Membrane heterogeneous catalysis

In this chapter, we Hmit ourselves to the topic of zeolite membranes in catalysis. Many types of membranes exist and each membrane has its specific field where it can be appHed best. Comparing polymeric and inorganic membranes reveals that for harsher conditions and high-temperature applications, inorganic membranes outperform polymeric membranes. In the field of heterogeneous catalYsis, elevated temperatures are quite common and therefore this is a field in which inorganic membranes could find excellent applications. 

The favourable properties which mark out vesicles as protocell models were confirmed by computer simulation (Pohorill and Wilson, 1995). These researchers studied the molecular dynamics of simple membrane/water boundary layers the bilayer surface fluctuated in time and space. The model membrane consisted of glycerine-1-monooleate defects were present which allowed ion transport to occur, whereby negative ions passed through the bilayer more easily than positive ions. The membrane-water boundary layer should be particularly suited to reactions which are accelerated by heterogeneous catalysis. Thus, the authors believe that these vesicles fulfil almost all the conditions required for the first protocells on earth ... 

There are significant challenges for catalysis to realize these objectives, as detailed in this chapter. There is the need for better integration between bio-, homo- and heterogeneous catalysis, and to foster cross-fertilization between these and contiguous areas (reaction engineering, membrane, etc.). New tools for the synthesis and understanding of these catalysts are need to be developed. 

J. A. Dalmon, Catalysis in membrane reactors, in Handbook of heterogeneous catalysis, Eds. G. Ertl, H. Knozinger, J. Weitkamp, Wiley-VCH, Weinheim, Chapter 9.3, 1997. 

The term reactive filtration may be used in a variety of applications. A simple search of the internet provides results such as reactive filter paper [1], adsorption filters for removing heavy metals from water [2], solid matrices used in organic synthesis [3], membranes for wastewater treatment, or even dialysis machines, filters for deep-frying pans and devices for the dechlorination of shower water by reaction with vitamin C. Most of the applications termed reactive filtration would be named heterogeneous catalysis or adsorption from a chemical engineer s point of view. 

A number of enzymes appear therefore to be localized in a specific micro-environment, which can influence. their biocatalytic activity. Because of the complexity of biological membranes, our understanding of the influence of micro-environmental effects on membrane-bound enzyme is minimal. An important contribution to better understanding the mode of action of membrane-bound enzyme has been the development of the concept of heterogeneous catalysis by enzymes synthetically bound to water-insoluble supports. These immobilized enzymes were viewed as models for the cellular bound enzyme (3,A). 

Coupling two operations like membrane separation and a catalytic reaction or adsorption in a given process of synthesis, purification, or decontamination of effluents is intrinsically interesting from a general technical-economical point of view. Ceramic membranes are ideal solid-fluid contactors, which can be efficiently used to couple separation and heterogeneous catalysis for membrane reactor applications. ... 

Dalmon JA. Catalytic membrane reactors. In Handbook of Heterogeneous Catalysis. In Ertl G, Knbzinger H, and Weitkamp J (eds.), VCH Pub. 1997 Chapter 9.3 1387-1398. 

Even if not really new, the concept of a membrane reactor has not been applied so intensively in homogeneous catalysis as in heterogeneous catalysis [20]. Nevertheless, the application of this concept in this area seems to be an exciting new topic, which opens up enormous possibilities of research and development. 

An understanding of the properties of liquids and solutions at interfaces is very important for many practical reasons. Some reactions only take place at an interface, for example, at membranes, and at the electrodes of an electrochemical cell. The structural description of these systems at a molecular level can be used to control reactions at interfaces. This subject entails the important field of heterogeneous catalysis. In the discussion which follows in this chapter the terms surface and interface are used interchangeably. There is a tendency to use the term surface more often when one phase is in contact with a gas, for example, in the case of solid I gas and liquid gas systems. On the other hand, the term interface is used more often when condensed phases are involved, for example, for liquid liquid and solid liquid systems. The term interphase is used to describe the region near the interface where the structure and composition of the two phases can be different from that in the bulk. The thickness of the interphase is generally not known without microscopic information but it certainly extends distances corresponding to a few molecular diameters into each phase. 

Solid-state electrochemistry is an important and rapidly developing scientific field that integrates many aspects of classical electrochemical science and engineering, materials science, solid-state chemistry and physics, heterogeneous catalysis, and other areas of physical chemistry. This field comprises - but is not limited to - the electrochemistry of solid materials, the thermodynamics and kinetics of electrochemical reactions involving at least one solid phase, and also the transport of ions and electrons in solids and interactions between solid, liquid and/or gaseous phases, whenever these processes are essentially determined by the properties of solids and are relevant to the electrochemical reactions. The range of applications includes many types of batteries and fuel cells, a variety of sensors and analytical appliances, electrochemical pumps and compressors, ceramic membranes with ionic or mixed ionic-electronic conductivity, solid-state electrolyzers and electrocatalytic reactors, the synthesis of new materials with improved properties and corrosion protection, supercapacitors, and electrochromic and memory devices. 

The catalytic behavior of enzymes in immobilized form may dramatically differ from that of soluble homogeneous enzymes. In particular, mass transport effects (the transport of a substrate to the catalyst and diffusion of reaction products away from the catalyst matrix) may result in the reduction of the overall activity. Mass transport effects are usually divided into two categories - external and internal. External effects stem from the fact that substrates must be transported from the bulk solution to the surface of an immobilized enzyme. Internal diffusional limitations occur when a substrate penetrates inside the immobilized enzyme particle, such as porous carriers, polymeric microspheres, membranes, etc. The classical treatment of mass transfer in heterogeneous catalysis has been successfully applied to immobilized enzymes I27l There are several simple experimental criteria or tests that allow one to determine whether a reaction is limited by external diffusion. For example, if a reaction is completely limited by external diffusion, the rate of the process should not depend on pH or enzyme concentration. At the same time the rate of reaction will depend on the stirring in the batch reactor or on the flow rate of a substrate in the column reactor. 

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