Microwave-Assisted Pyrolysis Fishbone Catalyst Turns Agricultural Plastic Waste into Olefin-Rich Bio-Oil

A catalyst produced from discarded fish bones could provide a low-cost route for recycling agricultural plastic waste. In laboratory tests, the material converted mulch film into a high yield of olefin-rich bio-oil through microwave-assisted pyrolysis.

A research team developed an acid–metal bifunctional catalyst made from discarded fish bones that efficiently converted waste agricultural film into olefin-rich bio-oil through microwave-assisted pyrolysis. Under optimized conditions, the catalyst produced a bio-oil yield of 89.32 wt.%, with olefins accounting for 84.03% of the oil and C6–C12 compounds dominating the hydrocarbon fraction. By simultaneously valorizing fish-processing residues and difficult-to-recycle plastic mulch, the approach could support more economical plastic recycling and provide a sustainable route to high-value hydrocarbon feedstocks.

Low-density polyethylene mulch film is widely used to conserve soil moisture, regulate temperature, suppress weeds, and improve agricultural productivity. However, residual films can accumulate in soil, generate microplastics, and cause persistent environmental pollution. Pyrolysis can recover their carbon and hydrogen as fuels or chemicals, but conventional thermal processes often require high temperatures and produce poorly controlled mixtures of gases, oils, waxes, and coke. Although acidic and metal-supported catalysts can improve plastic cracking, simultaneously achieving high liquid yields, strong olefin selectivity, low coke formation, and catalyst stability remains challenging. Low-cost catalyst supports derived from biological waste may offer a solution, but the combined effects of acid modification and metal loading require further investigation.

A study published by Yunfeng Zhao's team, Shihezi University, reports that a phosphoric-acid-treated, iron-loaded fishbone catalyst enabled high bio-oil production and selective formation of light olefins.

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The researchers prepared the catalyst from whitefish bones by first immersing the pretreated material in phosphoric acid solutions ranging from 0 to 40 wt.%. After washing and drying, they loaded the modified bones with 0–30 wt.% iron using ferric nitrate and carbonized the resulting precursors at 600 °C under nitrogen using 800-W microwave heating. The catalysts were characterized through X-ray diffraction, X-ray photoelectron spectroscopy, electron microscopy, infrared spectroscopy, nitrogen adsorption, temperature-programmed desorption and reduction, and elemental analysis. These tests showed that phosphoric acid disrupted the crystalline hydroxyapatite structure and introduced Brønsted and Lewis acid sites, while iron loading created highly dispersed nanoscale iron species and stable Fe–O–P interfacial bonds.

The team then tested the catalysts in a two-stage fixed-bed reactor containing separate pyrolysis and catalytic zones. Low-density polyethylene mulch film was heated under nitrogen for 40 minutes, and the resulting vapors passed through the catalyst before condensation. The researchers compared untreated fishbone char, phosphoric-acid-modified char, iron-loaded acid-modified char, and non-catalytic pyrolysis. They also varied the pyrolysis temperature from 450 to 650 °C, catalytic temperature from 300 to 500 °C, and catalyst-to-feedstock ratio from 0:1 to 3:1. Non-catalytic pyrolysis produced 72.3 wt.% bio-oil with 43.78% olefin selectivity. Acid modification increased olefin selectivity to about 80%, while the optimized 20Fe–30P@FC catalyst raised the bio-oil yield to 89.32 wt.% and olefin selectivity to 84.03%. Of the hydrocarbons produced, 99.78% fell within the valuable C6–C12 range, while coke yield was only 0.43%. The optimal operating conditions were a pyrolysis temperature of 550 °C, catalytic temperature of 350 °C, and catalyst-to-feedstock ratio of 1:2. After five regeneration cycles, bio-oil yield remained at 86.52 wt.%, olefin selectivity at 82.32%, and C6–C12 selectivity at 99.11%, demonstrating relatively stable performance.

Overall, the study shows that phosphoric acid and iron work cooperatively to regulate plastic cracking and dehydrogenation. Acid sites generate carbocation intermediates and promote controlled carbon–carbon bond cleavage, while neighboring iron sites facilitate hydrogen transfer, olefin formation, and rapid product desorption. This interaction suppresses excessive cracking, aromatization, and coke formation. Although further work is needed to improve long-term resistance to carbon deposition and evaluate larger-scale operation, the catalyst provides a promising, low-cost strategy for transforming both fishbone waste and discarded agricultural films into valuable light hydrocarbons.

Original Article: Iron and phosphoric acid co-modified fishbone char for olefin-rich bio-oil production from waste agricultural films via microwave-assisted pyrolysis; Sustainable Carbon Materials; https://doi.org/10.48130/scm-0026-0021

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