Highly efficient alkylation of phenol with tert-butyl alcohol using environmentally benign Bronsted acidic ionic liquids

• Phenol alkylation using Bronsted acidic ionic liquids as catalyst.
• Kinetics study on phenol alkylation using synthesized ionic liquids.
• High phenol conversion (94.2%) obtained at 80 °C within 2 h of reaction time.
• DFT studies to correlate acidity and catalytic activity trends of the ionic liquids.
• Estimation of activation energy (34.8 kJ mol−1) from kinetic modeling.

A series of SO3-H functionalized environmentally benign Bronsted acidic ionic liquids were prepared using triethylamine, 1-methyl imidazole, pyridine, diethanolamine, 1,4-butane sultone and methane sulfonic acid as the starting materials. These synthesized ionic liquid catalysts were characterized using spectroscopic tools like 1H NMR, 13C NMR. The catalytic activity of the synthesized ionic liquids was investigated in tert-butylation of phenol with tert-butyl alcohol. The effect of kinetic parameters like reaction time, reaction temperature, reactant mole ratio, reactant to catalyst mole ratio, on the conversion of phenol and selectivity to 2-tert-butyl-phenol and 2,4-di-tert-butyl-phenol were investigated. Phenol conversion upto 94.2% was achieved at temperature as low as 80 °C and low alcohol to phenol molar ratio of 2:1, ionic liquid to phenol ratio of 1:1 and reaction time of 2 h. The catalyst was found to retain its activity even after 8 recycles and was completely recoverable. The plausible reaction mechanism was formulated based on product distribution. The kinetic data was found to agree well with classical phenomenological models and the kinetic parameters were estimated. The experimental results were further validated by Density Functional Theory (DFT) optimizations of the ionic liquid structures performed at the B3LYP/6-311G** level using Gaussian 09 package.

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