Reversible complexation reactions membranes

Reversible complexation reactions can be utilized to facilitate the transport of molecules from the gas phase across liquid membranes resulting in a selective separation. The effectiveness of the transport can be related to key physical properties of the systan. Results for several systems are compared to the predictions of mathematical nxidels. Advemtages and difficulties associated with the use of ion-exchange membranes are discussed. Several areas for future research are suggested. 

Due to their extensive use in the polymer industry and as solvents, there is a continuing need for better separation processes for alkenes and other unsaturated organic compoimds from alkanes. Perfluorosulfonic acid (PFSA) membranes, such as Nafion (1), that have been ion-exchanged with silver(I) ion exhibit large transport selectivities for many unsaturated hydrocarbons with respect to saturates with similar physical properties. These selectivities are the result of reversible complexation reactions between the unsaturated molecules and Ag+ (2-4), which results in facilitated transport through the membranes (5). 

Non-covalent bonds in molecular complexes and membranes are formed only under defined conditions (e.g., in water at pH 7 and at room temperature). Under different conditions (e.g., in ethanol or at pH 4 in water or at 70°C in water) complete decomposition may occur. Such decomposition is, however, usually fully reversible. Since reversible reactions have small activation energies, synki-... 

Reversible complexatlon reactions have long been used to improve the speed and selectivity of separation processes, especially those Involving the separation or purification of dilute solutes (j ). Such reactions are the basis of a multitude of separation unit operations Including gas absorption, solvent extraction, and extractive distillation. When a reversible complexatlon reaction (carrier) Is Incorporated into a membrane, the performance of the membrane can be improved through a process known as facilitated transport. In this process, shown schematically In Figure 1, there are two pathways available for the transport of the solute through the membrane. The solute can permeate through the membrane by a solution-diffusion mechanism and by the diffusion of the solute-carrier complex. Other solutes are not bound by the carrier due to the specificity of the complexatlon reaction this Increases the selectivity of the process. 

Figure 5.4.4. Various liquid membrane permeation mechanisms. (After Marr and Kopp (1982).) (a) Simple permeation of sp des A (b) simple permeation enhanced by reaction of A with an agent E in permeate (c) facilitated transport with a reversible complexing agent B in the membrane (d) facilitated transport in the presence of a reactive agent E in permeate (e) countertransport (f) cotransport.

Numerical solutions of equations of this type are included in the programme of complex reactions reported by Sel6gny et al. [68] giving the concentration of species in solutions in contact with multi-enzyme membranes. The evolution of substrate-concentration-profiles with symmetrical boundary conditions in an originally void membrane and reversible reactions has been calculated according to the same technique in cooperation between Thomas and Kernevez and is reported in different contexts by both of them in their respective theses [44, 45]. At a given time interestingly two minima separated by a maximum in this profile are predicted. 

The PSII complex contains two distinct plastoquiaones that act ia series. The first is the mentioned above the second, Qg, is reversibly associated with a 30—34 kDa polypeptide ia the PSII cote. This secondary quiaone acceptor polypeptide is the most rapidly tumed-over proteia ia thylakoid membranes (41,46). It serves as a two-electron gate and connects the single-electron transfer events of the reaction center with the pool of free... 

The mitochondrial complex that carries out ATP synthesis is called ATP synthase or sometimes FjFo-ATPase (for the reverse reaction it catalyzes). ATP synthase was observed in early electron micrographs of submitochondrial particles (prepared by sonication of inner membrane preparations) as round, 8.5-nm-diameter projections or particles on the inner membrane (Figure 21.23). In micrographs of native mitochondria, the projections appear on the matrixfacing surface of the inner membrane. Mild agitation removes the particles from isolated membrane preparations, and the isolated spherical particles catalyze ATP hydrolysis, the reverse reaction of the ATP synthase. Stripped of these particles, the membranes can still carry out electron transfer but cannot synthesize ATP. In one of the first reconstitution experiments with membrane proteins, Efraim Racker showed that adding the particles back to stripped membranes restored electron transfer-dependent ATP synthesis. 

Cholanic acid also possesses the ability of transporting cations across a lipophilic membrane but the selectivity is not observed because it contains no recognition sites for specific cations. In the basic region, monensin forms a lipophilic complex with Na+, which is the counter ion of the carboxylate, by taking a pseudo-cyclic structure based on the effective coordination of the polyether moiety. The lipophilic complex taken up in the liquid membrane is transferred to the active region by diffusion. In the acidic region, the sodium cation is released by the neutralization reaction. The cycle is completed by the reverse transport of the free carboxylic ionophore. 

In 1977, Parshall and co-workers published their work on the separation of various homogeneous catalysts from reaction mixtures.[46] Homemade polyimide membranes, formed from a solution of polyamic acid were used. After reaction the mixture was subjected to reverse osmosis. Depending on the metal complex and the applied pressure, the permeate contained 4-40% of the original amount of metal. This publication was the beginning of research on unmodified or non-dendritic catalysts separated by commercial and homemade membranes. 

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