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.

The involvement of lattice oxygen species is important toward oxidative coupling of the methane reaction (OCM) over supported Mn-Na 2 WO 4 /SiO 2 catalysts, but there is no consensus regarding the types, role, and origin of lattice oxygen species present in supported Mn-Na 2 WO 4 /SiO 2 catalysts, which hinders the understanding of the OCM reaction network. In the present study, by utilizing the temporal analysis of products technique, we show that supported Na 2 WO 4 /SiO 2 catalysts possess two different types of oxygen species, dissolved O 2 and atomic O, at an OCM-relevant temperature. The addition of Mn-oxide to this catalyst increases the total amount and release rate of dissolved O 2 species and improves C 2 selectivity of both dissolved O 2 and atomic lattice O species. KEYWORDS: Mn-Na 2 WO 4 /SiO 2 catalyst, oxidative coupling of methane (OCM), lattice oxygen, dissolved oxygen, molten salt, temporal analysis of products (TAP)

The nature of isolated tungsten oxide (WO x ) sites in the dispersed phase on the surface of a β-cristobalite (β-SiO 2 ) support in undoped and Na-or Mn-doped WO x /SiO 2 model catalysts used for the oxidative coupling of methane (OCM) has not been explored previously. This work provides a computational model for isolated surface WO x sites (doped and undoped) supported on β-cristobalite (β-SiO 2 ) and computationally explores their molecular structure, degree of hydration, and energetics over a wide range of OCM-relevant temperatures from 500 to 1300 K. Ab initio thermodynamic analysis showed that the most stable molecular configuration of the surface sites in all cases (WO x , Na-WO x , Mn-WO x ) was the digrafted, dioxo pseudotetrahedral WO 4 . The thermal stability of the surface WO 4 sites was in the order Na-WO 4 ≫ WO 4 > Mn-WO 4 in the OCM-relevant temperature range of 850−1300 K. A spin analysis of an Mn-WO 4 isolated surface site indicates a paramagnetic high-spin Mn 2+ −O−W 6+ electronic state, in line with literature reports. The computed frequencies for isolated surface WO 4 sites agree well with the experimental in situ Raman spectra of the corresponding model catalysts, proving their existence. Finally, steady-state OCM studies showed that the C 2 selectivity was highest for Na-WO 4 sites, followed by Mn-WO 4 and WO 4 , a trend mimicking the degree of distortion in the molecular geometry of each site. Na-WO 4 exhibited the lowest reactivity toward CH 4 and the highest degree of WO bond elongation.

Molecular-level understanding of the structure–activity relationships for oxidative coupling of methane (OCM) by the supported Mn2O3–Na2WO4/SiO2 (also written as Mn-Na-WOx/SiO2) catalyst system is currently based on hypotheses presented several decades ago that proposed that Mn–O bonds of the surface MnO x sites are directly involved in activation of CH4 and O2. The current studies, employing in situ Raman, UV–vis, and transient TPSR spectroscopies and steady-state OCM catalytic studies with nonstoichiometric SiO2-supported catalysts, however, reveal that the oligomeric MnO x surface sites and poorly crystalline Mn-WO3 and MnWO4 nanoparticles only play a minor role during OCM and essentially behave as spectator sites. The catalytic active sites responsible for activation of both CH4 and O2 for the formation of C2 products are the isolated, pseudotetrahedral, Na-coordinated WO4 surface sites (Na-WO4) on the SiO2 support. The Na-WO4 surface sites are thermally robust and do not restructure during the OCM reaction. These results indicate the need to critically re-evaluate the role of Mn-promoter for the catalytic OCM reaction, utilizing evidence-based experimental data obtained under OCM reaction conditions with nonstoichiometric catalysts.

In this work, we study temperature-induced state change of an aqueous solution of triblock copolymer composed of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), PEO-PPO-PEO (Pluronic F127), at different concentrations using rheology. While this temperature-dependent state change visually appears like a liquid–soft solid transition, and the soft solid state has been termed as a gel in the literature, there is a debate regarding the precise microstructure of the soft solid state. We observe that over a concentration domain of interest, an aqueous solution of F127 overwhelmingly demonstrates all the characteristic rheological features of not just a sol–gel–glass transition at low temperatures and glass–liquid transition at high temperatures, but also that associated with the individual states, such as sol, post-gel, and glass. The temperature at which the gel–glass transition is observed decreases while the temperature associated with glass–liquid transition increases with an increase in the concentration of F127. Based on the observed behavior, we propose a mechanism that considers the change in micelle volume fraction and alteration of the hydrophilicity of PEO corona as a function of temperature. Finally, we construct a phase diagram and discuss the similarities and differences with respect to various phase diagrams of F127 solution available in the literature.