Role of homogeneous reactions

Contrary to the case of olefins, homogeneous catalytic oxidations of light alkanes occur at temperatures similar to those of the catalytic reaction. This certainly led to misinterpretation of supposedly catalytic data in certain cases. Two examples will illustrate the role of homogeneous reaction the oxidative dehydration of propane and the reactions of pentane with oxygen. 

Role of homogeneous reactions in the control of the selectivity to maleic and phthalic anhydrides in the oxidation of n-pentane... 

In conclusion, in the case of Pt and the other high temperature catalysts examined so far, several pieces of evidence have been presented in the literature in favour of the prevailing role of homogeneous reactions in the production of olefins at high temperature and short contact time. According to this picture, the role of the catalytic phase is essentially that of providing thermal ignition. In turn, the role... 

The role of mixing in heterogeneous reactions is obvious. In multiphase processes mixing imposed by a stirrer or an external pump is necessary to increase the interface through which reactants pass to meet their partner in the other phase and/or to intensify mass transfer between phases. Mixing can also play a significant role in the case of homogeneous reactions. Chemical reactions occur at the molecular level. Reactant molecules introduced into a reactor encounter the environment in the vicinity of the inlet. The composition of the mixture there is obviously... 

Certain crude approaches are available to predict overall results, that is, nonequilibrium compositions. More refined techniques are available for the analysis of simplified models. Solution of the reaction kinetics of homogeneous gas phase combustion is possible through numerical solution of the rate equations. With the exception of the role of an overall highly exothermic reaction, the procedures are similar to those described in the preceding section on nozzle processes. The solution of the droplet burning problem including the role of chemical reaction rates, while not particularly tractable, is feasible. 

As a further development in the exploration of the role of solvent in hydroformylation systems, the use of supercritical carbon dioxide (SCCO2), ionic liquids and fluorous systems has been reported. A review by Jessop, Ikariya, and Noyori presents information about the utility of SCCO2 in a variety of homogeneous reactions. ... 

W. A. R. Slegeir, R. S. Saplenra, R. Rayford, L. Lam, Role of homogeneous formate complexes in the water gas shift reaction catalyzed by the Group VI metal carbonyls, Organrane-tallics 1 (1982) 1728-1730. 

The important role in astrochemistry of homogeneous reactions involving charged species (e.g., ion-molecule reactions and ion-electron recombinations) is reflected in the proceedings of this meeting. However, it has been recognized for some time [1] that neutral-neutral reactions may also play some role in astrochemistry, even at the exceedingly low temperatures of interstellar clouds. 

In microreactors with FeCr alloy walls, is impacted more severely by the larger difference between gas and solid thermal conductivities, with the time required for steady state increasing by as much as 17.2% for Case 12 when gas-phase reactions are taken into account (compare with corresponding Case 25 in Table 8.2). Regardless of reactor material and operating conditions, the role of homogeneous chemistry in the start-up of methane-fueled catalytic microreactors is to elongate the steady-state times. 

The final step of the whole reaction process is the desorption of the products. This step is essential not only for the practical purpose of collecting and storing the desired output, but also for the regeneration of the catalytic active sites of the surface. Most reactions have at least one rate-hmiting step, which frequently makes the reaction prohibitively slow for practical purposes when, e.g., it is intended for homogeneous (gas or fluid) media. The role of a good solid-state catalyst is to obtain an acceptable... 

Another problem that required solving was the moderate yield obtained in the cyclopropanation reaction when only one equivalent of styrene was used. By increasing the amount of styrene up to its use as the reaction solvent, a noticeable effect on the selectivities was observed when laponite was used as the support [58]. The active role of the clay support was definitely estabhshed when the results in homogeneous and heterogeneous phases were compared (Table 9). These effects involved the reversal of the trans preference in solution to the cis preference with the laponite-supported catalyst in styrene, and also a reversal in the absolute configuration of the major cis enantiomer ob-... 

In situ infrared observations show that the primary species present during the reduction of NO by CH4 over Co-ZSM-5 are adsorbed NO 2 and CN. When O2 is present in the feed NO2 is formed by the homogeneous and catalyzed oxidation of NO. In the absence of O2, NO2 is presumed to be formed via the reaction 3 NO = NO2 + N2O. The CN species observed are produced via the reaction of methane with adsorbed NO2, and transient response studies suggest that CN species are precursors to N2 and CO2. A mechanism for the SCR of NO is proposed (see Figure 10). This mechanism explains the means by which NO2 is formed from adsorbed NO and the subsequent reaction sequence by which adsorbed NO2 reacts with CH4 and O2 to form CN species. N2 and CO or CO2 are believed to form via the reaction of CN with NO or NO2. CH3NO is presumed to be formed as a product of the reaction of CH4 with adsorbed NO2. The proposed mechanism explains the role of O2 in facilitating the reduction of NO by CH4 and the role of NO in facilitating the oxidation of CH4 by O2. 

In this chapter the potential of nanostructured metal systems in catalysis and the production of fine chemicals has been underlined. The crucial role of particle size in determining the activity and selectivity of the catalytic systems has been pointed out several examples of important reactions have been presented and the reaction conditions also described. Metal Vapor Synthesis has proved to be a powerful tool for the generation of catalytically active microclusters SMA and nanoparticles. SMA are unique homogeneous catalytic precursors and they can be very convenient starting materials for the gentle deposition of catalytically active metal nanoparticles of controlled size. 

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