Dual-function catalysis

For hydrocarbon reactions, metals (particularly platinum and its alloys) are frequently applied to acidic supports to catalyse hydrogen transfers. Thus platinum on a chlorinated alumina support accelerates the acid catalysed isomerization of n-alkanes (at about 150°C). In hydrocracking, the metal catalyses hydrogenation of heavy aromatic and polyaromatic components the resulting cycloparaffins (cycloalkanes) undergo zeolitic cracking, with 

Metallic components have also been added to a variety of heterogeneous oxide catalysts, to introduce additional hydrogenation, dehydrogenation and hydrogen transfer processes during aldolization, ketonization or Tishchenko reactions. Examples include acetone (propanone) to 4-methyl-pentan-2-one, ethanol to acetone and methanol to methyl formate (methyl methanoate), e.g. 

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Olefins are formed by dehydrogenation of the n-paraffin feed over the metallic hydrogenation-dehydrogenation function and are adsorbed on the acidic surface of the catalyst as carbonium ions by proton addition. After skeletal isomerization they are desorbed as isoolefins and subsequently hydrogenated to the corresponding isoparaffins. The net result (i.e., the formation of carbonium ions) of the action of metal and acid in dual function catalysis is, on pure Friedel-Crafts type catalysts, described by the scheme ... 

In studies of dual-functional catalysis the mixed catalyst technique has many advantages, two of which are mentioned. (1) It allows separate and independent preparation of each component for example, a platinum preparation can be made in any manner desired in order to obtain a certain platinum activity without regard to what such procedures might do to the acidic properties of the oxide base, this interdependence always being a matter of concern in conventional direct impregnation techniques. (2) A component s relative activity contribution can be flexibly varied in a perfectly known and controllable manner by simply varying its bulk amount in admixture with the other. 

Third, ions introduced into cavities through ion exchange have separate activities of their own. Opportunities for dual functional catalysis involving acidity tviih other activities are greatly facilitated. These three factors have made zeolites a powerful addition to the arsenal of catalytic... 

Enhanced catalytic activity has also been observed for the hydroformylation of oct-l-ene and dec-l-ene with water-soluble phosphine-caltK[4]arene—rhodium complexes (Figure 32). " These organometallic compounds behave, not only as homogeneous metal catalysts but also as inverse phase-transfer catalysts, that is, they perform a dual functional catalysis. The olefin is believed to be included in the hydrophobic cavity and to simultaneously interact with a catalytic transition metal center coordinated to the phosphine moieties. 

Rg.2 (a) Catalytic reaction on the surface (b) dual-function catalysis (c) mixed catalytic cycles involving the contribution of leaching and surface reactivity. Reproduced with the permission of ACS Publishing. (V. P. Ananikov and I. P. Beletskaya, OrganometaUics, 2012, 31, 1595). 

Calabro, D.C., Vartuli, J.C., and Santiesteban, J.G. The characterization of tungsten oxide modified zirconia snpports for dual functional catalysis. Top. Catal 2002,18, 231-242. 

Since qualitative yields were described, it is difficult to conclude the quantitative nature of the promotions. The total reaction time in the presence of barium chloride was reported to be much shorter as well. This dual-function catalysis warrants further investigation. 

Heterogeneous tandem catalysis involving at least one of the components being supported has also been reported [178, 179]. For example, calcosilicate has recently been used as an effective carrier for simultaneous immobilisation of a dual-functional system based on a bis(imino)pyridine iron compound and a zirconocene to form a heterogeneous catalyst precursor. On activation with triethylaluminium, ethylene was converted to LLDPE the layered structure of the calcosilicate was used to account for the improved thermal stability and higher molecular weights of the LLDPE formed [179],... 

Dual-function catalysts possessing both metallic and acidic sites bring about more complex transformations. Carbocationic cyclization and isomerization as well as reactions characteristic of metals occurring in parallel or in subsequent steps offer new reaction pathways. Alternative reactions may result in the formation of the same products in various multistep pathways. Mechanical mixtures of acidic supports (silica-alumina) and platinum gave results similar to those of platinum supported on acidic alumina.214,215 This indicates that proximity of the active sites is not a requirement for bifunctional catalysis, that is, that the two different functions seem to operate independently. 

Among the many proteins that bind to RNA molecules437 39 are the aminoacyl-tRNA synthetases, a variety of other well known enzymes,440 the ribosomal proteins discussed in Chapter 29, and various proteins with dual functions of catalysis and regulation of... 

In addition, in the field of dewaxing (gas oils, HDC residues, lubricating oil, etc.), synthesis of novel molecular sieves with better adsorption and separation abilities is highly desired. In the past 20 years, thanks to the discovery of many molecular sieves with new compositions and structural features [secondary building units (SBUs) and pores], there have appeared a number of new application fields for molecular sieves, such as basic catalysis, extra-large microporous molecular sieve catalysis, redox catalysis, asymmetric catalysis, and dual- and multi-functional catalysis.1-201 All of these will lay a further solid foundation for the development of molecular sieves in catalysis, adsorption, and separation. 

