Reduction selectivity enhancement

The reaction was studied in the temperature range of 300-430°C.26 It was found that the increase in the rate of NO reduction to N2 is typically 103-105 times larger than the rate of supply of Na and up to 30 times larger than the unpromoted rate. Sodium coverages of 0.06 cause up to 1450% increase in the total rate of NO reduction and enhance the selectivity to nitrogen up to 75% vs 30% on the Na-free surface, as already shown in Figures 4.17 and 4.18. 

Biosynthetic production of thymidine is overall a complex process combining the controlled introduction of a novel biotransformation step into a biological system with selective enhancement or knock-out of a series of existing metabolic steps. Metabolic engineering to enhance cofactor recycling at both ribonucleotide reduction and dUMP methylation steps has important parallels in other systems, as whole-cell biotransformations are frequently employed as a means to supply, in situ, high-cost and usually labile cofactors. 

The liquid holdup of the packing section decreases, which leads to a lower conversion of the kinetically controlled reactions of C02 and a reduction in the C02 absorption rate. As a consequence, the solvent mole fractions of HC()3 and carbamate decreases whereas the relative fraction of HS increases. The selectivity of the absorption process toward the H2S and HCN reduction is enhanced by minimizing the liquid holdup of the column. At the same time, a larger interfacial area improves the performance of the plant. Therefore, modem industrial sour gas scrubbers should be equipped with structured packings. 

It also seems to be noteworthy that in the presence of phospholipids the nitrogen reduction catalyzed by an artificial nitrogenase system is very selectively enhanced [45], Ding et al. [46] gave examples of the use of chiral amphiphilic ligands in hydrogenation reactions but directed attention to their application in biphasic systems. Usually, the presence of water causes loss of activity and, in the case of enantioselective hydrogenation, loss of enantioselectivity [47]. 

The use of both types of modifier to influence the selectivity of heterogeneous catalysts is not new. It has long been known, for example, that modifiers can have a powerful selectivity-enhancing effect in catalytic hydrogenation the Rosenmund reduction of acid chlorides to aldehydes is an early example of this. Another well-known modifier effect is rate and selectivity enhancement by bismuth in precious metal-catalyzed oxidations (Section 9.3). We feel, however, that the enormous po-... 

Suppression of Combustion Activity - A remarkable alteration of the typically high combustion activity of Pd occurs in the presence of low concentrations of NO, e.g. a few thousand ppm, typical of exhausts. In a recent study, the methane combustion activity with and without NO in the feed has been compared on a series of Pd catalysts on various supports and with different Pd loadings. It was observed that supports containing Bronsted acidity exhibited a high selectivity towards the selective reduction of NO by methane, and that this selectivity enhancement was accompanied by a drastic decrease of the methane combustion rate. 

The citral conversion and the selectivity to unsaturated alcohols increased with the Ge content, this effect being most obvious on alumina supported catalysts (Fig. 9.8). The selectivity enhancement on alumina in comparison with silica was explained by Ge deposited on the alumina support in the close vicinity of Rh nanoparticles during the catalytic redox reaction. These Ge species would be partially reduced during the reduction step of the catalyst preparation and would promote the activation of the carbonyl group for hydrogenation. Then, the catalytic properties of the bimetallic Rh-Ge/Al203 catalysts prepared by the redox reaction strongly depended on their reduction temperature, which must be... 

To enhance the enantioselective reduction of acetophenone, several heterogeneous variations of the chiral rhodium complex were investigated. Included in the study was a polyurea-supported complex in which the rhodium was deposited on a polyurea polymer and a polymerized diamine-rhodium complex that incorporates the rhodium centers within a cross-linked polyurea support. In addition, a rhodium MIP in which l-(S) -phenylethoxide is used as a template was also prepared. Examination of these heterogeneous examples was aimed to provide insight into the role of secondary ligand structure around the rhodium center on the reduction selectivity of aryl ketones. 

The ratios of difiusivities for a smies of penetrants in the 50% crystalline polyethylene (i>o.s) m the hypothetical amorphous sanqile (0. o) are shown in Fig. 20.4-2. In addition to the substantial nondis-criminating reductions in mobility, diere is an additional size-dependent reduction in mobility due to chain restriction effects. Note that since the solubilities of all the components are proportional to the amorphous fraction (Fig. 20.4-1), selectivity enhancements resulting from the introduction of crystallinity rely solely on die chain restriction effect. 

Table 2.4 Selectivity enhancement of yeast reductions via L-enzyme inhibition (for formulas see Scheme 2.123)...

Noncnzymc-Catalyzcd Reactions The variable-time method has also been used to determine the concentration of nonenzymatic catalysts. Because a trace amount of catalyst can substantially enhance a reaction s rate, a kinetic determination of a catalyst s concentration is capable of providing an excellent detection limit. One of the most commonly used reactions is the reduction of H2O2 by reducing agents, such as thiosulfate, iodide, and hydroquinone. These reactions are catalyzed by trace levels of selected metal ions. Eor example the reduction of H2O2 by U... 

Optically Active Acids and Esters. Enantioselective hydrolysis of esters of simple alcohols is a common method for the production of pure enantiomers of esters or the corresponding acids. Several representative examples are summarized ia Table 4. Lipases, esterases, and proteases accept a wide variety of esters and convert them to the corresponding acids, often ia a highly enantioselective manner. For example, the hydrolysis of (R)-methyl hydratropate [34083-55-1] (40) catalyzed by Hpase P from Amano results ia the corresponding acid ia 50% yield and 95% ee (56). Various substituents on the a-carbon (41—44) are readily tolerated by both Upases and proteases without reduction ia selectivity (57—60). The enantioselectivity of many Upases is not significantly affected by changes ia the alcohol component. As a result, activated esters may be used as a means of enhancing the reaction rate. 

---