Catalytic pyrolysis of biodegradable plastic in CO2 atmosphere using MSW incinerator bottom ash for PLA monomer recovery

Abstract

This study proposes a method for recovering value-added monomers from abandoned biodegradable plastics such as used biodegradable straws (UBSs) by utilizing a catalyst made of bottom ash generated from municipal solid waste (MSW) incinerators. The MSW-derived bottom ash (MSW-BA) catalyst primarily comprises alkaline metal oxides such as CaO (58.88 wt%). Thermogravimetric and single-shot pyrolysis analyses show that polylactic acid (PLA) monomers—lactide and lactic acid—are the main products of UBS pyrolysis. We conducted catalytic pyrolysis of UBS at 500 °C under different atmospheres (N₂ and CO₂), with and without the MSW-BA catalyst. Compared with non-catalytic pyrolysis, the MSW-BA catalyst-based pyrolysis significantly increased the yield of PLA monomers in both N₂ and CO₂ environments. This is because the base sites present on the MSW-BA catalyst promoted the polymeric bond cleavage of PLA. The MSW-BA catalyst in CO₂ achieved the highest recovery yield of PLA monomers (20.84 wt% per feedstock mass basis), which was approximately 17.75 % higher than that during non-catalytic UBS pyrolysis. The base sites present on the MSW-BA catalyst promoted polymeric bond cleavage of PLA, and the base-catalyzed pyrolysis of PLA was enhanced by the more reactive cleavage of the bond linkages in the presence of CO₂. The proposed approach not only reuses biodegradable plastic waste but also utilizes the MSW treatment byproducts.