Pyrolysis can turn plastics recovered from end-of-life vehicles (ELVs) into a liquid fuel in the diesel range, and the catalyst used has a large effect on how much oil the process yields — but much less on what that oil actually contains. That is the central finding of a study that compared thermal and catalytic pyrolysis of automotive plastic waste (APW) recovered directly from vehicle components, using a low-cost iron-pillared clay as the catalyst.
Recycling plastics from ELVs is an important part of improving resource recovery and supporting more circular use of plastics. Among the available technologies, pyrolysis has emerged as a promising route for recovering fuels and chemicals from waste polymers. The process thermally decomposes plastics at elevated temperatures without oxygen, breaking polymer chains into liquid, gaseous and solid products.
Previous studies have shown that pyrolysis can convert common automotive plastics — low-density polyethylene (LDPE), high-density polyethylene (HDPE) and polypropylene (PP) — into valuable hydrocarbons at relatively high yields. Much of that research, however, focused on automotive shredder residue, a heterogeneous mixture containing plastics, metals, textiles and other materials. That leaves less information about how plastics recovered directly from components such as dashboards, bumpers and headlamps behave during pyrolysis.
The distinction matters because APW can also contain mineral fillers, flame retardants, fiberglass and other additives that affect degradation behavior and product composition, shaping both how the material breaks down and what the resulting liquid looks like.
Characterizing the feedstock
To address that gap, the researchers characterized plastics recovered directly from ELVs before performing pyrolysis, rather than relying on mixed shredder residue. The feedstock contained acrylonitrile butadiene styrene (ABS), high- and low-density polyethylene (HDPE and LDPE), polypropylene (PP), polystyrene (PS) and fiberglass, and it also carried substantial volatile and ash fractions. Front and rear bumpers accounted for 86.6% of the sample by weight, and the fiberglass commonly used to reinforce automotive components contributed to the relatively high ash content.
Catalyst choice and experimental design
Catalysts can improve pyrolysis efficiency by accelerating reactions, altering product distributions and potentially reducing the required temperature or residence time. Commercial zeolites have been widely studied, but lower-cost clay-based alternatives are drawing increasing interest. Among them, pillared clays modified with metals such as iron, titanium or zirconium have shown potential to increase liquid yields and 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. That process introduces iron species into the clay structure, changing characteristics such as surface acidity and pore structure that can influence pyrolysis reactions. The team then compared thermal and catalytic pyrolysis across temperatures ranging from approximately 350°C to 521°C, at catalyst loadings of roughly 6% to 34% by weight. A statistical experimental design allowed them to evaluate the interaction between temperature and catalyst concentration.
Yield gains, limited compositional change
The catalyst strongly affected liquid yield. Catalytic pyrolysis produced an average oil yield of approximately 68%, compared with 41% for thermal pyrolysis alone. The highest oil yield, 78.6%, was achieved at approximately 416°C with a catalyst loading of about 24%.
Yet the catalyst had much less influence on the composition of the resulting oil. Thermal and catalytic pyrolysis produced similar proportions of hydrocarbons within the C8–C24 range associated with gasoline- and diesel-range compounds. In other words, the catalyst helped convert more of the plastic feedstock into liquid product, but it did not substantially shift that liquid toward a higher proportion of diesel-range molecules.
Temperature, by contrast, had a greater effect on product composition. Lower pyrolysis temperatures favored diesel-range organics, while temperatures above 450°C produced a heavier, waxier oil. That suggests operating temperature is a more useful lever than catalyst loading when the goal is to steer the molecular makeup of the product.
Fuel properties and their limits
The resulting pyrolysis oil showed several properties comparable to those of 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. Branched heptenes and heptanes dominated the oil, compounds also identified in previous studies of polypropylene and automotive-plastic pyrolysis.
However, not all properties met diesel specifications. Both density and viscosity exceeded the limits typically specified for diesel fuel, which the researchers attributed partly to heavier, long-chain hydrocarbon fractions remaining in the pyrolysis oil. That gap means additional processing could be required before the oil can be used as a diesel-range fuel. The researchers suggested approaches including distillation and centrifugation, as well as optimizing reaction conditions through lower temperatures and longer residence times, to reduce heavier fractions and improve fuel properties.
Path to wider application
The study positions pyrolysis as a technically promising route for recovering value from automotive plastics, while also highlighting the additional development required before the process can move toward broader commercial application. Future work could investigate 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.
Read together, the results suggest that a cheap, iron-modified clay can meaningfully increase how much liquid comes out of automotive plastic pyrolysis, but that turning that liquid into a specification-grade diesel substitute still depends on temperature control and downstream upgrading rather than on the catalyst alone.