Guest Lecturer Joachim Sauer Discusses C-H Bond Activation in Oxidative Coupling of Methane
Professor Sauer, who has been Professor of Theoretical Chemistry at Humboldt University in Berlin, Germany, has been an external member of the Fritz Haber Institute (Max Planck Society) since 2006. His research has explored the application of quantum chemical methods in chemistry, with emphasis on surface science, particularly adsorption and catalysis.
An abstract of his lecture follows:
With the raw material shift in chemical industry to natural gas, there is renewed interest in the formation of higher hydrocarbons, e.g., in the formation of ethylene by oxidative coupling of methane (OCM). The simplest catalysts for this reaction, among a large number of complex solid oxides, is Li-doped MgO. Early, Lunsford proposed that the active sites are O•– radicals neighbored to Li+ and that the C–H bond is activated by homolytic splitting involving hydrogen atom transfer to the O•– sites.
Density functional calculations yield energy barriers for H abstraction by oxygen radical sites in Li-doped MgO that are much smaller (12 ± 6 kJ/mol) than the barriers inferred from different experimental studies (80 – 160 kJ/mol). This raises doubts that the Li+O•– site is the active site as postulated by Lunsford. This is supported by temperature programmed oxidative coupling reaction experiments (Horn et al.) which show that the same sites are responsible for the activation of CH4 on both Li-doped and pure MgO catalysts, and by the synthesis of nano-structured pure MgO catalysts (Schlögl et al.). Further calculations showed that CH4 binds heterolytically on Mg2+O2- sites at steps and corners, but that the homolytic release of methyl radicals into the gas phase will happen only in the presence of O2.
Unlike transition metal oxide catalysts, the role of simple oxides is to bring together the reactants which exchange redox equivalents directly among them.
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More about Professor Joachim Sauer:
Dr. Sauer received the Dr. rer. nat. degree in Chemistry from Humboldt University in Berlin in 1974, and the Dr. sc. nat. degree from the Academy of Sciences in (East-)Berlin in 1985. From 1999 to 2011 he was chairman of the Collaborative Research Center of the German Research Foundation (DFG) "Aggregates of transition metal oxides – Structure, dynamics, reactivity" and he is co-founder and principal investigator of the DFG-funded Cluster of Excellence “Unifying concepts in catalysis” in Berlin. He is also a member of the Berlin-Brandenburg (formerly Prussian) Academy of Sciences, the German National Academy Leopoldina, and the Academia Europaea.
He has published more than 330 research papers, notably in the area of modeling the structure and reactivity of transition metal oxide catalysts and zeolites, and he has given more than 380 invited lectures.