Catalyst dependence formation

In order to rationalize the catalyst-dependent selectivity of cyclopropanation reaction with respect to the alkene, the ability of a transition metal for olefin coordination has been considered to be a key factor (see Sect. 2.2.1 and 2.2.2). It was proposed that palladium and certain copper catalysts promote cyclopropanation through intramolecular carbene transfer from a metal carbene to an alkene molecule coordinated to the same metal atom25,64. The preferential cyclopropanation of terminal olefins and the less hindered double bond in dienes spoke in favor of metal-olefin coordination. Furthermore, stable and metastable metal-carbene-olefin complexes are known, some of which undergo intramolecular cyclopropane formation, e.g. 426 - 427 415). 

The 1,3-dipolar cycloaddition of nitrones to vinyl ethers is accelerated by Ti(IV) species. The efficiency of the catalyst depends on its complexation capacity. The use of Ti( PrO)2Cl2 favors the formation of trans cycloadducts, presumably, via an endo bidentate complex, in which the metal atom is simultaneously coordinated to the vinyl ether and to the cyclic nitrone or to the Z-isomer of the acyclic nitrones (800a). Highly diastereo- and enantioselective 1,3-dipolar cycloaddition reactions of nitrones with alkenes, catalyzed by chiral polybi-naphtyl Lewis acids, have been developed. Isoxazolidines with up to 99% ee were obtained. The chiral polymer ligand influences the stereoselectivity to the same extent as its monomeric version, but has the advantage of easy recovery and reuse (800b). 

The photoreactivity of the involved catalyst depends on many experimental factors such as the intensity of the absorbed light, electron-hole pair formation and recombination rates, charge transfer rate to chemical species, diffusion rate, adsorption and desorption rates of reagents and products, pH of the solution, photocatalyst and reactant concentrations, and partial pressure of oxygen [19,302,307], Most of these factors are strongly affected by the nature and structure of the catalyst, which is dependent on the preparation method. The presence of the impurities may also affect the photoreactivity. The presence of chloride was found to reduce the rate of oxidation by scavenging of oxidizing radicals [151,308] ... 

Adducts of type 13, arising from the rearrangement of the allylic intermediate, have never been observed. The product distribution in methanol depends, however, on the reaction conditions. When the addition of XeF2 is carried out in the presence of boron trifluoride as a catalyst, the formation of the complex b has been suggested. This complex would react with 2,3-dimethylbutadiene as a positive oxygen electrophile to give, besides 1,2- and 1,4-difluoro derivatives, 1,4- and 1,2-fluoromethoxy products with a predominance of the anti-Markovnikov adduct (equation 26). 

As an aside, we should mention that the same principles apply to the formation of bimetallic clusters on a support. In the case of Pt-Re on AI2O3 it has been shown that hydroxylation of the surface favors the ability of Re ions to migrate toward the Pt nuclei and thus the formation of alloy particles, whereas fixing the Re ions onto a dehydroxylated alumina surface creates mainly separated Re particles. As catalytic activity and selectivity of the bimetallic particles differ vastly from those of a physical mixture of monometallic particles, the catalytic performance of the reduced catalyst depends significantly on the protocol used during its formation. The bimetallic Pt-Re catalysts have been identified by comparison with preparations in which gaseous Re carbonyl was decomposed on conventionally prepared Pt/Al203 catalysts. ... 

Microstructures of CLs vary depending on applicable solvenf, particle sizes of primary carbon powders, ionomer cluster size, temperafure, wetting properties of carbon materials, and composition of the CL ink. These factors determine the complex interactions between Pt/carbon particles, ionomer molecules, and solvent molecules, which control the catalyst layer formation process. The choice of a dispersion medium determines whefher fhe ionomer is to be found in solubilized, colloidal, or precipitated forms. This influences fhe microsfrucfure and fhe pore size disfribution of the CL. i It is vital to understand the conditions under which the ionomer is able to penetrate into primary pores inside agglomerates. Another challenge is to characterize the structure of the ionomer phase in the secondary void spaces between agglomerates and obtain the effective proton conductivity of the layer. 

Four reviews on allylic and vinyl substitution have been published.20-23 The use of pentamethylcyclopentadienylruthenium catalysts for the. S n reactions of allyl substrates has been reviewed.20 The Sn reactions of allyl substrates in the presence of ruthenium catalysts occur primarily at the most substituted position of the allylic group. All the catalysts involve formation of an intermediate where the allyl compound becomes associated with the Ru atom in the catalyst. The regiospecificity (50-98%) depends on the structure of the allylic substrate, the nucleophile, the solvent, the temperature, and the catalyst. These catalysts have also been used for protection of allylic alcohol and amino groups. Some of the reactions are stereospecific. 

The formation of CDT is suppressed if ethylene as well as butadiene is brought into contact with a naked-nickel catalyst. Depending on the reaction conditions, the product is a mixture of m,tram-1,5-cyclodecadiene (CDD) and 1,tram-4,9-decatriene (DT) (90). With equal concentration of butadiene and ethylene the co-oligomerization occurs some six times faster than the cyclotrimerization of butadiene to CDT. 

When metal centers act in conjunction with acid sites on the zeolite, bifunctional catalysis can occur (e.g., Pd/HY). This type of catalysis is used mainly for the hydrocracking and isomerization of long-chain n-alkanes. For example, the rates of formation of 2- and 5-methylnonane isomers obtained from n-decane isomerization over bifunctional zeolite catalysts depend on the size and structure of the zeolites used. This reaction has been developed as a test reaction to characterize zeolite structures (17-19). 

