Heterogeneous transfer dehydrogenation for catalytic upcycling of polyethylene

Promovendus/a: Tim de la Croix

Promotor(en): Prof. dr. ir. Dirk De Vos, Prof. dr. Dimitrios Sakellariou

Since the 1950s, production of plastics as cheap and versatile materials has grown rapidly, and enormous amounts of plastic waste are generated as a result. Landfilling or incineration of plastic waste should be avoided for a more sustainable world, focusing instead on recycling. Unfortunately, simple recycling techniques, based on shredding and re-melting plastics, tend to struggle with “downcycling”, where recycled materials are of poorer quality than the original plastics.

In this work, a chemical recycling route for polyethylene, a common plastic, is explored, with the goal of converting plastic waste into new building blocks for the plastic industry.

First, the concept of the transfer dehydrogenation was studied. The main problem with polyethylene when it comes to chemical recycling, is that it consists entirely of a chain of mostly inert C-C single bonds, saturated with hydrogen. However, by removing some hydrogen, more reactive C=C double bonds are created in the polymer chains, forming sites where the chains can be cut through further chemical reactions. However, the process of removing this hydrogen, the dehydrogenation reaction, is energetically very unfavorable and would require very high temperatures. To compensate for this, a hydrogen acceptor is added, allowing for hydrogen to be transferred in a far more favorable net reaction. For this reaction, heterogeneous, i.e. solid, platinum catalysts were used, allowing for easy separation of the catalyst from the reaction products.

Finally, the heterogeneous transfer dehydrogenation was applied to several polyethylene feeds, with further optimizations of the catalyst and conditions. The products of these reactions, polymer chains containing the reactive double bonds, were then subjected to metathesis, a reaction which breaks the double bonds in the chain and replaces them with a new chemical functional group, like esters. This yields shorter chains with functional ends, which can be used to make new polymers. This process thus promotes carbon circularity, and allows polyethylene waste to be reused as a resource in plastic production.