Deactivation Effects of Plastic Waste Impurities on the Catalytic Aromatization of <i>n</i> -Hexane to BTX

Catalytic pyrolysis of plastic waste is an emerging technology for the chemical recycling of contaminated, mixed plastic waste with high polyolefin contents, yielding valuable bulk chemicals, such as benzene, toluene, and xylene (BTX). However, the presence of other O- and N-containing polymers (e.g., PET, polyamides) and contaminants can have adverse effects on the performance of the zeolite as an aromatization catalyst. In this work, the impact of five contaminants (aniline, benzonitrile, benzoic acid, caprolactam, and phenol) identified in pyrolysis oils originating from contaminated plastic waste on the performance of H-ZSM-5 to produce BTX is studied using a model n-hexane feed. Catalyst performance is assessed over 20 h time on stream (TOS) at a WHSV of 25 h–1, with the impurity fed between TOS 5 and 10 h. Spent and regenerated catalysts are characterized to identify deactivation pathways, which were found to be varying degrees of coke formation and reversible absorption. Benzonitrile and aniline, originating from nitrile rubbers and polyurethanes, showed partial reversible deactivation due to reversible binding to acidic active sites. Phenol, caprolactam, and benzoic acid, originating from resins, polyamides (PA), and poly(ethylene terephthalate) (PET), respectively, showed more irreversible deactivation due to coke formation. Caprolactam, the model component for a nylon-6, was identified as the most problematic for catalytic pyrolysis in terms of catalyst deactivation, showing the most rapid deactivation and no recuperation of performance with n-hexane feeding. This work serves as a guideline for the upstream processing of plastic waste and highlights the importance of proper sorting and feedstock selection for the chemical recycling of waste plastics.