Separation/extraction catalysis

This chapter will thus not deal with the synthesis and characterization of hybrid structures or with the application of hybrid materials in classical fields, such as separation/extraction, catalysis, or physisorption. Instead, new functional sensing concepts having an improved level of performance or sophistication will be highlighted. " Special attention will be paid to hybrid systems that clearly show synergic functional effects that are not found in molecular-based systems or with unmodified nanoscopic solids alone. 

Membrane technology may become essential if zero-discharge mills become a requirement or legislation on water use becomes very restrictive. The type of membrane fractionation required varies according to the use that is to be made of the treated water. This issue is addressed in Chapter 35, which describes the apphcation of membrane processes in the pulp and paper industry for treatment of the effluent generated. Chapter 36 focuses on the apphcation of membrane bioreactors in wastewater treatment. Chapter 37 describes the apphcations of hollow fiber contactors in membrane-assisted solvent extraction for the recovery of metallic pollutants. The apphcations of membrane contactors in the treatment of gaseous waste streams are presented in Chapter 38. Chapter 39 deals with an important development in the strip dispersion technique for actinide recovery/metal separation. Chapter 40 focuses on electrically enhanced membrane separation and catalysis. Chapter 41 contains important case studies on the treatment of effluent in the leather industry. The case studies cover the work carried out at pilot plant level with membrane bioreactors and reverse osmosis. Development in nanofiltration and a case study on the recovery of impurity-free sodium thiocyanate in the acrylic industry are described in Chapter 42. 

Membrane reaction processes are systems where separation and reaction are carried out simultaneously, and the continuous extraction of one of the products can shift the equilibrium, enhancing yield and selectivity as compared with a traditional system. The development of membrane reactors has gone hand-in-hand with innovations in membrane materials and catalysts. Specifically, in the case of membranes, the same type of materials used to obtain them can also be adapted to support different catalysts. In terms of the separation and catalysis functions, porous membranes with permeance superior to dense membranes are the preferred candidates for use in membrane reactors these include porous oxide, zeolite, glass, metal, and, more recently, carbon membranes. Although carbon membranes are still in their infancy and have some serious challenges, such as weak mechanical strength as unsupported membranes and bad controllability and reproducibility of manufacture as supported membranes, they are believed to be promising... 

Transition metal catalysis in liquid/liquid biphasic systems principally requires sufficient solubility and immobilization of the catalysts in the IL phase relative to the extraction phase. Solubilization of metal ions in ILs can be separated into processes, involving the dissolution of simple metal salts (often through coordination with anions from the ionic liquid) and the dissolution of metal coordination complexes, in which the metal coordination sphere remains intact. 

The major advantage of the use of two-phase catalysis is the easy separation of the catalyst and product phases. FFowever, the co-miscibility of the product and catalyst phases can be problematic. An example is given by the biphasic aqueous hydro-formylation of ethene to propanal. Firstly, the propanal formed contains water, which has to be removed by distillation. This is difficult, due to formation of azeotropic mixtures. Secondly, a significant proportion of the rhodium catalyst is extracted from the reactor with the products, which prevents its efficient recovery. Nevertheless, the reaction of ethene itself in the water-based Rh-TPPTS system is fast. It is the high solubility of water in the propanal that prevents the application of the aqueous biphasic process [5]. 

Today SCFs are used for natural product extractions, chromatographic separations, pollution prevention, material processing and as solvents for chemical reactions.[75-77] Chemical applications include catalysis, polymerization, enzymatic reactions and organic synthesis. 

Cyclodextrins can solubilize hydrophobic molecules in aqueous media through complex formation (5-8). A nonpolar species prefers the protective environment of the CDx cavity to the hulk aqueous solvent. In addition, cyclodextrins create a degree of structural rigidity and molecular organization for the included species. As a result of these characteristics, these macrocycles are used in studies of fluorescence and phosphorescence enhancement (9-11), stereoselective catalysis (.12,13), and reverse-phase chromatographic separations of structurally similar molecules (14,15). These same complexing abilities make cyclodextrins useful in solvent extraction. 

In another procedure [522] the sample of seawater (0.5-3 litres) is filtered through a membrane-filter (pore size 0.7 xm) which is then wet-ashed. The nickel is separated from the resulting solution by extraction as the dimethylglyoxime complex and is then determined by its catalysis of the reaction of Tiron and diphenylcarbazone with hydrogen peroxide, with spectrophotometric measurement at 413 nm. Cobalt is first separated as the 2-nitroso-1-naphthol complex, and is determined by its catalysis of the oxidation of alizarin by hydrogen peroxide at pH 12.4. Sensitivities are 0.8 xg/l (nickel) and 0.04 xg/l (cobalt). 

Other applications of PVA are in areas of water and wastewater treatment (extraction, ultra-filtration, ion-exchange materials, etc.), catalysis, separation, etc. 

Although the history of rigid monolithic polymers is relatively short, a number of applications have already been explored. These applications cover a rather broad range of fields from heterogeneous catalysis and solid-phase extraction, to polymer-supported chemistry and a variety of separation processes. 

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