Theoretical insights into the selective oxidation of methane to methanol
- While the search for catalysts capable of directly converting methane to higher value commodity chemicals and liquid fuels has been active for over a century, a viable industrial process for selective methane activation has yet to be developed. Generally, this failure has been attributed to two primary difficulties: (1) the high barriers required to activate methane (activity), and (2) the higher activity of the C-H bonds in methanol compared to methane (selectivity). In this thesis, we delve into both of these problems and identify general frameworks and strategies that can be used in future research to identify promising catalyst materials. First, we show that, for the wide range of catalysts that proceed via a radical intermediate, a unifying framework for predicting C-H activation barriers using a single universal descriptor can be established. We combine this scaling approach with a thermodynamic analysis of active site formation to provide a map of methane activation rates, and successfully rationalize the available empirical data across catalyst classes. We then extend this analysis to catalysts that proceeed via a surface-stabilized transition state for methane activation. We present a framework for predicting the transition state's geometry (radical or surface-stabilized) and its approximate energy on a given active site. We then shift our attention to understanding and optimizing selectivity. First, we determine the principles guiding the selectivity of a related reaction, direct methanol to formaldehyde on ruthenium oxide catalysts, and find that limiting electrophilic surface oxygen coverage for this reaction is key to obtaining high selectivity. Next, recognizing that a precise and general quantification of the limitations of catalytic direct methane to methanol has not yet been established, we present a simple kinetic model to explain the selectivity-conversion tradeoff that hampers continuous partial oxidation of methane to methanol. Stemming from this analysis, we suggest several design strategies for increasing methanol yields under the constraint of the selectivity-conversion tradeoff. 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. Having identified these strategies, we work towards realizing a system capitalizing on (2), aqueous reaction conditions, by collaborating with experimental colleagues to develop a low-temperature electrochemical approach for methane oxidation. It is the general aim of this thesis to develop straightforward, accessible models with the help of Density Functional Theory that yield generalizable insights. We are hopeful that the models presented herein may provide a reliable framework for future researchers working to understand and evaluate new catalysts and processes for the direct oxidation of methane to methanol.
|Type of resource
|electronic resource; remote; computer; online resource
|1 online resource.
|Reed, Evan J
|Degree committee member
|Reed, Evan J
|Stanford University, Department of Chemical Engineering.
|Statement of responsibility
|Submitted to the Department of Chemical Engineering.
|Thesis Ph.D. Stanford University 2019.
- © 2019 by Allegra Latimer
- This work is licensed under a Creative Commons Attribution Non Commercial 3.0 Unported license (CC BY-NC).
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