(198d) Microkinetic Analysis of Methane Oxidative Coupling

Along with high throughput technology accelerating the collection of kinetic data for different catalysts, microkinetic modelling can be an effective tool for knowledge extraction from these data. In the present contribution, this has been demonstrated using methane oxidative coupling as example reaction. In particular, the microkinetic model for methane oxidative coupling includes so-called catalyst descriptors and, hence, can assist in catalyst design and development. Yields and selectivities exhibited by virtual methane oxidative coupling catalysts have been investigated by varying the catalyst descriptor values and operating conditions. The results illustrate the effect of the catalyst descriptor values on the energetics on the catalyst surface and indicate the maximum yield of the desired products as a function of the catalyst descriptors and the operating conditions.

The microkinetic model of methane oxidative coupling is based on a reaction network that contains gas phase kinetics and catalytic reactions and is implemented in a one-dimensional heterogeneous reactor model [1]. Interactions between the catalytic elementary steps and the gas-phase reactions are also accounted for. The gas phase kinetics contain 39 elementary reactions among 13 molecules and 10 radicals. The catalytic reaction network consists of 14 reactions. It describes the activation of methane on the catalyst surface by the dissociation of oxygen, hydrogen abstraction of methane and the regeneration of active site. It contains three parallel pathways for CO2 generation: the hydrogen abstraction from methoxy species on the catalyst surface in a sequence of elementary steps, CH3O→H2CO→HCO→CO→CO2; hydrogen abstraction of C2 surface species leading to radicals that are oxidized to CO2 in the gas phase; and the oxidation of adsorbed CO. The adsorption of CO2 and the quenching of hydroperoxy gas phase species are also included to account for the inhibition effect of CO2 and the quenching function of methane oxidative coupling catalyst.

The application of catalyst descriptors in the microkinetic model for methane oxidative coupling will accelerate catalyst development by reducing the number of experiments as well as the number of catalysts to be tested. A catalyst descriptor corresponds to a measurable physical or chemical property of the catalyst, preferably corresponding to an interaction between the latter and the reacting species. Based on the reaction mechanism adopted in the microkinetic model, a set of catalyst descriptors for methane oxidative coupling has been identified such as the adsorption enthalpy of oxygen, the reaction enthalpy of hydrogen-abstraction of methane, etc. Starting from the defined catalyst descriptors, the reaction enthalpies of the catalytic elementary reactions can be calculated by imposing thermodynamic consistency.

The elementary reactions of methane oxidative coupling involving the catalyst can be divided into 4 reaction families: ? hydrogen abstraction by Eley-Rideal reaction ? hydrogen abstraction by surface reaction ? CO catalytic oxidation to CO2 ? association of hydroxyl surface species to H2O Activation energies within a reaction family are obtained through Evans-Polanyi relationships, which relate the reaction enthalpy to the activation energy.

The microkinetic model with catalyst descriptors has been validated with existing experimental data on different catalysts. An illustration of the model adequacy is given for Sn/LiMgO. This shows that the microkinetic model with catalyst descriptors is a useful tool in the assessment of the large amounts of data provided by high throughput experimentations and is capable of providing guideline of the recipe for the synthesis of new catalyst. Of course, the latter requires also a quantitative relation between the synthesis conditions and the catalyst descriptors. It has been identified that the adsorption enthalpy of oxygen and the reaction enthalpy of hydrogen abstraction of methane play an important role in the yield of C2 components[2]. The effects of these two catalyst descriptors on the yield of C2 components have been investigated with the model developed in the present work. The selected ranges of the oxygen adsorption enthalpy and the reaction enthalpy of hydrogen abstraction of methane are 0-300 kJ/mol and 0 to 200 kJ/mol, respectively, corresponding to the range of values reported in literature. The values of the other descriptors are kept constant at those corresponding to the Sn/LiMgO catalyst. This hold also for the reaction condition. The latter are the conditions at which C2 yield of 14% was obtained on the Sn/LiMgO catalyst. The results shows one maximum yield and multiple local optimum yields in Figure 1.

Of course, the yield optimisation should not be limited to the variation of the 2 catalyst descriptors considered in the Figure 1. The microkinetic model allows to find the global optimal combination of catalyst properties and reaction conditions. This can be formulated as an optimisation problem, in which the objective function is defined as a function of C2 yields and selectivities and in which the optimisation variables include reaction conditions, i.e. temperature, pressure, and CH4/O2 inlet ratio, next to catalyst properties, i.e. catalyst surface area, catalyst porosity, and catalyst descriptors.

[1] Couwenberg P. M., Chen Q., Marin G. B., Ind. Eng. Chem. Res. 1996, 35 (2), 415 [2] Su, Y. S., Ying, J. Y., Green, W. H., Journal of Catalysis, 2003, 218(2), 321

Acknowledgement: The study is supported by the European research project ?TOPCOMBI? (contract NMP2-CT2005-515792).