Effect of reaction conditions on the oxidative coupling of methane over doped MnOx-Na2WO4/SiO2 catalyst

• Published in: Journal of catalysis 2019-08, Vol.376, p.25-31
• Main Authors: Hayek, Naseem S., Khlief, Grace J., Horani, Faris, Gazit, Oz M.
• Format: Article
• Language: English
• Subjects: Cristobalite; Ethylene; Manganese oxide; Metal oxide; Methane; OCM; Sodium tungstate
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/j.jcat.2019.06.042

[Display omitted] •The MnOx-Na2WO4/SiO2 catalyst doped with Ti, Nb, Ce, Sn, Ge or Fe was tested under full and differential conditions.•The strong catalyst deactivation process is shown to be masked at full conversion and excess catalysts.•The deactivation process is shown to be inherent to the undoped MnOx-Na2WO4/SiO2.•The effect of doping on OCM catalytic activity is discussed with respect to initial reaction performance and deactivation. The MnOx-Na2WO4/SiO2 mixed oxide is a highly promising catalyst for the oxidative coupling of methane to ethylene (OCM). But it seems that its catalytic performance still does not meet industrial requirements. Several additives were reported to promote its activity and selectivity such as Ti, Ce, La, etc. However, contradicting results were obtained in the different studies and the additives were not compared under the same reaction conditions. In this study, we tested several reported additives (Ti, Nb, Ce, and Sn) in OCM, which previously showed promising results, as well as additives (Ge and Fe) that were not examined before. Using these doped catalysts, it is shown that the improved performance and the long-term stability of these catalysts are a product of the specific reaction conditions, i.e. the extent of oxygen conversion and time on stream. We show that overall these catalyst systems are unstable and undergo severe deactivation that is masked if tested under conditions were the O2 is totally consumed. Finally, by characterizing spent catalysts, several possible deactivation mechanisms are briefly discussed.