Isomerization bifunctional

Two pieces of chemical evidence support the three-membered ring formulation. The bifunctional oxazirane prepared from glyoxal, tert-butylamine, and peracetic acid (6) can be obtained in two crystalline isomeric forms. According to the three-membered ring formula there should be two asymmetric carbon atoms which should allow the existence of meso and racemic forms. A partial optical resolution was carried out with 2-7i-propyl-3-methyl-3-isobutyloxazirane. Brucine was oxidized to the N-oxide with excess of the oxazirane. It was found that the unused oxazirane was optically active. 

Catalytic processes frequently require more than a single chemical function, and these bifunctional or polyfunctional materials innst be prepared in away to assure effective communication among the various constitnents. For example, naphtha reforming requires both an acidic function for isomerization and alkylation and a hydrogenation function for aromati-zation and saturation. The acidic function is often a promoted porous metal oxide (e.g., alumina) with a noble metal (e.g., platinum) deposited on its surface to provide the hydrogenation sites. To avoid separation problems, it is not unusual to attach homogeneous catalysts and even enzymes to solid surfaces for use in flow reactors. Although this technique works well in some environmental catalytic systems, such attachment sometimes modifies the catalytic specifici-... 

To illustrate how a bifunctional catalyst operates, we discuss the kinetic scheme of the isomerization of pentane [R.A. van Santen and J.W. Niemantsverdriet, Chemical Kinetics and Catalysis (1995), Plenum, New York]. The first step is the dehydrogenation of the alkane on the metal ... 

Hence, the rate depends only on the ratio of the partial pressures of hydrogen and n-pentane. Support for the mechanism is provided by the fact that the rate of n-pentene isomerization on a platinum-free catalyst is very similar to that of the above reaction. The essence of the bifunctional mechanism is that the metal converts alkanes into alkenes and vice versa, enabling isomerization via the carbenium ion mechanism which allows a lower temperature than reactions involving a carbo-nium-ion formation step from an alkane. 

We have explored rare earth oxide-modified amorphous silica-aluminas as "permanent" intermediate strength acids used as supports for bifunctional catalysts. The addition of well dispersed weakly basic rare earth oxides "titrates" the stronger acid sites of amorphous silica-alumina and lowers the acid strength to the level shown by halided aluminas. Physical and chemical probes, as well as model olefin and paraffin isomerization reactions show that acid strength can be adjusted close to that of chlorided and fluorided aluminas. Metal activity is inhibited relative to halided alumina catalysts, which limits the direct metal-catalyzed dehydrocyclization reactions during paraffin reforming but does not interfere with hydroisomerization reactions. 

As to the main limitation of MS vs. FTIR detection, namely the inability to distinguish closely related isomers, this rarely plays a role in additive analysis. Notable examples of isomeric additives are the bifunctional stabilisers C22H30O2S as 4,4 -thio-bis-(6-t-butyl-m-cresol), 2,2 -thio-bis-(4-methyl-6-f-butylphenol) and 4,4 -thio-bis-(2-methyl-6-f-butylphenol) (Section 6.3.6), the bisphenolic antioxidants C23H32O2 (Plastanox 2246 and Ethanox 720) and the phenolic antioxidants C15H24O (nonylphenol and di-f-butyl-p-cresol). 

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]. 

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. 

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],... 

The hydroisomerization of heavy linear alkanes is of a great interest in petroleum industry. Indeed, the transformation of long chain n-alkanes into branched alkanes allows to improve the low temperature performances of diesel or lubricating oils [1-3]. On bifunctional Pt-exchanged zeolite catalysts, n-CK, transformed into monobranched isomers, multibranched isomers and cracking products [4], The HBEA zeolite based catalyst was more selective for isomerization than those containing MCM-22 or HZSM-5 zeolites [4], This was explained on one hand by a rapid diffusion of the reaction intermediates inside the large HBEA channels, and on the other hand by the very small crystallites size of this zeolite (0.02 pm). 

The objective of this work is to determine the influence of the porous structure (size and shape) and acidity (number and strength of the acid sites) on isomerization selectivity during the conversion of ethylbenzene on bifunctional catalysts PLAI2O3/ 10 MR zeolite. The transformation of EB was carried out on intimate mixtures of Pt/Al203 (PtA) and 10 MR zeolites (ZSM-5, ZSM-22, Ferrierite, EU-1) catalysts and compared to Mordenite reference catalyst activity. 

The conversion of the monofunctional adducts into bifunctional lesions depends drastically on the structure of the Pt drug. Obviously, Pt compounds exhibiting trans geometry form different bisadducts than cisplatin and hence, a different spectrum of antitumor activity is expected. Mechanistically, the formation and possible isomerization of bisadducts are not well understood. The assumption that hydrolysis of the second leaving group controls the formation of bisadduct may be an oversimplification. Studies with model compounds as well as with oligonucleotides have indicated that a certain nucleobase may be a powerful nucleophile toward Pt(II) if spatially in a correct position. Unfortunately, our knowledge on these interactions is at present very limited. 

The enantioselective isomerization of bifunctional Cs-isoprenoid allylamines to optically active bifunctional aldehydes was developed by using Rh(l)+/( )-BIPHEMP as the catalyst (Scheme 2).17 These aldehydes are useful optically active bifunctional building blocks for isoprenoid homologation. 

Kubica et al45 also investigated the effect of platinum-modified zeolites on the decalin reaction. They found that the addition of Pt enhances the catalyst activity. The initial isomerization was increased 3 times, which can be interpreted in terms of a change in the reaction initiation. In addition to initiation by a PC step over Bronsted acid sites, as proposed for H-form zeolites, a bifunctional initiation path... 

Noble metals (e.g., Pt) can be introduced within the micropores of zeolites by exchange with a complex cation (e.g., Pt(NH3)4 ) followed by calcination and reduction. This mode of introduction generally leads to very small clusters of Pt (high Pt dispersion) located within the micropores. Pt supported on acid zeolites are used as bifunctional catalysts in many commercial processes. The desired transformations involve a series of catalytic and diffusion (D) steps, as shown in n-hexane isomerization over Pt acidic zeolite (Equation 12.1). 

Under the operating conditions, the reaction intermediates (w-hexenes and i-hexenes in n-hexane isomerization) are thermodynamically very adverse, hence appear only as traces in the products. These intermediates (which are generally olefinic) are highly reactive in acid catalysis, which explains that the rates of bifunctional catalysis transformations are relatively high. The activity, stability, and selectivity of bifunctional zeolite catalysts depend mainly on three parameters the zeolite pore structure, the balance between hydrogenating and acid functions, and their intimacy. In most of the commercial processes, the balance is in favor of the hydrogenation function, that is, the transformations are limited by the acid function. 

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