Na2WO4/Mn supported on all-silica delaminated zeolite for the optimal oxidative coupling of methane via the effective stabilization of tetrahedral WO4: Elucidating effects of support precursors with different crystal structures, Al-addition, and morphologies

[Display omitted] • SiO 2 -supported Na 2 WO 4 /Mn catalysts for oxidative coupling of methane are prepared. • Role of α-cristobalite formation in support is probed by assessing catalytic activity. • α-Cristobalite is shown to secure high activity by stabilizing WO 4 tetrahedrons. • Unprecedentedly high methane conversion and C 2+ selectivity are achieved. • Strategy for better catalyst design through zeolite support modification is provided. The oxidative coupling of methane (OCM) converts this abundant natural feedstock into value-added products and is typically performed in the presence of catalysts such as SiO 2 -supported Na 2 WO 4 /Mn to suppress the undesired deep oxidation affording CO and CO 2. Currently, the phase transformation of SiO 2 supports to α-cristobalite is known to be important for securing high OCM activity, although the underlying reasons of this influence remain debatable. Herein, Na 2 WO 4 /Mn catalysts supported on several SiO 2 -based materials including conventional amorphous SiO 2 and crystalline zeolites were prepared to close the above knowledge gap and thus promote the design of more efficient OCM catalysts. The best support was identified as calcined D -ITQ-1, which is a well-developed delaminated zeolite with a thin basal all-silica nanosheet structure facilitating the transition to the α-cristobalite phase. The corresponding catalyst retained high activity over 100 h and exhibited one of the highest yields (25.8%) of C 2+ hydrocarbons (paraffins and olefins) reported for dopant-free Na 2 WO 4 /Mn/SiO 2 catalysts to date. The results of XRD, Raman, and 29Si NMR indicated that this high performance can be ascribed to the stabilization of tetrahedral WO 4 units due to the transition from uncalcined silica to α-cristobalite. Thus, the present work advances our understanding of structural phase transitions and the concomitant formation of the desired active species in Na 2 WO 4 /Mn/SiO 2 catalysts and paves the way to the design of better catalysts through the modification of zeolite supports. [ABSTRACT FROM AUTHOR]