Homogeneous chemistry

Figure 4.3. Relative diffusion rates in HZSM5. The shaded areas are the pore walls, the unshaded parts the vertical pore system from Figure 4.1. As can be seen, the rate of diffusion varies enormously with only very small changes in molecular size and shape. This allows the zeolite to discriminate almost completely between the three molecules shown, a situation which is unprecedented in traditional, homogeneous chemistry.

Acidic, high area silica-almnina had received substantial attention in ICC 1, 52-58). Perhaps the most dramatic change in the subsequent catalytic literature was the debut of zeolites. Why acid catalyzed reactions are so much faster on zeolites than on silica-alumina has been extensively discussed but probably not conclusively. One should be able to know the exact structures of catalytic sites in zeolites, but initial hopes that this would do wonders for mechanistic imderstanding have not been fully realized. Super acids and carbonium ions came into heterogeneous catalysis from homogeneous chemistry and in special cases reaction via carbonium ions seems to occur. 

Additional sulfates continue to form after the eruption as gaseous S02 is oxidized to sulfuric acid and sulfates. While we shall focus here on the effects of these sulfate particles on the heterogeneous chemistry of the stratosphere, there may be other important effects on the homogeneous chemistry as well. For example, model calculations by Bekki (1995) indicate that this oxidation of S02 by OH leads to reduced OH levels, which alters its associated chemistry. 

Plot and discuss the Sherwood number as a function of for different values of the homogeneous chemistry. Specifically, consider three cases where Da/, = 0, Dah = 1, and Da/, = 10. In all cases assume the limit of fast surface chemistry with... 

Pichler and Schulz were the first to suggest that the initial step in metal-catalyzed CO/H2 reactions might be the migration of catalyst-bound hydride to CO to yield a formyl intermediate [Eq. (1)] (3). The alkyl ligand in many isolable metal carbonyl alkyls can similarly be induced to migrate to coordinated CO (4. ). Thus, Eq. (1) has some precedent in homogeneous chemistry. Other suggestions for the involvement of formyl intermediates have come from Wender (6) and Henrici-Olive and Olive (7). 

The above critical remarks do not diminish the enormous success in the development of oxidation reactions that seem close to impossible within the framework of homogeneous chemistry. The most successful process is the one-step reaction of butane to maleic anhydride (MA) ... 

Chemistry that takes place in a phase of uniform composition is termed homogeneous chemistry. In indoor environments, this chemistry takes place in air, water, coatings, and adhesives. 

Later [24], it was shown that the theory for the ErQ process under SECM conditions can be reduced to a single working curve. To understand this approach, it is useful first to consider a positive feedback situation with a simple redox mediator (i.e., without homogeneous chemistry involved) and with both tip and substrate processes under diffusion control. The normalized steady-state tip current can be presented as the sum of two terms... 

In the second chapter, Appleby presents a detailed discussion and review in modem terms of a central aspect of electrochemistry Electron Transfer Reactions With and Without Ion Transfer. Electron transfer is the most fundamental aspect of most processes at electrode interfaces and is also involved intimately with the homogeneous chemistry of redox reactions in solutions. The subject has experienced controversial discussions of the role of solvational interactions in the processes of electron transfer at electrodes and in solution, especially in relation to the role of Inner-sphere versus Outer-sphere activation effects in the act of electron transfer. The author distils out the essential features of electron transfer processes in a tour de force treatment of all aspects of this important field in terms of models of the solvent (continuum and molecular), and of the activation process in the kinetics of electron transfer reactions, especially with respect to the applicability of the Franck-Condon principle to the time-scales of electron transfer and solvational excitation. Sections specially devoted to hydration of the proton and its heterogeneous transfer, coupled with... 

The effects of gas and coal/char feeds and reactor geometries upon these internal processes and, hence, upon the performance of the reactor, can be simulated with this numerical model. The model incorporates representations of particle-particle and particle-gas interactions which account for finite rate heterogeneous and homogeneous chemistry as well as the hydrodynamical processes associated with particle collisions and drag between the particles and the gas flow. The important influences of multicomponent gas phase properties as well as solid particle properties, such as shape and size, are included in the representations. 

Once formed, the dihydrofuryl radicals may undergo a variety of reactions as known from homogeneous chemistry including disproportionation, addition to double bonds, electron transfer and dimerization. Surprisingly, the latter pathway is followed to about 90 % as indicated by the complete material balance [145]. It is therefore likely that C-C coupling occurs between radicals within the water-2,5-DHF surface layer. This is corroborated by the quadratic dependence of the initial... 

Platt U. and Moortgat G. K. (1999) Heterogeneous and homogeneous chemistry of reactive halogen compounds in the lower troposphere. J. Atmos. Chem. 34, 1-8. 

This chapter is divided into three parts. In the first, basic definitions and their consequences for homogeneous chemistry are presented. The second deals with the fundamental aspects of electrode phenomena, whereas the third discusses the problem of mass transfer at electrodes and its consequences for electrochemical kinetics. The particular problems and concepts associated with preparative-scale electrolysis are presented in a special chapter (Chapter 3). 

In homogeneous chemistry, pure electron transfer reactions are seldom encountered. Indeed, with the exception of a few examples, electron transfers are often associated with atom or group transfers. This usually results in a confused notion of the nature of oxidation-reduction reactions. For example, the reaction of a ketone with sodium in alcohol to afford the corresponding alcohol, via the sequence in Eq. (1) [2],... 

In homogeneous chemistry an electron transfer reaction involves two reactants, an acceptor A and a donor D. Thus, formally, an electron transfer reaction consists of the superimposition of two elementary chemical acts the reduction of the acceptor and the oxidation of the donor. Yet as discussed in the first part of this chapter, since more attention is given to one of these elemental reactions, the overall reaction is termed a reduction when the acceptor is the compound of interest or an oxidation when chemical transformation of the donor is favored. 

The second question is more difficult to answer. Indeed, since the electrical perturbation extends only over a few angstroms from the electrode surface, the medium in which the electrogenerated intermediates react is identical to the bulk of the solution, which favors a positive answer. On the other hand, they react under essentially nonisotropic conditions because of the existence of concentration profiles, that is, of concentration changes with distance of the electrode (see Fig. 15). Yet this is a situation identical to that encountered when heterogeneous reagents or polyphasic conditions are used in usual homogeneous chemistry. A more serious difference is related to the nature of the media... 

From an electrochemical point of view it is easily inferred that the solution in a cell near an electrode is separable into two parts a stagnant layer adjacent to the electrode in which no convective motions occur, and the remainder of the solution, which is homogeneous (bulk solution). Yet this is not a particularity of electrochemical methods since the same phenomena occur at any solid/liquid interface, as when metal particles (reductions by Zn or Na, for example) or any heterogeneous reagent is used in organic homogeneous chemistry, as well as in phase-transfer catalysis or related methods. 

---