Year: 2017
Type: article
Abstract: Shale gas has become an abundant source of natural gas in recent years; methane is one of its major chemical components. Meanwhile, benzene, which is applied into the manufacturing of complex chemicals, is an important chemical intermediate in petrochemical industry. Direct methane dehydroaromatization (DHA) under non-oxidative condition is an alternative approach to convert methane into benzene. The reaction mechanism is generally considered a synergistic result of metal sites and Bronsted acid sites of HZSM-5. Here, methane is activated by metal site while zeolite provides the shape-selective environment and the diffusion channel for reactive molecules. Among different bi-functional catalysts, Mo/HZSM-5 is the most studied catalyst for this reaction due to its strong catalytic performance. However, coke formation driven by thermodynamics will ultimately lead to catalyst deactivation. This problem is exacerbated by the fact that the catalyst regeneration for Mo/ZSM-5 is relatively lengthy and uneconomical due to its loss of activity after burn-off. Recent reports suggested that atomically dispersed iron on silica can have high conversion of methane to higher hydrocarbon and can potentially suppress coke formation. Our group previously found that highly dispersed iron on ZSM-5, H-(Fe)ZSM-5, also shows high benzene selectivity and reduced coke formation. However, severe coke formation still result from secondary reactions of product on Bronsted acid site of ZSM-5. The purpose of this research is to study how catalytic performance and coke formation are influenced by the catalyst structure of H-(Fe)ZSM-5 and how this structure can be affected by the ZSM-5 synthesis. Specifically, the impact of hydrothermal time, hydrolysis time and template ratio in ZSM-5 synthesis were investigated since these parameters can have influence on both crystallinity and particle size. It is found that both of these structural parameters can have significant influence on catalytic performance. In addition, we found that passivation of external Bronsted acid sites can improve catalyst stability by reducing coke formation but simultaneously also decreases the reactivity. Overall, we developed a well-defined catalyst with high selectivity (~86%) in DHA by optimizing only synthetic parameters without applying any post-treatment.
Authors: Yu‐Chieh Cheng, Götz Veser
Language: English
Subfield: Inorganic Chemistry
Field: Chemistry
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W2725256121