We have studied the direct and stepwise selective oxidation of methane to methanol over copper-mordenite, (Cu/MOR) under conditions of plug flow using is combined and quantitative time resolved Cu K-edge XANES and online mass spectrometry (MS). We delineate the kinetics and speciation involved in methane activation in Cu/MOR and then go on to understand aspects the character of product release through steaming in the temperature range 403 to 493 K. Time resolved Cu K-edge XAS, allied to Principal component analysis (PCA), reveals that two spectroscopically discrete Cu(I) species arise from the activation of methane. Moreover, we show that two distinct regimes for methane activation exist as function of methane exposure. We conclude that copper (I) hydroxyls, rather than being active species, are a direct product in both mechanisms. We show that the steaming of products (dimethyl ether, methanol, carbon dioxide, and carbon dioxide) is a process comprising three distinct phases:  zeolite hydration; product release, and formation of further copper (I) hydroxyls; and finally slow re-oxidation of Cu (I) to Cu (II). We further show that product release can be achieved on much shorter timescales than previously realized, and that different products are evolved on different timescales leading to temporal dependence in reaction selectivity within the stepwise process. Quantitative cross correlation of Cu K-edge XANES with mass spectrometry in a plug flow environment is shown to be extremely informative regarding fundamental copper speciation and reactive kinetics, but also aspects of process chemistry that are of significant import in respect of designing a viable application of this challenging but highly sought after chemistry.

Kinetics studies of the direct conversion of methane to methanol by Cu/mordenite zeolites: What can they tell us about fundamental and process related aspects of the “Holy Grail” of catalysis?

2019 Fall National Meeting

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