Ex-situ catalytic fast pyrolysis of waste polycarbonate for aromatic hydrocarbons: Utilizing HZSM-5/modified HZSM-5

Abstract

Catalytic pyrolysis has been proven as a highly efficient process for transforming waste polycarbonate (PC) into valuable aromatic hydrocarbons, especially benzene, toluene, ethylbenzene, and xylene (BTEX). In the present work, the ex-situ catalytic fast pyrolysis of PC on metal- or alkali-modified HZSM-5 was investigated using the double micro-fixed bed system. Experimental results indicate that the rapid pyrolysis of PC may follow a homolytic chain scission mechanism, resulting in pyrolysis products dominated by phenols. Metal- or alkali-modified HZSM-5 significantly enhances the cleavage of C-OH bonds in phenols, improving the yield of aromatics. The yield of aromatics in the liquid product is 91%, 72%, and 77% when loaded with nickel (Ni), iron (Fe), and cobalt (Co), respectively, all of which are higher than unmodified HZSM-5 with the yield of 48%. Among them, the Ni/HZSM-5 has significantly enhanced the yield of BTEX reaching 81.64%. When using alkali-modified HZSM-5 with a larger pore volume and higher specific surface area, the yield of aromatics can be increased to 60%. Furthermore, the dual modification of alkali treatment and Ni loading on HZSM-5 simultaneously enhanced catalytic activity and resistance to coke formation. The yield of BTEX reaches 94.47%. The BTEX formation mechanism can be explained by direct deoxygenation (DDO) of the pyrolytic volatiles of PC and the “phenol pool” mechanisms of primary products on the catalyst surface. This study enriches the understanding of catalytic pyrolysis of PC and provides a theoretical foundation for the harmless treatment of PC.