546235 Identification of the Reaction Pathway and Active Site for the Catalytic Conversion of Methane to Methanol over Copper-Exchanged Zeolites

Tuesday, June 4, 2019: 11:51 AM
Texas Ballroom A (Grand Hyatt San Antonio)
Kimberly Dinh1, Mark Sullivan1, Pedro Serna2, Randall Meyer3, Mircea Dinc─â4 and Yuriy Roman1, (1)Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA, (2)ExxonMobil, (3)ExxonMobil, Annandale, NJ, (4)Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA

The selective oxidation of methane to value-added chemicals remains a grand challenge in catalysis. Methane monooxygenase enzymes selectively convert methane to methanol at room temperature, but these enzymes suffer from limited temperature stability and are difficult to purify. As inorganic analogs of these enzymes, small metal clusters and single metal atoms (Fe, Cu, Rh) in zeolites are capable of converting methane to methanol directly. Recently, we reported the first instance of a continuous, gas phase process for the direct conversion of methane to methanol at high selectivity using only Cu-exchanged zeolites and gaseous CH4, O2, and H2O, but the system remains largely unexplored. This work represents the first kinetic investigation and characterization of Cu-based catalysts for the steady state conversion of methane to methanol in zeolitic catalysts.

Rigorous kinetic experiments on a series of copper-exchanged zeolites show that the rate-limiting step is the C-H bond scission of methane, carbon dioxide stems from the sequential overoxidation of methanol, re-oxidation of the Cu active site is kinetically irrelevant, and water is required for methanol desorption. We then utilized in situ NH3 titration experiments coupled with infrared and x-ray absorption spectroscopy to identify a [Cu-O-Cu]2+ motif as the active site for H-abstraction from CH4, and demonstrate the necessity of zeolitic protons for both methanol desorption and copper active site formation. We finally hypothesize that zeolitic protons facilitate the diffusion of isolated, hydrated Cu species throughout the zeolite framework, allowing for the formation of dimeric Cu active sites.

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See more of this Session: Methane to Methanol I
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