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

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 ¥ÄE_vac
of various metal-doped CeO₂, doped MgO, doped TiO₂, ZnO
and TbO_x. 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.