Catalyst Interactions

Chromium compounds decompose primary and secondary hydroperoxides to the corresponding carbonyl compounds, both homogeneously and heterogeneously (187—191). The mechanism of chromium catalyst interaction with hydroperoxides may involve generation of hexavalent chromium in the form of an alkyl chromate, which decomposes heterolyticaHy to give ketone (192). The oxidation of alcohol intermediates may also proceed through chromate ester intermediates (193). Therefore, chromium catalysis tends to increase the ketone alcohol ratio in the product (194,195). 

All lation of Phenols. The approach used to synthesize commercially available alkylphenols is Friedel-Crafts alkylation. The specific procedure typically uses an alkene as the alkylating agent and an acid catalyst, generally a sulfonic acid. Alkene and catalyst interact to form a carbocation and counter ion (5) which interacts with phenol to form a 7T complex (6). This complex is held together by the overlap of the filled TT-orbital of the aromatic... 

In the above cases, an optically active reducing agent or catalyst interacts with a prochiral substrate. Asymmetric reduction of ketones has also been achieved with an achiral reducing agent, if the ketone is complexed to an optically active transition metal Lewis acid. ... 

A chiral indene derivative, structure K, has been most commonly used.222 The catalyst interacts with the trialkylaluminum to generate a bimetallic species that is the active catalyst. 

A rationale for these results is that the catalyst interacts with the substrate through weak intermolecular association, and that the strength and nature of... 

Figure 6.1. Substrate-catalyst interactions favor a specific mode of alkene insertion into the zirconocene—alkene complex.

Specific control of the stereochemistry of the chemical reaction is better achieved using chiral phase-transfer catalysts. These catalysts interact specifically with the substrate and sterically hinder the approach of nucleophile to one face of the reactive site. Experimental procedures are essentially the same as those employed in reactions using achiral catalysts where there is no stereochemical control and, in subsequent sections, reference is made back to the appropriate Chapter unless variations in the procedure differ significantly. 

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. 

In 1973, the Arab oil embargo and the sudden escalation in crude oil prices (Figures 1,2) and availability placed refiners under great economic pressures to process more abundant, less expensive, metals-contaminated crude oils and residuum feedstocks. The need for metals-resistant FCC became apparent, and work on understanding metal-catalyst interactions became an area of intense research in industrial laboratories and in the academic community. 

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. 

The significant factors are catalyst, time, temperature,. and the time x catalyst interaction. Reference to the two-way tables will help interpret the relative magnitude of the effects. In the time x catalyst interaction, the effect of change in catalyst level depends on how long the reaction has been run. 

Since the calculated F does not exceed the critical value, we are not able to say that a real time x temperature X catalyst interaction exists ... 

This latter interpretation would mean that with the approach depicted in Fig. 10, the catalyst itself could be monitored. The authors reported that the silica-supported Nafion could not be observed in the beginning of their experiments and appeared in the spectra only after the catalyst interacted with octanol. This observation may indicate that the octyl groups promote the sticking of the catalyst particles onto the ATR probe, within the evanescent field. However, the example also shows that this approach may not be without problems, because it depends on the adsorption of the particles from the slurry reactor onto the ATR element. This process is accompanied by the adsorption of molecules on the catalyst surface and complicates the analysis. More important, as also indicated by the work of Mul et al. (74). this adsorption depends on the surface properties of the catalyst particles and the ATR element. These properties are prone to change as a function of conversion in a batch process and are therefore hardly predictable. 

If a third component (M), which can specifically stabilize one of the products of electron transfer, is introduced into the D -A system, the free energy change of photoinduced electron transfer is shifted to the negative direction, when the activation barrier of electron transfer is reduced to accelerate the rates of electron transfer, as shown in Figure 3, where M forms a complex with A ". It should be emphasized that there is no need to have an interaction of M with A and that the interaction with the reduced state (A ") is sufficient to accelerate the rate of photoinduced electron transfer. This contrasts sharply with the catalysis on conventional ionic or concerted reactions, in which the catalyst interacts with a reactant to accelerate the reactions. The initial interaction between M and A in the complex A-M, where charge is partially transferred from A to M, would also result in acceleration of the photoinduced electron transfer, since the reduction potential of A-M is shifted to the negative direction as compared to that of A. 

Vanadyl porphyrin interaction with the surface is a function of the catalyst. Adsorption through electron acceptor sites dominates on the oxide surface, whereas the sulfided catalyst interacts through electron donor sites (see Section IV,B,5). Heats of adsorption have been estimated to be 8 to 12 kcal/mole. Values in this range are indicative of weak adsorption interactions that are of reduced importance at hydroprocessing conditions. 

Detailed adsorption studies by Morales and co-workers (Morales and Galiasso, 1982 Andreu et al., 1981 Morales et al., 1984) are unique in their examination of the interaction of VO-porphyrins extracted from Boscan crude on y-ALOrSupported cobalt and molybdenum catalysts. Interaction with the oxide catalyst is primarily through the oxygen ligand of... 

In two instances are the electronic spectra of the bulk of the catalyst particles of interest in catalysis research first, when chemisorption gives rise to electron exchange that extends to large distances into the solid and second, when various components of a multiphase catalyst Interact so as to dope one phase with the chemical elements of another, resulting in new, enhanced, or reduced activity of the catalyst. 

Supramolecular catalysis can involve passive effects such as the confining of two reactive molecules within a cavity and active effects where the catalyst interacts with the substrate via an active site. The active site may be metal-based as in other kinds of homogeneous catalyst based on transition metals or Lewis acids, or my involve interactions such as hydrogen bonding to bring about both polarisation of the reactants and their mutual spatial organisation. 

Substrate-catalyst interaction is also essential for micellar catalysis, the principles of which have long been established and consistently described in detail [63-66]. The main feature of micellar catalysis is the ability of reacting species to concentrate inside micelles, which leads to a considerable acceleration of the reaction. The same principle may apply for polymer systems. An interesting way to concentrate the substrate inside polymer catalysts is the use of cross-linked amphiphilic polymer latexes [67-69]. Liu et al. [67] synthesized a histidine-containing resin which was active in hydrolysis of p-nitrophenyl acetate (NPA). The kinetics curve of NPA decomposition in the presence of the resin was of Michaelis-Menten type, indicating that the catalytic act was accompanied by sorption of the substrate. However, no discussion of the possible sorption mechanisms (i.e., sorption by the interfaces or by the core of the resin beads) was presented. 

Epoxidation of olefins by peroxy acetic acid, formed in the reaction mixture on catalytic (with H2S04 or lower alkylsulfonic acids as the catalysts) interaction between H202 and CH3COOH, is described [49] ... 

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