Multi-step separation

A number of other interesting examples of multi-step separations of complex mixtures can be found in the book by Karger et al. [10]. 

A multi-residue method has also been developed by Jansson et al. [38], This method includes a multi-step separation enabling the determination of several... 

A systematic basis for the combining of independent selectivity mechanisms can provide a major boost to the overall selectivity. The overall effect is multiplicative based on the separating power, or peak capacity, of each of the steps. Either implicitly or explicitly, this is the widely used basis for multi-step separation schemes. 

The topic of this paper is the modeling of events occurring in the recovery of proteins and in the conditioning of the product streams for further purification using precipitation. The typical goal of downstream processing is the recovery of a desired product from a very dilute stream while minimizing the loss of the material in what is usually a multi-step separation process. Precipitation enables an early concentration of the product and can simultaneously serve to remove contaminants that would interfere with subsequent purification steps. Further, the wide variety of potential... 

A further advancement in organic synthesis was the accomplishment of multi-step syntheses comprising 20 and more steps and the synthesis of rather unstable organic compounds. It was now feasible to do an experiment on a milligram scale and to separate and identify products from by-products in order to analyze them separately. The application of selective reagents or reaction conditions allowed the synthesis of enantiomerically or diastereomerically pure compounds. 

In processes involving whole cells the required product can often be formed in a single step, although the cells essentially carry out a multi-step synthesis. This means that only a single product purification is necessary. Conversely, in chemical synthesis of compounds, each step in the synthesis is usually carried out separately. Thus the product of one reaction must often be purified before it can be used in the next step in the synthetic sequence. This multi-step approach is expensive, time consuming and can require a complex process plant to handle the individual steps on an industrial scale. 

Around the world chemical professionals continually commercialize new products and processes. Much of this activity results in batch processing. Fine and custom chemicals can involve as many as ten to twenty batch reactions in series, sometimes with multi-step parallel paths, with various separation technologies between reaction steps. This paper is an attempt to reflect the experience of many individuals as seen through the author s eyes over almost four decades, with several very typical situations. 

One of the most interesting aspects of ion trap mass spectrometers is represented by the possibility of carrying out multi mass separation steps with only one analyzer (see Section 2.9). 

Sahoo HR, Kralj JG, Jensen KF (2007) Multi-step continuous flow microchem-ical synthesis involving multiple reactions and separations. Angew Chem Int Ed, DOI anie.200701434... 

This multi-step, one-pot process was taken further by integration of a third supported reagent for the sequential preparation of 3,5-diphenylpyrazole (Scheme 2.17). Following the previously established procedure, acetophenone was deprotonated and acylated to afford the 1,3-dicarbonyl species. This intermediate was easily separated from the spent polymers by filtration and passed without isolation into a suspension of the resin bound hydrazine salt (9), affording the desired pyrazole in 91% yield. In a subsequent publication, the authors reported that the depleted polymeric reagents from the first step of the conversion (i.e. (7) and (8)) were recovered and separated via a selective flotation procedure, enabhng them to... 

In the search to develop new materials for immobilization of homogeneous transition metal catalyst to facilitate catalyst-product separation and catalyst recychng, the study of dendrimers and hyperbranched polymers for application in catalysis has become a subject of intense research in the last five years [68], because they have excellent solubility and a high number of easily accessible active sites. Moreover, the pseudo-spherical structure with nanometer dimensions opens the possibility of separation and recycling by nanofiltration methods. Although dendrimers allow for controlled incorporation of transition metal catalysts in the core [69] as well as at the surface [70], a serious drawback of this approach is the tedious preparation of functionalized dendrimers by multi-step synthesis. 

Ultrafiltration has been used for the separation of dendritic polymeric supports in multi-step syntheses as well as for the separation of dendritic polymer-sup-ported reagents [4, 21]. However, this technique has most frequently been employed for the separation of polymer-supported catalysts (see Section 7.5) [18]. In the latter case, continuous flow UF-systems, so-called membrane reactors, were used for homogeneous catalysis, with catalysts complexed to dendritic ligands [23-27]. A critical issue for dendritic catalysts is the retention of the catalyst by the membrane (Fig. 7.2b, see also Section 7.5). 

The whole process is multi-step, and catalysed by the pyruvate dehydrogenase enzyme complex, which has three separate enzyme activities. Dnring the transformation, an acetyl group is effectively removed from pyruvate, and passed via carriers thiamine... 

TechXtract is an extraction technology that has been used to remove a variety of contaminants from the surfaces of concrete, steel, brick, and other materials. Target contaminant types include organics, heavy metals, radionuclides, and polychlorinated biphenyls (PCBs). The technology uses proprietary chemical formulations in successive steps to remove these contaminants. The process employs as many as 25 different components in 3 separate chemical formulations that are applied to the contaminated surface and then removed in a multi-step, multicycle sequence. TechXtract is commercially available and has been used at multiple sites. 

Moreover, as schematically shown in Fig. 2.15, three different types of ion-pairs, i.e. solvent-separated (A+SSB-), solvent-shared (A+SB-) and contact (A+B-) types, may be formed depending on the strength of ion-solvent interactions, complicating the treatment of ion association. If we consider the following multi-step equilibrium, the total ion-association constant (KA) is expressed by Eq. (2.21) ... 

The suitability of this model, at least for a non-too-wide conversion range, has been confirmed by several authors. However, a correct description of the maleic anhydride production obviously demands splitting of kB into individual rate coefficients for maleic anhydride formation and for combustion, while in fact a separate equation should be added for maleic anhydride combustion. Such multi-step redox models have not been reported in the literature. Sets of first-order rate equations, however, are widely used,... 

One well-known drawback with the Wittig reaction in solution is the formation of 1 equiv. of triphenylphosphine oxide, which is inherently difficult to separate from the final product. Thus, Westman23 developed a one-pot multi-step microwave-assisted... 

Since the multi-step swelling and polymerisation method is time consuming, in 2005 Chen et al. [97] prepared uniformly sized MIP beads for separation of... 

In search of new natural products, crude extracts are classically subjected to multi-step work-up and isolation procedures which include various separation methods (besides HPLC, for instance, column, gel or counter-current chromatography) in order to obtain pure compounds which are then structurally elucidated by using off-line spectroscopic methods such as nuclear magnetic resonance spectroscopy and mass spectrometry. 

Notwithstanding all its advantages, the principle of solid-phase synthesis cannot be applied to all kinds of chemical reactions. Although reactants are used in excess, reaction is not always quantitative. The resulting impurities cannot be separated on the solid phase, giving rise to separation problems particularly in multi-step systems. Moreover, only limited use can often be made of conventional analytical methods (NMR, MS). Recent methods of 13C-NMR spectroscopy on solid phases [21] or in gel phases [22] are ideally suited for solid-phase synthesis, but are not universally available owing to the expensive instrumentation. 

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