Strategies for Valorization of Methane: A Computational Perspective

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 an industrially viable process for direct conversion to methanol does not exist. Beginning with a universal descriptor model [1] that describes C-H activation energies for a range of catalysts (with special attention to cation-exchanged zeolites)[2-4], we will present a simple kinetic model to explain the selectivity-conversion tradeoff that hampers continuous partial oxidation of methane to methanol.[5] 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, ∆G^a (dictated by the relative reactivity of the C-H bonds in methane and methanol). Using this 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 ∆G^a. 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.

1. ACS Catal., 2018, 8, pp 6894–6907
2. ACS Catal., 2016, 6, 10, pp 6531–6536
3. ACS Catal., 2016, 6, 6, pp 3760–3766
4. Catal. Sci. Technol., 2018, 8, pp 114-123
5. ACS Catal., 2018, 8, pp 6894–6907