Title: Direct Methane to Methanol: The Selectivity–Conversion Limit and Design Strategies

Abstract

Currently, methane is transformed into methanol through the two-step syngas process, which requires high temperatures and centralized production. While the slightly exothermic direct partial oxidation of methane to methanol would be preferable, no such process has been established despite over a century of research. Generally, this failure has been attributed to both the high barriers required to activate methane as well as the higher activity of the CH bonds in methanol compared to those in methane. However, a precise and general quantification of the limitations of catalytic direct methane to methanol has yet to be established. Herein, we present a simple kinetic model to explain the selectivity–conversion trade-off that hampers continuous partial oxidation of methane to methanol. For the same kinetic model, we apply two distinct methods, using ab initio calculations and fitting to a large experimental database, to fully define the model parameters. We find that both methods yield strikingly similar results, namely, that the selectivity of methane to methanol in a direct, continuous process can be fully described by the methane conversion, the temperature, and a catalyst-independent difference in methane and methanol activation free energies, ΔG^a, which is dictated by the relative reactivity of the C–H bonds inmore » methane and methanol. Stemming from this analysis, we suggest several design strategies for increasing methanol yields under the constraint of constant ΔG^a. These strategies include “collectors”, materials with strong methanol adsorption potential that can help to lower the partial pressure of methanol in the gas phase, aqueous reaction conditions, and/or diffusion-limited systems. Here, by using this simple model to successfully rationalize a representative library of experimental studies from the diverse fields of heterogeneous, homogeneous, biological, and gas-phase methane to methanol catalysis, we underscore the idea that continuous methane to methanol is generally limited and provide a framework for understanding and evaluating new catalysts and processes.« less

Authors:

Latimer, Allegra A.  ^[1];

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Kakekhani, Arvin  ^[1];

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Kulkarni, Ambarish R. ^[1]; Nørskov, Jens K. ^[2]

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+ Show Author Affiliations

1. SUNCAT Center for Interface Science and Catalysis, Department of Chemical Engineering, Stanford University, 450 Serra Mall, Stanford, California 94305, United States
2. SUNCAT Center for Interface Science and Catalysis, Department of Chemical Engineering, Stanford University, 450 Serra Mall, Stanford, California 94305, United States, SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States

Publication Date:

2018-06-29

Research Org.:

SLAC National Accelerator Lab., Menlo Park, CA (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES)

OSTI Identifier:

1460114

Alternate Identifier(s):

OSTI ID: 1459595

Grant/Contract Number:  

32 CFR 168a; AC02-76SF00515

Resource Type:

Published Article

Journal Name:

ACS Catalysis

Additional Journal Information:

Journal Name: ACS Catalysis Journal Volume: 8 Journal Issue: 8; Journal ID: ISSN 2155-5435

Publisher:

American Chemical Society

Country of Publication:

United States

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; density functional theory; heterogeneous protecting groups; methane to methanol; methanol collector; partial oxidation; selective oxidation; selectivity−conversion limit