Improvement of Selectivity

From the selectivity point of view, LC-NMR coupling is especially suited to the analysis of compound classes such as nitroaromatics, phenols, aromatic amines, aromatic carboxylic acids, polyaromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and azo- and anthraquinone dyes. Another advantage of LC-NMR coupling for the investigation of aromatic compounds in environmental samples is that the position of substituents on the aromatic ring, e.g. in unknown metabolites or degradation products, can best be determined by NMR spectroscopy. 

For aliphatic compounds with longer alkyl chains, such as surfactants, the NMR detector can contribute little to an increased selectivity of the LC-NMR coupling since, in the range of aliphatic protons, the spectra are often complex. Moreover, analyte signals around 2 ppm can be suppressed or influenced by the solvent suppression when acetonitrile is used as the organic component of the eluent. Since surfactants are present in many environmental samples, they pose problems for non-target analysis, not only because of their complex spectra but also because they can influence the separating properties of the analytical column by their surface activity [2]. 

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Improvement of selectivity by using heteroatom functionalities for the control of the electrochemistry and reactivity during electrochemical transformations ... 

The discovery of crown ethers and cryptands in the late sixties opened new possibilities of cation recognition with improvement of selectivity, especially for alkali metal ions for which there is a lack of selective chelators. Then, the idea of coupling these ionophores to chromophores or fluorophores, leading to so-called chromoionophores and fluoroionophores, respectively, emerged some years later l9) As only fluorescent probes are considered in this chapter, chromoionophores will not be described. 

In the last twenty years, many of the developed and validated high performance liquid chromatography methods with conventional diode array or fluorescence detectors (DAD, FLD) were improved and substituted by new hyphenation with mass spectrometric instrumentation and/or NMR, especially for the analyses of raw materials derived from Natural sources. The main goal of this coupling is achieved by improvement of selectivity and sensitivity of new instrumental configurations [7], Furthermore, with these configurations it is possible to obtain, in only one analysis, the complete chemical structure elucidation, identification and quantification of targeted compounds. 

Riedel K (1991) Biochemical fundamentals and improvement of selectivity of microbial sensors - a minireview. Bioelectrochem Bioenerg 25 19-30... 

Accordingly, correlations are often found between activity, selectivity and the concentration of cations in specific oxidation states, e.g. V4+ in V205. The improvement of selective catalytic qualities of metal oxides by addition of modifiers, or by combination in mixed oxides may hence be explained by stabilization of the essential cation in the proper oxidation state. In some cases, stabilization of a (partially) reduced cation appears to yield the most effective catalyst however, more often it is the higher oxidation state that should be maintained, and accordingly the role attributed to a second cation often concerns facilitating the reoxidation, for instance, by direct electron transfer between the cations or, in general, by increasing electron conductivity. 

Tens of different specialized glass compositions have been published and patented. The main goals are improvement of selectivity and lower overall resistance. 

The catalytic dimerization of alkynes has led to the development of a variety of catalysts by new combinations of transition metals and ligands, and to a better understanding of the processes and mechanism involved, leading to improvement of selectivity and scope. In Table 1 the most relevant catalysts are compared with regard to phenylacetylene dimerization. The nature of the terminal alkyne has also a marked effect on the outcome of this reaction. 

When certain cyclodipeptides are used as catalysts for the enantioselective formation of cyanohydrins, an autocatalytic improvement of selectivity is observed in the presence of chiral hydrocyanation products [80]. A commercial process for the manufacture of a pyrethroid insecticide involving asymmetric addition of HCN to an aromatic aldehyde in the presence of a cyclic dipeptide has been described [80]. Besides HCN itself, acetone cyanohydrin is also used (usually in the literature referred to as the Nazarov method), which can be activated cata-lytically by certain lanthanide complexes [81]. Acetylcyanation of aldehydes is described with samarium-based catalysts in the presence of isopropenyl acetate cyclohexanone oxime acetate is hydrocyanated with acetone cyanohydrin as the HCN source in the presence of these catalytic systems [82]. 

Catalyst performance has of course been a permanent theme in industry. For example, the catalytic activity of oxo catalysts (in hydroformylation) has improved in the past 50 years by a factor of 10 000 change from diadic and triadic process technology to continuous plant operation, replacement of cobalt by rhodium, tailoring of the ligand sphere (phosphines), change of phase application (from mono- to two-phase processes). At the same time, an improvement of selectivity has been achieved, apart from the ease of product/catalyst separation [132]. A similar development seems to occur in the Monsanto acetic acid process [49]. 

