Zhengxi Yu

Synthesis of mesoporous ZSM-5 catalysts using different mesogenous templates and their application in methanol conversion for enhanced catalyst lifespan

In this work, two kinds of mesoporous ZSM-5 were synthesized successfully using a hydrothermal methodology by utilizing different soft templates, namely, dimethyl octadecyl [3-(trimethoxysilyl)propyl]ammonium chloride ([(CH3O)3SiC3H6N(CH3)2C18H37]Cl, TPOAC) and hexadecyl trimethyl ammonium bromide (C16H33(CH3)3NBr, CTAB). The obtained mesoporous ZSM-5 samples were compared with conventional ZSM-5, and the effects of different surfactant usages during the synthesis of mesoporous ZSM-5 on the physicochemical and catalytic properties were systematically investigated. Multiple techniques, such as XRD, SEM, N2 adsorption techniques, HP 129Xe NMR, 27Al MAS NMR, 29Si MAS NMR, and 1H MAS NMR, were employed for the characterization. Although the synthesized mesoporous ZSM-5 samples had equal surface areas, they presented different relative crystallinities, morphologies, pore-size distributions, micropore–mesopore interconnectivity, framework atom coordination states and acidities. When using these synthesized ZSM-5 samples as catalysts for methanol conversion, the mesoporous ZSM-5 templated with TPOAC exhibited an extremely long catalyst lifespan compared to conventional ZSM-5, while mesoporous ZSM-5 templated with CTAB showed no advantage in prolonging the catalyst lifetime during the reaction. The differences in the catalytic lifespan and the reduction of coke deposition were correlated to the variation of acidity and porosity with the mesopore generation in the ZSM-5 catalysts by the usage of different structure-directing agents. Compared to the mesopore structure-directing agent, CTAB, with the use of TPOAC as the template and part of the Si source, mesoporous ZSM-5 could be synthesized with good mesopore–micropore interconnectivity, which accounted for the improved catalytic performance in the reaction of methanol conversion.

Methanol aromatization over HZSM-5 catalysts modified with different zinc salts

Abstract HZSM-5 catalysts modified with various Zn salts, namely zinc sulfate, zinc acetate, zinc nitrate, and zinc chloride, were prepared using an impregnation method. The resultant catalysts were characterized by X-ray diffraction, N 2 adsorption, thermogravimetry-mass spectrometry, temperature-programmed desorption of NH 3 , and infrared spectroscopy using pyridine as the probe molecule. The methanol-to-aromatic (MTA) performance over the modified catalysts was investigated. The results showed that the type of Zn species in the catalyst significantly influenced the catalyst surface acidity. The distribution of acid sites and Zn species in the HZSM-5 catalyst modified with zinc sulfate effectively improved the MTA performance.

Improved Selectivity toward Light Olefins in the Reaction of Toluene with Methanol Over the Modified HZSM‐5 Catalyst

Light olefins and para‐xylene were produced with high selectivity in the reaction of toluene with methanol over a modified HZSM‐5 catalyst. Ordered mesoporous structure and weaker acidic sites led to significant contribution to the selectivity toward para‐xylene. By means of a single‐pulse reaction and the temperature‐programmed surface reaction, it was confirmed that the presence of toluene and as‐produced xylene shortened the reaction induction period and reduced the formation temperature of ethylene and propylene, which was beneficial to improve the selectivity toward light olefins, particularly ethylene.

Synthesis of mesoporous ZSM-5 using a new gemini surfactant as a mesoporous directing agent: A crystallization transformation process

A new gemini surfactant, [C18H37(CH3)2–N+–(CH2)3–N+–(CH3)2C18H37]Cl2 (C18-3-18), has been successfully used as the mesopore directing agent in the hydrothermal synthesis of mesoporous ZSM-5 (MZSM-5). The synthesis of MZSM-5 was realized with a low temperature crystallization process at 130 °C. The amount of C18-3-18 used in the synthesis affected the relative crystallinity and the textural properties of the obtained MZSM-5. Detailed investigation showed that the formation of MZSM-5 followed a crystallization transformation process. The use of C18-3-18 resulted in the formation of mesoporous material during the early stage of the synthesis, which was converted into MZSM-5 crystals templated by tetrapropylammonium bromide to form the MFI phase. As the synthesis proceeded, the MZSM-5 crystals aggregated into particles by weak interactions. This work shows that C18-3-18 can be used as a mesopore directing agent, which could provide a route for the synthesis of other mesoporous zeolites.