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

The literature for the oxidative coupling of methane (OCM) on supported Mn/Na2WO4/SiO2 catalysts is systematically and critically reviewed. The influence of the precursors, starting SiO2 support crystallinity, synthesis method, calcination temperature, and OCM reaction conditions on the catalyst structure is examined. The supported Mn/Na2WO4/SiO2 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 SiO2 (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 CH4 and CD4. Although the reaction kinetics follow a Langmuir–Hinshelwood type mechanism, r = [CH4]1[O2]1/2, isotopic 18O2–16O2 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/Na2WO4/SiO2 catalysts that would allow for the rational design of improved catalysts.