Heterogeneous single-atom based catalysts to chemo/photo-catalytic depolymerization: a route to circular polymer economy

Abstract

The majority of plastic waste is not recycled. Impediments to the recycling of commodity polymers include separation, impurities and degradation of the macromolecular structures, all of which can negatively affect the properties of recycled materials. An attractive alternative is to transform polymers back into monomers and purify them for repolymerization - a form of chemical recycling we term chemical recycling to monomer (CRM). In general, chemicals are employed to promote the depolymerization, however, catalysts and photocatalysts also can be employed.This proposal aims to promote the depolymerization of commonly used polymers by chemo and/or photo-catalytically employing carbon nitride-based materials as heterogeneous catalysts. Recently, poly(heptazine imides) were applied to host single atoms of different metals. These highly crystalline type of carbon nitrides can be synthesized via a reaction with a carbon and nitrogen source (melamine, urea, cyanamide) and an alkali salt (NaCl, LiCl, KCl). This process generates crystalline carbon nitride structures with alkali ions (Na+, K+, Li+) accommodated within the carbon nitride units. Among different synthesis alkali salts in literature, NaCl was used to produce highly crystalline carbon nitrides with ordered poly(heptazine imide) (PHI) structures with sodium ions (Na-PHI). Prof. Teixeira's group has developed a simple step to exchange these Na+ cations for desirable transitions metals like Ni, Ru, Fe, Pt or protons (acid catalyst).5 Unlike Na+ cations, transitions metals are chemically bonded to N atoms in a carbon nitride structure and cannot be replaced by other elements. Recent results have shown that these transition metals can lead to single-atom sites highly active for a range of reactions. Here, we aim to employ these materials (M-PHI) in the depolymerization of polyesters, polycarbonates or polyamides. All these polymers can be depolymerised by employing strategies, such as glycolysis, alcoholysis and aminolysis, and these processes can be catalysed. This project's main objective is to evaluate the catalytic effect of the M-PHI catalysts in the depolymeration of polyesters, polycarbonates or polyamides. (AU)