Exploring catalysis for selective depolymerisation of polyolefins

Promovendus/a: Brent Smeyers

Promotor(en): Prof. dr. ir. Bert Sels

Plastics are indispensable materials in our everyday life. They are widely used to package our food and beverages, but also play important roles in our houses, cars, and electronic devices. While plastics used in construction or electronics can last for decades, packaging plastics are often used once and disposed. These single-use plastics, mostly made of polyethylene (PE) and polypropylene (PP), consist of a long chain of smaller building blocks, named ethylene and propylene, respectively. Nowadays, these building blocks are mainly produced from crude oil, a fossil resource that is slowly running out.

The large volumes of plastic packaging waste should be managed properly. Unfortunately, plastic waste often ends up in landfills or is burned, releasing harmful greenhouse gases like carbon dioxide (CO₂) and other toxic gases. This type of waste management is part of an unsustainable linear economy where the material is lost. More appealing alternatives are found in recycling. Mechanical recycling, where plastics are cleaned, melted, and reshaped into new products, only works for relatively clean plastic streams. Alternatively, chemical recycling can convert contaminated plastics to their original building blocks, which can then be reused to make new plastics. One promising method of chemical recycling is catalytic pyrolysis. In this process, plastic is heated to high temperatures (up to 500 °C) in the absence of air, which causes the plastic chain to break into smaller compounds. A catalyst, which is a material that speeds up chemical reactions, assist in breaking the plastic chains, which makes the process more efficient, reducing energy consumption. Besides, the catalyst increases the selectivity to desired products.

Although plastics show similarities with crude oil, they behave differently during processing. This brings up the need for new catalysts specifically tailored to plastics. Development of new catalysts is preferably performed at milligram-scale to save time, reduce costs, and allow rapid tests. Therefore, thermogravimetric analysis (TGA) and micropyrolysis are typically used. Unfortunately, research so far has been scattered and is inconsistent, with studies using different methods, making it hard to compare results. This study revisits the milligram-scale methodologies to evaluate catalysts for converting PE and PP. Catalyst performance is captured by introducing descriptors that assist the researcher with the interpretation of results. The aim is to align data quality over the worldwide community. Moreover, selected catalysts are also tested in a gram-scale reactor (x1000 in scale) to verify if the trends from milligram-scale tests can be translated to larger scale setups.

The work starts with screening more than sixty potential catalysts, from which two case studies are selected. The first case illustrates the relevance of the methodology in a context of catalyst design and optimization, where the aim is to make the catalyst more active, while maintaining high stability and selectivity to the plastic building blocks. The second case study focuses on two other catalyst classes, where the methodology is used to assist in selecting a suitable catalyst for making the same building blocks, but this time via a two-step process.

This research demonstrates how small-scale testing, when done in a consistent and thoughtful way, can play a major role in the development of smarter recycling technologies. It supports the transition towards a circular plastic economy, where materials are reused instead of discarded, protecting our environment and reducing reliance on fossil fuels.