Polymeric dense membranes

Tetra(o-aminophenyl)porphyrin, H-Co-Nl TPP, can for the purpose of electrochemical polymerization be simplistically viewed as four aniline molecules with a common porphyrin substituent, and one expects that their oxidation should form a "poly(aniline)" matrix with embedded porphyrin sites. The pattern of cyclic voltammetric oxidative ECP (1) of this functionalized metal complex is shown in Fig. 2A. The growing current-potential envelope represents accumulation of a polymer film that is electroactive and conducts electrons at the potentials needed to continuously oxidize fresh monomer that diffuses in from the bulk solution. If the film were not fully electroactive at this potential, since the film is a dense membrane barrier that prevents monomer from reaching the electrode, film growth would soon cease and the electrode would become passified. This was the case for the phenolically substituted porphyrin in Fig. 1. 

Dense membranes are a special type of polymeric membranes. Jacobs et al. published on the use of polydimethylsiloxane (PDMS) dense membranes in the hydrogenation of dimethylitaconate and acetophenone using standard homogeneous catalysts (see Section 4.6.1)[48]. The membranes were homemade from a PDMS solution in hexane, which was cross-linked in a vacuum oven at 100°C. The membranes were able almost completely to retain unmodified Ru-BINAP dissolved in isopropanol. However, as mentioned earlier, these applications will strongly depend on the size, i.e. molecular weight, of the substrate to be converted in order to guarantee a sufficient difference in size of the product and the catalyst to be retained. 

A completely different approach was taken by Koresh and Soffer (1980, 1986, 1987). Their preparation procedure involves a polymeric system like polyacrylonitrile (PAN) in a certain configuration (e.g. hollow fiber). The system is then pyrolyzed in an inert atmosphere and a dense membrane is obtained. An oxidation treatment is then necessary to create an open pore structure. Depending on the oxidation treatment typical molecules can be adsorbed and transported through the system. 

In this chapter, gas-solid systems, with an emphasize on inorganic permeable materials, to produce dense and porous membranes for chemical, sustainable energy, and pollution abatement applications, are considered. However, since the most important membranes currently in use are the polymeric porous membranes, then these are discussed at the end of the chapter. 

As discussed earlier, many composite porous membranes have one or more intermediate layers to avoid substantial penetration of fme particles from the selective layer into the pores of the bulk support matrix for maintaining adequate membrane permeability and sometimes to enhance the adhesion between the membrane and the bulk support The same considerations should also apply when forming dense membranes on porous supports. This is particularly true for expensive dense membrane materials like palladium and its alloys. In these cases, organic polymeric materials are sometimes used and some of them like polyarilyde can withstand a temperature of up to 350X in air and possess a high hydrogen selectivity [Gryaznov, 1992]. 

Pervaporation have been considered an interesting alternative process for the current industrial options for aroma recovery, distillation, partial condensation, solvent extraction, adsorption, or a combination thereof. It is considered a basic unit operation with significant potential for the solution of various environmental and energetic processes (moderate temperatures). This separation process is based on a selective transport through a dense membrane (polymeric or ceramic) associated with a recovery of the permeate from the vapour phase. A feed liquid mixture contacts one side of a membrane the permeate is removed as a vapour from the other side. Transport through... 

The theory presented here for the formation of porous polymeric membranes will also be applicable to the formation of dense membranes. In the latter membrane formation, the solution at stage (b) In Figure 2 (or at most between stages (b) and (c)) Is dipped Into a nonsolvent bath and rapid coagulation and gelation occur. If this is true, the theory predicts the existence of heteogenelty on order of some tens of nanometres, even In the dense polymer membrane. 

Since the majority of UF membranes have dense surface layers, it is difficult to characterize them with a true pore size distribution. Therefore, polymeric UF membranes are described by their ability to retain or allow passage of certain solutes. The MWCO values for UF membranes can range from as low as 1000 dalton (tight UF) to as high as 200,000 Dalton (loose UF). This roughly corresponds to an equivalent pore diameter range from about 1 nanometer (nm) to 100 nm (0.1 pm) as described in Ref 10. 

J.P. Robinson, E. S. Tarleton, C. R. Millington, A. Nijmeijer, Solvent flux through dense polymeric nanofiltration membranes, J. Memhr. Sci. 230 (2004) 29-37. 

Synthetic polymers are made by polymerization of one monomer or by the co-polymerization of two different monomers. A broad range of structures has been produced, from linear chain polymers, snch as polyethylene, to cross-linked structnres, snch as bntyl mbber [167]. Polymer membranes can have symmetric or asymmetric strnctnres. The former, which is considered the less important type, can be porons or microporons [168]. Dense membranes, such as silicone rubber, are non-porous on a macroscopic scale [124] therefore, permeating species mnst dissolve into the polymer and then diffuse through the membrane, making them highly selective. However, mass transfer rates were much lower than those observed in porons membrane, due to the dominating solution-diffusion mechanism [167]. 

