Bifunctional operations

Supported zeolite Y, silicalite-1, A and a composite of silicalite-1 on A can be synthesised in a membrane configuration in a reproducible manner. Synthesis techniques using seeds were applied in the membrane preparation. The double layer type membrane has great potential in the bifunctional operation in one integrated unit. 

If an RFC system has a separate FC and electrolyzer, it is possible to choose different materials for the electrolyzer and for the FC electrodes. The materials can be adapted in terms of ideal performance and high durabihty. If a URFC system is considered, the electrodes have to be suitable for both hydrogen consumption and evolution or oxygen consumption and evolution. This bifunctional operation subjects the electrodes to considerable stress and can reduce their performance and durability. In addition to the type of operation (conventional or bifunctional), the composition of the electrodes is greatly dependent on the electrolytes used. In the following subsections, the different electrolytes (alkaline, PEM, SOE) are described and also suitable electrodes for each type of operation. 

Apart from the hardness and softness, two reactivity-related features need to be pointed out. First, iron salts (like most transition metal salts) can operate as bifunctional Lewis acids activating either (or both) carbon-carbon multiple bonds via 71-binding or (and) heteroatoms via a-complexes. However, a lower oxidation state of the catalyst increases the relative strength of coordination to the carbon-carbon multiple bonds (Scheme 1). 

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 ... 

If we regard it as a cross-linked polymer, we require ywj the weight average degree of polymerization if all cross-linkages were severed. This operation would amount to replacing each tetrafunctional unit, or pair of cross-linked units, with two bifunctional units. The extent of reaction p would not be affected. Hence, according to Eq. (VIII-8)... 

The most important progress in the last decade has been in the design and synthesis of [RuCl2(diphosphine)(l,2-diamine)] catalysts exploiting the metal-ligand bifunctional concept developed by Noyori and co-workers.29-31 The Noyori catalysts seem to possess all of the desired properties, such as high turnover number (TON), high turnover frequency (TOF), and operationally simple, safe, and environmentally friendly reaction conditions. 

A monohydride mechanism is not operating in reactions catalyzed by these complexes. Noyori observed that the presence of an NH or NH2 in the auxiliary ligands was crucial for catalytic activity, the corresponding dialkylamino analogs being totally ineffective. These findings indicate a novel metal-ligand bifunctional cycle (Scheme 28) KOH reacts with the pre-catalyst (87)... 

Pennline et al.32 used bifunctional Co/ThO/ZSM-5 catalysts at 280°C, 21 bar, H2/CO = 1 in the FTS. XRD of the used catalyst indicated that cobalt carbide is present. They found that the relative amount of the carbide species is larger on the used catalyst operated at 280°C than on the used catalyst operated at 320°C. They argued that this is because cobalt carbide begins to decompose around 300°C. Since this catalyst lacked high water gas shift activity, and a low feed gas ratio of... 

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]. 

Prominent co-catalysts for the Pt-on-carbon anode catalyst in the oxidation of polyhydric alcohols are Ru or Ce02 [54, 60]. Their increased resistance to poisoning with mainly CO during operation is associated with the existence of a bifunctional mechanism (Scheme 11.6). 

Li YQ, Bricks JL, Resch-Genger U et al (2006) Bifunctional charge transfer operated fluorescent probes with acceptor and donor receptors. 2. Bifunctional cation coordination behavior of biphenyl-type sensor molecules incorporating 2, 2 6, 2"-terpyridine acceptors. J Phys Chem A 110 10972-10984... 

On the basis of the combined weight of the above results, we believe that bifunctional electrocatalytic properties may be operative for both MOR and ORR on the AuPt bimetallic nanoparticle catalysts depending on the nature of the electrolyte. For ORR in acidic electrolyte, the approaching of both the reduction potential and the electron transfer number for the bimetallic catalyst with less than 25%Pt to those for pure Pt catalyst is indicative of a synergistic effect of Au and Pt in the catalyst. For MOR in alkaline electrol)he, the similarity of both the oxidation potential and the current density for the bimetallic catalyst with less than 25%Pt to those for pure Pt catalyst is suggestive of the operation of bifunctional mechanism. Such a bifunctional mechanism may involve the following reactions ... 

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. 

The recognition of consonant bifunctional relationships in the target molecule allows their disconnection by a retro-Claisen, a retro-aldol or a retro-Mannich condensation or by retro-Michael addition [equivalent, according to Corey s formalisation, to the application of the corresponding transforms (= operators) to the appropriate retrons]. 

HP-3 FGI, in order to introduce a C=0 group, modify a double bond or to proceed to a "reactivity inversion" operation Umpolung) in dissonant bifunctional relationships (see below iii-a). 

Fumeaux, Davidson and Ball (1987). The location of the catalyst had a remarkable influence on the reaction process. The membranes could operate as bifunctional catalysts. 

The main feature of bifunctional catalysts is not only the appearance of novel catalytic functions but also its mode of operation various functions may (and sometimes should) participate alternately in the transformation of one single molecule. Consequently, various multistep pathways may lead to the same end product. In this situation it can be extremely difficult to distinguish between the role of individual catalytic functions. Three experimental approaches have been offered ... 

The authors suggested that 222 operates in a bifunctional mode by hydrogenbonding activation of the nitroalkane and subsequent a-deprotonation through the basic oxazoline nitrogen providing a nucleophilic nitronate, which attacks the imine and give the observed aza-Henry adduct (Scheme 6.183) [345]. 

Synthesis of dihydroartemisinin polyethylene-glycol dimers 130 may be achieved easily in one chemical operation starting with dihydroartemisinin 29a and the corresponding bifunctional alcohol or thiol under acidic conditions (Table 7) <1997BMC1257>. 

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