Membrane-assisted catalysis

Because there are many different ways to combine a catalyst with a membrane, there are numerous possible classifications of the CMRs. However, one of the most useful classifications is based on the role of the membrane in the catalytic process we have a catalytically active membrane if the membrane has itself catalytic properties (the membrane is functionalized with a catalyst inside or on the surface, or the material used to prepare the membrane is intrinsically catalytic) otherwise if the only function of the membrane is a separation process (retention of the catalyst in reactor and/or removal of products and/or dosing of reagents) we have a catalytically passive membrane. The process carried out with the second type of membrane is also known as membrane-assisted catalysis (a complete description of the different CMRs configurations will be presented in a specific chapter). 

Particularly interesting is the case of his-trp, which is also condensed into oligomers by the same mixed-liposome system, affording a peptide which, possessing histidine residues, may be provided with catalytic activity. This links to the next argument I would like to discuss in this review, namely the membrane-assisted catalysis. 

Membrane-assisted catalysis has already been applied in a number of processes... 

Solid mixed ionic-electronic conductors (MIECs) exhibit both ionic and electronic (electron-hole) conductivity. Naturally, in any material there are in principle nonzero electronic and ionic conductivities (a i, a,). It is customary to limit the use of the term MIEC to those materials in which a, and 0, 1 do not differ by more than two orders of magnitude. It is also customary to use the term MIEC if a, and Ogi are not too low (o, a i 10 S/cm). Obviously, there are no strict rules. There are processes where the minority carriers play an important role despite the fact that 0,70 1 exceeds those limits and a, aj,i< 10 S/cm. In MIECs, ion transport normally occurs via interstitial sites or by hopping into a vacant site or a more complex combination based on interstitial and vacant sites, and electronic (electron/hole) conductivity occurs via delocalized states in the conduction/valence band or via localized states by a thermally assisted hopping mechanism. With respect to their properties, MIECs have found wide applications in solid oxide fuel cells, batteries, smart windows, selective membranes, sensors, catalysis, and so on. 

PV-assisted catalysis in comparison with reactive distillation has many advantages the separation efficiency is not limited by relative volatility as in distillation in pervaporation only a fraction of the feed is forced to permeate through the membrane and undergoes the liquid- to vapor-phase change and, as a consequence, energy consumption is generally lower compared to distillation. 

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. 

Part III Beyond the Fundamentals presents material not commonly covered in textbooks, addressing aspects of reactors involving more than one phase. It discusses solid catalyzed fluid-phase reactions in fixed-bed and fluidized-bed reactors, gas-solid noncatalytic reactions, reactions involving at least one liquid phase (gas-liquid and liquid-liquid), and multiphase reactions. This section also describes membrane-assisted reactor engineering, combo reactors, homogeneous catalysis, and phase-transfer catalysis. The final chapter provides a perspective on future trends in reaction engineering. 

In 1988 Dr. Bouwmeester was appointed assistant professor at the University of Twente, where he heads the research team on Dense Membranes and Defect Chemistry in the Laboratory of Inorganic Materials Science. His research interests include defect chemistry, order-disorder phenomena, solid state thermodynamics and electrochemistry, ceramic surfaces and interfaces, membranes, and catalysis. He is involved in several international projects in these fields. 

Pervaporation-assisted catalysis is a typical example of an operation eflide-ntly carried out in extractor-type catalytic membrane reactors. Esterification is by far the most studied reaction combined with pervaporation. " Esters are a class of compounds with wide industrial appUcation, from polymers to fragrance and flavour industries. Esterification, a reaction between a carboxylic acid and an alcohol with water as a by-product, is an equilibrium-limited reaction. So, this is a typical reaction that can be carried out advantageously in a extractor-type membrane reactor. By selectively removing the reaction product water, it is possible to achieve a conversion enhancement over the thermodynamic equilibrium value based on the feed conditions. 

The use of polymeric catalytic membranes in pervaporation-assisted catalysis applied to non-esterification readions has also been referred in recent reviews. ... 

Figueiredo, K. C. D. S., Sahm, V. M. M., Borges, C. P. (2008). Synthesis and characterization of a catalytic membrane for pervaporation-assisted esterification reactors. Catalysis Today, 133-135, 809-814. 

Gobina, E. and Hughes, R. (1996) Reaction assisted hydrogen transport during catalytic dehydrogenation in a membrane reactor. Applied Catalysis A General, 137,119-127. 

Fig. 3. The structure of four sRAFs used in this section, (a) a sRAF whose substrates can cross through the membrane and therefore are continuously provided by the environment (sRAF A) (b) a sRAF where one of the reactions uses as substrate its catalyzer (a suicidal process ) (sRAF B) (c) a SRAF composed by one condensation and one cleavage (a collectively suicidal process where we assist to the continuous creation (reaction R2) and destruction (reaction Rl) of species AAAB - in this case both actions being catalyzed by the same catalyst AAB) (sRAF C) (d) a SRAF composed by 5 reactions (all condensations) whose substrates are continuously provided by the environment (sRAF D). Solid lines represent materials production/consumption, whereas dotted lines represent catalysis if not differently indicated on the text, all the kinetic constants of the reactions have the same values...

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