Preparation Of 2,6-Dimetylnaphtalene By The Disproportionation Of 2-Metylnaphtalene OverIonic Liquids

2,6-Dimethylnaphthalene(2,6-DMN) is the important fine chemical raw material used for the production of new polyester material-PEN with good heat-resistance, excellent mechanic & chemical property, and UV radiatio n-resis tance. With the development of the PEN application field, 2,6-DMN will be wanted increasingly. The disproportionation of β -methylnahthalene(β-MN) is the important means of preparation of 2,6-DMN. In this paper, study on the disproportionation of β-MN catalyzed by ionic liquids to prepare 2,6-DMN was carried out.The disproportionation of β-MN was usually catalyzed by zeolites and Lewis acid..However, there were some problems, such as esay formation of coke, short catalysis life for zeolites. And it was very difficult for Lewis acid catalyst to reuse and handle. Ionic liquids (ILs) are regarded as relatively clean catalysts and solvents. Recently ILs has been applied in many organic reactions because of its non-volatile & non-flammable property, high thermalstability, potential for recy -cling, compatibility with various organic compounds and organometallic catalyst. In this paper, it is the first time that ILs were applied in the disproportionation of β-MN to prepare 2,6-DMN. The experiment results show that ILs have better catalytic activity and are separated, recovered from reaction products more easier, and reused with activity relative to the former catalysts. The experiment results is summarized as the following:(1) AlCl₃ type ionic liquid has the best catalytic activity among the constitution of MCly (FeCl₃, AlCl₃, CuCl);(2) The orthogonal experiments were carried out for optimizing reaction cond itions of ILs-1,3-dialkylimidazolium chloride-aluminum chloride, N-alkylpyrod ium chloride-aluminum chloride and alkylammonium-aluminum chloride. The results show that the 1,3-dialkylimidazolium chloride-aluminum chloride is the best IL system for disproportionation reaction of β-MN. The most suitable conditions were 160℃, mole ratio of AlCl₃[bmirn]Cl 4:1, catalyst mass fraction 14Wt%, reaction time3.5h;(3) The ionic liquids can be recovered & reused for 4 times. The main deactivated reason of ionic liquids is due to the little amount of water by means of FT-IR;(4) For l-methyl-3-alkylimidazolium chloride-aluminum chloride (alkyl group: butyl-, hexyl-, octyl-) Us, it was concluded that with the increase carbon chain length, the catalyst activity is better. From the investigation of FT-IR, the carbon chain length increase can't improve the acid strength, but can increase the catalyst's solubility in reactant,(5)For l-methyl-3-octylimidazolium halides-aluminum chloride( halides: Cl, Br, I), it was concluded that bromides ([Cgmim]Br-AlCl3) showed outstanding activ -ity, because that it showed the higher inherent acidity relative to others and formed the anion [A^C^Br]" which showed a strong acidity based on FT-IR characterization, and this anion can react with hydrogen atoms at the 2-position of an imidazolium ion to form Bronsted acid.(6)Under optimum experimental conditions, ionic liquid showed lower conversion and higher selectivity of 2,6-DMN by comparision of [Cgmim]Br-AlCl3 and solid acid;It is concluded that (3-MN disproportionation reaction over ionic liquid shows better activity and selectivity of 2, 6-DMN and the catalysts are easily separated from reactant product and recoverable.