Using phosphoric acid-activated walnut shell biochar as a catalyst, the study found that a catalytic bed temperature of 350 °C produced the highest olefin selectivity, reaching 69%, while 400 °C promoted more aromatic formation and generated carbon deposits that were easier to remove.
Waste plastic mulch film is widely used in agriculture but is difficult to manage after use because it persists in soil, degrades slowly, and can release pollutants if treated improperly. Chemical recycling, especially pyrolysis, has attracted growing interest because it can convert long-chain polyolefin plastics into fuels, monomers, and other useful chemicals. Microwave-assisted pyrolysis offers additional advantages for low-thermal-conductivity materials such as low-density polyethylene films, including rapid heating and improved energy transfer. However, catalyst deactivation caused by tar and coke deposition remains a major obstacle, and previous studies have not fully linked catalytic temperature, product selectivity, tar chemistry, and regeneration behavior.
A study (DOI:10.48130/scm-0026-0004) published in Sustainable Carbon Materials on 05 March 2026 by Haiyan Yan’s & Yunfeng Zhao’s team, Shihezi University, reports that catalytic temperature governs the trade-off between maximizing olefin-rich pyrolysis oil and forming carbon deposits that are easier to regenerate.
The researchers first prepared a walnut shell-derived biochar catalyst by washing, drying, grinding, phosphoric acid impregnation, microwave-assisted carbonization, neutral washing, and drying. Waste plastic mulch film collected from farmland was then processed in a microwave-assisted ex-situ catalytic pyrolysis system. In the reactor, 5 g of waste plastic film was heated under nitrogen, while 2.5 g of catalyst was placed in a downstream catalytic reforming zone. The pyrolysis zone was heated to 500 °C, and the catalytic bed was separately regulated at 300, 350, or 400 °C to test how temperature affected product formation and catalyst deactivation. The resulting pyrolysis oil, deposited tar, and spent catalysts were analyzed using gas chromatography–mass spectrometry (GC–MS), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), surface area and porosity measurements, and kinetic modeling. At 300 °C, the catalyst was insufficiently activated, and the reaction was dominated by random cracking of plastic chains. The oil contained a high proportion of alkanes and alkenes, while the deposited tar was mainly composed of heavy hydrocarbons. This condition produced the largest amount of tar, about 0.63 g, and the deposits were relatively stable, indicating pore blockage caused largely by physically retained heavy fragments. At 350 °C, the catalyst reached its most favorable window for producing target liquid chemicals. Olefin selectivity in the pyrolysis oil reached 69%, and the oil yield peaked at 72.0 wt%. The results suggest that acid sites on the catalyst promoted carbocation-mediated β-scission, generating lighter olefin products. However, this same temperature also encouraged side reactions, including esterification, producing oxygen-containing tar that could chemically deactivate active sites. At 400 °C, the process shifted toward deeper catalytic conversion. Aromatic content increased to 18%, gas yield rose to 30.4 wt%, and tar deposition decreased to 0.28 g. Kinetic analysis showed that the tar formed at this temperature had the lowest apparent activation energy, generally in the 40–50 kJ/mol range, meaning the carbon deposits were more reactive and easier to remove during regeneration.
Overall, the study reveals that catalyst performance in plastic mulch film pyrolysis cannot be judged only by the highest yield of desired products. A lower catalytic temperature favors physical tar accumulation, an intermediate temperature maximizes olefin production but causes chemical deactivation, and a higher temperature sacrifices some olefin selectivity while improving catalyst regenerability. By connecting oil composition, tar chemistry, carbon-deposit kinetics, and catalyst deactivation pathways, the work provides a useful framework for designing more selective, stable, and regenerable catalytic systems for agricultural plastic waste upcycling.
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References
DOI
10.48130/scm-0026-0004
Original Source URL
https://doi.org/10.48130/scm-0026-0004
Funding Information
Part of the financial support comes from the Xinjiang Production and Construction Corps Guidance Project (KX009303), the High-level Talent Project of Shihezi University (2022ZD060), the Training Project of Shihezi University (CXPY202212), the Tianchi Talent Young Doctor Project (CZ0023406), and the Key Field Innovation Team Construction Project of Xinjiang Production and Construction Corps (2019CB The financial support was provided by the Research Platform Project of Shihezi University (Project Number: KYPT201904) and the Key Laboratory of Nutrition and Special Food Safety of the Eighth Division of Shihezi City (2022PT02).
About Sustainable Carbon Materials
Sustainable Carbon Materials is a multidisciplinary platform for communicating advances in fundamental and applied research on carbon-based materials. It is dedicated to serving as an innovative, efficient and professional platform for researchers in the field of carbon materials around the world to deliver findings from this rapidly expanding field of science. It is a peer-reviewed, open-access journal that publishes review, original research, invited review, rapid report, perspective, commentary and correspondence papers.