Hydrogenation aqueous catalysis

Whereas most hydrogenation catalysts function very well in water (see for example Chapter 38 for two-phase aqueous catalysis), scattered instances are known of inhibition by water. Laue et al. attached Noyori s transfer hydrogenation catalyst to a soluble polymer and used this in a continuous device in which the catalyst was separated from the product by a membrane. The catalyst was found to be inhibited by the presence of traces of water in the feed stream, though this could be reversed by continuously feeding a small amount of potassium isopropoxide [60]. A case of water inhibition in iridium-catalyzed hydrogenation is described in Section 44.6.2. 

The sucrose inversion has been extensively studied from the viewpoint of electrolyte effects (Guggenheim and Wiseman, 2), the application of the Arrhenius equation to the reaction (Leininger and Kilpatrick, 3), and the catalytic effects of acid molecules (Hammett and Paul, 4). It is probable that, in aqueous solution, we are dealing with a case of specific hydrogen ion catalysis and can postulate the equilibrium (Gross, Steiner, and Suess, 5)... 

Water solubility of all the aforementioned catalysts was due to the hydrophilic nature of ligand. In a second class, the aqueous nature originates from direct interaction of water molecules with the metal center [61]. Although this area of aqueous catalysis is not as extensive, there are several representative examples illustrating its importance and potential, as well as the variation of the metal center. One of the earlier examples involved the hydrogenation of maleic and fumaric acids with [RhCl (H20)6 ]3-" and [RuCl (H20)6 n]3" [62]. These simple catalysts revealed key mechanistic information that is applicable to many other systems, such as the requirement of alkene complexation prior to H2 activation. After this important discovery, many advances have been made in the areas of hydrogenation and polymerization reactions using these types of catalysts. 

Lowry s theory that the substrate is attacked simultaneously by acids and bases leads to a variety of possibilities of hydroxide ion and hydrogen ion catalysis in aqueous solution according to the values of Xa and Xb in Bronsted s equations. With Xb large and Xa very small, the reaction is catalyzed by base but not by acid with Xa large and Xb very small, the reaction is catalyzed by acid but not by base with Xa and Xb of intermediate magnitudes, the reaction is catalyzed by both acids and bases and with both Xa and Xb either very large or very small, the reaction is not apparently faster in the presence of either acids or bases. 

Various reactions are catalyzed by substances in the same phase as the reactants. A number of reactions in aqueous solutions are catalyzed by acids or bases. In general acid catalysis the rate depends on the concentration of unionized weak acid. In specific hydrogen-ion catalysis the rate depends on the concentration of hydrogen ions. Acid... 

Disselkamp, R.S., Flarris, B.D., Hart, T.R., 2008. Hydroxy acetone and lactic add synthesis from aqueous propylene glycol/hydrogen peroxide catalysis on Pd-black. Catalysis Communication 9,2250-2252. 

The role that acid and base catalysts play can be quantitatively studied by kinetic techniques. It is possible to recognize several distinct types of catalysis by acids and bases. The term specie acid catalysis is used when the reaction rate is dependent on the equilibrium for protonation of the reactant. This type of catalysis is independent of the concentration and specific structure of the various proton donors present in solution. Specific acid catalysis is governed by the hydrogen-ion concentration (pH) of the solution. For example, for a series of reactions in an aqueous buffer system, flie rate of flie reaction would be a fimetion of the pH, but not of the concentration or identity of the acidic and basic components of the buffer. The kinetic expression for any such reaction will include a term for hydrogen-ion concentration, [H+]. The term general acid catalysis is used when the nature and concentration of proton donors present in solution affect the reaction rate. The kinetic expression for such a reaction will include a term for each of the potential proton donors that acts as a catalyst. The terms specific base catalysis and general base catalysis apply in the same way to base-catalyzed reactions. 

An example of a reaction that is subject to homogeneous catalysis is the decomposition of hydrogen peroxide in aqueous solution ... 

This ease with which we can control and vary the concentrations of H+(aq) and OH (aq) would be only a curiosity but for one fact. The ions H+(aq) and OH (aq) take part in many important reactions that occur in aqueous solution. Thus, if H+(aq) is a reactant or a product in a reaction, the variation of the concentration of hydrogen ion by a factor of 1012 can have an enormous effect. At equilibrium such a change causes reaction to occur, altering the concentrations of all of the other reactants and products until the equilibrium law relation again equals the equilibrium constant. Furthermore, there are many reactions for which either the hydrogen ion or the hydroxide ion is a catalyst. An example was discussed in Chapter 8, the catalysis of the decomposition of formic acid by sulfuric acid. Formic acid is reasonably stable until the hydrogen ion concentration is raised, then the rate of the decomposition reaction becomes very rapid. 

New materials are also finding application in the area of catalysis reiated to the Chemicals industry. For example, microporous [10] materials which have titanium incorporated into the framework structure (e.g. so-calied TS-1) show selective oxidation behaviour with aqueous hydrogen peroxide as oxidizing agent (Figure 5). Two processes based on these new catalytic materials have now been developed and commercialized by ENl. These include the selective oxidation of phenol to catechol and hydroquinone and the ammoxidation of cyclohexanone to e-caproiactam. 

The durability of the catalytic system was investigated by employing it in five successive hydrogenations. Similar TOFs were observed due to the water solubihty of the protective agent which retains nanoparticles in aqueous phase. The comparative TEM studies show that (i) the average particle size was 2.2 0.2 nm (ii) the coimter anion of the surfactant does not allow a major influence on the size and (iii) nanoparticle suspensions have a similar size distribution after catalysis. 

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