Preparation Of Modified Zsm-12 Zeolite And Its Catalytic Alkylation Reaction

Large production and commercialization of polyethylenenaphthalate (PEN) was restricted due to the high cost and low availability of 2,6-DMN which is a key intermediate in the synthesis of PEN. Therefore, preparation and modification of catalyst able to produce 2,6-DMN by the alkylation of naphthalene and methanol in high selectivity using one-step method is becoming one of the hot spots. ZSM-12 zeolite showns better catalytic properties in the alkylation of naphthalene and methanol than other zeolites.In this paper, ion exchange, isomorphous substitution and addition of F- into the synthetic system were used to modify self-made HZSM-12 zeolite. Analytical technique including XRD, FT-IR, SEM, N2 physisorption, AAS, ICP, XRF, 27Al NMR, XPS, NH3-TPD and Py-IR were used to characterize the structure and acidity of HZSM-12 zeolite before and after modification. Catalytic performance of zeolites were evaluated by the the alkylation of naphthalene and methanol for synthesis of 2,6-DMN in the fixed-bed microreactor experimental device. The rule of different modified method to the influence of structure, acidity and catalytic performance for HZSM-12 zeolite were studied.Aqueous solution of Mg(NO3)2 and La(NO3)3 were used to modify HZSM-12 zeolite by ion-exchange method. The results show that pore structure, acid and acid strength of HZSM-12 zeolite can be modulated and catalytic performance improved by modification of Mg2+ and La3+. The degree of carbon deposition resistance reduced and ability of inactivation enhanced after modification of Mg2+. The selectivity to 2,6-DMN and ratio of 2,6-/ 2,7-DMN were increased after modification of La3+.Isomorphous substitution to modify HZSM-12 zeolite were carried out by Ga, Ti, B and Si. The results show that the type of heteroatoms, methods and conditions have a greater impact on the structure and acidity of HZSM-12, secondary mesoporous were produced, mesoporous volume increased to varying degrees and the acid strength for the strong acid sites decreased after modification. The conversion of naphthalene, selectivity to 2,6-DMN and ratio of 2,6-/2,7-DMN were significantly increased on the ZSM-12 zeolite in which framework Al atoms were isomorphously substituted partially by gallium atoms. Catalytic performance of HZSM-12 was improved by modification of (NH4)2TiF6, NH4BF4 and (NH4)2SiF6 with optimum concentration. A obvious increase of selectivity to 2,6-DMN was observed on the premise that catalytic activity was similar to that of HZSM-12.ZSM-12 zeolite was firstly synthesised in F- system. The effects of amount of F ions and crystallization time to zeolite structure were studied. The results show that ZSM-12 zeolite with high crystallinity can be synthesised when the F/Si in the range of 0.025 ~ 0.10. Addition of F- not only significantly reduced crystallization time of zeolite, but also changed the appearance of the crystal, pore structure characteristics and acidity. the acid strength for the strong acid sites increased, the number of acid sites in outer surface reduced, and catalytic performance improved obviously. The selectivity to 2,6-DMN and ratio of 2,6-/2,7-DMN were increased at the same time catalytic activity to the alkylation of naphthalene and methanol and anti-inactivating capacity improved. In comparison between HZSM-12 zeolite and F(0.1)-Z-4 samples synthesised in F- system, conversion of naphthalene increased frome 43.7% to 55.3% , selectivity to 2,6-DMN and ratio of 2,6-/2,7-DMN increased from 25.3% and 1.83 to 26.8% and 1.90, respectively.