Organocatalysed depolymerisation of PET in a fully sustainable cycle using thermally stable protic ionic salt

Abstract

An innovative temperature-resistant organocatalyst allows the depolymerisation of PET through a solvent-free process, opening routes to green recycling of common plastics.

Abstract

The world's plastic production is continuously and exponentially increasing, creating millions of tons of short-lived items that end as waste and accumulate in the environment. Poly(ethylene terephthalate) (PET) provides one of the best examples as it is a non-biodegradable polymer that is mainly used as raw material for a wide range of packaging applications, making degradation of PET a subject of great interest for researchers. Herein we report a sustainable process for the chemical recycling of PET from waste to a new polymer using an innovative protic ionic salt. Using a simple solvent-free process, post-consumer PET bottles are degraded into bis(2-hydroxyethyl) terephthalate (BHET) monomer. The catalyst, formed by an equimolar quantity of triazabicyclodecene (TBD) and methanesulfonic acid (MSA), completely depolymerises PET in less than 2 h, producing 91% of highly pure BHET. Due to the unusual thermal stability of the TBD : MSA salt, the catalyst can be recycled at least 5 times to depolymerise more PET waste. In addition, we demonstrate that the monomer obtained from the degradation reaction can be used to synthesise new PET with similar thermal properties to that produced using a conventional polycondensation method. The protic ionic salt catalyst combines the excellent catalytic ability of organocatalysts with the thermal stability of metal catalysts, resisting degradation up to >400 °C, thus for the first time presenting an industrially-relevant organocatalyst for high-temperature polymer degradation and recycling.