Selective conversion of guaiacol to substituted alkylphenols in supercritical ethanol over MoO₃

Selective conversion of guaiacol over MoO₃ to produce various alkylphenols, including ethylphenols, isopropylphenols, butylphenols (tert-, sec-) and tert-amylphenol is investigated in ethanol without the addition of gaseous hydrogen. A high conversion of 99% is achieved at 280 °C for 4 h and the total alkylphenols account for up to 94% in the quantified products. Six molecules, i.e. 2,5-diethylphenol, 2,6-diisopropylphenol, 2,4-diisopropylphenol, 2,6-ditertbutylphenol, 2,4-ditertbutylphenol and 2,6-ditertbutyl-4-ethylphenol, are the main outcomes. The higher alkylphenols in the aforesaid products are verified to form via a novel reaction step in which hydrogen atom at the α-carbon of the lower alkylphenol is substituted consecutively with methyl or ethyl groups. Further examination reveals that catechol is formed as the intermediate via demethylation of guaiacol and followed by direct conversion to low alkylphenols without the formation of phenol. Post-catalytic analysis indicates that an oxycarbohydride phase (MoO_xC_yH_z) with Mo⁵⁺ developed in the supercritical alcohol batch system played the role of active species. Ethanol is found to be the most effective solvent for the conversion. The MoO₃ catalyst undergoes a gradual deactivation resulted from the consumption of Mo⁵⁺ and carbon deposition but can be regenerated with a simple calcination.