Preparation Of Acidic Ionic Liquids And Study On Catalytic Alkylation Reactivity

Preparation of 2,6-dimethylnaphthalene(2,6-DMN) through transalkylation reaction of methylnaphthalene(MN) and 1,2,4,5-tetramethylbenzene(TeMB) is one of methods with excellent exploitation value and potentiality. Searching high effectively and environmental benign catalysts for synthesis of 2,6-DMN with high selectivity in the transalkylation reaction is the key technology due to products complexity.In this paper, Alkylimidazolium Chlorine-aluminate Ionic Liquids, Alkylpyridinium Chlorine-aluminate Ionic Liquids, triethylamine hydrochlorine-aluminium(â…¢) Chloride Ionic Liquids(denoted:[C_nmim]X-xAlCl₃, [C_nPy]X-xAlCl₃ and Et₃NHCl-xAlCl₃, respectively) were preparation. Forms of the existence and acidic properties of ionic liquids were characterized by Py-IR, acetonitrile-IR and ²⁷Al NMR. The results showed that ionic liquids had both Lewis and Bronsted acid sites; the molar fraction of AlCl₃ determined the acidic properties and forms of inorganic anions species of ionic liquids, on the other hand the structure of cation have affect on the acidic properties to some extent. The main content of this paper as follows:The acidic strength, reaction temperature, reaction time of Chlorine-aluminate Ionic Liquids with different structure of cation on the activity of transalkylation of MN with TeMB and the selectivity of 2,6-DMN were studied. The optimum reaction condition was obtained: the molar fraction of AlCl₃ is 0.71, molar ratio of 2-MN and TeMB is 1, the amount of ionic liquid is 32wt%, reaction temperature is 20℃and reaction time is 8h using [C_nmim]X-xAlCl₃ and [C_nPy]X-xAlCl₃ as catalysts; the optimum reaction time is shortened 6h using Et₃NHCl-xAlCl₃ as catalyst. Under the same reaction condition, 48.7% conversion of 2-MN, 48.1% selectivity of 2,6-DMN but lower ratio of 2,6-DMN to 2,7-DMN(2.3) were obtained when the Et₃NHCl-xAlCl₃ used as catalyst. On contrast, the highest selectivity of 2,6-DMN and 2,6-DMN /2,7-DMN(80.5% and 4.1, respectively) were obtained when the[C₄Py]Cl-xAlCl₃ used as catalyst. The catalysts activity and selectivity of 2,6-DMN of [C₄mim]Cl-xAlCl₃ were between two above catalysts. The high selectivity of 2,6-DMN also can be obtained by transalkylation reaction using a mixture of MNs.Under the optimal reaction condition, the deactivation of multiple reaction were researched, then we analysed the causes for ionic liquids deactivation. It was found that moisture in the system of reaction is main reason that lead to deactivation of ionic liquids.The reaction mechanism of transalkylation reaction of methylnaphthalene and 1,2,4,5-tetramethylbenzene catalyzed by chlorine-aluminate ionic liquids has been proposed. It was found that the course of ionic liquids catalytic reaction is proton acid catalytic reaction as well as the main reason of high selectivity of 2,6-DMN is the formation of transition state of C⁺ with the feasible acidic properties and steric hindrance.