Pyrolysis of plastics recovered from end-of-life vehicles (ELVs) can yield a liquid whose properties come close to those of diesel, but the catalyst used to drive the reaction appears to influence how much oil is formed more than what that oil contains. A study on thermal and catalytic pyrolysis of automotive plastic waste, using an iron-pillared clay instead of a commercial zeolite, examined that trade-off directly.
Recycling plastics from ELVs is seen as an important route to better resource recovery and more circular use of plastics, and pyrolysis — which thermally decomposes polymers at elevated temperatures in the absence of oxygen, breaking chains into liquid, gaseous and solid products — has emerged as a promising way to recover fuels and chemicals from waste polymers. Earlier work showed that common automotive plastics such as low-density polyethylene (LDPE), high-density polyethylene (HDPE) and polypropylene (PP) can be converted into valuable hydrocarbons at relatively high yields. Much of that research, however, focused on automotive shredder residue, a heterogeneous mix of plastics, metals, textiles and other materials.
Component plastics behave differently from shredder residue
That leaves less information about how plastics recovered directly from components such as dashboards, bumpers and headlamps behave during pyrolysis. Automotive plastic waste (APW) can also carry mineral fillers, flame retardants, fiberglass and other additives that affect degradation behaviour and product composition. In the study, the researchers characterised plastics taken directly from ELVs rather than relying on mixed shredder residue. The feedstock contained acrylonitrile butadiene styrene (ABS), HDPE, LDPE, PP, polystyrene (PS) and fiberglass, along with substantial volatile and ash fractions. Front and rear bumpers made up 86.6% of the sample by weight, and fiberglass, used to reinforce automotive components, contributed to the relatively high ash content.
Iron pillaring as a lower-cost catalyst option
Catalysts can raise pyrolysis efficiency by accelerating reactions, shifting product distributions and potentially cutting the required temperature or residence time. Commercial zeolites have been widely studied, but lower-cost clay-based alternatives are attracting growing interest. Among these, pillared clays modified with metals such as iron, titanium or zirconium have shown potential to increase liquid yields and to influence the formation of fuel-range hydrocarbons. Iron-modified clays are particularly attractive because they may improve oil production without relying on more expensive commercial catalysts. Rather than using bentonite clay in its original form, the researchers modified it through iron pillaring, introducing iron species into the clay structure and altering characteristics such as surface acidity and pore structure that can shape pyrolysis reactions.
Yield rises sharply while composition barely moves
The team compared thermal and catalytic pyrolysis across temperatures from roughly 350°C to 521°C and catalyst loadings of about 6% to 34% by weight, using a statistical experimental design that allowed them to evaluate the interaction between temperature and catalyst concentration. The catalyst had a strong effect on liquid yield. Catalytic pyrolysis produced an average oil yield of about 68%, compared with 41% for thermal pyrolysis alone, and the highest oil yield, 78.6%, was reached at approximately 416°C with a catalyst loading of about 24%.
The catalyst's influence on the oil's composition was far weaker. Thermal and catalytic runs produced similar proportions of hydrocarbons in the C8–C24 range associated with gasoline- and diesel-range compounds, meaning the catalyst helped convert more feedstock into liquid but did not substantially shift that liquid toward a higher share of diesel-range molecules. Temperature mattered more for composition: lower pyrolysis temperatures favoured diesel-range organics, while temperatures above 450°C produced a heavier, waxier oil. Branched heptenes and heptanes dominated the oil, compounds also identified in earlier studies of polypropylene and automotive-plastic pyrolysis.
Fuel properties and the gaps that remain
The resulting pyrolysis oil showed several properties comparable to commercial diesel. The researchers reported a calorific value of 49.85 MJ/kg, a flash point of 113°C and total aromatics of 3.55% by area — values within or comparable to ranges specified for conventional fuels. Not all properties met diesel specifications, however:
- Density exceeded the limit typically specified for diesel fuel.
- Viscosity also exceeded diesel limits, which the researchers attributed partly to heavier, long-chain hydrocarbon fractions remaining in the oil.
Additional processing would therefore be needed before the oil could serve as a diesel-range fuel. The researchers suggested distillation and centrifugation, as well as optimising reaction conditions through lower temperatures and longer residence times, to reduce heavier fractions and improve fuel properties.
Toward industrial-scale operation
The study positions pyrolysis as a technically promising route for recovering value from automotive plastics while underscoring the development still required before broader commercial application. Future work could examine different iron concentrations in the catalyst, improve reactor designs and heat transfer, and combine pyrolysis with downstream distillation to separate gasoline- and diesel-range fractions. Improved collection and sorting of automotive plastics will also be important: without systematic removal and classification of plastic components from end-of-life vehicles, reliable data on the volume and composition of available automotive plastic waste remain limited. Better information on these waste streams would help researchers and recyclers assess feedstock availability and plan potential industrial-scale pyrolysis operations.