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.
Catalysts with only dispersed phase Na–WO4 sites where Na/W < 2 are slightly less active but significantly more C2 selective than the traditional Na2WO4/SiO2 catalysts that contain a crystalline phase where Na/W = 2.
The literature on methane dehydroaromatization (MDA) to benzene using ZSM-5 supported, group V–VIII transition metal-based catalysts (MOx/ZSM-5) is critically reviewed with a focus on in situ and operando molecular insights.
The complex structure of the catalytic active phase, and surface‐gas reaction networks have hindered understanding of the oxidative coupling of methane (OCM) reaction mechanism by supported Na2WO4/SiO2 catalysts. The present study demonstrates, with the aid of in situ Raman spectroscopy and chemical probe (H2‐TPR, TAP and steady‐state kinetics) experiments, that the long speculated crystalline Na2WO4 active phase is unstable and melts under OCM reaction conditions, partially transforming to thermally stable surface Na‐WOx sites. Kinetic analysis via temporal analysis of products (TAP) and steady‐state OCM reaction studies demonstrate that (i) surface Na‐WOx sites are responsible for selectively activating CH4 to C2Hx and over‐oxidizing CHy to CO and (ii) molten Na2WO4 phase is mainly responsible for over‐oxidation of CH4 to CO2 and also assists in oxidative dehydrogenation of C2H6 to C2H4. These new insights reveal the nature of catalytic active sites and resolve the OCM reaction mechanism over supported Na2WO4/SiO2 catalysts.
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