(218e) First-Principles Based Design of Reaction Conditions for the Catalytic Conversion of Methane to Methanol over Cu-Exchanged SSZ-13

Authors: 
Göltl, F., University of Wisconsin-Madison
Mavrikakis, M., University of Wisconsin-Madison
Due to its perfect symmetry, methane is the most stable hydrocarbon and its selective conversion to methanol is one of the long-standing challenges in catalysis. Cu containing zeolites are able to catalyze this reaction under relatively mild conditions but a variety of CuxOyHz clusters have been suggested as active sites. Results in the literature indicate that the type of active site does not only vary with the zeolite framework, but different types of active sites have been observed for the same zeolite structure prepared by different groups or under varying conditions and clear rules for the formation of these sites have not been established so far.

In this contribution, we construct a thermodynamic model for Cu in zeolite SSZ-13 based on RPA Density Functional Theory, and show that under almost all conditions the formation of a distribution of CuxOyHz clusters is favorable. We identify the chemical potential of O and H (μO and μH), the local Al distribution, the reference state and the temperature as important parameters in predicting the exact stoichiometry of these sites. We furthermore model the CuxOyHz distribution of several systems studied in the literature; comparing these results to experimental studies for the catalytic conversion of methane to methanol significantly narrows down candidates for the most active sites. In a subsequent step we then use our model to predict conditions, which lead to equally as good or better catalytic performance.

The systematic work presented here shows that the Cu speciation in zeolites is condition-dependent and that there is not one single Cu species present. Furthermore, we establish basic rules for choosing the proper conditions to optimize the performance of this catalytic system.

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