Reactions in heterogeneous phase

In general, bioreactions can occur in either a homogeneous hquid phase or in heterogeneous phases including gas, hquid, and/or sohd. Reactions with particles of catalysts, or of immobilized enzymes and aerobic fermentation with oxygen supply, represent examples of reactions in heterogeneous phases. 

Fuentes, A., Sinisterra, J. V. Single electron transfer mechanism of the Cannizzaro reaction in heterogeneous phase, under ultrasonic conditions. Tetrahedron Lett. 1986, 27, 2967-2970. 

Recently, ESR techniques have been applied to study polymerization reactions in heterogeneous phase, for example, emulsion polymerization. The development of the direct measurements of radical concentration by ESR represents a major advance in obtaining reliable data on important parameters in emulsion copolymerization modeling, snch as propagation rate coefficients or termination rate coefficients as functions of chain length and conversion [140,141],... 

N-Acylnitroso compounds 4 are generated in situ by periodate oxidation of hydroxamic acids 3 and react with 1,3-dienes (e.g. butadiene) to give 1,2-oxazines 5 (Scheme 6.3). The periodate oxidation of 4-O-protected homo-chiral hydroxamic acid 6 occurs in water in heterogeneous phase at 0°C, and the N-acylnitroso compound 7 that is generated immediately cyclizes to cis and tranx-l,2-oxazinolactams (Scheme 6.4) [17a, b]. When the cycloaddition is carried out in CHCI3 solution, the reaction is poorly diastereo-selective. In water, a considerable enhancement in favor of the trans adduct is observed. 

In particular, reactions in heterogeneous catalysis are always a series of steps, including adsorption on the surface, reaction, and desorption back into the gas phase. In the course of this chapter we will see how the rate equations of overall reactions can be constructed from those of the elementary steps. 

The similarities of above experimental results inspired us to investigate the role of SE in heterogeneous catalytic enantioselective hydrogenation reactions. In heterogeneous catalytic reaction the SE means that a given template molecule interacts with the prochiral substrate in the liquid phase in such a way that one of the prochiral sites is preferentially shielded. If the substrate is shielded then its adsorption onto the metal can take place with its unshielded site resulting in ED. 

At even milder conditions, gold nanoparticles were found to be effective heterogeneous catalysts for this reaction in aqueous phase. A yield of acetic acid of 92% after eight hours and at the considerably lower temperature of 180 °C and 3.5 MPa air pressure was achieved, employing Au/MgAl204 as the catalyst, and starting from ethanol concentrations comparable to those... 

There are many more types of elementary processes in heterogeneous catalysis than in gas phase reactions. In heterogeneous catalysis the elementary processes are broadly classified as either adsorption-desorption or surface reaction, i.e., elementary processes which involve reaction of adsorbed species. Free surface sites and molecules from the fluid phase may or may not participate in surface reaction steps. 

It is clear from the above that the main concepts used in the mass balance for one-phase systems can be extended to heterogeneous system by simply writing mass-balance equations for each phase and taking the interaction (mass transfer) between the phases into account. The same applies when there are reactions in both phases. Then the rate of reaction terms must be included in the phase mass balances as shown earlier. 

A two-phase heterogeneous system with multiple inputs, multiple outputs, and multiple reactions in each phase and with mass transfer between the two phases... 

A two-phase heterogeneous system with single input, single output, and a single reaction in each phase Figure 6.9... 

Advances in Homogeneous, Heterogeneous, and Biphasic Metal-Catalyzed Reactions in Dense-Phase Carbon Dioxide... 

Heterogeneous reactions involve two or more phases. Examples are gas-liquid reactions, solid catalyst-gas phase reactions and products, and reactions between two immiscible liquids. Catalytic reactions as illustrated in Chapter 1 involve a component or species that participates in various elementary reaction steps, but does not appear in the overall reaction. In heterogeneous systems, mass is transferred across the phase. 

In this context, heterogeneity refers to the occurrence of the chemical reaction in two phases—liquid (solvent) and solid (cellulose). The reaction is homogeneous if the two reacting phases are completely miscible. 

A process for the preparation of fluorobcnzencs comprises the heating of fluorobenzaldehydes in the presence of a catalyst. Suitable catalysts are transition metals from the B groups 1, 11. VI. VII and VIII. The best catalytical properties seem to be held by rhodium and the metals of the platinum group, e.g. formation of 1.3-difluorobenzene (5). The reaction maybe carried out in homogeneous solution with soluble rhodium catalysts (Wilkinson s catalyst) or in heterogeneous phase with the catalyst fixed on a carrier. ... 

