Polyethylene accounts for 36 % of all plastics and is resistant to high temperatures, pressure, mechanical force, and chemical corrosion. These properties make it a useful material, but they also make it difficult to recycle. Nickel catalysts can help the recycling process, but refining the metal from its ore causes greenhouse gas emissions, pollutes the environment, and poses health risks to workers. However, nickel can be extracted from metal-contaminated soils that cannot be used for agriculture.
James H. Clark, University of York, UK, and colleagues have used alyssum and willow grown in nickel-rich conditions to make a biologically bound catalyst (Ni-phytocat) using a one-step microwave-assisted pyrolysis process. The resulting catalyst can break down polyethylene or mixed plastic waste into hydrocarbons and hydrogen at around 250 °C, using microwaves instead of thermal depolymerization processes that require temperatures upwards of 400 °C. The method offers greater selectivity and the amount of hydrogen and small aromatic molecules can be adjusted by changing the temperature, time, and amount of nickel.
The single-step process typically takes up to 70 s to transform a sample of low-density polyethylene into liquid hydrocarbons (40–60 % oil yield), hydrogen (11–30 % gas yield), and filamentous carbon (25–37 % solid yield). Ni-phytocat enhanced the production of C6–C12 aliphatics (up to 56 % selectivity) and favored the aromatization of linear alkanes to monocyclic aromatics (up to 33 % selectivity), thereby releasing more H2 (up to 74 % selectivity) as gaseous fractions.
The catalyst is highly effective because the plant takes up nickel from the soil in a nanoparticulate form. This is good for catalysis and difficult to achieve synthetically.
The team tested plants grown on soil where nickel occurs naturally. They would like to investigate how the process could be used on soil contaminated with a mixture of chemicals. They are interested in whether the plants can selectively take up certain chemicals and still end up with a viable catalyst.
- Biologically bound nickel accelerated de-polymerization of polyethylene to high value hydrocarbons and hydrogen,
Parul Johar, Elizabeth L. Rylott, C. Robert McElroy, Avtar S. Matharu, James H. Clark,
RSC Sustainability 2023.