Catalysts, polynuclear sites

In conclusion, unlike heterogeneous processes with commercial HDS catalysts, single-site catalysts have been found to desulfurize thiophenes (T, BT, DBT) exclusively after these have been converted to saturated thiols or thiolates. No example of catalytic desulfurization of THT or DHBT by a single-site catalyst has ever been reported, although stoichiometric reactions assisted by both mononuclear and polynuclear complexes are known for THT and other cyclic thioethers [2 b, 32]. 

True examples of single-site catalysts are enzymes, where active sites are made mainly by metalhc centers (mono- or polynuclear species) whose coordination sphere is completely defined by ligands [1-4]. The strength of enzymes is the combined effect of metal center activity with the specific behavior of metal coordination sphere hgands. These species play a key role, being optimized to create an environment suitable for (i) metal centers approaching and coordinating by reactants (ii) product removal from the catalytic centers at the end of the reaction in order to avoid further reactions. 

A metal cluster can be considered as a polynuclear compound which contains at least one metal-metal bond. A better definition of cluster catalysis is a reaction in which at least one site of the cluster molecule is mechanistically necessary. Theoretically, homogeneous clusters should be capable of multiple-site catalysis. Many heterogeneous catalytic reactions require multiple-site catalysis and for these reasons discrete molecular metal clusters are often proposed as models of metal surfaces in the processes of chemisorption and catalysis. The use of carbonyl clusters as catalysts for hydrogenation reactions has been the subject of a number of papers, an important question actually being whether the cluster itself is the species responsible for the hydrogenation. Often the cluster is recovered from the catalytic reaction, or is the only species spectroscopically observed under catalytic conditions. These data have been taken as evidence for cluster catalysis. 

It has been our goal for some time to run photochemical energy storage reactions without relay molecules or separate catalysts. We have concentrated on the photochemistry of polynuclear metal complexes in homogeneous solutions, because we believe it should be possible to facilitate multielectron transfer processes at the available coordination sites of such cluster species. 

Homogeneous catalysts have now been reported for hydrogenation of carbon monoxide, a combustion product of coal (see Section VI,B). More effective catalysts will undoubtedly be discovered in the near future. Polynuclear or, at least, binuclear sites are favored for reduction of the triple bond in carbon monoxide (see Section VI,B), and this together with the popular parallelism to heterogeneous systems, has renewed interest in metal clusters as catalysts (see Section VI). A nickel cluster is the first catalyst reported for mild (and selective) hydrogenation of the triple bond in isocyanide (see Section VI,A). The use of carbon monoxide and water as an alternative hydrogen source is reattracting interest (see Section VI,C). 

On heating deactivated parent H-mordenite (80 to 33% cumene conversion), quantities of desorbate are so low (30.6 g/gram catalyst) that the desorbable deactivants, and hence the catalyst activity, must be at the pore mouth in the deactivated material. Non-desorbable polynuclear aromatics fill the mordenite tube. On the other hand, aluminum-deficient H-mordenite did not deactivate significantly for the same cumene treatment. Activity of this catalyst could be throughout the tube, but because of the disperse nature of the alumina sites, the high activity of parent H-mordenite, only active at its mouth, is not approached. 

It is now well established that a variety of organic molecules such as polynuclear aromatic hydrocarbons with low ionization energies act as electron donors with the formation of radical cations when adsorbed on oxide surfaces. Conversely, electron-acceptor molecules with high electron affinity interact with donor sites on oxide surfaces and are converted to anion radicals. These surface species can either be detected by their electronic spectra (90-93, 308-310) or by ESR. The ESR results have recently been reviewed by Flockhart (311). Radical cation-producing substances have only scarcely been applied as poisons in catalytic reactions. Conclusions on the nature of catalytically active sites have preferentially been drawn by qualitative comparison of the surface spin concentration and the catalytic activity as a function of, for example, the pretreatment temperature of the catalyst. Only phenothiazine has been used as a specific poison for the butene-1 isomerization on alumina [Ghorbel et al. (312)). Tetra-cyaonoethylene, on the contrary, has found wide application as a poison during catalytic reactions for the detection of active sites with basic or electron-donor character. This is probably due to the lack of other suitable acidic probe or poison molecules. 

These results indicate that the isolated copper species on ZSM-5 have an activity for the decomposition reaction of NO different from that of the dimeric or polynuclear copper species, probably because there are different reaction mechanisms (237). The results obtained with the Cu(ll)ZSM-5 catalyst also suggest that Cu " ions promote the spontaneous low -tempera-ture dehydroxylation of nearby Brpnsted sites or the elimination of lattice oxygen anions which play a vital role in the decomposition of NO. When the dimeric or polynuclear species of Cu are present, the spontaneous elimination of the lattice oxygen bridging the two Cu"+ sites does not occur at low temperatures however, this reaction occurs at high temperatures. The activity for the decomposition of NO is nearly zero at about 573 K, but in the presence of O2 a different reaction mechanism is initiated and this results in the enhancement of NO conversion. Moreover, the presence of stronger Brpnsted sites in ZSM-5 can explain why only the CuZSM-5 catalyst exhibits much higher activity for the reduction of NO in NO-NH3-O2 reaction systems. 

Abramo GP, Li L, Marks TJ (2002) Polynuclear catalysis Enhancement of enchainment coop-erativity between different single-site olefin polymerization catalysts by ion pairing with a binuclear cocatalyst. 1 Am Chem Soc 124 13966... 

Isolated V oct. sites are probably responsible for the selective behavior in toluene oxidation to benzaldehyde as indicated by the characterization of Vx-HMS catalysts. It should be remarked that unsupported or supported vanadium oxides oxidize toluene to benzoic acid [12] or a mixture of benzaldehyde and benzoic acid [2], whereas all V-containing zeolites tested form only benzaldehyde. The nature of the zeolite influences the nature of isolated species, as well as the ratio between isolated to polynuclear vanadium species. 

Models for metal sites in supported catalysts. - An example of this is the use of some model polynuclear complexes in an investigation into the structure of Co-Mo/AI2O3... 

This paper will describe our use of polynuclear manganese complexes of the type believed to be present at the active site of the oxygen evolving complex (OEC) of photosystem II (PSII) as catalysts for hydrocarbon oxygenation. 

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