Predictability of catalytic activity

On the other hand, Lewis acids are catalytically inactive for skeletal transformations unless proton donors are available at the same locality.1 Lewis acids consist of incompletely coordinated surface ions aluminum ion is the most frequently cited example. Since the relevance of acidity measurements for the prediction of catalytic activity is what we are trying to emphasize, we have concentrated on the determination of Brpnsted acidity in this critique. The problem of finding the most relevant method for acidity measurement has therefore been treated as an evolutionary process in which successive methods have been used more and more successfully for the characterization of a relatively small number of strong Br0nsted acids that are frequently accompanied by a multitude of other surface acids. 

During the early years of development of the multiplet theory, attention was paid chiefly to the correspondence of the structure of reacting molecules and catalyst, especially in relation to the sextet model of dehydrogenation of six-membered cycles on metal catalysts. This work permitted the determination of the group of metals that can act as catalysts for the dehydrogenation of cyclohexane (the so-called Blandin s square of activity ) and the prediction of catalytic activity, e.g., for Re which was unknown as a catalyst for this reaction. 

In the case of this concept, too, empirical findings are of greater interest than exact theoretical predictions of catalytic activity. It is particularly useful for explaining many chemisorption effects and for oxidation reactions. 

Some pessimism in assessing the situation in the field of electrocatalysis may also derive from the fact that one of the final aims of work in this held, setting up a full theory of electrocatalysis at a quantum-mechanical level while accounhng for all interactions of the reacting species with each other and with the catalyst surface, is still very far from being reahzed. So far we do not even have a semiempirical— if sufficiently general—theory with which we could predict the catalytic activity of various catalysts. 

The lack of correlation between isosteric heats of adsorption and catalytic activity illustrates the point we have been stressing the strength of attachment of a base to a catalyst surface is not a valid index of catalytic activity when a substantial portion of the base is strongly bonded to inactive portions of a catalyst surface. Putting it more explicitly, isosteric heats of adsorption are useful indexes for the prediction of acid-catalyzed... 

The rapid development of combinatorial screening methods has been accompanied by the development of ever more efficient high-throughput analysis technologies. These not only enable analysis of catalytic activity but also the determination of enantiomeric excess [2, 21]. Taking these developments together, research in this field can be expected to yield highly active and selective catalysts with structures that could have not been predicted by conventional means. 

Fan H-J, Hall MB (2001) Recent theoretical predictions of the active site for the observed forms in the catalytic cycle of Ni-Fe hydrogenase. J. Biol. Inorg. Chem. 6 467 173... 

These relationships, when incorporated into microkinetics models of catalytic reaction cycles, enable remarkable new predictive insights into the control of heterogeneously catalyzed reactions. Predictive models of catalytic activity as a function of catalyst composition as well as reaction conditiorvs have been constructed (22-24). The resultant volcano curves can be considered to be an application of the Sabatier principle (25,26). 

The simple idea involved in the electrostatic theory was very effective in predicting the catalytic activity of various cation-exchanged X and Y catalysts. However, it failed to explain quantitatively the difference in activity between cation-exchanged X and Y, alkaline earth cation X being less active than expected. It did not explain the cause for the similar behavior between cation-exchanged and decationized zeolites, and it did not offer satisfactory chemical evidence for the suggested reaction mechanism. 

In the application of Brewer-Engel concepts of electronic-structure influences on the properties of transition metal alloys by Jaksic et ai, it is not clear how these ideas predict improved catalytic activity for Lewis base-acid" pairs of the metals in the alloy. Only on an empirical basis, from the observed experimental behavior, are such effects indicated. Strong intermetallic binding is, however, indicated, and it would have to be supposed that this effect spills over in causing strong H adsorption at the surfaces of these alloys... 

For optimal use of Pt, a preferred size of Pt crystal can be predicted. For catalytic activity of exposed atoms (STY), this would take place around point C (Fig. 6.7 d). The opposite trends of dispersion and catalytic activity (related to metal character of atom in the dispersion) give the resulting curve C for overall catalysis observed as function of cluster size. Catalysts prepared by different methods will, generally, yield different crystallite size distribution and shape of metal crystals. This will result in dissimilarities in sorptive and catalytic behavior, even if allowance is made for the difference in active surface area285. ... 

Theoretical density functional calculations on the possibility of addition of imidazolium salts to electron-rich palladium centers predicted an exothermic enthalpy for such a process [36]. These results suggested that, under appropriate reaction conditions and with the use of a proper carbene precursor, this reaction should present a feasible synthetic path to carbene/palladium complexes. Only recently, the addition of the C(2)-H bond of an imidazolium salt, in the form of an ionic liquid, to a Pd(0)/NHC complex with the formation of a stable Pd-H bond has been reported [41]. These complexes bear three carbenes per metal center, the fourth coordination position being occupied by hydrogen. The isolation of these complexes has proven that the beneficial role of ionic liquids as solvent can lead to the formation of catalytically active palladium-carbene complexes (see Scheme 7). 

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