413843 oxidative coupling of methane: Descriptors for activity and selectivity

Gaurav Kumar, Sai Lap Jacky Lau, Michael J. Janik

Research output: Chapter in Book/Report/Conference proceedingConference contribution

Abstract

Increasing natural gas reserves in the recent years motivate further development of processes to convert methane into value-added products. Since the initial description [1] of catalytic oxidative coupling of methane (OCM) to form C2 hydrocarbons, various pure/doped metal-oxide catalysts have been examined for their methane conversion and C2 selectivity. This ongoing search to find high performing catalysts has been pre-dominantly empirical, and the relationship between active site properties and catalyst activity and selectivity remains unclear. A known dependence of the OCM activity and selectivity on material property (s) enables us to tune active site properties to optimize catalyst performance. This study uses density functional theory (DFT) methods and the established OCM reaction mechanism [2] to relate activity and selectivity to the surface electronic and structural properties of the catalyst. C-H activation and °nCH3 radical adsorption determine the activity and selectivity of a catalyst, respectively. Since both of these elementary processes reduce the catalyst surface, we hypothesize that the activity (measured by C-H activation) and selectivity measured by °oCH3 adsorption) of the catalyst correlate with surface reducibility. C-H activation energy and QCH3 adsorption energy have been plotted against the oxygen vacancy formation energy AEvac of various metal-doped CeO2, doped MgO, doped TiO2, ZnO and TbOx. Computational results (Figure 1) show a linear relation of the vacancy formation energy (surface reducibility), with the C-H activation energy and nCH3 adsorption energy. Ceria has a lower C-H activation energy which suggests that it is highly active and would offer better methane conversion. However, it also binds °nCH3 strongly leading to over-oxidation of methane and thus gives low C2 selectivity. Conversely, high C-H activation energy and weak °nCH3 adsorption imply that pure/doped MgO catalysts will have better C2 selectivity but low activity. Therefore, a trade-off between activity and selectivity is inherent in the active site. Sub-correlations to that shown in Figure 1 demonstrate that this correlation holds across a single material with increasing state of reduction as well as with change in U parameter used in the DFT+U method. Methods on finding an optimal trade-off in activity and selectivity, as well as potential approaches to breaking this correlation, will be discussed.

Original languageEnglish (US)
Title of host publicationLiaison Functions 2015 - Core Programming Area at the 2015 AIChE Annual Meeting
PublisherAIChE
Pages160-161
Number of pages2
ISBN (Electronic)9781510818637
StatePublished - Jan 1 2015
EventLiaison Functions 2015 - Core Programming Area at the 2015 AIChE Annual Meeting - Salt Lake City, United States
Duration: Nov 8 2015Nov 13 2015

Publication series

NameLiaison Functions 2015 - Core Programming Area at the 2015 AIChE Annual Meeting

Other

OtherLiaison Functions 2015 - Core Programming Area at the 2015 AIChE Annual Meeting
CountryUnited States
CitySalt Lake City
Period11/8/1511/13/15

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All Science Journal Classification (ASJC) codes

  • Renewable Energy, Sustainability and the Environment
  • Chemical Engineering(all)
  • Engineering(all)
  • Modeling and Simulation

Cite this

Kumar, G., Lau, S. L. J., & Janik, M. J. (2015). 413843 oxidative coupling of methane: Descriptors for activity and selectivity. In Liaison Functions 2015 - Core Programming Area at the 2015 AIChE Annual Meeting (pp. 160-161). (Liaison Functions 2015 - Core Programming Area at the 2015 AIChE Annual Meeting). AIChE.