The study on citric acid-modified HUSY catalyzed alkylation of phenol with cyclohexanol

Lignin-derived oxygenated aromatics, particularly phenols and aromatic ethers obtained through depolymerization, represent promising feedstocks for synthesizing high-density and high-heat-sink aviation fuels via alkylation-hydrogenation processes. This study systematically evaluates the catalytic performance of various zeolites (Hβ, HZSM-5, MCM-41 and HUSY) in the alkylation reaction of phenol with cyclohexanol. Characterization results demonstrate that HUSY zeolite showed superior catalytic activity compared to other zeolites, attributable to its favorable pore architecture and well-balanced acid site distribution that synergistically facilitate molecular diffusion and catalytic transformations. To further enhance the catalytic properties, HUSY zeolite was modified with citric acid at various concentrations and compared with those treated with NaOH and oxalic acid. The results revealed that citric acid treatment preserved the crystallinity of the zeolite while modulating its acid distribution and pore structure. All modified zeolites enhanced phenol alkylation activity. Notably, the HUSY-0.5M catalyst, which exhibited the highest medium-strong acid to total acid ratio, achieved superior catalytic performance, 80.4% conversion of phenol and 99.6% selectivity for alkylation products. The catalyst also exhibited high activity in the alkylation of various lignin-derived compounds, demonstrating its broad applicability. This work provides a new strategy for the valorization of lignin-derived phenols into high-value fuel precursors through alkylation.