Synthesis, Modification And Catalytic Performance Of ZSM - 12 Molecular Sieve Disproportionation Reaction

In this paper, ZSM-12 molecular sieves with different Si/Al ratios were synthesized by hydrothermal method and further used as catalysts for disproportionation of 1,2,4-TMB on fixed-bed microreactor to investigate their catalytic performance. The structure and acid properties of the catalysts were characterized by XRD, SEM, NH3-TPD, and Py-IR. The influence of reaction temperature and reaction space velocity on the catalytic performance was investigated when the sample (Si/Al=65) was chosen as catalyst. The influence of Si/Al ratio of ZSM-12 zeolite on the catalytic performance was also investigated.The ZSM-12 zeolite (Si/Al=65) was modified by vapor deposition method and liquid phase deposition method. The effect of deposition temperature, deposition time and flows of propylene on the structure and acidity was investigated. And the catalytic performance of modified ZSM-12 zeolite was investigated too. The results revealed that samples treated with chemical deposition from propylene can reduce the acidity of ZSM-12 zeolite. The optimized deposition conditions were as follows:temperature 400℃, time 3 h and the flows of propylene 7 ml/min. Although the conversion of 1,2,4-TMB was decreased, selectivity of 1,2,4,5-TEMB was improved to a certain extent when used modified ZSM-12 zeolite as the catalyst for disproportionation of 1,2,4-TMB. Besides, PDMS was chosen as the substitute for propylene to deposit on the surface of ZSM-12 zeolite. The results revealed that SiO2 deposition mainly covered the B acid sites of ZSM-12 and block the isomerization of 1,2,4-TMB which result in a higher D/I ratio. The D/I ratio raised from 2.54 to 3.10 when the amount of SiO2 deposition was 8%.The ZSM-12 zeolites (Si/Al=65) were next modified by citric acid and alkaline treatment with different solution concentration and treated time. After modification, the structure and acid properties of ZSM-12 zeolites with different modified methods were investigated. The ZSM-12 zeolites generated more secondary mesoporous after treated with citric acid, and alkaline and diffuse abilities of reactant molecules were remarkably enhanced which resulted in a higher catalytic performance. Under the optimized reaction conditions, the D/I ratio was arrived at 3.01 and the conversion of 1,2,4-TMB was 65.67% when the sample modified by alkaline treatment (0.6 mol/1,65 ℃,0.5 h) were employed as the catalyst which showed the best catalytic performance.ZSM-12 zeolites with initial gel of different Si/Al ratio were successfully synthesized by seed-induced method. The synthetic conditions (crystallization time of the seed, amount of the added seed crystals, and crystallization time) of the ZSM-12 zeolite were optimized. The structure and acidity properties of the zeolites under different conditions were determined by XRD, N2 physical adsorption, SEM, NH3-TPD and Py-IR. Compared with hydrothermal method, seed-induced method could reduce the crystallization time significantly, and the grain size of ZSM-12 turned out to be smaller. What’s more, the catalytic performance of these samples in disproportionation of 1,2,4-TMB was also effectively improved.