A study is conducted to demonstrate catalytic dehydrogenation of light alkanes on metals and metal oxides. The study provides a complete overview of the materials used to catalyze this reaction, as dehydrogenation for the production of light olefins has become extremely relevant. Relevant factors, such as the specific nature of the active sites, as well as the effect of support, promoters, and reaction feed on catalyst performance and lifetime, are discussed for each catalytic Material. The study compares different catalysts in terms of the reaction mechanism and deactivation pathways and catalytic performance. The duration of the dehydrogenation step depends on the heat content of the catalyst bed, which decreases rapidly due to the endothermic nature of the reaction. Part of the heat required for the reaction is introduced to the reactors by preheating the reaction feed, additional heat being provided by adjacent reactors that are regenerating the coked catalysts.

Catalytic dehydrogenation of light alkanes on metals and metal oxides.

Publisher Summary This chapter describes the interaction between metal and support, which may complicate the interpretation of data obtained with supported metals, the critical problem of determining the surface area of the metal, and illustrates the use of supported metals for the elucidation of the problem of catalyst specificity. Work on the series of platinum catalysts suggests that many reactions of hydrogenation, dehydrogenation, and hydrogenolysis are structure-insensitive. If, these same catalysts are now used in reactions involving oxygen—decomposition of hydrogen peroxide, oxidation of methanol and ethanol, oxidation of acetaldehyde—the picture is substantially altered: while specific rates are again almost the same for highly dispersed samples with percentages of dispersion between 80 and 100%, these specific rates change by at least one order of magnitude on samples with a smaller dispersion (50%) and on platinum black.

Catalysis by Supported Metals

Catalytic processes on surfaces have long been studied by probing model reactions on single-crystal metal surfaces under high vacuum conditions. Yet the vast majority of industrial heterogeneous catalysis occurs at ambient or elevated pressures using complex materials with crystal faces, edges and defects differing in their catalytic activity. Clearly, if new or improved catalysts are to be rationally designed, we require quantitative correlations between surface features and catalytic activity--ideally obtained under realistic reaction conditions. Transmission electron microscopy and scanning tunnelling microscopy have allowed in situ characterization of catalyst surfaces with atomic resolution, but are limited by the need for low-pressure conditions and conductive surfaces, respectively. Sum frequency generation spectroscopy can identify vibrations of adsorbed reactants and products in both gaseous and condensed phases, but so far lacks sensitivity down to the single molecule level. Here we adapt real-time monitoring of the chemical transformation of individual organic molecules by fluorescence microscopy to monitor reactions catalysed by crystals of a layered double hydroxide immersed in reagent solution. By using a wide field microscope, we are able to map the spatial distribution of catalytic activity over the entire crystal by counting single turnover events. We find that ester hydrolysis proceeds on the lateral {1010} crystal faces, while transesterification occurs on the entire outer crystal surface. Because the method operates at ambient temperature and pressure and in a condensed phase, it can be applied to the growing number of liquid-phase industrial organic transformations to localize catalytic activity on and in inorganic solids. An exciting opportunity is the use of probe molecules with different size and functionality, which should provide insight into shape-selective or structure-sensitive catalysis and thus help with the rational design of new or more productive heterogeneous catalysts.

Spatially resolved observation of crystal-face-dependent catalysis by single turnover counting

“Indeed, the catalytic activity, for a valid comparison, must be referred to the number of exposed surface atoms of a specified kind. Thus a convenient way to express catalytic activity is by means of a turnover number equal to the number of reactant molecules converted per minute per catalytic site for given reaction conditions.” With these words of Boudart the first definition of what later was called the Turnover Frequency (TOF) entered into the realm of heterogeneous chemistry. It was a term borrowed from enzymatic kinetics, and slowly passed to homogeneous catalysis. Nowadays it is a ubiquitous term, focusing strictly on the catalytic center, as distinct from the classical term “rate of reaction”, which emphasizes the generation of products or the consumption of reactants. Despite its utility and common use, the TOF concept is still not well-defined and leads to confusion. IUPAC’s gold book, the most authoritative source of chemical terminology, has a very concise definition of the turnover frequency: “Commonly called the turnover number, N, and defined, as in enzyme catalysis, as molecules reacting per active site in unit time.” This description of the TOF has two main problems. The first is the difficulty of providing a one-to-one correspondence between name and function, since (as appeared in Boudart’s paragraph) the terms “turnover frequency” (TOF) and “turnover number” (TON) seem to have one and the same meaning. However, in typical catalytic jargon, both expressions have very different connotations. Sometimes also the terms “turnover rate” and “catalytic constant” (kcat) are used interchangeably in the literature with the same meaning. To make matters worse, the TOF is occasionally considered a rate-constant, since the rate of reaction (r = TOF × [Cat]) depends on the catalysts concentration. However, the TOF itself can depend on the concentration of reactants and products even at saturation, and in this sense it is closer to a rate than to a kinetic constant. In spite of this, from a strict terminological stance the TOF is a frequency, with units of time−1. All this debate evidently resembles the biblical story of the Tower of Babel and the confusion of languages. The second problem of IUPAC’s and Boudart’s definitions is a recurring expression of the TOF as a function of the number of reactants consumed, or even of the products generated. In most cases it is indeed an accurate way to derive the TOF, but for instance in bimolecular reactions this is not the case. Moreover, and from a philosophical perspective, when expressing the TOF as a function of produced or consumed molecules, the focus of the measure goes back to those molecules instead of emphasizing the role of the catalyst (see Scheme 1).

“Turning Over” Definitions in Catalytic Cycles

Publisher Summary Metals constitute a wide class of catalysts, and because catalysis occurs on the surface, there is an economic incentive, especially for precious metals, to obtain catalysts in the form of small metal particles. This, however, raises two main problems. One is fundamental in nature and addresses the question as to below which particle size the metallic properties are lost. The other is more practical and concerns the preparation and characterization of very small particles and their catalytic activity. The other problem concerns the practical ways of producing metals with particle sizes around the critical value calculated above. The easiest and most practical way to achieve this goal is to deposit the metal in low concentration on a high-surface-area support. The main purpose of using a carrier is, of course, to achieve a high dispersion of the metal and to stabilize it against sintering. In a number of reactions, however, the metal support is not inert and the overall process is actually a combination of two functions: that of the metal and that of the catalytically active support.

C. Girvin Harkins

Papers

On the specific activity of platinum catalysts