing tar that deposited on the catalyst surface, gradually blocking its pores and reducing its effectiveness.

The temperature trade-off

At 400 degrees Celsius, the product mix shifted. Aromatic compounds became more prevalent in the liquid output, and olefin yield dropped. But the tar deposits formed at this temperature were less stubborn. Kinetic analysis showed their decomposition required an activation energy of only 40 to 50 kilojoules per mole, meaning the catalyst could be regenerated more easily through heat treatment.

This creates a practical dilemma for anyone trying to build a working system. The temperature that produces the most valuable chemicals is also the temperature that fouls the catalyst fastest. The temperature that keeps the catalyst cleaner produces less valuable output. Designing a process that balances these competing demands, perhaps by cycling between temperatures or developing catalysts more resistant to tar deposition, is the next engineering challenge.

Walnut shells as catalyst feedstock

Using walnut shell biochar as the catalyst material is itself noteworthy. Conventional pyrolysis catalysts, such as zeolites, can be expensive and energy-intensive to produce. Walnut shells are an abundant agricultural by-product with little existing economic value. If biochar-based catalysts can approach the performance of synthetic alternatives, the economics of plastic waste pyrolysis improve considerably, and the process gains a satisfying circularity: agricultural waste catalyzing the breakdown of agricultural plastic waste.

Laboratory results, not a factory yet

The study is a laboratory demonstration. Real-world agricultural plastic mulch is contaminated with soil, moisture, and sometimes pesticide residues, all of which could affect catalyst performance and product purity in ways the controlled experiments did not test. Scaling pyrolysis from bench to industrial operation introduces challenges in heat management, continuous feed systems, and product separation that the paper does not address.

The catalyst deactivation problem also needs longer-term testing. Showing that tar deposits can be removed at 400 degrees Celsius is different from proving that a catalyst can survive hundreds of regeneration cycles without degrading.

Still, the core finding is valuable. It links product chemistry, tar composition, and kinetic behavior in a way that gives future engineers concrete parameters to work with. The ideal catalytic pyrolysis system for farm plastic waste may not exist yet, but this study maps the terrain it will need to navigate.