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

Abstract

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 alpha-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 alpha-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 alpha-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.