(257g) High Temperature Characterization of Reaction Active Sites in Mn?Na2WO4/SiO2 Catalyst for the Oxidative Coupling of Methane

Accurate quantification of catalytic active sites is of great importance to properly reference the properties of a catalyst, to fundamentally understand the reaction kinetics, and to advance catalyst design. Oxidative coupling of methane (OCM) is a promising single-step process for conversion of methane to ethylene and, while the Mn‐Na₂WO₄/SiO₂ catalyst is one of the most widely, the structural characterization of this complex industrial catalyst is far more challenge than single atom catalyst as the Na₂WO₄ surface phase becomes a molten at temperatures near 700 ^oC. As a result, the commonly used low temperature chemisorption method cannot provide accurate reference for the active sites in use at reaction temperatures.

In this study, we apply recently developed pulse response protocols [1] to quantify active sites on OCM catalysts at reaction temperatures. Methane is reversibly adsorbed 1.5Mn-5Na₂WO₄/SiO₂ surfaces as indicated in CH₄/Ar pulse experiment shown in Figure 1A. From multiple time-dependent gas and surface concentration series at increasing pulse intensity, a point of momentary equilibrium point is followed. Figure 1B shows the dependency of methane gas and surface concentration and its isotherm regression. Based on their dependency, the number of active sites for methane adsorption is estimated to be 3.3·10¹² sites/cm². The same methodology was also applied to study oxygen activation (1.6·10¹⁴ sites/cm²). We hence find significantly more active sites for oxygen activation compared to methane. The distinct oxygen and methane activation sites over Na₂WO₄/SiO₂ were 50 and 7 times less, respectively, as compared to the Mn‐Na₂WO₄/SiO₂ catalyst.

This unique feature of time-dependent experiments fills a critical gap for precise catalyst characterization and renders more accurate determination of turnover frequency.

1. Redekop, et al. Industrial & Engineering Chemistry Research, 52 (2013) 15417.