Reactions Producing a New Phase

In applied electrochemistry, reactions are very common in which a new phase is formed (i.e., gas evolution, cathodic metal deposition, etc.). They have a number of special features relative to reactions in which a new phase is not formed and in which the products remain part of the electrolyte phase. 

The first step in reactions of the type to be considered here is the usual electrochemical step, which produces the primary product that has not yet separated out to 

These primary electrochemical steps may take place at values of potential below the eqnilibrinm potential of the basic reaction. Thns, in a solntion not yet satnrated with dissolved hydrogen, hydrogen molecnles can form even at potentials more positive than the eqnilibrinm potential of the hydrogen electrode at 1 atm of hydrogen pressnre. Becanse of their energy of chemical interaction with the snbstrate, metal adatoms can be prodnced cathodically even at potentials more positive than the eqnilibrinm potential of a given metal-electrolyte system. This process is called the underpotential deposition of metals. 

Snbseqnent steps are the formation of nnclei of the new phase and the growth of these nnclei. These steps have two special featnres. 

The nnclei and the elements of new phase generated from them (gas babbles, metal crystallites) are macroscopic entities their nnmber on the surface is limited (i.e., they emerge not at all surface sites but only at a limited number of these sites). Hence, the primary products should move (by bulk or surface diffusion) from where they had been prodnced to where a nucleus appears or grows. 

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Any of the steps listed can be rate determining formation of the primary product, its bulk or surface diffusion, nucleation, or nucfeus growth. Hence, a large variety of kinetic behavior is typical for reactions producing a new phase. 

Two types of reactions producing a new phase can be distinguished (1) those producing a noncrystalline phase (gas bubbles liquid drops as, e.g., in the electrolytic deposition of mercury on substrates not forming amalgams), and (2) those producing a crystalline phase (cathodic metal deposition, anodic deposition of oxides or salts having low solubility). 

Nucleus Growth The basic difference between reactions producing a new crystalline phase and reactions producing a gas or liquid phase is the step of nucleus growth. Difficulties exist in the incorporation of primary reaction products into the lattice. 

For a reaction to produce a new phase, the new phase must first form (nucleate) from an existing phase or existing phases. Nudeation theory deals with how the new phase nucleates and how to predict nudeation rates. The best characterization of the present status of our understanding on nudeation is that we do not have a quantitative understanding of nudeation. The theories provide a qualitative picture, but fail in quantitative aspects. We have to rely on experiments to estimate nudeation rates, but nudeation experiments are not numerous and often not well controlled. In discussion of heterogeneous reaction kinetics and dynamics, the inability to predict nudeation rate is often the main obstacle to a quantitative understanding and prediction. The nudeation theories are... 

Figure 3.13. Reactions involving mixture of two W/O microemulsions containing reactants in the droplets to produce a new phase. Depending on experimental conditions, one of the micelles may become vacant after the reaction and product formation, as shown here.

The two reactants belong to two different sohd phases, and the reaction produces a new, third solid phase, different from the first two, with possible release of a gas. 

Since Corey s group first reported 0(9)-allyl-N-(9-anthracenylmethyl) cinchonidi-nium bromide as a new phase-transfer catalyst [13], its application to various asymmetric reactions has been investigated. In particular, this catalyst represents a powerful tool in various conjugated additions using chalcone derivatives (Scheme 3.2). For example, nitromethane [14], acetophenone [15], and silyl eno-lates [16] produce the corresponding adducts in high enantioselectivity. When p-alkyl substrates are used under PTC conditions, asymmetric dimerization triggered by the abstraction of a y-proton proceeds smoothly, with up to 98% ee [17]. 

Farneth et al. have investigated the mechanism of the solid-state conversion of a series of II-VI precursors of general formula (R4N )4[S4Mio(SPh)i6]" (R = Me, Et M = Gd, Zn) to the bulk metal sulphide structure. The transformation, as followed by combined TGA and mass spectroscopy, proceeds in two discrete reaction steps. In the case of cadmium derivative, the loss of countercations around 200 °G produces a new molecular solid, which was characterized (X-ray) to be GdioSi6Phi2. This intermediate composition gave a broad X-ray diffraction pattern that indicated very small (<25 A) sphalerite-phase (cubic) crystals of GdS. The second decomposition reaction eliminates S6Phi2 around 350 °G and produces phase-pure GdS (wurtzite) (Equation (5)). 

Physical blends of bisphenol-A polycarbonate (PC) and a poly-arylate (PAr) exhibit by thermal analysis two amorphous phases a pure PC phase and a PAr-rIch miscible mixed phase. On controlled thermal treatment, transreaction between PC and PAr takes place mainly in the mixed phase, producing a new copolymer. Reaction progression from block to random copolymers has been traced by DSC, 13c NMR and CPC. The final product of transreaction is an amorphous copolymer showing a single T depending on the original binary composition. ... 

Recent industry advances have been reported such as the Quick Contact (QC) reaction system of the Stone and Webster Eng. Co. (Gartside, 1989). The QC system consists of a new fluidized solids unit which combines patented mixing and separation devices to produce a dilute phase reaction system capable of operating at short residence times (200 ms) and plug flow. 

With heterogeneous reactions, we only discussed the case of reactions of [13.R15] type. The most common are those that produce a new solid phase. The study of these types of reaction will be the topic of Chapter 14. 

In 1987, Toray Industries, Inc., announced the development of a new process for making aromatic nitriles which reportedly halved the production cost, reduced waste treatment requirements, and reduced production time by more than two-thirds, compared with the vapor-phase process used by most producers. The process iavolves the reaction of ben2oic acid (or substituted ben2oic acid) with urea at 220—240°C ia the presence of a metallic catalyst (78). 

From diese various estimates, die total batch cycle time t(, is used in batch reactor design to determine die productivity of die reactor. Batch reactors are used in operations dial are small and when multiproducts are required. Pilot plant trials for sales samples in a new market development are carried out in batch reactors. Use of batch reactors can be seen in pharmaceutical, fine chemicals, biochemical, and dye industries. This is because multi-product, changeable demand often requues a single unit to be used in various production campaigns. However, batch reactors are seldom employed on an industrial scale for gas phase reactions. This is due to die limited quantity produced, aldiough batch reactors can be readily employed for kinetic studies of gas phase reactions. Figure 5-4 illustrates die performance equations for batch reactors. 

The second half of the glycolytic pathway involves the reactions that convert the metabolic energy in the glucose molecule into ATP. Altogether, four new ATP molecules are produced. If two are considered to offset the two ATPs consumed in phase 1, a net yield of 2 ATPs per glucose is realized. Phase II starts with the oxidation of glyceraldehyde-3-phosphate, a reaction with a large... 

Diphenol carbonate is produced by the reaction of phosgene and phenol. A new approach to diphenol carbonate and non-phosgene route is by the reaction of CO and methyl nitrite using Pd/alumina. Dimethyl carbonate is formed which is further reacted with phenol in presence of tetraphenox titanium catalyst. Decarbonylation in the liquid phase yields diphenyl carbonate. 

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