Phenolic transalkylation over Y zeolites: Brønsted–Lewis acid balance and suppression of polycondensation

Transalkylation of dimethylphenols with phenol is an atom-economical route to upgrade coal-tar phenolics to cresols, yet poor isomer separation and limited molecular insight into competing pathways hinder optimization. We developed a GC method using a diisodecyl phthalate (DIIDP) capillary column that achieves baseline separation of cresol, dimethylphenol, and ethylphenol isomers for quantitative mechanistic analysis. FAU-type Y zeolite was identified as the optimal catalyst for 2,4-dimethylphenol (2,4-DMP) transalkylation. Tuning the silica-to-alumina ratio (SAR) revealed a relationship between Brønsted/Lewis acid balance, cresol selectivity, and coke formation. The combined acid-site characterization and in situ DRIFTS results support complementary roles of Brønsted acid sites in 2,4-DMP adsorption and Lewis acid sites in phenol adsorption and activation on Y60. A proposed dual-site pathway involving a plausible phenol-derived keto-type intermediate could favor methyl transfer to cresols. Conversely, Y5.3 (SAR = 5.3) exhibits greater carbonaceous deposition, with 9-methylene-9H-fluorene identified in the extract from the spent catalyst. These findings reveal how acid-site composition and strength influence transalkylation and competing coking pathways, offering rational design principles for zeolite-catalyzed phenolic upgrading toward industrial coal-tar utilization.