(385x) Acid-Catalyzed Decomposition of Polypropylene into Naphtha in Hydrocarbon Solvent

Focusing on the chemical recycling of waste plastics, especially polyolefins such as polyethylene and polypropylene, we aim to develop a catalytic conversion process of waste plastics into petrochemical feedstock. The challenges in chemical recycling of waste plastics are the extremely high viscosity of melted plastic, which makes it difficult to handle as a fluid in the process, the associated poor heat conduction and mass transfer in the reactor, and the need to remove various foreign substances such as fillers. In addition, it is generally known that pyrolysis produces a large amount of light oil components. To address these issues, we propose the use of hydrocarbon solvents to improve flowability as a fluid, removal of solid foreign substances, and conversion to petrochemical feedstock (C5-C9 products) by catalytic cracking using zeolites, assuming that the petroleum refining process would be utilized.

In this study, polypropylene (PP) was used as model plastic and zeolite beta as the catalyst. Hydrotreated light gas oil (RBC-LGO) and light cycle oil (RBC-LCO) were used as petroleum solvents. RBC-LGO had a higher content of aliphatic hydrocarbons, while RBC-LCO had a higher content of aromatic hydrocarbons. In addition, n-cetane (n-C16) and 1-methylnaphthalene (1-MN) were used as solvents representing linear alkanes and aromatic hydrocarbons. The decomposition behavior of PP in these petroleum solvents, e.g., PP conversion, was studied.

Decomposition test was conducted in a batch reactor at 673 K for 60 min using 20 g of solvent, 5 g of PP, and 1 g of beta zeolite. As shouwn Figure 1, while the levels of PP conversion was 97.0% and 58.4% when RBC-LGO and RBC-LCO were used as solvents, those were 96.9% and 68.2% when n-C16 and 1-MN were used as solvents, suggesting that aromatic hydrocarbons had a negative effect on the PP decomposition because the high aromatic hydrocarbon content in LCO inhibited the reaction of aliphatic hydrocarbons within the micropores of zeolite beta. The yields of C2-C9 petrochemical feedstock were 60-70%, depending on the reaction conditions, indicating that zeolite is a suitable catalyst for producing petrochemical feedstock. As a comparison, amorphous silica-alumina was also used as a catalyst, but the yield of petrochemical feedstock was low. Therefore, it was found that aliphatic hydrocarbon-rich solvents such as LGO are suitable for the cracking of PP.