Synthesis and molecular structure of model silica-supported tungsten oxide catalysts for oxidative coupling of methane (OCM)

Abstract

The molecular and electronic structures and chemical properties of the active sites on the surface of supported Na₂WO₄/SiO₂ catalysts used for oxidative coupling of methane (OCM) are poorly understood. Model SiO₂-supported, Na-promoted tungsten oxide catalysts (Na–WO_x/SiO₂) were systematically prepared using various Na- and W-precursors using carefully controlled Na/W molar ratios and examined with in situ Raman, UV-vis DR, CO₂-TPD-DRIFT and NH₃-TPD-DRIFT spectroscopies. The traditionally-prepared catalysts corresponding to 5% Na₂WO₄ nominal loading, with Na/W molar ratio of 2, were synthesized from the aqueous Na₂WO₄·2H₂O precursor. After calcination at 800 °C, the initially amorphous SiO₂ support crystallized to the cristobalite phase and the supported sodium tungstate phase consisted of both crystalline Na₂WO₄ nanoparticles (Na/W = 2) and dispersed phase Na–WO₄ surface sites (Na/W < 2). On the other hand, the catalysts prepared via a modified impregnation method using individual precursors of NaOH + AMT, such that the Na/W molar ratio remained well below 2, resulted in: (i) SiO₂ remaining amorphous (ii) only dispersed phase Na–WO₄ surface sites. The dispersed Na–WO₄ surface sites were isolated, more geometrically distorted, less basic in nature, and more reducible than the crystalline Na₂WO₄ nanoparticles. The CH₄ + O₂-TPSR results reveal that the isolated, dispersed phase Na–WO₄surface sites were significantly more C₂selective, but slightly less active than the traditionally-prepared catalysts that contain crystalline Na₂WO₄ nanoparticles (Na/W = 2). These findings demonstrate that the isolated, dispersed phase Na–WO₄ sites on the SiO₂ support surface are the selective-active sites for the OCM reaction.