Metals separation and recovery is always of importance for industry and the environment. The theoretical and fundamental studies on metal transport through SLMs are advanced but still conducted toward implementation of laboratory-scale parameters to industrial applications [191]. This is related to the increased attention to improvement of selectivity and stability... 

Another area in which SLMs have interesting prospects is sample preparation in analytical chemistry. Unlike in other fields where a high active area is required, the new trends are toward miniaturization, which in the case of SLM is represented by the hquid-phase microextraction (LPME) method based on porous hollow fibers [231]. The other issues being addressed are to improve automation and high throughput of the analytical methods, in which the SLM system is used as an analyte enrichment method [167, 232-234]. Additionally, the improvement of selectivity of SLM extraction is one of the main areas of interest see the immuno-SLM method example described earher. Several improvements in this methodology have recently been reported [35, 36, 235-237]. 

In another attempt. Song et al. reported that ionic liquids [bmim]PF act as powerful media in scandium triflate catalyzed Diels-Alder reactions not only for the facilitating of catalyst recovery but also for the accelerating of the reaction rate and improving of selectivity (Scheme 17.5). Various dienes and dienophiles provided excellent yields and selectivity at room temperature in 4 h [47]. 

Table 3 shows the catalytic properties of these samples. One can see from the data, that all catalysts synthesized on the basis of the mechanochemically treated V2O5 show an increased selectivity towards maleic anhydride and higher specific rate of n-butane and n-pentane oxidation as compared to those obtained in traditional synthesis. The best effect in the improvement of selectivity can be reached by increase of the relative exposure of (001) plane at the VOHPO4.O.5.H2O surface which is known to be transformed into (200) plane of (VO)2P207. The low paraffins conversion over VPO-E samples at the given reaction conditions can be directly connected with their low specific surface area. The comparison made between samples VPO-El and VPO-E2 shows that the precursor synthesis using V2O5-E needs to be optimized in order to improve the catalytic performance. 

The main advantage of the two step process is a higher selectivity as compared to the conventional process. Burch and Swamakar [3] established this for the oxydehydrogenation of ethane just as Mtiller-Erlwein and Guba [4] for the oxidation of isobutyraldehyde to methacrolein. Emig et al. [5] applied the two step approach to the butane-oxidation to maleic anhydride. DuPont intends to install the two step process for butane oxidation to maleic anhydride on a technical scale [6-9]. Weismantel et al. [10] obtained an improvement of selectivity with a two step process for the oxidation of isobutyric acid. 

De Groot G, Koops R. Improvement of selectivity and sensitivity by column switching in the determination of glycyrrhizin and glycyrrhetic acid in human plasma by highperformance liquid chromatography. J Chromatogr 1988 456 71-81. 

Favorable difference of enthalpy of the reactions leading to the isomers. Improvement of selectivity by performing the reaction at lower temperature (temperature effect)... 

An important point which is often misinterpreted in high pressure chemistry, is the effect of temperature on selectivity. Thus, in many cases the observed improvement of selectivity under high pressure conditions is not due to a favorable AAV, but results from carrying out the reaction at a lower temperature. For transformations where a large difference in reaction enthalpy (AAH ) exists leading to the different isomers, lowering the temperature has a strong effect on selectivity. The pressure and enthalpy effect of the reaction can be cooperative but it can also be opposed. An accurate differentiation of these two influences can only be achieved by a separate determination of the AV and AH for the different reaction channels. 

For the synthesis of compactin, Konoike et al. performed an intramolecular cycloaddition of the chiral (Z)-129 to give the dccalin derivatives 130 and 131 [62] (Scheme 8.31). The best results were obtained under high pressure at 1.0 GPa and 20 °C resulting in 53 % of 130 and 8 % of 131. Whereas the Lewis acid-induced reaction gave a lower selectivity of 42 % of 130 and 16 % of 131, the thermal reaction showed much worse results with 20 % of 130 and 5 % of 131. However, it is not clear whether these results are due to a pressure-induced improvement of selectivity or due to an isomerization of the starting material caused by the Lewis acid or the higher temperature. 

L. Weismantel, J. Stockel. G. Emig. Improvement of Selectivity with a Two-Step Process for the Oxidation of Isobutyric. 4cid,. Appl. Catal. A 137 (1996) 129... 

Minimisation of impurities by the improvement of selectivity in the all three reactors chlorination, thermal cracking and oxy-chlorination. 

Imamura, M Nuruyu, T Kawasaki. T Teranishi, T Sakata, Y. Improvement of selectivity for partial hydrogenation of benzene by rare earth nitride upon NHi ireatincni. Catalysis Letters, 2004 96, 185-187. 

Fig. 6.6 Improvement of selectivity to 6 using counter-current reactive separator over conventional reactors...

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