Catalysis-related uses

Recently, a series of chemical substances called functional dyes have attracted considerable attention. Because such dyes have long conjugated pi-electron systems and in many cases possess intramolecular charge transfer structures, functional dyes are expected to show interesting optical and electronic properties. Among functional dyes, phthalocyanine compounds have been extensively investigated because of their excellent physical, chemical and coloristic properties, as mentioned above. For example, about 1000 related US patents, published from 1990 onwards, are retrievable from the World Patent Index data base of Derwent, and more than 30 % of these are classified in such non-colorant applications as electrophotography, catalysis, and infrared radiation absorption. 

This biphenyl model compound was shown to possess the so-called primary optical specificity of a-chymotrypsin. That is, only the enantiomer related to L-phenylalanine was hydrolyzed by the enzyme. Generally speaking, there are three more levels of specific substrate recognition in enzyme catalysis. Let us consider a peptide bond in a polypeptide chain. The lateral side chain R2 is responsible for the normal specificity of the enzyme. For a-chymotrypsin, R2 is an aromatic side chain and the hydrophobic cavity a aromatic hole in the active center is there to accommodate the amino acid to be recognized by the enzyme. This is referred to as the primary structural specificity. 

Kinetics provides the frame vork for describing the rate at which a chemical reaction occurs and enables us to relate the rate to a reaction mechanism that describes how the molecules react via intermediates to the eventual product. It also allows us to relate the rate to macroscopic process parameters such as concentration, pressures, and temperatures. Hence, kinetics provides us with the tools to link the microscopic world of reacting molecules to the macroscopic world of industrial reaction engineering. Obviously, kinetics is a key discipline for catalysis. 

Seeing the surface of a catalyst, preferably in atomic detail, is the ideal of every catalytic chemist. Unfortunately, optical microscopy is of no use for achieving this, simply because the rather long wavelength of visible light (a few hundred nanometers) does not enable features smaller than about one micrometer to be detected. Electron beams offer better opportunities. Development over the past 40 years has resulted in electron microscopes which routinely achieve magnifications on the order of one million times and reveal details with a resolution of about 0.1 nm [1], The technique has become very popular in catalysis, and several reviews offer a good overview of what electron microscopy and related techniques tell us about a catalyst 12-6],... 

The similarity in the rate laws does not allow a clear choice to be made between mechanisms, but Mechanism A is required in H20 by the observation of general base catalysis. However, the relative stability of the (red) T° intermediate in Me2SO (this is dependent on the nature of the AA side chain, cf. Section III,C) in the absence of proton-ated amine makes us prefer Mechanism B for reaction in this solvent, since the solvent is unable to assist the departure of MeOH. The similar catalytic rate constants found for B = imidazole, Af-methylimidazole (26) suggest that transfer of the proton from T+ to the alcohol function remains stepwise (i.e., via T°) since N-methylimidazole cannot carry out a concerted transfer. Such general acid-catalyzed loss of MeOH from T° supports a suggestion made many years ago by Burnett and Davies relating to purely organic esters (62). 

In commencing a discussion of coordination chemistry aspects of catalysis, it is well to look more closely at what is meant by the term catalyst in relation to homogeneous catalysis. In general the word catalyst is applied to that complex which is added to the reaction mixture. Our current knowledge of the mechanisms of such reactions tells us that this complex rarely, if ever, takes part in the operating catalytic cycle which converts substrate to product. This complex is therefore much better termed the catalyst precursor . 

Growing ultrathin metallic layers on a (chemically different) single crystal metal surface allows us to explore the changes in morphology and electronic structure that occurs as strain relief processes develop and relate them to the changes in reactivity, a subject of immense importance in catalysis. 

One of us has reviewed catalysis by metal clusters through to mid-1979. Emphasis was placed on the potential applications of metal clusters in catalysis and on the problem area of characterization especially as it relates to our knowledge of systems under reaction conditions. The difficulty of estab-hshing beyond reasonable doubt that the catalytically active entity in a given reaction is indeed a cluster compound, and not a product of dissociation or aggregation, was also stressed. We now complement the earlier review by a consideration of subsequent developments and of attempts that have been... 

This quite provocative representation of micellar catalysis may allow us to speculate on a possible relation to the origin of life and evolution, as the authors have done. However, in our context it gives clear evidence of the... 

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