Modifier-catalyst interactions

Reaction of 3 with 1 equivalent of a phosphine results in formation of "phosphine-modified catalysts (4). The complex formed from 7r-allyl-nickel chloride, tricyclohexylphosphine, and methylaluminum dichloride (4a) has been isolated and its structure determined crystallographically (see Fig. 1) (57) The phosphine is bonded to the nickel atom, and interaction with the Lewis acid takes place via a chlorine bridge. The bridging chlorine atom is almost symmetrically bound to both the nickel... 

One of the oldest mechanisms of interaction between adsorbed reactant and adsorbed TA has been proposed by Klabunovskii and Petrov [212], They suggested that the reactant adsorbs stere-oselectively onto the modified catalyst surface. The subsequent surface reaction is itself nonstere-ospecific. Therefore, the optically active product is a result of the initial stereoselective adsorption of the reactant, which in turn, is a consequence of the interactions between reactant, modifier, and catalyst. The entities form an intermediate chelate complex where reactant and modifier are bound to the same surface atom (Scheme 14.4). The orientation of the reactant in such a complex is determined by the most stable configuration of the overall complex intermediate. The mechanism predicts that OY only depends on the relative concentrations of keto and enol forms of the reactant,... 

The resolution of ( ) amino acids isn t really a synthetic method, but it s certainly useful in the production of a particular amino acid from a racemic mixture. In the resolution of ( ) amino acids, an enzyme (a biological catalyst) interacts with only one enantiomer. (Why, you ask Because enzymes are stereoselective.) The enzyme leaves one enantiomer unchanged and modifies the other into a different compound, which makes it possible to separate the enantiomer from the other compound by a number of techniques. After the enantiomer has been separated, all that s left is to reverse the process induced by the enzyme. 

These results suggest that oxidation state is not solely responsible for catalyst deactivation but that other factors such as V location and mobility may play an important role. Basic alkaline earth oxide passivators such as MgO, admixed to the catalyst, interact strongly with vanadium during the regeneration period. Although the oxidation state of vanadium is essentially unaffected, MgO structurally modifies V as evidenced by a unique X-ray absorption spectrum. 

These examples illustrate that biomolecules may act as catalysts in soils to alter the structure of organic contaminants. The exact nature of the reaction may be modified by interaction of the biocatalyst with soil colloids. It is also possible that the catalytic reaction requires a specific mineral-biomolecule combination. Mortland (1984) demonstrated that py ridoxal-5 -phosphate (PLP) catalyzes glutamic acid deamination at 20 °C in the presence of copper-substituted smectite. The proposed pathway for deamination involved formation ofa Schiff base between PLP and glutamic acid, followed by complexation with Cu2+ on the clay surface. Substituted Cu2+ stabilized the Schiff base by chelation of the carboxylate, imine nitrogen, and the phenolic oxygen. In this case, catalysis required combination of the biomolecule with a specific metal-substituted clay. 

The temperature of calcination of a support before impregnation understandably modifies the interaction between the precursor and the support, as shown in the above case and that of alumina-supported nickel catalysts [68]. 

There are plateau propellants that exhibit an intermediate range of pressure over which m is practically independent of p and mesa propellants for which m(p) achieves a maximum at a particular value of p then a minimum at a higher value. These effects may be produced in conventional double-base propellants by suitable addition of burning-rate catalysts (typically certain metal-organic salts) to the propellant formulation. It has been shown experimentally that these catalysts usually operate by modifying the interaction between the condensed-phase and dispersed-phase reaction zones [57], [58]. Thus dispersion phenomena are of importance to the deflagration of homogeneous propellants in a number of ways. 

So calcination greatly modifies the interaction between the metals in catalysts prepared by catalytic reduction. 

Pure and NaP-modified MnOx-catalysts were used in our study. Due to easy visualization by AFM, the MnOx layer was placed on a Si-wafer substrate (1 cm x 1 cm plate), by a reactive deposition technique. The sample preparation was carried out in a vacuum installation equipped with an resistance evaporator. Metallic manganese (99.8%) as a source and a Si wafer with a surface orientation (111) and resistivity of 7.5 ohm/cm as support, were used. During MnOx deposition, an oxygen partial pressure of ca 10 torr, in dynamic mode, was maintained. Before used for the catalytic purpose, MnOx samples were calcined in air at 700°C for 60min. In order to prepare the NaP-modified catalyst, the MnOx samples were impregnated in a diluted Na4P20 solution (5 wt %), dried and finally calcined at 500° C, in air during 30 min. The interaction with methane was performed in a quartz reactor in a methane atmosphere at 700° 5° C. 

The emergence of new prochiral substrates and chiral modifiers and new enantioselective reactions have created a need for identifying common mechanistic features in all of these reactions the structural, reactivity- and affinity characteristics of the effective modifiers, the interactive functional groups of the prochiral substrates and the appropriate catalysts. 

One general observation is that the chiral effect of both the cinchona and vinca type alkaloids appears to change, in most cases to decrease, if the reaction is started as a racemic hydrogenation compared with the case when the chiral modifier is added at the beginning of the hydrogenation. But no clear conclusion can be made as to whether or not the modifier molecules interact less with the catalyst surface, which is covered by hydrogen and chiral product molecules, and as a result exert less asymmetric effect. 

Our results clearly show that the behaviour of the same modifier-substrate system strongly depends on the catalyst used (the same metals on different supports or Adams Pt or Pd black), indicating that the catalyst-modifier-substrate interaction on the catalyst surface is a crucial factor in the process of enantioselection and the observed rate acceleration of pyruvate hydrogenation. On the other hand, it has been shown by CD and NMR measurements that the... 

An enhancement of the regioselectivity was observed when the catalyst modifier concentration was increased (Figure 3). The enhancement might be caused by 1) the modifier-reactant interaction in the liquid-phase, 2) the modifier-reactant interaction on the catalyst surface, 3) the coverage dependent adsorption modes of the reactant and modifier or a combination of factors 1-3. 

The modifier-reactant interaction on the catalyst surface is the basis of the model proposed by Baiker et al. [28] for the rate enhancement and enantioselectiv-ity in the case of ethylpymvate hydrogenation over Ft catalysts modified by cin-chonidine. The maximal in enantiomeric excess and rs were observed close to the 1 1 ratio of surface Pt-to-modifier (Figure 3) which supports the hypothesis that, the adsorption of the modifier on the catalyst surface would be the origin of the enantioselectivity and the enhancement in rs. 

To be used as modified catalysts or ion exchangers, the incorporation of d or f block elements into zeohte structures has been extensively investigated (the ion exchange of cations in crystaUine aluminosihcates and related materials is a standard method used for modifying their catalytic and sorption properties. For example, the solid-state interaction of lanthanum(III) chloride with zeolites under anhydrous conditions was investigated [2]. 

On the other hand, Tungler et al. considered the selective characteristics of heterogeneous chiral modified catalysts in the enantioselective and diastereselective hydrogenation of C=C, C=0, and C=N double bonds. Because of the absence of interaction between these groups and the tertiary A-atom of the Troger s base (Scheme 5.31.), rate acceleration in hydrogenation could not be observed and resulted in an ee of only 38.3%. 

A number of examples investigating alternative strategies for catalyst immobilization were also described. These, too, are important, as they can alleviate the time and cost barriers toward modifying catalyst architectures for attachment onto a solid support. Ideally, these systems should display many of the same benefits attributed to the covalently immobilized systems however, catalyst leaching is a potential concern due to the lack of a covalent interaction between the catalytic moiety and the supporting matrix. This is more problematic when substrates containing polar functional groups are examined, or the use of polar solvents are required for the process. 

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