Applications of CMRs

No application of CMR substances or other highly dangerous substances by cotmnercial users outside industrial installations, as experience demonstrates that the required protective measures are not observed in too many instances. Risk reduction by substitution is needed. In the case of closed industrial installations, especially for chemical synthesis, minimisation of technical emissions (without substitution) may be the better strategy. 

As pointed out in Section 9.3.2.1, the most common application of CMRs concerns equilibrium-restricted reactions [1-12]. The selective removal of a reaction product from the reaction zone through a membrane will shift the equilibrium, leading to higher conversions when compared to conventional (nonmembrane) reactors. 

The application of CMRs appears of particular interest in several areas such as hydrogen production, oxidation reactions, and enantiomeric productions. 

The first applications of CMRs have concerned high temperature reactions. The employed inorganic membranes, characterized by higher chemical and thermal stability with respect to polymeric membranes, still today suffer from some important drawbacks high cost, limited lifetime, difficulties in reactor manufacturing (delamination of the membrane top-layer from the support due to the different thermal expansion coefficients). 

The application of CMRs appears of particular interest in several areas such as hydrogen production [59, 64], oxidation reactions [60, 65], and enantiometric productions [8, 66]. The SLM reactors in which the membrane defines the reaction volume, (providing a contacting zone for two immiscible phases, phase-transfer catalysis) make the process environmentally more attractive. 

Unfortimately, quantitative applications of CMR spectroscopy are less straightforward than those of PMR because, under the usual experimental conditions, the relative intensities of signals do not reflect accurately their relative abundances. This is due principally to relaxation phenomena and makes proton rather than carbon NMR the method of choice for the analysis of mixtures. 

FIGURE 11.22 Applications of CMRs as (a) contactors using opposing reactant mode, (b) interfacial contactors for triphasic reactions, and (c) efficient gas-solid contactor using forced flow mode. 

Khasianine has been isolated from berries of Solanum khasianum. By application of CMR spectroscopy the structure has been elucidated... 

One of the most studied applications of Catalytic Membrane Reactors (CMRs) is the dehydrogenation of alkanes. For this reaction, in conventional reactors and under classical conditions, the conversion is controlled by thermodynamics and high temperatures are required leading to a rapid catalyst deactivation and expensive operative costs In a CMR, the selective removal of hydrogen from the reaction zone through a permselective membrane will favour the conversion and then allow higher olefin yields when compared to conventional (nonmembrane) reactors [1-3]... 

This account has been written from a personal perspective and thus has focused almost exclusively on the development and applications of the CMR and MBR. In that context, the numerous significant contributions of others are beyond the present scope and have been discussed comprehensively by the author elsewhere [6, 33, 34]. Nonetheless, they shall not be allowed to pass here without mention. 

Researchers at Degussa AG focused on an alternative means towards commercial application of the Julia-Colonna epoxidation [41]. Successful development was based on design of a continuous process in a chemzyme membrane reactor (CMR reactor). In this the epoxide and unconverted chalcone and oxidation reagent pass through the membrane whereas the polymer-enlarged organocatalyst is retained in the reactor by means of a nanofiltration membrane. The equipment used for this type of continuous epoxidation reaction is shown in Scheme 14.5 [41]. The chemzyme membrane reactor is based on the same continuous process concept as the efficient enzyme membrane reactor, which is already used for enzymatic a-amino acid resolution on an industrial scale at a production level of hundreds of tons per year [42]. 

Catalytic membrane reactors (CMRs) are reactors which couple, in the same unit, a conversion effect (catalyst) and a separation effect (membrane). These reactors, besides the obvious interest of concentrating two classical steps of chemical processes in the same unit, have already shown various potential benefits for a range of reactions, and the concept of CMRs is a matter of continuous investigation for a large number of applications. 

The purpose of this article is not to provide an exhaustive review of CMRs, as a number of papers have already been published in this area [1-9]. Rather, its goal is to describe the different types of CMRs and their potential applications. Some relevant references which divulge further detail are also given. Furthermore, the present limitations of CMRs and the areas which require further work are described. 

This third type of CMR is probably less developed than the previous ones, but the field of applications appears very large. The main role of the membrane here is to improve and manage the contact between the reactants and the catalyst. The membrane (which is not necessarily a permselective one) is generally catalyti-cally active and separates the two reactants the reactive interface between the reactants and the catalyst being localized in (porous) or on (dense) the membrane. Gas or gas-liquid reactions may be involved. 

Equilibrium-restricted reactions (Section A9.3.3.1) have until now been the main field of research on CMRs. Other types of application, such as the controlled addition of reactants (Section A9.3.3.2) or the use of CMRs as active contactors (Section A9.3.3.3), seem however very promising, as they do not require permselective membranes and often operate at moderate temperatures. Especially attractive is the concept of active contactors where the membrane being the catalyst support becomes an active interface between two non-miscible reactants. Indeed this concept, initially developed for gas-liquid reaction [79] has been recently extended to aqueous-organic reactants [82], In both cases the contact between catalyst and limiting reactant which restricts the performance of conventional reactors is favored by the membrane. 

Potential non- threshold toxicity N/A Any exposure Extends current ban on CMR substances (i.e., categories 1 and 2) in consumer products [302] to include substances with potentially low DNEL or non-threshold effects (e.g., penta and octa-BDE [486]) follows restrictions on diffusive applications of chloroform and tetracholoroethane [302] Not allowed as consumer substances or preparations... 

An innovative potential application of membrane technology in catalysis and in CMRs might be the possibility to produce catalytic crystals with a well-dehned size, size distribution, and shape by membrane crystallization [19,20] (Figure 43.5). Membrane crystallization is particularly attractive for the preparation of heat-sensitive catalysts such as enzymes. 

---