Oxidative Coupling of Methane (OCM) by SiO₂-Supported Tungsten Oxide Catalysts Promoted with Mn and Na

Abstract

The literature for the oxidative coupling of methane (OCM) on supported Mn/Na 2 WO 4 /SiO 2 catalysts is systematically and critically reviewed. The influence of the precursors, starting SiO 2 support crystallinity, synthesis method, calcination temperature, and OCM reaction conditions on the catalyst structure is examined. The supported Mn/Na 2 WO 4 /SiO 2 catalyst system is found to be dynamic with the catalyst structure quite dependent on the set of variables. Although almost all of the reported studies have determined the catalyst crystalline structures under ambient conditions (room temperature and air exposed), recent in situ/operando characterization study under OCM reaction conditions revealed that all previously detected crystalline phases of the active Mn−Na−W−O components are not present because the reaction temperature is above the melting points of their oxides. The presence of Na also induces the crystallization of the silica support to SiO 2 (cristobalite) at elevated temperatures. The nature of the surface active sites under OCM reaction conditions is still not known because of the absence of in situ/operando surface spectroscopy characterization studies under relevant reaction conditions. Consequently, the proposed structure−activity models in the literature are highly speculative since they are lacking supporting data. The rate-determining-step involves activation of the methane C−H bond by atomic surface O* as demonstrated by a kinetic isotope effect (KIE) between CH 4 and CD 4 . Although the reaction kinetics follow a Langmuir− Hinshelwood type mechanism, r = [CH 4 ] 1 [O 2 ] 1/2 , isotopic 18 O 2 − 16 O 2 studies have shown that the catalyst lattice also provides O* for the OCM reaction suggesting involvement of a Mars−van Krevelen mechanism. Recommendations are given regarding the experimental investigations that could establish the fundamental reaction aspects of OCM by supported Mn/Na 2 WO 4 /SiO 2 catalysts that would allow for the rational design of improved catalysts.

Catalyst screening for the oxidative coupling of methane: from isothermal to adiabatic operation via microkinetic simulations

“…It can be observed that almost all these catalysts fall in the 2 nd quadrant, being characterized by above-median rankings. This is in agreement with the fact that many of the top-performing catalysts reported in literature belong to this catalyst family 61 . However, promoted NaMnW/SiO2 catalysts have proven to be thermally unstable and, thus, prone to deactivation 55 and alternatives for adiabatic operation shall thus be explored.…”

“…However, promoted NaMnW/SiO2 catalysts have proven to be thermally unstable and, thus, prone to deactivation 55 and alternatives for adiabatic operation shall thus be explored. Therefore, even though not specifically addressed in the present work, the screening and selection procedure should not disregard issues such as phase transformation, thermal stability and potential deactivation, measured via, for instance, operando characterization 61 . If catalysts such as the ones in the 1 st quadrant would prove to be more suitable than nd quadrant catalysts in this respect, the former would be of higher interest.…”

Effect of thermal treatment on the stability of Na–Mn–W/SiO2 catalyst for the oxidative coupling of methane

“…Among them, the Na-Mn-W/SiO 2 is generally accepted to be one of the most promising catalyst materials showing high catalytic performance (> 20 % yield) and long-term stability [15][16][17] . Numerous studies and reviews [18][19][20] have focused on investigating the nature of active phases/species as well as on determining the optimal catalyst composition, preferred preparation method and the role of catalyst promoters. It should be noted though that there is still much debate regarding the phases/species of the Na-Mn-W/SiO 2 catalyst including two main theories regarding the nature of active site(s).…”

Analysis of Updated Literature Data up to 2019 on the Oxidative Coupling of Methane Using an Extrapolative Machine‐Learning Method to Identify Novel Catalysts

“…Oxidative coupling of methane (OCM) reactions, in which methane (CH 4 ) is partially oxidized to C 2 hydrocarbons such as ethane (C 2 H 6 ) and ethylene (C 2 H 4 ) in a single step, represent a long‐standing challenge in the field of catalysis [29–31] . Successful OCM is difficult because the C 2 hydrocarbon products are more reactive than CH 4 and readily undergo nonselective oxidation to CO x [32–34] .…”