A Theoretical Study of Cr/oxide Catalysts for Dehydrogenation of Short Alkanes
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Natural gas reserves are being produced at increasing rates world wide, thus providing an abundance of short-alkane feedstock for the petrochemical industry. Effective routes to selective activation of short alkanes may represent a bottleneck of this development. In this respect, processes of catalytic dehydrogenation have significant economic potential. Several catalysts have been investigated and some are implemented industrially. At the core of one such catalyst are species of chromium oxide supported on the surface of amorphous alumina. Since the original report by Frey and Huppke in 1933, this system has been under study, the literature expanding with the development of experimental methods. However, due to the heterogeneity of the Cr/alumina surfaces a structure-activity relationship has not been determined to date. Opinions are converging regarding the oxidation state of active chromium, but little is known about the nano-range surroundings of the active site. The involvement of oxygen ligands is a popular issue related to the mechanism of catalysis. Other remaining questions concern the chromium nuclearity of the active surface site and the role of the oxide support. With the development of quantum chemical tools and powerful hardware, computational modelling emerges as a manageable complement to experiment. The combination may lead to novel insight into the structure-activity relationship on Cr/oxide surfaces. Ultimately this may contribute to more effective catalysts by rational design. For this thesis, model catalysts have been investigated computationally with respect to (i) mechanisms of catalytic dehydrogenation, and (ii) surface reactions of chromium precursor species leading to the active catalyst. The work includes simulated annealing to generate realistic surface models, and reactivity studies based on cluster and embeddedcluster approaches. Based on these computations, two mechanisms of catalytic dehydrogenation appear viable depending on the nano range surrounding the active site. One mechanism is found to involve cleavage and reformation of a Cr–O bond. However, the activation energy of this mechanism is found prohibitively high unless the Cr–O bond is weakened by structural strain. Alternatively, a pre-catalytic step of C–H activation in alkane involving irreversible cleavage of a Cr–O bond is found leading to formation of a hydridochromium moiety, which appears highly active according to a second mechanism of catalytic dehydrogenation. Regarding the role of the support, this is proposed one of providing precursor species and sustaining the reactive sites. Moreover, it appears that polynuclear sites of the active chromium species are formed in the typical steps of catalyst preparation. These results form the basis for a hypothesis on which novel experiments are suggested.
Paper I: Journal of Physical Organic Chemistry 17(11), Lillehaug, Sindre; Børve, Knut J.; Sierka, Marek and Joachim Sauer, (2004), Catalytic Dehydrogenation of Ethane over Mononuclear Cr(III)/silica Surface Sites. Part I. C–H Activation by σ-bond metathesis, pp. 990-1006. Copyright John Wiley & Sons Limited. Reproduced with permission. Published version available at: http://dx.doi.org/10.1002/poc.842Paper II: Journal of Physical Organic Chemistry 19(1), Lillehaug, Sindre; Jensen, Vidar R. and Knut J. Børve, (2006), Catalytic Dehydrogenation of Ethane over Mononuclear Cr(III)/silica Surface Sites. Part II. C-H Activation by Oxidative Addition, pp. 25-33. Copyright John Wiley & Sons Limited. Reproduced with permission. Published version available at: http://dx.doi.org/10.1002/poc.990Paper III: Lillehaug, Sindre and Knut J. Børve, The Role of the Carrier Oxide in Cr/oxide Catalysts for Dehydrogenation of Ethane. Pre-print.Paper VI: Lillehaug, Sindre and Knut J. Børve, Oxidation of Cr(II) to Cr(III) on the Surface of Amorphous Silica, with Implications for Catalytic Dehydrogenation of Ethane. Pre-print.