Catalysts catalytic reaction conditions

In this study we have shown that the catalytic method—carbon deposition during hydrocarbons conversion—can be widely used for nanotubule production methods. By variation of the catalysts and reaction conditions it is possible to optimize the process towards the preferred formation of hollow... 

An important future goal of catalytic surface science is to monitor the structure of surfaces and adsorbates at the molecular level in situ under catalytic reaction conditions, to model the more complex technical catalysts, and to undertake the design and tuning of new catalyst surfaces. 

This situation is termed pore-mouth poisoning. As poisoning proceeds the inactive shell thickens and, under extreme conditions, the rate of the catalytic reaction may become limited by the rate of diffusion past the poisoned pore mouths. The apparent activation energy of the reaction under these extreme conditions will be typical of the temperature dependence of diffusion coefficients. If the catalyst and reaction conditions in question are characterized by a low effectiveness factor, one may find that poisoning only a small fraction of the surface gives rise to a disproportionate drop in activity. In a sense one observes a form of selective poisoning. 

In catalytic reaction conditions (H2 pressure), by interaction of a solvent such as THF or acetone, the 16-electron cationic [Rh(diphos)(NBD)]+ affords a 12-electron unsaturated diphosphine intermediate, which is the real active species. The catalytic cycle begins with alkene binding, followed by oxidative addition of H2. These cationic catalysts can reduce alkenes to... 

Dan Resasco (with colleagues Phuong Do, Steven Crossley, Malee Santikuna-porn University of Oklahoma) examine strategies for improving important fuel properties catalytically—e.g., cetane number and threshold soot index. They show that proper choice of catalysts and reaction conditions can significantly improve these widely used measures of fuel performance. 

Figure 8.7 Structural changes of ReOx species in HZcvd catalyst preparation and the catalytic reaction conditions, and a proposed structure of active [Re6017] cluster in the ZSM-5 pore channel, where the [Re6013] cluster is bound to the pentagonal rings of the zeolite inner wall via three lattice oxygen atoms, and the oxygen atoms are tentatively arranged on the Re6 octahedron.

The proposed Re6 cluster (8) with terminal and bridged-oxygen atoms acts as a catalytic site for selective propene oxidation under a mixture of propene, Oz and NH3. When the Re6 catalyst is treated with propene and Oz at 673 K, the cluster is transformed back to the inactive [Re04] monomers (7), reversibly. This is the reason why the catalytic activity is lost in the absence of ammonia (Table 8.5). Note that NH3, which is not involved in the reaction equation for the acrolein formation (C3H6+02->CH2=CHCH0+H20) is a prerequisite for the catalytic reaction as it produces the active cluster structure under the catalytic reaction conditions. 

The catalytic reaction conditions required some optimization. This was due to competing reaction pathways. The interception of trans-11 results in the formation of the organotitanium intermediate 44, as shown in Scheme 17. Thus, 2 equiv. of Cp2TiCl are consumed and a complete conversion in the presence of 10 mol% Cp2TiCl2 cannot be achieved because catalyst regeneration is prevented. Similar considerations apply for czs-11. 

Catalyst Preparation, Reaction Conditions and Catalytic Cycle... 

Supported ruthenium catalysts prepared from Ru3(CO),2 have been used in CO hydrogenation because of the highly dispersed metallic phase achieved when this carbonyl-precursor is used [70,107-109]. However, under catalytic reaction conditions the loss of ruthenium from the support could take place, ft has been reported that at low temperatures it takes place through the formation of Ru(CO)s species, whereas at high temperature dodecarbonyl formation occurs [110]. Decarbonylation of the initial deposited carbonyl precursor under hydrogen could minimize this problem [107]. 

Further reduction is achieved by catalytic hydrogenation using different catalysts and reaction conditions [78, 407] and by lithium in ethylenediamine... 

