Facilitated transport membranes complex

Theory. The relationship of the chemical aspects of complexatlon reactions to the performance of facilitated transport membranes Is discussed by Koval and Reyes (108). They describe a procedure which can be used to predict and optimize the facilitated transport of gases, Including measurement of the appropriate equilibrium, transport, and kinetic parameters and structural modification of the carrier to Improve the performance of the membrane. Examples of this procedure and carrier modification are given for derivatives of Fe(II) tetralmlne complexes which reversibly bind CO In nitrile solvents (118). Experimental challenges In the measurement of the appropriate properties for other membrane configurations such as reactive Ion exchange membranes and reactive polymer membranes are also discussed. 

The use of facilitated transport membranes for gas separation was first introduced by Ward and Robb [54] by impregnating the pores of a microporous support with a carrier solution, and a separation factor of 1500 was reported for CO2/O2. These membranes, or supported liquid membranes (SLMs), are discussed by several, and initially very good separation properties are observed [55-58]. They are however known to have serious degradation problems like loss of carrier solution due to evaporation or entrainment with the gas stream, and the complexing agent (carrier) can be deactivated. These problems have restricted further development... 

Promising results have been obtained with the facilitated transport membranes in which an oxygen-complexing carrier compound acts like a shuttle to transport the oxygen selectively throngh the membrane [44],... 

In a facilitated transport membrane process, carriers react or coordinate reversibly with a solute which is transported through the membrane. The principles for such a membrane process for separation of ethene from ethane are shown in Figure 1. On the feed side of the membrane, ethane and ethene are dissolved in the membrane surface. Only ethene can form a complex with the silver ions within the membrane. Diffusion of the ethene-silver ion complex across the membrane to the permeate side takes place according to a concentration gradient. On the permeate side the partial pressure of ethene must be low so that a decomplexation reaction occurs and ethene is released from the membrane surface. Ethane can only diffuse through the membrane according to the concentration gradient across the membrane. 

Villamo, O., Barboiu, C., Barboiu, M., Yau-Chun-Wan, W. and Hovnanian, N. (2002) Hybrid organic-inorganic membranes containing a fixed thioether complexing agent for the facilitated transport of silver ions. Journal of Membrane Science, 204, 97-110. 

Barboiu, M., Guizard, C., Luca, C., Albu, B., Hovnanian, N. and Palmeri, J. (1999) A new alternative to amino add transport Facilitated transport of L-phenylalanine by hybrid siloxane membrane containing a fixed site macrocydic complexant. Journal of Membrane Science, 161, 193-206. 

Facilitated transport of organics of biological interest II. Selective transport of organic adds by macrocydic fixed site complexant membranes. Journal of Membrane Science, 174, 277—286. 

A series of model siderophore molecular recognition studies coupled with host-guest carrier facilitated model membrane transport studies was reported (198-202). Three approaches were taken which incorporate (i) second coordination shell host-guest complexation, (ii) ternary complex formation, and (iii) a combination of ternary complex - second coordination shell host-guest complex formation. Examples of these approaches are described below. 

Solid PVA-Co2+ composite asymetric membranes have been prepared starting from PVA and two different salts Co(N03)2 and Co(CH3COO)2, respectively, in order to separate cyclohexene/cyclohexan mixtures. A facilitated transport mechanism has been evidenced, due to the capacity of Co2+ ions to coordinate the olefin molecules [82], The authors reported stronger complexation of Co2+ ions with cyclohexene in the case of PVA/ Co(CH3COO)2 mixtures then in the case of PVA/ Co(N03)2 mixtures. It was found that for a concentration ratio of ([Co2+]/[OH]) by 0.75 mol/mol, the permeation flux of PVA membrane containing Co2+ increases 2-3 times and the separation factor increses 50 times compared with pure PVA membrane. 

The theories referred to above, dating from around 1974, have been extended since then, as regards both refinements of the basic models and other types of mechanism, e.g. the important case of the transport of ions facilitated by their complexing with membrane-soluble carriers [109, 111]. A quite general formalism has been proposed by Rangarajan et al. [112]. 

In facilitated transport (also known as carrier-mediated membrane transport), a substance combines with a specific carrier protein on the membrane, and the resultant protein-sub-stance complex diffuses to the other side of the membrane, where it dissociates to release the substance. The absorption of glucose from the intestines into the blood, for example, requires facilitated transport of glucose across the cellular membranes of the epitheleal lining of the intestines. Many amino adds cross cellular membranes by fadlitated transport. 

Carrier facilitated transport involves a combination of chemical reaction and diffusion. One way to model the process is to calculate the equilibrium between the various species in the membrane phase and to link them by the appropriate rate expressions to the species in adjacent feed and permeate solutions. An expression for the concentration gradient of each species across the membrane is then calculated and can be solved to give the membrane flux in terms of the diffusion coefficients, the distribution coefficients, and the rate constants for all the species involved in the process [41,42], Unfortunately, the resulting expressions are too complex to be widely used. 

Both techniques shown in Figure 11.20 increase the complexity of the separation process significantly, and neither has advanced to a commercial process. The focus of much of the recent work on facilitated transport has been to produce membranes that are inherently stable and can be used in conventional gas separation systems. Laciak has recently reviewed this work [38],... 

One promising approach to facilitated transport pioneered by Nishide and coworkers at Wasada University is to chemically bind the oxygen carrier to the polymer backbone, which is then used to form a dense polymer film containing no solvent [28], In some examples, the carrier species is covalently bonded to the polymer matrix as shown in Figure 11.29(a). In other cases, the polymer matrix contains base liquids which complex with the carrier molecule through the base group as shown in Figure 11.29(b). Because these films contain no liquid solvent, they are inherently more stable than liquid membranes and also could be formed into thin films of the selective material in composite membrane form. So far the selectivities and fluxes of these membranes have been moderate. 

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