Pyrolysis mechanism of engineering plastic waste under carbon dioxide

Abstract

This study explored carbon CO2-assisted catalytic pyrolysis as a sustainable method for converting polybutylene terephthalate (PBT) waste into syngas under moderate-temperature conditions (≤700 ˚C), providing a more energy-efficient alternative to conventional gasification. CO2 was adopted as a reactive medium to enhance gas-phase carbon conversion and reduce the environmental footprint. The results showed that CO2-assisted catalytic pyrolysis effectively suppressed the formation of condensable pyrogenic products, reducing the liquid yields from 67.91 wt% to 10 wt%, and boosted CO production by up to 65-fold compared to that of one-stage pyrolysis under inert conditions. To elucidate the fundamental reaction pathways, terephthalic acid (TPA) was utilized as a model compound representing the core aromatic backbone of PBT. Mechanistic analysis showed that CO2 activation on the Ni catalyst surface generated reactive intermediates (CO2•⁻ and M-COO⁻), which promoted selective bond cleavage and deoxygenation reactions, efficiently converting heavy oxygenates into syngas. Moreover, CO2 played a dual role by also suppressing coke formation, thereby preserving the catalyst performance. An environmental assessment indicated that the catalytic pyrolysis of 1.8 million tonnes of PBT waste could lead to a net reduction of 6 million tonnes of CO2 emissions compared to that under incineration, which would release nearly 4 million tonnes. These findings highlight the use of CO2-assisted catalytic pyrolysis as a carbon-negative and circular solution that transforms plastic waste and CO2 into valuable products.