It is now considered, by most groups working in this area, that vanadyl pyrophosphate (VO)2P207 is the central phase of the Vanadium Phosphate system for butane oxidation to maleic anhydride (7 ). However the local structure of the catalytic sites is still a subject of discussion since, up to now, it has not been possible to study the characteristics of the catalyst under reaction conditions. Correlations have been attempted between catalytic performances obtained at variable temperature (380-430 C) in steady state conditions and physicochemical characterization obtained at room temperature after the catalytic test, sometimes after some deactivation of the catalyst. As a consequence, this has led to some confusion as to the nature of the active phase and of the effective sites. (VO)2P207, V (IV) is mainly detected by X-Ray Diffraction. 

In addition to the development of new catalysts and reaction conditions for aerobic oxidative heterocycUzation, considerable effort has been directed toward asymmetric transformations. Hosokawa and Murahashi reported the first example of asymmetric Pd-catalyzed oxidative heterocycUzation reactions of this type [157,158]. They employed catalytic [(+)-(Ti -pinene)Pd (OAc)]2 together with cocatalytic Cu(OAc)2 for the cycUzation of 2-allylphenol substrates however, the selectivity was relatively poor (< 26% ee). 

In situ dynamic ETEM studies in controlled environments of oxide catalysts permit direct observations of redox pathways under catalytic reaction conditions and provide a better fundamental understanding of the nucleation, growth and the nature of defects at the catalyst surface and their role in catalysis (Gai 1981-1982 92). The following paragraphs describe the methods of observation and quantitative analyses of the surface and microstmctural changes of the catalyst, and correlation of microstmctural data with measurements of catalytic reactivity. We examine examples of pure shear and crystallographic (CS) shear defects that occur under catalytic conditions. 

Metal Hydrides. The simplest reactions in this group are the various catalytic reduction reactions of carbon monoxide. Methane or higher hydrocarbons, methanol or higher alcohols, and a variety of other oxygenated organic compounds may be formed, depending upon the catalyst and reaction conditions (23). There is little evidence about the mechanism of these reactions, but the initial step in every example is probably a carbon monoxide insertion into a metal hydride, followed by reduction reactions. 

Stepwise catalytic reduction of pyran-2-ones to the dihydro (300) and tetrahydro (301) pyran-2-ones is achieved by suitable choice of catalyst and reaction conditions (Scheme 17). Anhydrous copper(II) sulfate simultaneously dehydrates the alcohol (78JHC1153). 

In 1977, Murai and co-workers described the catalytic addition of hydrosilane and carbon monoxide to an internal olefin to give enol silyl ethers in which one molecule of CO is incorporated.103-105 During the time period covered by this review, the transition metal-catalyzed reaction of HSiR3/ CO has been reported for many substrates. The catalytic system provides a facile route to a number of materials that are valuable in organic synthesis. The hydrosilane/CO system is very interesting, as different products can be obtained depending on substrate, catalyst, and reaction conditions employed. 

Mossbauer spectra were then taken of the small iron particles after various pretreatments, with the catalyst under reaction conditions (765). For increased sensitivity the velocity-offset mode was used (Section II, B, 1), and the magnetically split spectral area versus temperature curves after the various pretreatments are shown in Fig. 30. It is therein seen that the ammonia treatment, which increases the catalytic activity, decreases the magnetically split spectral area at a given temperature this is the result of a decrease in the magnetosurface anisotropy energy barrier. While the effects of these pretreatments are in themselves interesting, the important point for surface... 

The synthesis of the C20—C26 fragment started with a 4-alkylation of methyl aceto-acetate The first stereocentre was introduced by enantioselecuve catalytic hydrogenation with Noyort s (S)-binap rhodium complex (cf p 102f.) Stereoselective Frater-Seebach alkylation with allyl bromide introduced the second stereocentre in 90% yield (cf p 27) Stereospecifid introduction of the stereocentres C24 and C2 was achieved by a chelation controlled addition of an allylstannane to an aldehyde (see p 66f) After some experimentation with Lewis acid catalysts and reaction conditions a single diastereomer of the desired configuration was ob-... 

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