Selective guaiacol ethanol alkylation to produce pentaethylphenol on a WO₃/HY catalyst

The alkylation of lignin-derived phenolic monomers with alcohols is a key step in upgrading the lignin oil from lignin depolymerizations, yielding stable alkylphenols as final products with high heating value. In this work, ethanol alkylation of guaiacol is investigated with a Y zeolite-supported tungsten oxide catalyst, achieving a complete conversion of guaiacol and a 52 % total yield of alkylphenols. Based on monitoring the product distribution, a reasonable mechanism is proposed. Lower-grade ethyl-alkylated guaiacol and 2-ethoxyphenol are firstly produced, and their Ar-O bonds are readily cleaved when the number of ethyl groups attached to the benzene rings reaches or exceeds three, leading to the formation of higher alkylphenols. The catalyst characterization indicates that weak and medium-strong Lewis acidic sites are active sites, which promote the formation of carbocation intermediate for alkylation. An adsorption-desorption mechanism facilitated by the Lewis acid sites is also proposed. The interaction between oxygen functional groups and acid sites contributes to adsorption, while steric effects from ethyl groups promote desorption. The number of oxygen functional groups and ethyl groups significantly influences the adsorption-desorption process.