Catalysis 2025 Issue

polymer chains in the process. An energy-efficient pro - cess, mechanical recycling boasts a low-carbon footprint, minimal environmental impact, and helps reduce landfill disposal. In cases of mixed and/or contaminated plastic streams, these must be sorted and cleaned thoroughly to make a product of good quality – a process that ends up being both time-intensive and costly. However, mechanical recycling is feedstock specific, only accepting polyethylene terephthalate (PET), high-density polyethylene (HDPE), polypropylene (PP), or low-density polyethylene (LDPE) in most cases. Its impact is also limited on a global scale, as it cannot be utilised for hard-to-recycle plastics, which is where chemical recycling comes in. Chemical recycling Also known as advanced recycling, chemical recycling is the process of converting polymeric waste by altering its chem- ical structure and returning it to substances that can be used as raw materials. While introduced to industry decades ago, interest in these recycling technologies and the possibilities they present has been renewed in recent years. Complementing existing plastic recycling methods, chem- ical recycling can better deal with mixed plastic waste streams, like films and laminates, that would otherwise result in incineration or landfill. Examples of these methods include gasification, depolymerisation, hydrocracking, and pyrolysis. Pyrolysis has the potential to convert used plastic waste streams unsuitable for mechanical recycling into high-quality feedstock for even the most sensitive petrochemical industry applications A transformative chemical recycling technology, pyroly- sis has the potential to convert used plastic waste streams unsuitable for mechanical recycling into high-quality feed- stock for even the most sensitive petrochemical industry applications. The process sees plastics collected at the end of their product life cycle and heated to high temperatures (300- 900⁰C) in an inert atmosphere without oxygen. Thermal degradation causes these plastic materials to break down into smaller molecules, in turn, transforming plastic waste into pyrolysis (pyrolytic) oil or gas, which can be repurposed and utilised in the form of reusable crude oils. Suitable for multiple applications, pyrolysis oil can reduce dependence on fossil fuels, presenting a lower carbon solution for hard-to-abate sectors and diversifying energy materials. In petroleum refineries, it can be used as a more sustainable and high-quality feedstock alternative to fos- sil naphtha, including ethylene and propylene production, which are core monomer building blocks of most plastics. Pyrolysis oil can also be used as a fuel to power vehicles and machinery (once refined and blended with conventional fuels), which is particularly beneficial for industries that still

rely heavily on crude oil and natural gas, such as shipping, construction, and manufacturing. It can even replace diesel with regard to engine performance and energy output in certain instances. Primary drivers of increasing pyrolysis capacity are oil and gas corporates, which are expected to utilise most pyrolysis oil as a fuel replacement. Global interest in pyrolysis as a means of managing plas- tic waste can be seen in the development of significant commercial pyrolysis technologies for the processing of plastic waste. In the US, the American Chemistry Council advocates for state and federal policies that support these technologies for recycling and has emerged as a prominent research area in Europe. Large chemical companies are starting to invest in pyrol- ysis oil production. Initial commercial plants vary between 10 and 50 kta; as of 2023, at least nine 10 kta capacity pyrolysis or hydrothermal chemical recycling units were scheduled to come online in Europe. Of those, however, only two are known to have achieved production by the end of the same year. Since 2021, the global input capacity for pyrolysis plants has increased by more than 60%. Even so, the true potential of pyrolysis is still predominantly untapped. Approximately five million tons of plastic waste is currently mechanically recycled in Europe, compared to the 50,000 tons of plastic waste that is chemically recycled. There is clearly a signif- icant opportunity to increase these rates, provided certain challenges can be overcome. Meeting specifications Pyrolysis oil contamination is one such hurdle, affecting purity and composition. Mixed waste plastics are often a complex combination of polymers. The final composition of such products can differ due to regional and country- specific factors. Plastics such as PET and polyvinyl chloride (PVC) can yield oxygenated and chlorinated compounds. These chlorides and their complexity pose an additional issue. Tending to exist in roughly equal concentrations through- out the boiling point range of pyrolytic oil, they attach to hydrocarbons of varying chain lengths and have differing levels of steric hindrance. This can cause corrosion issues in steam cracking furnaces in petrochemical plants and result in plant and equipment breakdown. Steam crackers feature very tight specifications that need to be satisfied if the oil is added as a feedstock. As such, these impurities need to be removed in an economical and sustainable way. Currently, the amount of plastic pyrolysis oil that can be fed into a steam cracker is less than 10%. Therefore, it is not possible to use the oil in steam crackers on any commercially significant scale. Additionally, the products formed from pyrolysis are heavily dependent on both the composition of the feed- stock and the process conditions. Impacts include type of reactor, heat transfer, residence time, heating rate, and temperature. When looking specifically at commercial cat - alytic pyrolysis, a major challenge is improving selectivity, promoting deoxygenation reactions, and reducing catalyst degradation through coke formation.

Catalysis 2025