Mechanisms of hydrocarbon conversion reactions on heterogeneous catalysts: analogies with organometallic chemistry

Abstract

Catalysis is central to most industrial processes for chemical manufacturing. As catalytic processes have become more complex and more demanding, selectivity has become the central issue in their design. Selectivity is defined by the relative rates of competing reaction pathways available to crucial intermediates, and can be controlled by subtle changes in the nature of the catalyst, the reactants, and/or the reaction conditions. In order to be able to do this in a systematic manner, a good understanding of the catalytic reaction mechanisms is needed. Here a connection is drawn between the key elementary steps comprising hydrocarbon conversion reactions on surfaces and those known to occur on discrete organometallic complexes. This way, the hydrogenation, dehydrogenation, hydrogenolysis, chain growth, and isomerization reactions typical in heterogeneous catalysis are redefined in terms of hydride elimination, oxidative addition, reductive elimination, migratory insertion, and . 1, 2-shift elementary steps, among others. It is suggested that the knowledge already available from organometallic chemistry can be used to further advance the understanding of the surface science involved in heterogeneous catalysis. Thanks to the commonality of the chemistry involved, a better synergy could also be established between homogeneous and heterogeneous catalytic development. These ideas are discussed in this article in a critical and personal way.

Kinetic measurements of hydrocarbon conversion reactions on model metal surfaces

“…3,31,75 In the example of Fig. 2, total dehydrogenation and H 2 desorption are seen exclusively for doses up to 1.0 L, but significant alkene (from b-hydride elimination) 8,35 and alkane (from reductive elimination with coadsorbed hydrogen) 31,76,80 production is observed at coverages close to monolayer saturation. It should also be said that the detection of even minority products in TPD experiments may be sufficient to speculate on possible catalytic processes provided that the proper reaction conditions are identified.…”

“…66 By relying on the lability of carbon-halogen bonds (especially of carboniodine bonds), haloalkanes have been often used to prepare alkyl surface intermediates on surfaces; 30,31,[67][68][69][70][71] alkyl surface species are the initial intermediates during both the activation of alkanes 31,[72][73][74] and the hydrogenation of alkenes. 31,75,76 The data in Fig. 2, which address the case of n-butyl moieties on Pt (111), are quite representative of the surface chemistry of alkyls adsorbed on transition metal surfaces.…”

“…1,[26][27][28][29] We have been working on this problem for quite some time, with the specific goal to better understand reactions involving hydrocarbons on metals. [30][31][32][33][34][35] In this review we offer a personal perspective of the state of the art in this field. The emphasis here will be on the methods available for measuring kinetic parameters on small flat surfaces, typically single crystals, foils, or thin films.…”

Organometallic chemistry and catalysis on gold metal surfaces

“…Despite the widespread use of these heterogeneous catalysts in industrial applications, very little is known about the mechanistic details of the reactions that occur on the metal surfaces. Proposed mechanisms for surface metal-catalyzed reactions [1,2] are typically based on reactions of organic molecules that occur in solution-soluble transition metal complexes, where experimental methods are able to yield more details of reaction pathways. Often the coordination mode of the reacting organic molecule can be established by X-ray diffraction and NMR studies.…”

Bond Forming Reactions Involving C1 Moieties: Late Versus Early Transition Metal Surfaces

“…Many surface elementary steps have already been identified, and their relative rates evaluated, on a variety of metal surfaces [13][14][15]. However, the majority of that work has focused on late transition metals; early transition metals are usually very reactive and therefore considered of limited use for subtle catalytic processes designed to yield thermodynamically unstable products.…”