(247a) Oxidative Coupling of Methane: Descriptors for Activity and Selectivity

Authors: 
Kumar, G. - Presenter, Pennsylvania State University
Lau, S. L. J. - Presenter, Pennsylvania State University
Janik, M. J. - Presenter, Pennsylvania State University

Oxidative Coupling of
Methane: descriptors for activity and selectivity

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

Department of
Chemical Engineering, Pennsylvania State University, University Park, PA

*mjanik@engr.psu.edu

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 °¤CH3 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 °¤CH3 adsorption) of the catalyst correlate with surface
reducibility.
C-H activation energy and °¤CH3 adsorption energy have
been plotted against the oxygen vacancy formation energy ¥ÄEvac
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 °¤CH3 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 °¤CH3
strongly leading to over-oxidation of methane and thus gives low C2
selectivity. Conversely, high C-H activation energy and weak °¤CH3
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.

Figure 1. Correlation
between surface reducibility and activity/selectivity

References

1.       
G. E. Keller, M. M. Bhasin, Journal
of Catalysis
73 (1982) 9.

2.       
T. Ito, J.H. Lunsford, Nature 314
(1985) 721.