Catalysis, homogeneous activity

Shi, F., Tse, M.K., Pohl, M.M., Bruckner, A., Zhang, S.M. and Beller, M. (2007) Tuning catalytic activity between homogeneous and heterogeneous catalysis improved activity and selectivity of free nano-Fe203 in selective oxidations. Angewandte Chemie International Edition, 46 (46), 8866-8868. 

Mechanistic studies are of great importance in homogeneous catalysis. The active catalyst, often prepared in situ, is not easily isolated and, even when it is, can prove misleading (17). The comprehension of the catalytic process is therefore not an easy matter. In addition to producing kinetic measurements,... 

In conclusion, even if many efforts must be still devoted to face the four above rcpt>rtcd items, some encouraging results have been obtained such as the possibility to obtain under heterogeneous catalysis comparable activity with respect to the homogenous one and the etllcicni reusability of the supported guanidine. These aspects allow to feel moderately confident about the future of a possible large scale application of this kind of solid catalysts. 

At the dawn of this 21st century we have to try to fill the gap between homogeneous and heterogeneous catalysis, and the first step is to better understand the various mechanisms involved. For instance, now several questions are still not solved such as (i) Have we in heterogeneous catalysis, as active site, an ensemble of surface atoms, deduced from the kinetic process, instead of a monoatomic site as proposed in inorganic chemistry. and (ii) Whatever the metal used, can we always find new catalytic properties when the mean metallic particle size is decreased ... 

In addition to the wide range of metal oxide catalysts that can cany out oxidation via redox catalysis, there are a host of other materials that can carry out oxidation over non-reducible metal oxides. The oxidation mechanisms over non-reducible metal oxides are quite different and typically involve the production of free radical intermediates. The mechanisms tend to contain both heterogeneous and homogeneous activation and functionality. The oxide is used to activate a free radical process that can then proceed in the gas phase or at the surface. Li-substituted MgO and the rare earth metal oxides are two classes of materials that are considered non-reducible oxidation catalysts. Here we wiU specifically focus on the activation of alkanes over non-reducible metal oxides. 

The preexponential or the frequency factor A is catalyst dependent, that is, it varies with the extent of surface and has the same units as the rate constant k. On the basis of the collision theory, it can be estimated that the frequency factor of a unimo-lecular heterogeneous reaction is smaller than that of its homogeneous counterpart by a factor of 10. It follows that, for efficient catalysis, the activation energy ,4 of the catalyzed reaction should be at least 80 kj/mol lower than that of the uncatalyzed one at 298 K. At higher reaction temperatures, the difference in must also be higher in order to keep the advan-... 

Key words Catalytic oxidation, homogeneous catalysis, dioxygen activation, dioxygen conplexes, biomimetic oxidation, functional metaUoenzyme models, oxidation mechanisms, oxidative dehydrogenation, oxygen insertion, aUcene epoxidation, catecholase reaction... 

While all of these Ru NPs displayed suitable performances in catalysis, different activities and selectivities were observed. This highlighted that supramolecular interactions on the metallic surface in the presence of a CD control the catalytic reactivity of the nanocatalysts. Interestingly the CD acts as a phase-transfer promotor, which increases the activity and affects the selectivity. As evidenced via NMR studies, the influence of CD on the selectivity results from the formation of an inclusion complex between the CD and the diphosphine hgand at the Ru NP surface as reported in homogeneous catalysis. This original work takes advantage of the supramolecular properties of a CD to modulate the surface reactivity of diphosphine-stabilized Ru NPs, which is expected to offer novel opportunities to the field of nanocatalysis. 

Catalysis in a single fluid phase (liquid, gas or supercritical fluid) is called homogeneous catalysis because the phase in which it occurs is relatively unifonn or homogeneous. The catalyst may be molecular or ionic. Catalysis at an interface (usually a solid surface) is called heterogeneous catalysis, an implication of this tenn is that more than one phase is present in the reactor, and the reactants are usually concentrated in a fluid phase in contact with the catalyst, e.g., a gas in contact with a solid. Most catalysts used in the largest teclmological processes are solids. The tenn catalytic site (or active site) describes the groups on the surface to which reactants bond for catalysis to occur the identities of the catalytic sites are often unknown because most solid surfaces are nonunifonn in stmcture and composition and difficult to characterize well, and the active sites often constitute a small minority of the surface sites. 

In summary, the groups of Espenson and Loh observe catalysis of Diels-Alder reactions involving monodentate reactants by Lewis acids in water. If their observations reflect Lewis-acid catalysis, involvirg coordination and concomitant activation of the dienophile, we would conclude that Lewis-acid catalysis in water need not suffer from a limitation to chelating reactants. This conclusion contradicts our observations which have invariably stressed the importance of a chelating potential of the dienophile. Hence it was decided to investigate the effect of indium trichloride and methylrhenium trioxide under homogeneous conditions. 

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