Enhancement of selectivity

S. Palmarsdottir and L. E. Edholm, Enhancement of selectivity and concentration sensitivity in capillary zone electrophoresis by on-line coupling with column liquid chromatography and utilizing a double stacking procedure allowing for microliter injections , 7. Chromatogr. 693 131-143 (1995). 

It is noteworthy that a clear enhancement of selectivity for the linear hydro-formylation product is observed only with cdpp (Table 5.2-1, entry e). With all other ligands, the n/iso ratios are in the 2 to 4 range. While this is in accordance with known results in the case of PPI13 (entry a) and dppe (entry c) (in comparison to monophasic hydroformylation [69]) and also with reported results in the case of Natppts (entry b in comparison to the biphasic hydroformylation of 1-pentene in [BMIM][PF(3] [46]), it is more remarkable for the bidentate metallocene ligand dppf... 

Li+ (0.77) < Na+ (1.0) < K+ (1.48) < Rb+ (1.55) < Cs+ (1.56). While indeed the order is the same, the significant point is that the range of discrimination has been expanded by almost an order of magnitude. The concern then becomes the mechanism whereby the Gramicidin channel effects this enhancement of selectivity. An important source of selectivity that must be considered is the relative solvation energy of water... 

Figure 10.5 shows the basic concept of the particle-level MR that gives (i) selective addition of reactants to the reaction zone and (ii) selective removal of products from the reaction zone. In the first case, if the diffusivity of one reactant (A) is much higher than that of the other components (B), the reactant (A) selectively diffuses into a catalyst particle through a membrane. Undesired reactions or the adsorption of poisons on the catalysts can be prevented. In the second case, the reaction has a hmited yield or is selectivity controlled by thermodynamics. The selective removal of the desired product from the catalyst particle gives enhancement of selectivity when the diffusivity of one product (R) is much greater than that of the other products (S). 

The preparation of iron impregnated activated carbon as catalysts and the catalytic performance of these catalysts were studied in benzene hydroxylation with hydrogen peroxide as oxidant. 5.0Fe/AC catalyst containing 5.0 wt% iron on activated carbon yielded about 16% phenol. The addition of Sn on 5.0Fe/AC catalyst led to the enhancement of selectivity towards phenol. 

The use of water-soluble ligands was referred to previously for both ruthenium and rhodium complexes. As in the case of ruthenium complexes, the use of an aqueous biphasic system leads to a clear enhancement of selectivity towards the unsaturated alcohol [34]. Among the series of systems tested, the most convenient catalysts were obtained from mixtures of OsCl3 3H20 with TPPMS (or better still TPPTS) as they are easily prepared and provide reasonable activities and modest selectivities. As with their ruthenium and rhodium analogues, the main advantage is the ease of catalyst recycling with no loss of activity or selectivity. However, the ruthenium-based catalysts are far superior. 

Fully oxidized vanadia is a poor, nonselective catalyst for benzene oxidation. Reduction to lower oxides brings about a large increase in catalytic activity and some improvement in selectivity. Substantial enhancement of selectivity can be achieved by the addition of molybdena,1009 reaching a maximum at a M0O3 content of about 30%. Promoters (Ag, Ti, Ni, Co) lead to further improvements.1006... 

Steady-state extrapolation Fourier analysis Gating and signal filtering Transient signals Speeding up of response Spatial and temporal information Enhancement of selectivity Increasing order of measurement... 

Results obtained by ES/MS confirm that the stability of calixarene/cation complexes depends upon the medium. The calixarene in solution presents a strong affinity for cesium, whereas in the gas phase, it displays a stronger affinity for sodium. Moreover, the stability of calixarene/Na+ complexation in a solvent phase is increased by the presence of water in the dilution system (up to 40% in acetonitrile), whereas other alkali complexes are destabilized by the presence of water. Finally, affinity for sodium, which is weak in the solution for calixarenes bearing benzo moieties, considerably increases in the gas phase. These results confirm the interpretation of the MD simulations in an aqueous phase, which lead the authors to conclude that cesium-over-sodium selectivity is governed by the hydration of the sodium cation in the complex, and by the higher hydrophobicity of the complexation site leading to an enhancement of selectivity for cesium over sodium 49... 

As in the other 3 sections, catalytic processes controlled by specific reaction pathways have several similar requirements. In many of the above systems the degree of electron transfer between the metal and support was proposed to be important. Various surface structures were also believed to play an important role in enhancement of selectivity. In this particular section it is clear that the type of input energy such as photochemical, thermal or other energy forms may markedly influence product distributions of catalytic reactions. 

Branched and linear polyethylene imine), and poly(vinylimidazole), were similarly cross-linked in the presence of template Cu2+ or Co2+ ion, but no enhancement of selectivity towards the same metal ions was achieved 37>. 

Traditionally, nitration has been performed with a mixture of nitric and sulfuric acids (mixed acid method). However, the method is highly unselective for nitration of substituted aromatic compounds and disposal of the spent acid reagents presents a serious environmental issue. In order to address these problems several alternative methods for aromatic nitration have been developed recently. For example, lanthanide triflates catalyse nitration with nitric acid, which avoids the use of large volumes of sulfuric acid but provides no enhancement of selectivity.6 Selectivity of nitrations with alkyl nitrates,7 acyl nitrates,8 or even nitric acid itself9,10 can, however, be enhanced by zeolites. 

The use of substances that form micelles as mobile phase additives continues to serve as an area of academic and practical interest. Often touted as a new form of chromatography, micelle chromatography should perhaps be considered as a fascinating example of the incorporation of secondary equilibria for the enhancement of selectivity and the adjustment of retention. in terms of practical chromatography, it is not yet clear that micelle chromatography solves any problems that cannot be solved by conventional means. Hhat is more clear is that micelle chromatography may provide a new route to the study of micelle phenomena."... 

In a continuation of their work, however, Heilmann and Maier found that their imprinted silica gels exhibited selective transesterification and lactonisation that was no better than control materials containing phosphoric acid [55]. In fact, the catalytic activity could be removed by rinsing the gel in water. The TSA apparently turns into phosphoric acid during the high temperature treatment and then remains on the gel as a Bronsted acid catalyst (Fig. 8.15). The enhancement of selectivity... 

Kirsch N, Alexander C, Lubke M, Whitcombe MJ, and Vulfson EN. Enhancement of selectivity of imprinted polymers via postimprinting modification of recognition sites. Polymer 2000 41 5583-5590. 

Boudart and Ptak (77) confirmed this enhancement of selectivity in isomerization on the (111) faces by comparing the reactions of neopentane on two 1% Pt/carbon black catalysts of exactly the same dispersion, 0.35. One of the catalysts had been fired at high temperature (900°C), and such a treatment, according to the work of Lyon and Somorjai (188), is expected to favor the formation of randomly distributed (111) facets. It is remarkable that the selectivity in isomerization was five times larger on the sintered catalyst than on the nonsintered one. 

This results show that the side reaction is promoted by the presence of water in the solution. But the application of water-free solvents can not completely resolve the problem of enhancing of selectivity, because on the hydrogenation of nitro-group two H2O molecules appear, that, in their turn, can participate in the side reaction. The promising approach to the resolution of this problem is to clear up the peculiarities of the side reaction occurring and then to find out the reaction conditions to suppress the side process. 

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