Highly selective synthesis of 2,6-Dimethylnaphthanlene by green catalysts—N-alkyl-pyridinium halides-aluminum chloride ionic liquids Original Research Article

The transalkylation of 2-methylnaphthanlene (2-MN) with 1,2,4,5-tetramethylbenzene (TeMB) for highly selective synthesis of 2,6-dimethylnaphthalene (2,6-DMN) was performed in N-alkyl-pyridinium halides-aluminum chloride ionic liquids ([CnPy]Cl-AlCl3ILʹs) as acid catalysts. The influences of length of alkyl group at organic cation, of acid strength of the ionic liquids, as well as reaction time and temperature were studied. Due to its appropriate acidity, the [BuPy]Cl-AlCl3 ILʹs (x = 0.71) reveals higher activity and selectivity to 2,6-DMN in the reaction under mild conditions when both isomerization of 2,6-DMN and disproportionation of 2-MN are substantively restrained. The selectivity to 2,6-DMN and 2,6-/2,7-DMN ratio reach maximum values of 80.5% and 4.13, respectively, relatively to the thermodynamic value of 2,6-/2,7-DMN ratio ≈ l usually obtained with other acid catalysts. After 3 h of the reaction the selectivity to 2,6-DMN reaches 100% at 7.9% conversion. The mechanism of transalkylation of 2-MN with TeMB in [CnPy]Cl-AlCl3 ILʹs is proposed. It is believed that Al2Cl7− is probably the active catalytic species of the reaction. The re-using of the acidic ionic liquids has been investigated. The main reason for deactivation of the catalysts is found to be a leaching of the active Al2Cl7− species as a result of the hydrolysis.