Catalysis, bifunctional

Bifunctional zeolitic catalysts are also used for hydrocracking, in which the heaviest fraction of crude oil is cracked in the presence of hydrogen to give 

Although zeolitic solid acids have replaced supported mineral acids in many reactions, they cannot be used universally. For some reactions their acidity is too strong and their pore structure is readily blocked (etherification, alkene hydration) for others they are not strong enough acids. An example where zeolites have not been applied successfully is in the alkylation of alkanes with alkenes. Hydrocarbon fuels with high octane numbers are in constant demand for the majority of automobile engines, and branched alkanes of the gasoline fraction possess the required properties. Isoalkanes such as these are currently 

The discovery that the protonated forms of zeolites could be used as active, stable and shape selective catalysts in hydrocarbon transformations has been of immense benefit to the refining and petrochemicals industry. The need for optimised microporous acid catalysts will continue as fuel specifications change and the requirements of the chemicals market shift. The likely growth in demand for synthetic fuels, including diesels, is one expected trend that could involve zeolite catalysts. Diverse feedstock chemicals and fine chemicals synthesis involving zeolite catalysts are also being developed. 

As a result of this importance, great efforts have been made to understand the interplay between structure and chemistry to produce optimised acid catalysts for processes such as cracking, alkylation and isomerisation. It is now well established that zeolites are not superacidic, so that the apparent carbenium ion controlled conversions are thought to pass through carbenium-ion-like transition states stabilised by the zeolite framework. For methanol-to-hydrocarbon reactions, elegant in situ NMR has demonstrated that a reactive hydrocarbon pool that forms within the pores is observed to be responsible for the formation of the first C-C bonds, and it is likely that reactive hydrocarbon intermediates have a greater role in add-catalysed reactions than previously spelt out. 

As well as the crystal structure of the microporous catalysts, the secondary mesoporosity is also important, because molecular transport to and from the active sites is favoured in these materials. In steamed Y the mesoporosity and extra-framework aluminium results in a very active catalyst for cracking. Designed hierarchical structures, in which nanoparticles of zeolites are joined together to and connected by a secondary mesopore system for the same reason are discussed further in Chapter 10. 

in Catalysis , ed. G. C. Bond and G. Webb (Specialist Periodical Report), The Royal Society of Chemistry, London, 1982, Vol. 5, p. 172. 

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Another type of bifunctional catalysis has been noted with a,cn-diamines in which one of the amino groups is primary and the other tertiary. These substituted diamines are from several times to as much as 100 times more reactive toward imine formation than similar monofunctional amines. This is attributed to a catalytic intramolecular proton transfer. 

Zollinger s bifunctional catalysis (Sections II, D, 2, b and III, A) is probably further evidence in favor of the intermediate c-complex... 

Bifunctional catalysis in nucleophilic aromatic substitution was first observed by Bitter and Zollinger34, who studied the reaction of cyanuric chloride with aniline in benzene. This reaction was not accelerated by phenols or y-pyridone but was catalyzed by triethylamine and pyridine and by bifunctional catalysts such as a-pyridone and carboxylic acids. The carboxylic acids did not function as purely electrophilic reagents, since there was no relationship between catalytic efficiency and acid strength, acetic acid being more effective than chloracetic acid, which in turn was a more efficient catalyst than trichloroacetic acid. For catalysis by the carboxylic acids Bitter and Zollinger proposed the transition state depicted by H. 

Bifunctional catalysis has also been observed by Pietra and Vitali35 for a more typical nucleophilic aromatic substitution reaction, that of 2,4-dinitrofluorobenzene and piperidine in benzene. For this reaction triethylamine does not have an... 

Gomez GA, Morisseau C, Hammock BD, Christianson DW. Structure of human epoxide hydrolase reveals mechanistic inferences on bifunctional catalysis in epoxide and phosphate ester hydrolysis. Biochemistry 2004 43 4716-23. 

With some transition-metal complexes, the ligand is not only an ancillary ligand. Similar to the transition-metal, it takes directly part in the hydrogen transfer process. Such ligand-metal bifunctional hydrogenation catalysis is dramatically changing the face of reduction chemistry (Scheme 9) (for reviews of ligand-metal bifunctional catalysis, see [32, 37 0]). 

Figure 9.12. Bifunctional catalysis in the reforming of n-hexane. [After J.H. Sinfelt, Adu. Chem. Eng. 5 (1964) 37.]...

RARE EARTH MODIFIED SILICA-ALUMINAS AS SUPPORTS FOR BIFUNCTIONAL CATALYSIS... 

Feedstock Reactions Catalyst References Acetone Condensation-hydrogenation (bifunctional catalysis) Pd on sulfonated PS-DVB [6] Methanol, Raffinate II Condensation, hydrogenation Pd on sulfonated PS-DVB [61] Dioxygen dissolved in water Hydrogenation Pd on sulfonated PS-DVB [8]... 

For Se and Te, oxidation of the adatom takes place at potentials higher than that of CO oxidation. The adatom is always in its reduced state, and no bifunctional catalysis through the transfer of oxygen from the adatom to the CO molecule can take place. 

The three cycles have to turn over in the same range of temperature. This catalytic approach of the DeNOx reaction is not new. There is the same process for isomerization of alkanes, where there are also 3 catalytic cycles which have to turn over simultaneously (bifunctional catalysis). The kinetics of isomerization is given by only one cycle, the other two turning over very rapidly and are near equilibrium [13]. 

A major question has been that of bifunctional catalysis . For example, if a micelle contains both nucleophilic groups and groups which can transfer protons one might hope to achieve high rates of deacylation by having concerted nucleophilic attack and proton transfer (Scheme 6). Such concerted processes are well established in enzymic reactions, but evidence in... 

Catalytic Hydrogenations with Metal-Ligand Bifunctional Catalysis... 

The concerted delivery of protons from OH and hydride from RuH found in these Shvo systems is related to the proposed mechanism of hydrogenation of ketones (Scheme 7.15) by a series of ruthenium systems that operate by metal-ligand bifunctional catalysis [86]. A series of Ru complexes reported by Noyori, Ohkuma and coworkers exhibit extraordinary reactivity in the enantioselective hydrogenation of ketones. These systems are described in detail in Chapters 20 and 31, and mechanistic issues of these hydrogenations by ruthenium complexes have been reviewed [87]. 

Dehydrogenation of Imines and Alcohols by Shvo Complexes 191 Catalytic Hydrogenations with Metal-Ligand Bifunctional Catalysis 193... 

Wang, L-H. Zipse, H. Bifunctional Catalysis of Ester Aminolysis - A Computational and Experimental Study Liebigs Ann 1996,1501-1509. 

Bicycloguanidinium receptor, 16 787 Bidentate chelants, 5 709 Bidentate ligands, 9 396, 397 13 443 Bidentate phosphates ligands, 25 433 Bifunctional catalysis, 5 246-248 ... 

Catalysis by polymer-bound nucleophiles 455 Bifunctional catalysis 456 Stereoselective catalysis 459 Sulfate cleavage 463... 

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