A combined computational and experimental study of methane activation during oxidative coupling of methane (OCM) by surface metal oxide catalysts

Abstract

The experimentally validated computational models developed herein, for the first time, show that Mn-promotion does not enhance the activity of the surface Na2WO4 catalytic active sites for CH4 heterolytic dissociation...

Machine Learning-Aided Catalyst Modification in Oxidative Coupling of Methane via Manganese Promoter

“…Because such multiple synergy is expected to occur only slightly when using a conventional method, this new finding is anticipated as a predominant protocol for ML-aided catalyst investigation. Moreover, the role of Mn still remains an open question in OCM studies. ,, For instance, there has been reports on not only the positive effect in Mn-Na 2 WO 4 /SiO 2 and K x MnO y /SiO 2 but also the negative effect in the La 2 O 3 - or MgO-based catalyst . Because under the OCM the following occur: (i) C 2 production is influenced not only via the surface of catalyst but also via the outer surfaces (gas-phase) reaction. − (ii) The reaction usually performs under a high reaction temperature above melting points of metal components. − (iii) Synergy of components might be different from kinds of support, ,, and the effect of Mn in OCM under operation conditions is still a controversial subject.…”

“…Moreover, the role of Mn still remains an open question in OCM studies. 39,43,44 For instance, there has been reports on not only the positive effect in Mn-Na 2 WO 4 /SiO 2 45 and K x MnO y /SiO 2 46 but also the negative effect in the La 2 O 3 -or MgO-based catalyst. 17 Because under the OCM the following occur: (i) C 2 production is influenced not only via the surface of catalyst but also via the outer surfaces (gas-phase) reaction.…”

Structure–Activity Relationships in Lewis Acid–Base Heterogeneous Catalysis

“…MOs are well-established catalysts for methane activation and conversion. − Methane activation on MOs takes place through two competing mechanisms: radical and surface-stabilized, as illustrated in Figure . , The former is characterized by the formation of methyl radical and surface hydroxyl species . In the surface-stabilized mechanism, methane can either dissociate into methyl-hydroxy (M–CH 3 , O–H) or methoxy-hydride (O–CH 3 , M–H) surface intermediate pairs. − However, the TS of the latter has higher energy than that of the methyl-hydroxy pathway. , This can be attributed to the electrostatic interaction between the dipoles of the C–H and M–O bonds of the methoxy-hydride pathway, inducing a charge distribution that results in a negatively charged hydrogen (H δ− ). , In the methyl-hydroxy pathway, the electrostatic interaction at the TS is favored, and no additional charge distribution occurs, leading to a TS with lower energy. , Additionally, Lewis basic oxygen atoms have higher binding affinity to hydrogen than to methyl group, resulting in a more stable methyl-hydroxy surface intermediate pair . Generally, the initial C–H bond activation of methane is postulated to be the rate-limiting step in oxidative coupling of methane (OCM), methane combustion, and methane reforming. − Consequently, the ability of the catalyst to activate the methane C–H bond is an essential property of an ideal methane conversion catalyst.…”

“…52−56 Methane activation on MOs takes place through two competing mechanisms: radical and surfacestabilized, as illustrated in Figure 3. 57,58 The former is characterized by the formation of methyl radical and surface hydroxyl species. 57 In the surface-stabilized mechanism, methane can either dissociate into methyl-hydroxy (M− CH 3 , O−H) or methoxy-hydride (O−CH 3 , M−H) surface intermediate pairs.…”

High‐Performance Mn2O3‐Na2WO4/SiO2‐TiO2 Catalyst for the Oxidative Coupling of Methane: TiO2‐Modulated MnTiO3 Formation for Enhanced Low‐Temperature Performance

“…Thus, it is essential for the ideal catalyst to have selective surface oxygen ion radicals with suitable mobility, [21][22][23] enabling them to function as active sites for methyl radical generation while avoiding deep oxidation on the surface. [24,25] For this purpose, various simple, complex, and mixed oxides of alkaline, alkalineearth, and rare-earth elements have been investigated, with some of these showing high activity. [26][27][28][29][30][31] One such typical early catalyst is Li/MgO, with the Li + O À species on the surface able to generate CH 3 • from CH 4 efficiently; however, these are also subject to rapid deactivation owing to the loss of Li.…”