Study On The Methylation And Isopropylation Of 2-methylnaphthalene Over Modified ZSM-5 And Mordenite Zeolite Catalysts

Linear polynuclear aromatic hydrocarbons, such as 2,6-dialkylnaphthalene(2,6-DAN) are raw materials of advanced polymer materials. The corresponding acid produced by their oxidation is an important precursor used for thermotropic liquid crystal polymers, heat-resistant polymers, synthetic fibers and engineering plastics with superior properties. Amongβ,β'-alkylated isomers, the methylated 2-methylnaphthalene(2-MN) has attracted more and more attention because its oxidation to the corresponding carboxylic acid proceeds under mild conditions and without carbon loss. Because of the steric hindrance, the isopropylated 2-MN can gain higher 2,6-selectivity than the methylated one.In this paper, the methylation of 2-MN to 2,6-dimethylnaphthalene(2,6-DMN) over zeolite catalysts was studied. It was found that HZSM-5 was the appropriate catalyst for this reaction. Reaction temperature of 360℃, WHSV 2 h-1, atmospheric pressure were the optimal reaction conditions. With increasing reaction pressure, the selectivity of 2,6-DMN and 2,6-/2, 7-DMN ratio reduced. Here, the isopropylation of 2-MN to 2-methyl-6-isopropylnaphthalene (2,6-MIPN) over zeolite catalysts was also studied. It was found that HM was the appropriate catalyst for this reaction. The optimal reaction conditions were reaction temperature 200℃, reaction pressure 2.0MPa,2-methylnaphthalene:isopropanol:cyclohexane= 1:3:70.In order to improve the catalytic stability of HZSM-5 catalyst, the parent catalyst was firstly modified by hydrothermal treatment with acid leaching, it was found that the catalytic stability was improved with 550℃hydrothermal treated catalyst. It can be concluded that pore structure of HZSM-5 catalyst was modified by hydrothermal treatment, the increased mesopore volume enhanced diffusion of molecules, so that more 2,6-DMN emerged; at the same time, the acid strength and the number of strong acid sites of HZSM-5 catalyst were both reduced by hydrothermal treatment, especially for the reduction of non-shape-selective, the selectivity to 2,6-DMN increased; furthermore, as the reduction of the number of strong acid sites, the coke formation and the isomerization of 2-MN and DMNs were both decreased. After 8 h, the conversion of 2-MN was still 13.4%,2,6-DMN can reach 59.1%, among the 8 h, the maximum of 2,6-DMN was 7.1%. Secondly, the parent catalyst was modified by NH4F and Pt. On one hand, the coke formation was restrained through hydrogen overfall of Pt, On the other hand, the acid strength and the number of strong acid sites of HZSM-5 catalyst were reduced by NH4F, so that the catalytic activity and stability were improved. Thirdly, It can be seen that coke formation was restrained by enhancing reaction pressure and using H2 as carrier gas, MgO and CeO2 modified HM catalyst can improve the selectivity of 2,6-DMN.The isopropylation of 2-MN was studied on the HM catalyst, due to the steric hindrance, the selectivity of 2,6-MIPN was better over the parent catalyst. The external acid sites of HM catalyst was mainly deactivated by CeO2, MgO can enter into the inside of catalyst, modify the internal acid sites, and narrow the pore dimension, enhance the selectivity of 2,6-MIPN. After 8 h, the 2,6-MIPN selectivity can reach 89.5% over 3%MgO/20%CeO2/HM catalyst.