In a recent article published in the journal Sustainable Carbon Materials, researchers defined operational parameters that maximize olefin production, a useful chemical feedstock, while minimizing catalyst fouling, thereby advancing sustainable plastic waste valorization.
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Background
Previous efforts in plastic waste conversion have shown that catalytic pyrolysis can significantly alter product profiles compared to thermal pyrolysis alone.
Biochar derived from agricultural residues has emerged as a promising catalyst owing to its porosity, surface chemistry, and functionalization potential.
Nonetheless, catalyst deactivation, mainly due to tar deposition, remains a major obstacle. Tar, a complex mixture of heavy hydrocarbons and oxygen-containing species, accumulates on catalyst surfaces and blocks active sites.
Understanding the relationship between catalytic conditions, particularly temperature, and tar formation is crucial for optimizing these processes. The current study leverages kinetic modeling and comprehensive chemical analyses to elucidate the mechanisms underlying tar evolution and catalyst performance under varying thermal regimes.
The Current Study
Waste plastic mulch film and walnut shells were prepared as feedstock and catalyst precursor, respectively.
The biochar catalyst was activated with phosphoric acid to increase the density of acid sites. Ex-situ catalytic pyrolysis was carried out at three different bed temperatures: 300, 350, and 400 degrees Celsius.
Product gases, pyrolysis oils, and deposited tars were collected and characterized. Gas chromatography-mass spectrometry identified chemical species within the oils and tars, while Fourier transform infrared spectroscopy provided insights into surface functional groups on the catalyst. Thermogravimetric analyses assessed the thermal stability and decomposition behavior of deposited tars.
Pyrolysis kinetics were studied using model-free methods - Flynn-Wall-Ozawa, Kissinger-Akahira-Sunose, and Starink - to calculate apparent activation energies and develop a mechanistic understanding. Surface area and porosity measurements monitored catalyst pore blockage post-reaction. Ultrasonic extraction was employed to recover tar from spent catalysts for detailed chemical examination.
Results and Discussion
Pyrolysis at 300 degrees Celsius yielded a pyrolysis oil rich in aliphatic hydrocarbons - mainly alkanes and alkenes constituting 81 percent - while producing a smaller aromatic fraction of about 7 percent.
The tar deposited on the catalyst under these conditions was dominated by heavy hydrocarbons, including long-chain alkenes and waxes, physically retained within catalyst pores, resulting in substantial pore clogging and deactivation. This outcome reflected limited catalyst activation at this lower temperature, with the process largely controlled by free-radical chain scission from thermal cracking.
Increasing the temperature to 350 degrees Celsius shifted the product distribution favorably toward olefins, reaching a maximum olefin content of 69 percent in the pyrolysis oil, accompanied by a moderate 15 percent aromatics presence.
At this intermediate temperature, the catalyst acid sites were sufficiently activated to promote carbocation-mediated β-scission reactions, generating light olefins predominantly in the C5–C12 range. However, this condition also fostered in-situ esterification and condensation reactions leading to oxygen-containing tar.
The tar composition showed an intermediate carbon-number distribution and oxygenated compound content between those observed at 300 and 400 degrees Celsius. Although the catalyst’s cracking activity improved, chemical deposition of oxygenated macromolecular species began to contribute to deactivation, balancing the benefits of high target product selectivity against catalyst fouling.
At 400 degrees Celsius, a pronounced increase in gas yield was recorded, along with a shift in pyrolysis oil composition: olefin selectivity decreased to 60 percent, but aromatic compounds increased to 18 percent - highest among the tested temperatures.
The catalyst exhibited enhanced cracking, aromatization, oligomerization, cyclization, and dehydrogenation activities, attributable to stronger acid site activation at this elevated temperature. Importantly, the tar deposited under these conditions contained relatively more reactive oxygenated esters, formed through catalyst-surface interactions.
Thermogravimetric analysis revealed that this tar had the lowest apparent activation energy (40-50 kJ/mol), indicating it was more amenable to thermal decomposition and catalyst regeneration than tars formed at lower temperatures. Despite more chemical deactivation pathways triggered by oxygenates, the nature of deposits at 400 degrees Celsius improved the catalyst's ability to be regenerated effectively through coke oxidation.
Conclusion
This research underscores the pivotal role of catalytic bed temperature in directing product distribution and catalyst deactivation during the pyrolysis of waste plastic mulch films over phosphoric acid-activated walnut shell biochar.
While moderate temperature conditions optimally produce light olefins via activated acid sites, they also induce oxygen-rich tar formation that chemically poisons the catalyst.
Elevated temperatures enhance aromatization and generate coke deposits that, though chemically complex, are more readily oxidized and removed, preserving catalyst function.
The study reveals an inherent compromise between maximizing target chemical production and sustaining catalyst performance, thereby guiding future strategies for designing more selective and regenerable catalytic systems for plastic waste valorization. By integrating chemical composition, thermal stability, and kinetic assessments of tar deposits, the investigation advances understanding of the intricate interplay between reaction conditions, product formation, and catalyst longevity, contributing to improved sustainable plastic recycling technologies.
Journal Reference
Tan C., Lan X., et al. (2026). Controlling catalyst deactivation: temperature regulation for the directed synthesis of easily regenerable and refractory tar in the pyrolysis of waste films. Sustainable Chemistry & Materials 2: e008. DOI: 10.48130/scm-0026-0004, https://www.maxapress.com/article/doi/10.48130/scm-0026-0004