Shape-Selective Catalytic Synthesis Of 2, 6-dimethylnaphthalene

2,6-dimethylnaphthalene(2,6-DMN) is a precursor of 2,6-naphthalene dicarboxylic acid in the manufacture of high performance polyester resins such as polyethylene naphthalate polymer(PEN). PEN is too expensive to expand its market completely because of few effective processes for 2,6-DMN commercialization. Therefore, it has become an important subject to find an efficient process for 2,6-DMN synthsis to promote application of PEN. A shape-selective zeolite catalytic process for 2,6-DMN synthesis by methylation of 2-methylnaphthalene was employed in this paper.Compared with the reactive ability over Hβ HMCM-22 HZSM-5 and HZSM-12 zeolite catalysts, the results showed that the pore channel structure and size was the main factor affecting the reaction performance except for the acidic properties. The initial activity order of those zeolites was Hp>HMCM-22>HZSM-12>HZSM-5. Both HMCM-22 and HZSM-12 were effective catalysts for alkylation of 2-methylnaphthalene.The effect of reaction parameters such as reaction temperature, space velocity, mole ratio of 2-MN and methanol were investigated on catalyst of HMCM-22 and HZSM-12. As for HMCM-22 zeolite catalyst, the experiment was conducted at 400 ℃ using 1,3,5-trimethylbenzene as reaction medium and methanol as alkylation reagent. 2-MN, methanol and 1,3,5-trimethylbezene were premixed in mole proportion of 1:5:5 before reaction. Nitrogen was used as carrier gas at the flow rate of 10 ml/min. For HZSM-12, the feed ratio was in 1:3:3.Appropriate modification of zeolite can decrease isomerization reaction on its external surface, and increase the 2,6-DMN selectivity. HMCM-22 zeolite catalyst was modified via isomorphic substitution with NH4BF4. When B partly replaced Al in the framework of MCM-22, the selectivity of isomerization and disproportionation dropped dramatically with time, meanwhile, the yield of aim product 2,6-DMN enhanced, but the selectivity of 2,6-DMN didn't be improved on the modified catalyst.Steam treatment is an important modified way for catalyst and can restrain isomerization of reactant and improve selectivity of DMNs and 2,6-DMN. Compared the reaction capability over HMCM-22 with different steam treatment temperature, 450℃ was the best. After 8.25 h reaction, the conversion of 2-MN was 68.4%, the selectivity of 58.6% DMNs could be achieved, and the yield of 2,6-DMN was as high as 6.0%.Modification via Zr implanted in ZSM-12 framework, the conversion of 2-MN could be retained at 10% after 8.25 h, and the selectivity of 2,6-DMN could reach 36.8%, and the ratio of 2,6- to 2,7-DMN increased up to 1.7. Using magnesium salts as impregnant to modify HZSM-12, the initial activity was only 16.4%, but catalyst deactivation could be reduced greatly. Higher than 75% p,p-DMN selectivity, 40.1% 2,6-DMN selectivity, and 4.3% 2,6-DMN yield could be obtained. In addition, the isomerization and disproportionation decreased greatly.