Basu and coworkers [30,31] have reported that the chemoselectivity of di-ene-ene RCM reactions can be catalyst-dependent. In a systematic study it was found that in the RCM of 15, increasing the chain length (and hence product ring size) led to a divergence of catalyst behaviour, with 2 favouring diene products 17, and 4 favouring the formation of monoene heterocycles 16 (Scheme 4). This was explained in terms of the reversibility of RCM the stabler and more active catalyst 4 promotes the formation of the kinetic product 17 initially, which then equilibrates to 16 over time. Similar catalyst-dependent selectivity had previously been observed [32], Using 4, diene versus monoene product temperature dependence has also been reported [33]. 

The formation of coke on acid zeolite catalysts depends on i) the characteristics of the acid sites and of the pore structure of the zeolite and ii) the nature of the feed and the operating conditions (T,P). 

A frequent reason for the dependence of catalyst structure on the chemical potential in the gas phase containing all the reactants is the incorporation of molecules or atoms from the reaction mixture into the catalyst phases. Formation of subphases, often only in the near-surface region of the solid, fails to create phases with individual reflections but modifies the reflections of the starting precatalyst phase notably (see previous sections). This complication presents a massive problem in the analysis of working catalysts when significant partial pressures of products are important to the phase formation and when the necessary conversions cannot be reached in the experimental cell. The investigation of ammonia synthesis catalysts when insufficient partial pressures of the product ammonia prevent the formation of the relevant nitride phases is a prominent example of this limitation (Herzog et al., 1996 Walker et al., 1989). 

The activity of the catalyst depends on the conditions of preparation and purity of the titanium trichloride. In high activity catalysts the initial crystal aggregates are broken down completely and most of the metal atoms have been claimed to become involved in the polymerization [17]. The formation of the active sites at the crystal edges is considered to involve complexation or alkylation by the organo-metal compounds [18], viz. 

Carbon deposition is one of the luost serious problems of the steam reforming catalyst process (ref 1). The deposition of carbon on naphtha steam reforming catalysts depends ori the chemical composition of the hydrocarbon oil, the steam/carbon ratio in the feedstock, as well as the pi ocesa temperature and pressure, it is also affected by tlie presence of sulfur poisons Our past research of SNG catalysts ejiamined the nature of the carbon deposits as a function of the sulfur level on the catalyst (refs, 2 4). A small amount of sulfur was found to promote the formation of carbon that is non-reactive with steam and hydrogen under steam reforming reaction conditions. The continuous accumulation of this less reactive carbon [continuous carbon deposition (CCD)l on the catalyst surface leads to coke fouling Studies of the occurrence of CCD in our laboratory tests allow ua to predict, that coke fouling is likely to occur on the same catalyst used in real Indusl.rlal applications. 

The position of the double bond inside each structure depends on the catalyst composition. Formation of 2-methyl-2-pentene and 2,3-dimethyl-2-butene results from a consecutive isomerization reaction and thus depends not only on the catalyst composition but also on propene conversion. The relative reactivities of isomeric dimers for a double-bond shift are given in [6]. 

The Knoevenagel condensation is a base-catalyzed aldol-type reaction, and the exact mechanism depends on the substrates and the type of catalyst used. The first proposal for the mechanism was set forth by A.C.O. Hann and A. Lapworth Hann-Lapworth mechanism) In 1904." When tertiary amines are used as catalysts, the formation of a p-hydroxydlcarbonyl Intermediate is expected, which undergoes dehydration to afford the product. On the other hand, when secondary or primary amines are used as catalyst, the aldehyde and the amine condense to form an Imlnlum salt that then reacts with the enolate. Finally, a 1,2-ellmlnatlon gives rise to the desired a,p-unsaturated dicarbonyl or related compounds. The final product may undergo a Michael addition with the excess enolate to give a bis adduct. 

Catalysts based on CuO-ZnO are of great industrial interest because they exhibit high activity for the low temperature-pressure methanol synthesis and the water-gas-shift reactions. It is known that the activity and useful life of catalysts depend mainly on the activation process and the thermal history they experience during the operation. In the low temperature water gas shift (LTWGS) process, prior to reaction, the catalyst is activated by gas reduction to convert copper oxide into metallic copper [1]. It has been observed that reduction conditions affect the activity and the stability of Cu-ZnO catalysts. For instance, sintering and formation of alloys must be avoided in the reduction step because they deactivate the catalyst [2-3] for the water-gas-shift reaction. 

The deactivation of acid zeolite catalysts is mainly due to deposition within the pores and on the outer surface of the crystallites of heavy secondary products. The deactivation and regeneration of zeolites was discussed in more detail by Guisnet et al. [10]. The formation of coke on metal zeolite-alumina catalysts depends on the distribution of the acid sites, metal-to-acid ratio and of the texture. 

The synthesis mixture for mesoporous materials contains four major components inorganic precursors, organic template molecules, solvent, and acid or base catalyst. The formation of a material with a desired stmcture and morphology depends on a delicate interplay between several basic processes, whose relative rates determine the... 

Centi et al. [67] found that the activity of the Cu-ZSM-5 in SCR was higher than that of V2O5 on Ti02. The activity over Cu-ZSM-5 catalysts depends on the Si Al ratio [68], Decreasing the Si Al atomic ratio increases the activity. The active site consists of two adjacent copper ions according to Moretti [68]. It is well known that the activity of copper-exchanged ZSM-5 catalysts decreases in the presence of water at high temperatures. Moreover, NO2 formation is necessary but not a sufficient condition for the SCR of NO over Cu-ZSM-5 [69]. 

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