There are many examples of the alkylation of aromatics with olefins to produce alkylbenzene In textbooks, the open literature, and In numerous patents. This reaction Is catalyzed by both proton and Lewis acids In a homogeneous phase and In heterogeneous phases. The latter systems are characterized by both proton (H FO ) and Lewis acids (BF ) on supports and the amorphous and crystalline alumina silicates. And, the reaction has been studied extensively. However, up until the start of this Investigation (1969) there had not been a systematic investigation of the kinetic parameters nor an adequate catalyst aging study on the alkylation of benzene with propylene over a crystalline alumina silicate. 

One of the most significant points that we must consider in scientific studies, not limited to studies on photocatalysis, is distinction between evidence and consistency, as least as far as the author thinks. In other words, it is necessary to recognize every fact to be a necessary condition but not a sufficient condition in a strict scientific sense. For example, the fact that a reaction rate obeys the first-order rate law giving a linear relation in a plot of data as in Fig. 6 is only a necessary condition for a monomolecular reaction in homogeneous phase and also a necessary condition for heterogeneous photocatalytic reaction in diffusion-limited conditions or that in surface-reaction limited conditions with a Henry-type adsorption or a Langmuir-type adsorption in the lower-concentration region. 

Since the development of nonequilibrium thermodynamics in the late 1940s, initiated by the work of Prigogine (7), numerous reports have appeared dealing with the possibility of oscillations in reaction systems far from equilibrium. Initially the main focus of these studies was the Belouzov-Zhabotinskii liquid-phase reaction (2), but since the discovery of oscillating reactions in heterogeneous catalysis in the late 1960s (3-7), over 300 publications have described research in this field as well. This review focuses on this emerging and important area of research. 

Compare the half-factor in Eq. (205) or the half-exponent in Eq. (206.] This effect, which arises from the heterogeneous nature of the electrochemical process (i.e., a surface reaction vis-a-vis a volume reaction in homogeneous phases ), is the basis of the efficiency of redox catalysis or mediated electron transfer (see Sec. III.E.3 and also Chapter 28 mainly devoted to this topic). Thus for a given redox system, as in the sequence in Eqs. (190) and (191), the use of a redox mediator M in Eq. (207) allows the reduction of R to be performed at potentials less cathodic than x/i in Eq. (205) (or the R oxidation at potentials less anodic than E1/2) for the same electrochemical setup (i.e., an identical mass transfer rate). 

Cellulose esters are used primarily in structural applications, such as films, fibers, coatings, etc. Since they are processed in solution or in melt state, their resistance to flow (i. e., melt viscosity) represents a distinct handicap. Commercial cellulose esters are all generated by heterogeneous modification reaction [80,81,82,83,84]. However, reactions in homogeneous phase (with various solvents) are increasing in number and variety, and they have recently been reviewed by Heinze et al. [82]. Cellulose esters are usually classified as inorganic and organic esters. 

Of all the reaction variables involved in a heterogeneously catalyzed reaction, the most important is the nature of the catalyst to be used. Factors associated with catalyst preparation and selection will be discussed in Sections II and III. The relative importance of the other reaction parameters will depend on a number of factors. Reactions that run in a continuous or flow system have different requirements from those run in a batch mode. Generally, parameters such as the quantity of catalyst, the size of the catalyst particles, the temperature of the system, the concentration of the substrate(s), and, when gaseous reactants are used, the reaction pressure, are important variables in heterogeneously catalyzed reactions. In flow reactions the catalyst substrate contact time can frequently have a significant impact on the outcome of the reaction. In liquid phase batch processes catalyst agitation can also play an important role. The one constant parameter in almost all liquid phase reactions is the presence of a solvent, the nature of which is an important factor in heterogeneously catalyzed liquid phase reactions. 

In addition to these mass transport steps, heat conduction can also be important in heterogeneously catalyzed processes. For exothermic reactions the heat generated at the catalytic site must be dissipated away from the catalyst and into the reaction medium while heat must be supplied to the active sites for endothermic reactions. In liquid phase processes heat transport is generally not a significant factor since the liquid tends to equalize the temperature throughout the reaction medium and, thus, facilitate temperature control. In vapor phase processes, however, heat transport can be a significant problem. 

Angeletti, E., Tundo. P, and Venturello, P., Gas-liquid phase-transfer catalysis. Wittig-Horner reaction in heterogeneous conditions, 7. Chem. Soc., Perkin Trans. 1, 713, 1987. 

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