Bifunctional catalysts components

The metallic component of HCK catalysts provides hydrogenation, dehydrogenation, hydrogenolysis, and isomerization. The number and nature of reactive hydrogen species created by the interaction of a bifunctional catalyst with hydrogen is not well understood [103], on the other hand, neither the action of those species on the catalytic sites is understood. The main limitation in this understanding is the dynamic character of the interaction however, now that in situ characterization techniques are becoming available, research would soon defeat the limitations. 

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... 

In contrast to monofunctional (thio)urea organocatalysts, bifunctional catalyst structures enable simultaneous coordination, activation, and suitable relative orientation of both reaction components (the electrophile and the nucleophile) resulting in high... 

Support. In multiphase catalysts, the active catalytic material is often present as the minor component dispersed upon a support sometimes called a carrier. The support may be catalyticaliy inert but it may contribute to the overall catalytic activity. Certain bifunctional catalysts ( 1.2.8) constitute an extreme example of this. In naming such a catalyst, the active component should be listed first, the support second and the two words or phrases should be separated by a solidus, for example, platinum/silica or platinum/silica-alumina. The solidus is sometimes replaced by the word on, for example, platinum on alumina. 

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. 

Attempts have also been made to use bifunctional catalysts in which a C2-symmetric sulfide component is linked to the copper catalyst [220]. Although less catalyst could be used without reducing the yield (5 mol% compared with 20 mol% in the reactions already described) the enantioselectivity of these bifunctional catalysts did not exceed 24% ee. 

The results from the publications mentioned are of interest because they can help in the creation of effective catalytic systems containing porphyrins, which combine functions typical of multienzyme systems. The task in hand is the possible synthesis of bifunctional catalysts based on metalloporphyrin systems, when with the help of manganese porphyrins, for example, or SOD mimic, hydrogen peroxide is accumulated in the system. Afterwards, the accumulated hydrogen peroxide is used in oxidation reactions of various substrates with iron porphyrin components of the catalyst. 

Otherwise, by impregnating a Pd precursor onto a basic K-exchanged FAU zeolite a highly selective bifunctional catalyst is obtained for the low-pressure one-step synthesis of 2-ethylhexanal (a component of perfumes and fragrances) from M-butyraldehydc and H2 in a fixed-bed reactor/12,131 Under optimum reaction... 

As stated above, the aromatization of short alkanes is carried out in presence of bifunctional catalysts, in where the dehydrogenating function is given by the metal component (Ga, Zn, Pt) and the H-ZSM-5 zeolite carries the acid sites. Although there is still some uncertainty concerning the initial activation of the alkane, probably both the metal and the zeolite acid sites are involved in this step. Metal sites can dehydrogenate the alkane to give the corresponding alkene, which can then be protonated on the Bronsted acid sites of the H-ZSM-5 zeolite to produce the carbocation. 

The current theory of bifunctional catalysis assumes that paraffin isomerization is induced by olefin formation at the metal surface, followed by a typical acid-catalyzed reaction of the olefin at the active centers of the acidic component. Consequently, similar skeletal conversions must be found with olefins and an acid catalyst, and paraffins and a bifunctional catalyst. Our findings substantiate this theory. If these results (Figs. 2 and 3) are put together and compared to the predictions of the carbonium mechanism (Fig. 4), one can see that all the expected structures have been obtained in our experiments. 

Investigations of the isomerization of alkanes in recent years have provided evidence that the reaction can occur on certain metals, notably platinum, in the absence of a separate acidic component in the catalyst (20-22). While it has been shown that a purely metal-catalyzed isomerization process can occur, the findings do not challenge the commonly accepted mode of action of bifunctional reforming catalysts in which separate metal and acidic sites participate in the reaction. The available data at conditions commonly employed with commercial reforming catalysts indicate that a purely metal-catalyzed process does not contribute appreciably to the overall isomerization reaction on a bifunctional catalyst. 

Medium pore aluminophosphate based molecular sieves with the -11, -31 and -41 crystal structures are active and selective catalysts for 1-hexene isomerization, hexane dehydrocyclization and Cg aromatic reactions. With olefin feeds, they promote isomerization with little loss to competing hydride transfer and cracking reactions. With Cg aromatics, they effectively catalyze xylene isomerization and ethylbenzene disproportionation at very low xylene loss. As acid components in bifunctional catalysts, they are selective for paraffin and cycloparaffin isomerization with low cracking activity. In these reactions the medium pore aluminophosphate based sieves are generally less active but significantly more selective than the medium pore zeolites. Similarity with medium pore zeolites is displayed by an outstanding resistance to coke induced deactivation and by a variety of shape selective actions in catalysis. The excellent selectivities observed with medium pore aluminophosphate based sieves is attributed to a unique combination of mild acidity and shape selectivity. Selectivity is also enhanced by the presence of transition metal framework constituents such as cobalt and manganese which may exert a chemical influence on reaction intermediates. 

We are investigating bifunctional catalysts in which one component of the catalyst adsorbs or oxidizes CO and the other component dissociates water. Our present research is focusing on metal-support combinations to promote this bifunctional mechanism. The metallic component is chosen to adsorb CO at intermediate adsorption strengths (platinum [Pt], Ru, palladium [Pd], PtRu, PtCu, cobalt [Co], ruthenium [Ru], silver [Ag], iron [Fe], copper [Cu], and molybdenum [Mo]). The support is chosen to adsorb and dissociate water, typically a mixed-valence oxide with redox properties or oxygen... 

ATO and AFO type structures. At present the industrial process of lube oil dewaxing (ChevronTexaco) is realized on bifunctional catalyst with acidic SAPO-11 (AEL) component. Few examples in the literature devoted to comparative study of AEL-, ATO- and AFO-SAPO materials in hydroisomerization reaction are based on a single specimen of each catalyst, sometimes not phase-pure and often prepared by exotic or undefined method. Recently the authors found a new method for selective and reproducible synthesis of SAPO-31 (ATO type structure) materials in the presence of di-n-pentylamine and showed hydroisomerization efficiency of catalysts based on these systems [3,4]. 

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