Catalytic selectivity, changing

When Mn(salen) is substituted with ethoxy-groups in position 3 of the phenol moiety, selective hydroxylation catalysts for 1-naphlhol or phenol are obtained with the NaY encapsulated version [71]. However, when in Mn(saldp) (see scheme) methoxy-groups are placed in the same position, no acceptable (cyclohexene) epoxidation catalysts are obtained [72]. The abundant formation of allylic oxidation products points to the existence of highly distorted Mn complexes or of free Mn(II). Only the Mn(3-methoxy-salen) entrapped in zeolite X shows medium epoxidation selectivity [72], It is clear that subtle changes in host-guest size / configuration can cause drastic catalytic selectivity changes. 

Presently the catalytic selective NOx reduction by ammonia is efficient and widespread through the world for stationary sources. The remarkable beneficial effect of 02 for the complete reduction of NO into nitrogen is usually observed between 200 and 400°C. However, such a technology is not applicable for mobile sources due to the toxicity of ammonia and vanadium, which composes the active phase in vanadia-titania-based catalysts. Main drawbacks related to storing and handling of ammonia as well as changes in the load composition with subsequent ammonia slip considerably affect the reliability of such a process. On the other hand, the use of urea for heavy-duty vehicles is of interest with the in situ formation of ammonia. 

As a general phenomenon, observed already by Fischer and coworkers, activity and FT synthesis selectivity develop in the initial time of a run in a process of Formierung (formation)16—in modem terms self-organization and catalyst restructuring. In order to achieve high performance of synthesis with cobalt as catalyst, the temperature had to be raised slowly up to the temperature of steady-state conversion. A distinct thermodynamically controlled state of the Co surface, populated with reactants and intermediates, can be assumed. This state depends on temperature and particularly on CO partial pressure, and its catalytic nature changes with changing conditions. 

A selectivity model was proposed based on a theory that the product spectrum consists of two separate products formed on two different catalytic surfaces. Chemical reactivity arguments were used to show that the two surfaces (carbide and oxide) are responsible for the production of different groups of products. Separation of the product spectrum in this way enabled us to explain and sometimes predict changes in selectivity and gave us a handle on tracking the full selectivity changes by following the C02 and CH4 selectivity. 

Weakly adsorbed molecules (A in Figure 8. lb), which can exist only under catalytic reaction conditions, play an important role in surface catalytic reactions even if the adsorption of the promoter is very weak or is undetectable at the surface. Surface intermediates (Cat-X in Figure 8. lb) under the ambient gas molecules behave in a different way from those under vacuum, showing rate enhancement and selectivity change of the surface reaction in the presence of ambient gas. 

In practice, of course, it is rare that the catalytic reactor employed for a particular process operates isothermally. More often than not, heat is generated by exothermic reactions (or absorbed by endothermic reactions) within the reactor. Consequently, it is necessary to consider what effect non-isothermal conditions have on catalytic selectivity. The influence which the simultaneous transfer of heat and mass has on the selectivity of catalytic reactions can be assessed from a mathematical model in which diffusion and chemical reactions of each component within the porous catalyst are represented by differential equations and in which heat released or absorbed by reaction is described by a heat balance equation. The boundary conditions ascribed to the problem depend on whether interparticle heat and mass transfer are considered important. To illustrate how the model is constructed, the case of two concurrent first-order reactions is considered. As pointed out in the last section, if conditions were isothermal, selectivity would not be affected by any change in diffusivity within the catalyst pellet. However, non-isothermal conditions do affect selectivity even when both competing reactions are of the same kinetic order. The conservation equations for each component are described by... 

The assays are reported in Table I. The selectivity of the reaction is lower when rhodium rather than cobalt is used, and it is lower still when rhodium is used in the presence of triphenylphosphine. Thus, the catalytic precursor changes the selectivity to the possible isomeric aldehydes markedly. In each case it is possible to obtain one of the three aldehydes in good yields and in a high state of purity. 

