Upcycling of epoxy resin from waste wind turbine blades: Pyrolysis-methylation tandem reaction to methylated phenols and hexamethylbenzene

The pyrolysis products of epoxy resin consist of mixed phenolic compounds, which can be transformed into methylated phenolic compounds, pentamethylbenzene, hexamethylbenzene, and other high-value chemicals through methylation. However, the mechanism of methylation for pyrolysis products of epoxy resin remains unclear. In this study, using thermosetting epoxy resin and waste wind turbine blades as feedstocks, we investigated the methylation performance of different solid acid catalysts (γ-Al2O3, α-Al2O3, ZSM-5, MCM-41) through a pyrolysis-methylation cascade reactor. The study unveils the impact of reaction temperature and reaction time on the pyrolysis reaction and the product distribution. The optimal reaction parameters are determined to be 500 °C and a reaction time of 20 min. Under these conditions, the reforming products exhibited the highest selectivity for methylated phenolic compounds and hexamethylbenzene (42.57 wt.%). The proposed methyl substitution reaction pathway for isopropenylphenol was supported by a13C methanol isotope experiment. On nanoscale γ-Al2O3 catalyst, the methylation reaction displays a non-position-selective methylation pattern. Methyl substitution occurs by directly replacing the methylating agent on the isopropyl/isopropenyl group, leading to the breaking of the Carom-Caliph bond. Additionally, Density Functional Theory (DFT) simulations reveal that methyl serves as an intermediate in the methylation substitution reaction. The study also conducted a material flow assessment for the recycling of epoxy resin waste through the pyrolysis-methylation pathway.