From Waste Polyolefins to Benzene, Toluene, and Xylenes: Model Studies Using <i>n</i> -Hexane and Amines

Catalytic pyrolysis of waste polyolefins is an attractive technology for obtaining bulk chemicals such as benzene, toluene, and xylenes (BTX). However, impurities in the feed, such as nitrogen-containing compounds, may negatively affect the performance of acidic zeolites, which are typically used as catalysts for the aromatization of intermediates. Such nitrogen-containing compounds may arise from the presence of polyamides, polyurethanes, and protein residues in the waste polyolefin feed. We report here an experimental model study in a packed-bed reactor using n-hexane as the model feed and H-ZSM-5(23) as the catalyst (550 °C, WHSV = 20 h–1), along with comparative experiments with n-hexane in combination with 3 model amines (1-hexylamine, triethylamine, and aniline). The experiments with n-hexane showed some catalyst deactivation, as expressed by a reduction in n-hexane conversion (from 94 to 85% after 5 h of time on stream). Analyses of spent catalysts revealed that this is due to coke formation on the catalyst, resulting in reductions in the acidity and surface area. The presence of an amine (1 wt %) in the feed resulted in a lower initial catalyst activity (lower n-hexane conversion) and a substantial decrease in the BTX yield, the extent of which depends on the type of amine. Additional experiments starting with different feeding protocols (pure hexane, followed by feeding with n-hexane/amine mixtures and finally with pure n-hexane again) showed that the reduction in n-hexane conversion due to the addition of an amine is not because of irreversible catalyst deactivation but most likely arises from competitive adsorption of amines and hydrocarbons on active sites of the catalyst.