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Oxidative coupling of methane over Ce-based mixed oxides
J. Silva, J. Pereira, B.F. Machado, A.M.T. Silva and J.L. Faria
Laboratory of Catalysis and Materials (LCM), Associate Laboratory LSRE/LCM,
Chemical Engineering Department, Faculty of Engineering, University of Porto, Portugal.
One of the main advantages of the oxidative coupling of methane (OCM) is its ability to
produce higher hydrocarbons (ethane and ethylene, C2+) directly from natural gas. For this
reason, a great deal of effort has been devoted to enhance both methane conversion and C2
hydrocarbon yield in the OCM reaction.
The most efficient catalysts for the OCM reaction have been found to possess electrophilic
oxygen centers/defects (oxygen vacancies or hole O – -type centers), as these sites are thought to
be responsible for methane activation [1]. Furthermore, the selectivity to C2+ was also found to
vary according to the oxide band-gap: the higher the band gap, the more selective to C2+ in the
OCM reaction. Unfortunately, conversion of methane to C2+ hydrocarbons with high selectivity
is difficult to achieve because complete oxidation to CO2 and H2O often occurs. It is believed
that methyl radicals that are formed at the surface of the catalyst enter the gas phase where they
couple to form ethane and/or ethylene. In addition to coupling, the gas phase radicals may enter
into chain reactions that result in the formation of CO and subsequently CO2. Thus, one of the
challenges in catalyst development is to produce a material so that secondary reactions of C2+
are inhibited while the activation of CH4 still occurs [2].
Ce-based mixed oxides are synthesized by the solvothermal approach using the corresponding
nitrates as precursors, and methanol as solvent. The as prepared materials are introduced on a
quartz wool plug inside a continuous-flow reactor, used for the OCM experiments. Products
are analyzed by gas chromatography and the results of the reaction runs are analyzed in terms
of CH4 conversion and C2+ selectivity and yield.
In this work we compare and discuss the effect of Ce-based mixed oxides on both the activity
and selectivity to C2+ during the OCM reaction. These parameters are dependent on the
experimental conditions such as reaction temperature and methane/oxygen ratio.
Acknowledgments:
J.S. and J.P. gratefully acknowledge Fundação para a Ciência e a Tecnologia the BII (2009) grants.
References:
[1] Voskresenskaya, E.N., Roguleva, V. G., Anshits, A.G. (1995), Oxidant activation over structural
defects of oxide catalysts in oxidative methane coupling, Catalysis Reviews, 37 (1), 101-143.
[2] Lunsford, J.H. (2000), Catalytic conversion of methane to more useful chemicals and fuels: a
challenge for the 21st century, Catalysis Today, 63 (2-4), 165-174.
378 3 rd meeting of young researchers at UP