Performance of Ce-Added Na2WO4 Catalyst for Direct Conversion of Methane to Ethylene

The production of key petrochemical intermediates using methane as a raw material using direct coupling of methane (DCM) reaction has been studied for the past 30 years as a new technology replacing the existing indirect conversion processes which require tremendous capital investment and result in unfavorable project economics. Recently, as the mining unit price is lowered with the development of shale gas exploitation technology, the DCM becomes more interested by catalysis researchers. However, the optimal catalyst composition and structure have not been clearly presented to date. Also, the development of a catalyst having high performance and durability with a exceeding 30% ethylene yield, which is a commercially viable value, has not been reported yet. The reason is as follows: 1) thermodynamically limited conversion, 2) low selectivity to ethylene (by the oxidation of methane), 3) serious catalyst deactivation via hot spot and coke deposition. Numerous attempts have been made to overcome these problems in DMC reaction, and MnO₂-promoted Na₂WO₄/SiO₂ catalysts have been known to have the highest performance and durability so far. However, even in the case of MnO₂-promoted Na₂WO₄ catalyst, the commercially viable ethylene yield has not been satisfied and the optimal composition and structure of the catalyst and the deactivation mechanism of the catalyst are still under study.

In this study, we have developed catalysts with excellent catalytic performance with superior ethylene selectivity in oxidative coupling of methane (OCM) reaction. Our CeO₂-promoted Na₂WO₄ catalyst has shown high methane conversion with superior ethylene selectivity compared with MnO₂-promoted catalysts. However, as a result of 72 h-reaction, the deactivation rate of CeO₂-promoted catalyst was slightly faster than that of MnO₂-promoted catalyst. Therefore, several techniques have been applied for the long term stabilization of CeO₂-promoted Na₂WO₄ catalysts. Our research group has also been working on the deactivation mechanism of OCM catalysts using computational chemistry and atomic scale characterization techniques in order to secure the long-term stability as well as the high methane conversion in OCM using CeO₂-promoted Na₂WO₄ catalysts.