Catalytic performance of hierarchical H-ZSM-5/MCM-41 for methanol dehydration to dimethyl ether

Micro-mesoporous composite molecular sieves H-ZSM-5/MCM-41 were prepared by the hydrothermal technique with alkali-treated H-ZSM-5zeolite as the source and characterized by scanning electron microscopy,transmission electron microscopy,energy dispersive spectroscopy,X-ray diffraction,N2 adsorption-desorption measurement and NH3 temperature-programmed desorption.The catalytic performances for the methanol dehydration to dimethyl ether over H-ZSM-5/MCM-41 were evaluated.Among these catalysts,H-ZSM-5/MCM-41 prepared with NaOH dosage (nNa/nSi) varying from 0.4 to 0.47 presented excellent catalytic activity with more than 80%methanol conversion and 100%dimethyl ether selectivity in a wide temperature range of 170—300℃,and H-ZSM-5/MCM-41 prepared with nNa/nSi=0.47 showed constant methanol conversion of about 88.7%,100% dimethyl ether selectivity and excellent lifetime at 220℃.The excellent catalytic performances were due to the highly active and uniform acidic sites and the hierarchical porosity in the micro-mesoporous composite molecular sieves.The catalytic mechanism of H-ZSM-5/MCM-41 for the methanol dehydration to dimethyl ether process was also discussed.

Optimization of hydrothermal synthesis of H-ZSM-5 zeolite for dehydration of methanol to dimethyl ether using full factorial design

H-ZSM-5 zeolite was synthesized by hydrothermal method. The effects of different synthesis parameters, such as hydrothermal crystallization temperature （170-190 ℃） and Si/A1 molar ratio （100-150）, on the catalytic performance of the dehydration of methanol to dimethyl ether （DME） over the synthesized H-ZSM-5 zeolite were studied. The catalysts were characterized by N2 adsorption-desorption, XRD, NH3-TPD, TGA/DTA, and SEM techniques. The full factorial design of experiments was applied to the synthesis of H-ZSM-5 zeolite and the effects of synthesis conditions and their interaction on the yield of DME as the response variable were determined. Analysis of variance showed that two variables and their interaction significantly affected the response. According to the experimental results, the optimized catalyst prepared at 170℃ with the Si/A1 molar ratio of 100 showed the best catalytic performance among the tested H-ZSM-5 zeolite.

H-ZSM-5催化甲苯与碳酸二甲酯和甲醇甲基化的反应机理(英文)

对二甲苯是重要的石油化工产品之一,可以通过甲苯甲基化生产.本文采用"ourown-N-layered integrated molecular orbital+molecular mechanics"(ONIOM)和密度泛函理论(DFT)结合的方法,计算了H-ZSM-5催化甲苯与碳酸二甲酯(DMC)和甲醇甲基化反应机理.考察了反应物吸附和产物脱附.描述了主要的中间物种和过渡态的结构.用计算的速率常数来估计甲苯甲基化反应的动力学活性.H-ZSM-5催化的甲苯与DMC和甲醇甲基化的机理不同.甲苯和DMC甲基化包括DMC完全解离,接着甲基化生成二甲苯异构体.相比而言,在甲苯甲基化反应中,甲醇作为甲基化试剂的活性比DMC更好.甲苯和甲醇甲基化的分步反应路径和联合反应路径的本征活化能相似.在773K,分步反应路径的速率常数比联合反应路径更高.在甲苯和这两种试剂甲基化的反应中,生成对二甲苯为动力学优先,而间二甲苯为能量最低产物.我们的计算结果和实验观察到的现象一致.