Kinetics of phenol alkylation with tert-butyl alcohol using sulfonic acid functional ionic liquid catalysts

N-methyl imidazole, pyridine and triethylamine based Bronsted acidic ionic liquids with sulfonic acid (–SO3H) functionality were obtained using 1,4-butane sultone. They were tested in a batch reactor for catalytic activity in the tert-butylation of phenol with tert-butyl alcohol (TBA). The reaction time, reaction temperature and reactant mole ratio were optimized. Among the three ionic liquids, triethylammonium based sulfonic acid functionalized ionic liquid was found to be the most promising and gave the highest phenol conversion. A maximum phenol conversion of 86% with 57.6% selectivity to 4-tert-butylphenol (4-TBP) was achieved at a temperature of 70 °C at a low ionic liquid to phenol ratio. The catalyst retained its activity even after 5 recycles. A kinetic model was developed for the reaction mechanism based on the product distribution and the kinetic parameters were estimated. The activation energy for ionic liquid catalyzed alkylation of phenol was found to be 11.13 kcal/mol in the temperature range of 50–90 °C. Negative activation energy was observed for 2-tert-butylphenol (2-TBP) to 2,4-di-tert-butylphenol (2,4-DTBP) and the extended Arrhenius equation was used to interpret this trend.