Gas permeation through non-porous polymeric membranes is generally described by the solution-diffusion mechanism [2], This is based on the solubility of specific gases within the membrane and their diffusion through the dense membrane matrix. In turn, the solubility of a specific gas component within a membrane is a function of its critical temperature, as this is a measure of the gas condensability. Critical temperatures for a range of gas components are provided in Table 11.2. Conversely, the diffusivity depends upon the molecular size, as generally indicated by the kinetic diameter. Indeed, Robeson et al. [9] have recently postulated that the relationship between the ideal permeability of one species P, and that of another Pj are related by a simple function ... 

In the case of organic-inorganic manbranes, the continuous phase is typically the polymeric matrix and the transport behavior can be described through the mechanisms typical for dense membranes. On the other side, the mass transport in the dispersed phase can be described through the mechanisms characteristic of porous materials in accordance with the particular features of the phase. 

Yawalkar et al. (2001) has developed a model for a three-phase reactor based on the use of a dense polymeric composite membrane containing discrete cubic zeolite particles (Fig. 4.5) for the epoxidation reaction of alkene. Catalytic particles of the same size are assumed vdth a cubic shape and uniformly dispersed across the polymer membrane cross-section. Effects of various parameters, such as peroxide and alkene concentration in liquid phase, sorption coefficient of the membrane for peroxide and alkene, membrane-catalyst distribution coefficient for peroxide and alkene and catalyst loading, have been studied. The results have been discussed in terms of a peroxide effidency defined as the ratio of flux of peroxide through the membrane utilized for alkene oxidation to the total flux of organic peroxide through the membrane. The paper aimed to show that, by using an organophilic dense membrane and the catalysts confined in the polymeric matrix, the oxidant concentration (in that reaction peroxides) can be controlled on the active site with an improvement of the peroxide efficiency and selectivity to desired products. 

In a dense polymeric catalytic membrane the catalyst can be a thin layer on the membrane surface or distributed in the thickness of the polymeric matrix. An exhaustive review of methods for the preparation of catalytic polymeric membranes has been reported by Ozdemir et al. (2006). Vankelecom (2002) thoroughly reviewed the application of polymeric membranes in catalytic reactors. 

There seems to be a historical reason for the difference in these two approaches. The solution-diffusion approach was established for the permeation of liquid and gas through the membrane before reverse osmosis membranes of practical usefulness were developed by the phase-inversion technique. Dense membranes without asymmetricity were prepared from polymeric materials, and their transport properties were measured, assuming that the membranes were defect-free and the transport parameters so produced were the values intrinsic to the material. The membrane with the highest separation capacity for a given polymer was believed to be that which could exhibit the transport properties intrinsic to the polymer. The goal of membrane production engineering was to ensure the membrane intrinsic transport properties of the polymeric material. This approach is still popular in the membrane manufacturing industry. 

These membrane consists of a very dense top layer or skin with a thickness of 0.1 to 0.5 pm supported by a porous sublayer with a thickness of about 50 to 150 pm. These membranes combine the high selectivity of a dense membrane with the high permeation rate of a very thin membrane [9]. Composite membranes are, in fact, skinned asymmetric membranes. In composite membranes, the top layer and sublayer originate from different materials (polymeric or inorganic). Each layer can be optimized independently. In general, the support layer is already an asymmetric membrane on which a thin dense layer is deposited. 

Knowledge of crystalline morphology is essential in understanding the permeability and permselectivity of polymer membranes such as dense membrane, dialysis membrane and gas separation membrane. X-ray is very common to study the crystallinity in polymeric membranes. Crystalline structure of a polymer membrane includes dimensions of unit cell, percentage of crystallinity, crystallite size, and orientation. The most generally applicable technique that provides information is the X-ray diffraction method. 

The concentration gradient causes the diffusion in the direction of decreasing activity. Differences in the permeability in dense membranes are caused not only by diffusivity differences of the various species but also by differences in the physicochemical interactions of the species within the polymer. The solution-diffusion model assumes that the pressure within a membrane is uniform and that the chemical potential gradient across the membrane is expressed only as a concentration gradient. This mechanism controls permeation in polymeric membranes for separations. 

However, membranes can be symmetric or asymmetric. Many porous or dense membranes are asymmetric and have one or several more porous supporting layers and a thin skin layer which, in fact, gives selectivity. If these two layers are made of different materials, the membrane is a composite one. On some occasions, dense membranes have inclusions of other materials these are, of course, also composite membranes. In the case of gas separation membranes it has became usual to include inorganic charges in a polymeric membrane to get what is called a mixed matrix composite membrane. 

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