Several studies have been made of the effect of added metal ions on the pinacol/alcohol ratio. Addition of antimony(m) chloride in catalytic amounts changes the product of the electrochemical reduction of acetophenone in acidic alcohol at a lead electrode from the pinacol in the absence of added metal salt to the secondary alcohol in its presence53. Antimony metal was suspected to be an intermediate in the reduction. Conversely, addition of Sm(in) chloride to DMF solutions of aromatic aldehydes and ketones54 and manganese(II) chloride to DMF solutions of hindered aromatic ketones55 results in selective formation of pinacols in excellent yields. When considering these results one should keep in mind the fact that aromatic ketones tend to form pinacols in DMF even in the absence of added metal ions1,29,45. 

It is generally true that catalytic selectivity is not a good basis for mass sensors. Propose a scheme in which a highly selective interaction between the enzyme and the substrate could be used to detect interaction as a change in mass. 

From a series of experiments in this reactor, the deactivation effect of coke on a complex reaction mechanism may be obtained. This is illustrated for the catalytic cracking of n-hcxane on a US-Y zeolite catalyst. On a faujasite, the coke formation deactivates the main reactions, but not the coking reaction. Moreover, the coke formation induces selectivity changes, which can be explained by the distribution of acid site strength in Y-zeolites and the acid strength requirements of the various reactions. 

Time-resolved X-ray absorption spectra of an activated H5[PV2Moio04o] oxidation catalyst were recorded to determine correlations between the dynamic structure and the catalytic selectivity of the material (Ressler and Timpe, 2007). In addition to experiments carried out under steady-state conditions, time-resolved XAFS measurements at the Mo K-edge were performed under changing reaction conditions (with a time resolution of 30 s per spectrum) (Figure 52). Therefore, the gas-phase composition was isothermally switched from a reducing atmosphere (propene) to an... 

Find the expressions for the reaction rates in each of the stepwise channels and the selectivity of the ethylbenzene formation for the pro cess that is stationary with respect to catalytic intermediates Kj and K2. How will the selectivity change when DEB is fed to the reactor as an additional initial reactant ... 

Ultimately, further increases in x lead to a decrease in product molecular weight and C5+ selectivity because structural properties that limit olefin removal rates at low values of x also begin to limit the rate of arrival of reactants at catalytic sites as x increases (Fig. 28a). At higher values of x methane selectivity also reverses its initial trend and begins to increase. These results show that these transport restrictions and the resulting selectivity changes are caused by an increase in x, independent of how the value of this parameter is varied. 

Under standard conditions the tetrakis(allyl)neodymium complex, like the tris(allyl)neodymium complex, shows only moderate catalytic activity with a high trans selectivity, but in combination with appropriate Lewis acids such as alkylaluminum chlorides or methylalumoxane the activity can be increased considerably and the selectivity changes mainly to cis. Extremely active catalysts of very high cis selectivity are obtained with the chloro(allyl)neodymium compounds in combination with methylalumoxane in heptane. For more details see [39, 49, 106]. 

However, the electric potential of the electrocatalyst at its interface with the electrolyte (and thus the facility for charge transfer) can be easily and extensively altered at will to control rate and selectivity. For instance, a decrease of electrode potential by about 0.15 V can change the product selectivity for vinyl fluoride and chloride reduction on palladium by as much as 80% (31). In contrast, gas phase parallel reductions, with 5 kcal/mol difference in activation energies, would require a temperature increase from 500 K to 730 K for a comparable selectivity change. We should note here that the electrocatalytic specificity of the above reductions is quite similar to that of conventional heterogeneous catalytic reactions, but differs from that of conventional electrolytic reduction on noncatalytic electrodes (32). 

In terms of development, initial attention focused on the construction of relatively simple host-guest systems that were able to mimic certain aspects of enzymatic catalysis, for example, substrate binding in a cavity and conversion at a nearby catalytic center. The reactions studied were often taken from known enzymatic reactions, such as ester hydrolysis and aldehyde condensations, but Diels-Alder reactions have also led to nice examples of accelerations and selectivity changes. 

Changing Catalytic Selectivity Atom by Atom Polymerization of Acetylene on Palladium Nanocatalysts... 

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