There are situations where support acidity has a positive influence, influencing the main reaction. The support adds dual functionality to the overall catalysis, as best demonstrated with catalytic reforming/ The objective in this process is to convert low octane components of naphtha, typically normal paraffins and naphthenes, into high-octane iso-parafHns and aromatics. Low loadings of Pt type metals on AljOj are used for this purpose. Metallic Pt dehydrogenates naphthenes to aromatics but cannot isomerize or cyclize normal paraffins. This is accomplished through the acidic function of the support, as shown for n-hexane ... 

An interesting aspect of the elucidation of the structure and mechanisms of TPQ biogenesis and function is that for these amine oxidases certain amino acid residues have multiple roles during cofactor biogenesis and catalysis. This is clearly true for the tyrosine residue, which is converted into TPQ, It has also been demonstrated by site-directed mutagenesis that a second strictly conserved tyrosine residue, which is present in the active site is also required for TPQ biogenesis and influences catalysis. Furthermore, the bound copper serves dual functions in cofactor biosynthesis, as shown in Scheme 4 as well as during catalysis. The latter function appears to be in stabilization of transient intermediates in the oxidative half reaction. ... 

A series of quantitative data for solvent effect on the aminolysis of nitrophenyl esters attached to polyacrylamides have also been reported [41b]. These data are in broad agreement with the above-mentioned observations. However, the apparent solvent effects in chemical transformation of polymers must be interpreted in terms of a dual function, i.e. polymer solvation and solvent catalysis . For example, DMSO is a poor solvent for copol3 AOTq)-styrene), but a good solvent for polymers carrying amide residues. It should also be noted that alcohols and water are not usually suitable as solvent for chemical transformation of activated esters, because they may them lves enter the reaction as nucleophiles. 

An interesting and very practical example of these phenomena concerns catalysts composed of both types of materials—called dual function, or bifunctional (in general, polyfunctional) catalysts. A lucid discussion is provided by Weisz [16], and a few examples indicate the importance of these concepts, not only to catalysis, but also to the kinetic behavior. Much of the reasoning is based on the concept of reaction sequences involving the surface intermediates. Consider the scheme where the species within the dashed box are the surface intermediates. 

Like zeolites that combine shape selectivity with catalysis (Chapter 6), membranes combine separation with catalysis to enhance reaction rates. The dual functionality of zeolites derives from the nature of the catalytic material, whereas that of membranes derives from the nature of the reactor material. The catalyst in the membrane reactor can be a part of the membrane itself or be external to it (i.e., placed inside the membrane tube). The chief property of a membrane is its ability for selective permeation or permselectivity with respect to certain compounds. 

Barak, G., and Y. Sasson, Dual-Function Phase-Transfer Catalysis in the Metal-Assisted Oxidation by Hydrogen Peroxide of Styrene to Benzaldehyde and Acetophenon, Chem. Commun., 1266-1267 (1987). 

In all instances only starting material was isolated with the and C-NMR confirming the absence of any triazole signals. This clearly illustrates that the dipolar cycloaddition reaction only takes place in the presence of cucurbituril, which has a dual function to catalyze the cycloaddition reaction and in the process become inextricably threaded onto the polymer chain. The sequence of events involved in this catalytic self-threading process is presented in Fig. 1.45. The combination of using monomers with bulky in-chain stopper groups and the catalyst self-threading onto the polymer backbone limits this catalysis to a turnover of exactly 1. 

Wang Y, Liu XF, Deng L (2006) Dual-Function Cinchona Alkaloid Catalysis Catalytic Asymmetric Tandem Conjugate Addition-Protonation for the Direct Creation of Nonadjacent Stereocenters. J Am Chem Soc 128 3928... 

Fig. 4. (a) dual function of catalytic triad in serine proteases, where catalysis is performed (i) by the charge relay system, and (ii) by general acid-general base (the oxyanion hole is formed by and Si residues), (b) fimction and role of the catalytic residues C s, H , D158 and/ or Ni s (j bromelain) in cysteine proteases of papain family a hydrogen bond... 

Wang Y, Liu X, Deng L. Dual-function cinchona alkaloid catalysis catalytic asymmetric tandem conjugate addition— protonation for the direct creation of nonadjacent stereocenters. J. Am. Chem. Soc. 2006 128(12) 3928-3930. 

This equation gives (0) = 0, a maximum at =. /Km/K2, and (oo) = 0. The assumed mechanism involves a first-order surface reaction with inhibition of the reaction if a second substrate molecule is adsorbed. A similar functional form for (s) can be obtained by assuming a second-order, dual-site model. As in the case of gas-solid heterogeneous catalysis, it is not possible to verify reaction mechanisms simply by steady-state rate measurements. 

As far as we know, this is the first molecular probe that includes two different types of reporter units activated upon on a specific stimulus. The other option to achieve dual detection would be to use two separate probes. However, in this case there could be a problem of competitive catalysis (circumstances in which the Km of the two substrate is not identical). In our probe, 6-aminoquinoline and 4-nitrophenol, detected by fluorescence and absorbance spectroscopy, respectively, were used as reporter units. Due to the synthetic flexibility of our approach, other reporter molecules with different types of functional groups, like amine or hydroxyl, can be linked to our molecular probe. The two assays must be orthogonal to each other, in order to prevent disturbances in the detection measurement. Another advantage of the probe is the aqueous solubility... 

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