Mechanistic understanding of the C-C/C-O bonds cleavage-methylation tandem reaction for the conversion of phenolic resins to hexamethylbenzene using γ-Al2O3

The high-value recycling of discarded phenol-formaldehyde resins (PF) remains an unresolved challenge. Herein, we propose a novel approach leveraging γ-Al₂O₃ to convert PF into high-value hexamethylbenzene at a low temperature using a one-pot method. This study explores the degradation capability of PF, methylation reaction efficiency, and hydrodeoxygenation capacity among various cost-effective commercial catalysts: γ-Al₂O₃, ZrO₂, and TiO₂. It reveals the influence of different reaction times on PF pyrolysis and product distribution, and it was found that high value-added hexamethylbenzene exhibited the highest yield (73.33 wt%) with selectivity (75.83%) using γ-Al₂O₃ at 350 °C and 2 h of reaction. Experiments using PF models demonstrate the crucial synergy between γ-Al₂O₃ and C(aryl)-OH in the cleavage of C(aryl)-C(alkyl) bonds and methylation reactions. A pathway for PF C-C/C-O bonds cleavage-methylation tandem reaction is proposed, based on ¹³C methanol isotope experiments. PF undergoes C(aryl)-C(alkyl) bond cleavage to produce phenolic intermediates, which were then methylated; this is accompanied by the cleavage of C(aryl)-OH and C(aryl)-OCH₃, culminating in C-alkylation to form hexamethylbenzene. This research provides new insights into the high-value recycling of PF.