Bifunctional catalysts reaction mechanisms

Whatever the mechanistic explanation for this remarkable result, we hope that we have given the reader a taste of the fruits of considering both the fields of enzyme mechanism and organometallic chemistry. We are exploring the acceleration of other reactions using bifunctional catalysts, and these results will be described in due course. 

The classical HCK mechanism on bifunctional catalysts separates the metallic action from that of the acid by assigning the metallic function to the creation of an olefin from paraffin and the isomerization and cracking of the olefins to the acid function. Both reactions are occurring through carbenium ions [102],... 

Metal oxides possess multiple functional properties, such as acid-base, redox, electron transfer and transport, chemisorption by a and 71-bonding of hydrocarbons, O-insertion and H-abstract, etc. which make them very suitable in heterogeneous catalysis, particularly in allowing multistep transformations of hydrocarbons1-8 and other catalytic applications (NO, conversion, for example9,10). They are also widely used as supports for other active components (metal particles or other metal oxides), but it is known that they do not act often as a simple supports. Rather, they participate as co-catalysts in the reaction mechanism (in bifunctional catalysts, for example).11,12... 

Soualah, A., Lemberton, J.L., Pinard, L., Chater, M., Magnoux, P., and Moljord, K. (2008) Hydroisomerization of long-chain n-alkanes on bifunctional Pt/zeolite catalysts effect of the zeolite strucmre on the product selectivity and on the reaction mechanism. Appl. Catal. A., 336, 23-28. 

Industrial metal-zeolite catalysts undergo a bifunctional, monomolecular mechanism [1-5, 7]. Carbenium ions are the critical reaction intermediates to complete chain reactions. In the zeolite channels, carbenium ions likely exist as an absorbed alkoxyl species, rather than as free-moving charged ions [8], Figure 14.2 illustrates the accepted reaction mechanism, using hexanes as an example. 

In the present chapter, a classification of the hydrogenation reaction mechanisms according to the necessity (or not) of the coordination of the substrate to the catalyst is presented. These mechanisms are mainly classified between inner-sphere and outer-sphere mechanisms. In turns, the inner-sphere mechanisms can be divided in insertion and Meerweein-Ponndorf-Verley (MPV) mechanisms. Most of the hydrogenation reactions are classified within the insertion mechanism. The outer-sphere mechanisms are divided in bifunctional and ionic mechanisms. Their common characteristic is that the hydrogenation takes place by the addition of H+ and H- counterparts. The main difference is that for the former the transfer takes place simultaneously, whereas for the latter the hydrogen transfer is stepwise. 

This review is concerned with a discussion of the reactions of hydrocarbons over bifunctional catalysts, primarily from the viewpoint of mechanism and kinetics. Some discussion will also be given of the structure and properties of typical bifunctional reforming catalysts, since this is somewhat helpful in understanding how the catalyst functions in promoting the various reactions. In addition, at appropriate places in the article, the practical application of the principles of bifunctional catalysis in commercial reforming processes will be considered. 

As already mentioned, with time the mid-molecule cleavage typical of a bifunctional catalyst decreases over the molybdenum based catalyst and the demethylation reaction becomes dominant. Demethylation also increases with increasing pressure. Amir-Ebrahimi and Rooney proposed that the metallacyclobutane isomerization mechanism should have a significant methanation and homologation contribution.34 Homologation products were not analysed in this study but have been observed in studies of the C4 and C5 reactions 35 however, methane was an important component of the cracking products over the molybdenum catalysts. 

Push-pull acid-base catalysis has been proposed to account for the proton switch mechanism which occurs in the methoxyaminolysis of phenyl acetate (Scheme 11.14) where a bifunctional catalyst traps the zwitterionic intermediate. A requirement of efficient bi-functional catalysis is that the reaction should proceed through an unstable intermediate which has p values permitting conversion to the stable intermediate or product by two proton transfers after encounter with the bifunctional catalyst the proton transfer with monofunctional catalysts should also be weak. 

We examined the ability of our bis-imidazole cyclodextrin artificial enzymes to perform other bifunctionally-catalyzed reactions, where again the availability of the A,B and A,C and A.D isomers let us learn mechanistic details. As an important example, we examined three isomeric catalysts ability to promote the enolization of substrate 48, which binds into the cyclodextrin cavity in water [138]. Here there was again a strong preference among the isomers, but it was the A,D isomer 49 that was the effective catalyst It was also more effective than a cyclodextrin mono-imidazole that cannot use the bifunctional mechanism. 

A separate problem arises when the surface of the support bears non-metallic active centres. Such bifunctional catalysts are widely used in naphtha reforming and it has been suggested that a bifunctional mechanism may also operate in syngas reactions [35]. 

Job A, Janeck CF, Bettray W, Peters R, Enders D (2002) Tetrahedron 58 2253 Josephsohn NS, Kuntz KW, Snapper ML, Hoveyda AH (2001) Mechanism of enantioselective Ti-catalyzed Strecker reaction peptide-based metal complexes as bifunctional catalysts. J Am Chem Soc 123 11594—11599 Juhl K, Gathergood N, Jprgensen KA (2001) Catalytic asymmetric direct Man-nich reactions of carbonyl compounds with alpha-imino esters. Angew Chem Int Ed Engl 40 2995-2997... 

Fig. 2.1 Simplified reaction mechanism for the aromatization of propane on HZSM-5 and bifunctional (Ga, Zn, Pt)/HZSM-5 catalysts...

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