Producing aromatic-rich oil through microwave-assisted catalytic pyrolysis of low-density polyethylene over Ni/Co/Cu-doped Ga/ZSM-5 catalysts

Abstract

Microwave (MW)-assisted catalytic pyrolysis represents a promising method for transforming petroleum-based plastic waste into valuable chemicals, offering a pathway towards more sustainable circular economy. In this study, catalytic pyrolysis of low-density polyethylene (LDPE) was conducted under MW irradiation. The influence of various catalyst types (HZSM-5, Ga/ZSM-5, Ga/Ni/ZSM-5, Ga/Co/ZSM-5, and Ga/Cu/ZSM-5) on product yield and distribution was examined. The results revealed that the Ga/ZSM-5 catalyst yielded the maximum liquid oil, approximately 41%. Ga/Ni/ZSM-5 performed excellently in the production of long-chain olefins, constituting about 27% of the liquid fraction. However, Ga/Co/ZSM-5 led to the production of heavy pyrolysis oil containing nearly 25% long-chain paraffins, rendering it unsuitable for producing high-value chemicals. Conversely, the Ga/Cu/ZSM-5 catalyst yielded an aromatic-rich pyrolysis oil, with benzene derivatives constituting approximately 90% of the liquid oil fraction, thus proving to be a suitable catalyst for the intended application. The liquid product distribution was compared with a petroleum assay by SimDist, and this suggested that utilizing the HZSM-5 catalyst could yield an 86.4% naphtha fraction. The study also revealed that the Ga/Cu/ZSM-5 catalyst generated the largest amounts of hydrogen and syngas, as determined by a MicroGC analysis of the gas products. This catalyst also exhibited the maximum coke deposition (1.35%) postreaction, which was attributed to its high aromatic hydrocarbon content in the pyrolysis oil and maximal hydrogen release. A comparison of fresh and spent catalyst properties was conducted to gain insights into catalyst activity and to correlate the effects of metal doping on product distribution. These findings underscore the potential of MW-assisted catalytic pyrolysis, particularly with the Ga/Cu/ZSM-5 catalyst, for the efficient conversion of plastic waste into valuable chemicals, thereby contributing to sustainable resource utilization and environmental conservation.