(445h) Selective Oxidation of Methane to Methanol: How to Live with the Selectivity-Conversion Limit

Authors: 
Kakekhani, A., University of Pennsylvania
Latimer, A. A., Stanford University
Kulkarni, A. R., Georgia Institute of Technology
Nørskov, J., Stanford University and SUNCAT
Selective Oxidation of Methane to Methanol: How to Live with the Selectivity-Conversion Limit

Arvin Kakekhani [1],[a], Allegra Latimer [1],[a], Ambarish R. Kulkarni [1], Jens K. Nørskov [1],[2]

arvin.kake@gmail.com

[1] SUNCAT Center for Interface Science and Catalysis, Department of Chemical Engineering, Stanford University, 443 Via Ortega, Stanford, California 94305, United States

[2] SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States

[a] Authors with equal contributions

Selective partial oxidation of methane to methanol has long been one of the holy grails in catalysis. Currently, methane is transformed into methanol through the two-step syngas process that necessitates high temperatures and centralized production. Despite over a century of research no industrially viable and selective process exists for direct methane conversion to methanol. In this work,1 we first present a simple kinetic model to explain the selectivity-conversion tradeoff that hampers continuous partial oxidation of methane to methanol. We show that the selectivity of methane to methanol in a direct, continuous process can be fully described by the methane conversion, temperature, and the catalyst-independent difference in methane and methanol activation free energies, ∆Ga, which is dictated by the relative reactivity of the C-H bonds in methane and methanol. Using such model we successfully rationalize a large number of experimental studies from the diverse fields of heterogeneous, homogeneous, biological and gas-phase catalysis. Based on this understanding, we suggest several design strategies for increasing methanol yields under the constraint of constant ∆Ga. These strategies include (1) “collectors,” materials with strong methanol adsorption potential that can help to lower the partial pressure of methanol in the gas phase, (2) aqueous reaction conditions, and/or (3) diffusion-limited systems. These can provide experimentalists in the field some new insight and suggestions for designing more selective processes for direct methane to methanol conversion.

References

(1) Allegra A. Latimer, Arvin Kakekhani, Ambarish R. Kulkarni, Jens K. Nørskov ACS Catalysis (under-review)