Methane-to-aromatics catalysis: How does tungsten do it?

Abstract

Methane is flared on large scale leading to CO2 emissions in the upstream and downstream oil industry, due to the lack of utilization technologies on-site. Methane dehydroaromatization (MDA) is a reaction, in which the otherwise flared methane can be utilized resulting in direct production of benzene and hydrogen. Mo/ZSM-5 is the typical catalyst used for this reaction, but shows high deactivation rates under MDA conditions, due to the volatile character of molybdenum. Therefore, W/ZSM-5 might be an interesting alternative to replace Mo in the MDA reaction, because W/ZSM-5 lacks this issue. However, W/ZSM-5 displays a significantly longer activation time compared to molybdenum, and is not extensively studied leaving many open questions about the structure-performance relationships. This study found three ways to reduce this activation time, namely increasing the reaction temperature, increasing the calcination temperature and introducing a H2 pre-treatment. Higher reaction temperatures induced a faster formation and higher concentration of active tungsten species. The reduced activation time with increasing calcination temperature was presumably caused by the higher distribution of the tungsten species, which enables a faster access to the active site. The H2 pre- treatment produced pure tungsten species, which can readily convert to the presumed active site, tungsten carbide. In the activation time at high tungsten weight loading catalysts, tungsten sub-oxides, WO2.9 and WO2.72, were observed, due to the presence of WO3 particles at the start of the reaction. A reductive transition from WO2.9 to the more reduced WO2.72 was observed,, which indicate that tungsten species are reduced in the activation time. This further emphasizes the importance of a reduction in the activation time. This study demonstrates how the activation time is reduced and which aspects in the formation of the active species are relevant. Following this study, more advanced in situ characterization techniques should be used to establish a reaction pathway to the active tungsten species.