Enhanced Reaction Rates and Selectivities

In a very early study Patat (1945) investigated the hydrolysis of aniline to phenol in a water-based acidic solution in near-critical and supercritical water (Tc = 374.2°C, Pc = 220.5 bar). Phosphoric acid and its salts are used as the catalyst for this reaction. The reaction proceeds extremely slowly under normal conditions and reaches equilibrium at low conversion levels. For these reasons, Patat chooses to study the reaction in supercritical water to temperatures of 450°C and to pressures of 700 bar in a flow reactor. He finds that the reaction follows known, regular kinetics in the entire temperature and pressure space studied and the activation energy of the hydrolysis (approximately 40 kcal/mol) is the same in the supercritical as well as in the subcritical water. He suggests that the reaction is catalyzed by hydrogen ions formed from dissolution of phosphoric acid in supercritical steam. Very small amounts of phosphoric acid and the salts of the phosphoric acid are dissolved in the supercritical steam and are split into ions. Patat lists several dissolution constants for primary ammonium phosphates in supercritical steam. In this instance, the reaction performance is improved when the reaction is operated homogeneously in the mixture critical region and, thus, in intimate contact between the reactants and the catalyst. 

Using a batch reactor Blyumberg, Maizus, and Emanuel (1965) studied the oxidation of -butane at conditions near the critical point of butane (Tc = 152.1°C, Fc = 38.0bar). Both liquid phase and SCF phase oxidations 

The liquid phase oxidation has a long induction period, whereas the SCF phase oxidation has a much shorter induction time. Also, the liquid phase oxidation products are predominantly acetic acid and methyl ethyl ketone, whereas the SCF phase oxidation products are formaldehyde, acetaldehyde, methyl, ethyl, and propyl alcohols, and formic acid. The authors offer no explanation for the differences in product spectrum or induction periods for the reactions. 

Subramaniam and McHugh (1986) suggest that the increased reaction rates in the SCF phase may be associated with the more efficient production of free-radical pairs. When initiator molecule AB dissociates to form a geminate radical pair (A B ) it may either diffuse apart to form a free-radical pair or may recombine before it can diffuse apart in the so-called cage effect (Eckert, 1972)  

Since the resistance to diffusion will be lower in the mixture critical region than that in the liquid phase it is expected that the (A B ) radical pair should be more readily diffuse apart in the critical region. Although applied hydrostatic pressure favors the recombination of (A B ) to form AB, it seems reasonable to assume that the rate of diffusion dominates the pressure effect as long as the system pressure is maintained below approximately 1,000 bar. Therefore, the formation of free radicals should be facilitated in the SCF phase, as compared with the liquid phase, and shorter reaction times are to be expected. 

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Diels-Alder cycloaddition reactions have undergone impressive improvements, taking advantage of hydrophobic interactions existing between the essentially nonpolar reactants in the aqueous medium. The use of water as a solvent in Diels-Alder reactions leads to greatly enhanced reaction rates and selectivities. This remarkable result has been pioneered by Breslow et al. [801] and further explored by Grieco et al. [714] for reviews, see references [715-718]. 

Catalyst-membrane systems are promising structured catalysts. The perspectives for control of heterogeneous catalytic reactions by the combination of a catalyst and a membrane selectively permeable for one of the reactants have been discussed [1]. Catalyst-membrane systems enhance reaction rate and selectivity due to the directed transfer of reactants and energy. 

Carbonylation of nitrobenzene to phenylcarbamate was catalyzed by [PPN]2[RuRh4(CO)i5] in the presence of methanol. Addition of bipyridine greatly enhances reaction rate and selectivity. ... 

Because resolution is a kinetic phenomenon, an expression for the relative rates of conversion of the two enantiomers is desirable for a quantitative analysis of resolution. As the development and use of such an expression requires an understanding of the kinetics of enzymatic reactions, we defer considering this to Chapter 20 which deals with biochemical methods of enhancing reaction rates and selectivities. 

Recent NEMCA investigations have shown that J3"-A1203, a Na+ conductor, can be used as an active catalyst support to dramatically enhance the rate and selectivity of several enviromentally important reactions such as NO reduction by CO, H2 and C3H6, all catalyzed by Pt. Sodium supply to the catalyst has been found to enhance not only the catalytic activity, but also product selectivity to nitrogen. 

