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

Selective conversion of guaiacol over MoO3 to produce various allcylphenols, 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 degrees C for 4h 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 alpha-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 (MoOxCyHz) 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. (C) 2017 Elsevier B.V. All rights reserved.