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

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 SiO₂-supported Na₂WO₄/Mn to suppress the undesired deep oxidation affording CO and CO₂. Currently, the phase transformation of SiO₂ supports to α-cristobalite is known to be important for securing high OCM activity, although the underlying reasons of this influence remain debatable. Herein, Na₂WO₄/Mn catalysts supported on several SiO₂-based materials including conventional amorphous SiO₂ 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 C₂₊ hydrocarbons (paraffins and olefins) reported for dopant-free Na₂WO₄/Mn/SiO₂ catalysts to date. The results of XRD, Raman, and ²⁹Si NMR indicated that this high performance can be ascribed to the stabilization of tetrahedral WO₄ 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 Na₂WO₄/Mn/SiO₂ catalysts and paves the way to the design of better catalysts through the modification of zeolite supports.