CMR Applications

Dense Pd-based membranes have been first used for CMRs applications [4]. They are indeed highly selective for H2 permeation but are expensive, sensitive to ageing and poisoning and are strongly limited by their low permeabilities. 

A zeolite membrane, where the pores originate from the structure, presents only one type of (ultramicro)pore and therefore seems to be a good candidate for CMRs application. Moreover the structural origin of the pores should induce a much better thermal stability of the... 

This paper describes the morphological and transport properties of a composite zeolite (silicalite) - alumina membrane. Results in CMRs applications are also briefly given. 

Most porous membranes used in CMRs are made from oxide materials, although carbon membranes have also been used [1, 14, 22], However, although they possess very good separative properties, they have received less attention in CMR applications, probably due to their limited resistance to oxidative atmospheres. Vycor glass membranes also have certain drawbacks (brittleness, lack of high-temperature resistance) [9] for use in CMRs. Porous membranes in CMRs are, most often, made from ceramic materials or, more recently, from zeolites. 

A zeolite membrane is a membrane in which the transfer is controlled by the porous structure of the zeolite. Compared to sol-gel membranes, zeolite membranes can present some advantages for CMR applications. The most useful feature is that the pores of zeolites arc in the ultramicroporous range and have a very narrow size distribution (in this case pores are linked to the structure and not to the texture as in sol gel membranes). These characteristics of zeolite... 

LaMnOs is a prototype for a wide range of rare earth oxomanganates whose physical properties can be radically altered by partial substitution of alkaline earths for La or by similar replacement of Mn by a 3d or other small to medium sized cation. Lai j,Aj Mn03 (A = Ca, Sr, Ba) constitutes one of the more interesting systems for CMR applications. Pure stoichiometric LaMnOs is an insulator that orders antiferromagnetically at 135 K and it is only by introducing a critical amount of Mn+ into the stmcture that the CMR effect can be observed. Thus, some couuneut about the nature of the defect chemistry of LaMnOs is in order. 

The same reaction has been studied, in recent years, by Frisch and co-workers [2.43] using dense polymeric membranes. The dense polymeric membranes were prepared by blending polyethylacrylate with a 13X zeolite, which contained a dehydrogenation catalyst (Ti or Ni). They also prepared catalytic polymeric membranes by free radical polymerization of the monomer in the presence of the zeolite. These membranes were shown to be active for the cyclohexane dehydrogenation reaction at low temperatures. The recent development of thermally resistant polymeric membranes (Koros and Woods [2.44] and Rezac and Schoberl [2.45]) provides promise for the more widespread use of such membranes in CMR applications. 

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]... 

Beside their use in equilibrium-restricted reactions, CMRs have been also proposed for very different applications [6], like selective oxidation and oxidative dehydrogenation of hydrocarbons they may also act as active contactor in gas or gas-liquid reactions. 

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. 

So far we have studied the NMR ( or PMR) spectroscopy and its applications. In recent years a new technique called the 13C - NMR or CMR technique has been developed. 12C nucleus is not magnetically active because its / = 0 and does not give any NMR signal. 13C like H has a spin 1... 

Chloroform is used primarily in the production of chlorodifluoromethane (hydrochlorofluorocarbon-22 or HCFC-22) used as a refrigerant for home air conditioners or large supermarket freezers and in the production of fluoropolymers (CMR 1995). Chloroform has also been used as a solvent, a heat transfer medium in fire extinguishers, an intermediate in the preparation of dyes and pesticides, and other applications highlighted below. Its use as an anesthetic has been largely discontinued. It has limited medical uses in some dental procedures and in the administration of drugs for the treatment of some diseases. 

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. 

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]. 

Besides previously described examples of integrated membrane systems and much more reported in the literature, including applications in gas separation and the petrochemical industry [29], a special case of integrated or hybrid membrane systems, with a lot of interest in the logic of the sustainable growth, is represented by the catalytic membranes reactors (CMRs). 

Since the late 1950s PMR spectroscopy has contributed immensely to many areas of the chemistry of alkaloids (7). With the advent of Fourier transform spectrometers CMR has rapidly approached the level of PMR in its application to problems of structural elucidation and stereochemistry. In the case of the alkaloids many classes of the isoquinoline family have been studied. These alkaloids are of particular interest not only because of their widespread occurrence in nature but also because of their pharmacological activity (2-5). Wenkert et al. (6) were the first to review progress in this area. More recently, Shamma and Hindenlang (7) have made an extensive compilation of chemical shift data on amines and alkaloids that includes many... 

---