(560ek) Kinetics of O2 Activation over Cu-Exchanged Zeolites: Implications for Partial Methane Oxidation

Authors: 
Bregante, D. T., University of Illinois, Urbana-Champaign
Bregante, D. T., University of Illinois, Urbana-Champaign
Wilcox, L. N., Purdue University
Wilcox, L. N., Purdue University
Gounder, R., Purdue University
Gounder, R., Purdue University
Flaherty, D., University of Illinois, Urbana-Champaign
Flaherty, D., University of Illinois, Urbana-Champaign
The direct low temperature conversion of methane to methanol is considered a “holy grail” within the catalysis community, as current technologies are indirect routes that involve steam reforming of CH4 to form syngas which is subsequently converted to CH3OH over Cu-based catalysts [1]. Cu-exchanged zeolites (e.g., CHA, MFI, MOR) have received renewed attention as candidate materials for partial methane oxidation, often requiring stoichiometric cycles involving activation in O2 at high temperatures (~723 K) followed by reaction with CH4 at low temperatures (~473 K) to form chemisorbed methoxy-derived species that can be extracted by H2O [1]. Currently, Cu-exchanged zeolites do not give acceptable combinations of rates and selectivities for continuous catalytic operation, because the requirements for high selectivities (i.e., low temperature CH4 activation, stoichiometric step-wise reactions) are orthogonal to those that enable high rates (i.e., high temperature, isothermal continuous catalytic reaction). This is due, in part, to the lack of understanding of each step along the step-wise process (i.e., O2 activation, CH4 reaction, methanol extraction with H2O). Several studies have investigated the reaction between O2-activated Cu-zeolites with CH4 using in situ UV-vis spectroscopy, which have shown relatively fast kinetics (complete reaction in <0.5 h, 473 K) in comparison to the time required for the entire step-wise process (>6 h) [2]. The lack of precise details regarding the Cu-based active sites and mechanisms for O2 activation over Cu-zeolites have prevented extending empirical findings regarding optimizing CH3OH production on one material to broader classes of materials. Here, we utilize in situ resonance Raman spectroscopy and UV-Visible spectroscopy to monitor the kinetics of O2 activation over Cu-exchanged zeolite CHA (Cu-CHA) to develop a mechanistic understanding for this process.

The activation of O2 over Cu-CHA produces bis(μ-oxo) dicopper(II) and μ-(η2:η2)peroxo dicopper(II) intermediates that are Raman active and are often invoked as the intermediates responsible for CH4activation [2-5]. Multivariate curve resolution-alternative least squares was used to extract high-quality Raman spectra and time-resolved contributions from transient measurements to investigate the kinetics of O2 activation. The kinetics for the reaction between O2 and dimeric Cu active sites (Cu2) closely resemble a first-order reaction, because in all cases O2 is in large excess. The rates of O2 activationare independent of temperature above 698 K; yet, decrease exponentially with temperature below 698 K, which suggests that there may be two distinct reference states of Cu2 that depend strongly on temperature. On-going work seeks to extend this methodology for studying O2 activation to other zeolitic frameworks and explore the dependence on additional reaction conditions (e.g., O2 pressure, H2O pressure).

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