ORMOSIL are chemical sponges they adsorb and concentrate reactants at their surface, thereby enhancing reaction rates and sensitivity (in sensing applications). ORMOSIL-imprinted materials with a suitable chiral template such as a surfactant or a protein selectively adsorb (and detect) external reactants. A remarkable example is provided by thin materials that are generally enantioselective, namely where the chirally imprinted cavities can discriminate between enantiomers of molecules not used in the imprinting process, and completely different from the imprinting ones. 

The benefits of enhanced reaction rate and improved selectivity have to be seen in relation to the costs of working at higher pressures. The technical/economical limit for standard materials is somewhere around 300 bar. 

They provide a highly ionizing medium, which has enabled scientists to enhance reaction rates and increase reaction selectivity. The ionizing properties of ionic liquids have also proved to be valuable in electrochemical transformations and in battery uses. 

Vitamin B6 enzyme models that can catalyze five types of reactions - transamination, racemization, decarboxylation, P-elimination and replacement, and aldolase-type reactions - have been reviewed. There are also five approaches to construct the vitamin B6 enzyme models (i) vitamin B6 augmented with basic or chiral auxiliary functional groups (ii) vitamin B6 having an artificial binding site (iii) vitamin B6-surfactant systems (iv) vitamin B6-polypeptide systems (v) polymeric and dendrimeric vitamin B6 systems. These model systems show rate enhancement and some selectivity in vitamin B6-dependent reactions, but they are still primitive compared with the real enzymes. We expect to see improved reaction rates and selectivities in future generations of vitamin B6 enzyme models. An additional goal, which has not received ade-... 

Inorganic salts are another important additive used to enhance the rate and selectivity of Sml2-mediated reactions. The use of inorganic additives can be traced to the seminal studies of Kagan during which he used catalytic amounts of ferric chloride to accelerate the coupling reactions of alkyl iodides and... 

There is a class of nonporous materials called proton conductors which are made from mixed oxides and do not involve transport of molecular or ionic species (other than proton) through the membrane. Conduction of protons can enhance the reaction rate and selectivity of the reaction involved. Unlike oxygen conductors, proton conductors used in a fuel cell configuration have the advantage of avoiding dilution of the fuel with the reaction products [Iwahara ct al., 1986]. Furthermore, by eliminating direct contact of fuel with oxygen, safety concern is reduced and selectivity of the chemical products can be improved. The subject, however, will not be covered in this book. 

The use of small alternating voltage on a dc current, which has been claimed [6] to enhance the reaction rate and selectivity in some cases, does not require special considerations in the construction of the cell. 

Photochemistry on solid surfaces has unveiled the important role of sufaces as reactant media. Solid surfaces work as acids or bases sensitizers or quenchers reaction space for size-specific reactions seed crystals for epitaxial growth etc. Thus, the molecule-surface interaction enhances or reduces photoabsorption, reaction rates, and selectivities. Since there are a lot of parameters for surface reactions, such as adsorption, desorption, diffusion, nucleation etc., it has been difficult to control the photochemistry on solid surfaces. Recently, as it becomes possible to characterize the surface conditions with techniques of ESCA, SIMS, and STM and also to use new light sources, new research field appears as Surface Photochemisty ". 

A further technique to overcome the mass transport limitations in biphasic catalysis is the method to work in micellar [187] or reverse micellar [188] systems, that means to enhance the surface area decisively via addition of surfactants. Ren-ken found higher reaction rates and selectivities than in non-micellar systems and could hydroformylate also olefins with a long hydrocarbon chain up to C16 (see also Section 4.5). 

Later, the catalyst complex [Pd(PPh3)4] dissolved in [EMIM][Bp4] was successfully applied for the coupling ofheterocyclic chloroarenes with naphthaleneboronic acids (Scheme 5.3-27) [220]. Products were obtained in higher yields (43-81%), enhanced reaction rate and improved selectivity compared to conventional organic solvent. 

Varma moves away from the use of solvents altogether by demonstrating microwave expedited solvent-free synthetic processes. He exposes neat reactants to microwave (MW) irradiation in the presence of supported ret ents or catalysts on mineral oxides resulting in enhanced reaction rates, greater selectivity and experimental ease of manipulation. 

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