Catalytic hydrothermal conversion of polypropylene

Abstract

This work reports preliminary-stage studies demonstrating the highly selective conversion of polypropylene to short-chained olefins via hydrothermal catalytic processing. The production of such monomer species from waste polymers is highly desirable and has the potential to play a key role in plastics recycling and in the circular economy more broadly. Addition polymers such as polypropylene are, however, known to be particularly challenging to recycle into propene or other short-chained alkenes. Herein, we have compared and contrasted acid- and base-catalysed hydrothermal processing at a temperature of 360 °C, analysing the products of reaction by gas chromatography – mass spectrometry (GC-MS) and Fourier transform infrared (FTIR) spectroscopy. Employing the basic catalyst K2CO3, results in a 95 % yield of gas-phase products of which 52 % are propene and 9 % butenes on a mole basis. The solid acid catalyst HZSM-5 also selectively yields gaseous products, 21 % of which are propene and 22 % are butenes, the reaction proceeding via β-scission of the starting oligomers. These results represent a potential step-change in the production of monomer units from addition polymers, highlighting the potential value of catalytic hydrothermal processing in the field of polymer recycling.