Membrane solvent-stable

The permeation of binary mixtnre of ethanol/n-hexane showed that transport throngh dense membranes (solvent stable) occurs by couple diffusion, while for porous membranes transport has a convective natnre. It was shown that permeation through dense membranes is more affected by mutual affinities of membrane and solvent, whereas viscosity is the major transport parameter for porons membranes [33]. 

Kragl and Dreisbach (1996) have carried out the enantioselective addition of diethyl zinc to benzaldehyde in a continuous asymmetric membrane reactor using a homogeneous soluble catalyst, described in their paper. Here a,a-diphenyl-L-proline was used as a chiral ligand, coupled to a copolymer made from 2-hydroxy ethyl methacrylate and octadecyl methacrylate, which had a sufficiently high molecular weight to allow separation by ultra-filtration (U/F). The solvent-stable polyaramide U/F Hoechst Nadir UF PA20 retained more than 99.8% of the catalyst. The ee was 80 %, compared to 98 % for a noncoupled catalyst. 

Seen the list of demonstrated applications, numerous possibilities exist for the integration of homogeneous catalysis and a membrane separation. A complicating factor, however, is the relatively limited availability of solvent-resistant membranes. This will require a substantial development effort to obtain more solvent-stable membranes, including both polymeric and inorganic ones. 

The ultrafiltration of the microemulsion is a very useful operation for separating water and oil in these mixtures [117-120]. Because of the limited availability of solvent stable membranes, most of the work pubHshed so far was performed using ceramic membranes, which show a high adsorption of surfactant at the membrane surface and comparably low rejection rates of reverse micelles. Using electro ultrafiltration, where the concentration polarisation phenomenon of the reverse micelles (using the ionic surfactant AOT) at the membrane surface is depressed by asymmetric high voltage electrical fields, the rejection rates can be increased,but not to economical values [121,122]. 

Photoinduced Changes in Phase Boundary Potentials. The photoinduced membrane potentials were measured by using PVC matrix liquid membranes in contact with a polypyrrole-coated Pt electrode [dibutyl phthalate (DBP) as the membrane solvent]. The polypyrrole layer allows to obtain a stable and sample-in-dependent potential drop between Pt and the PVC membrane. The phase boundary potential at the interface of a membrane containing ionophore and an aqueous RbCl or KCl solution could be reversibly altered by UV and visible light irradiation, as shown for ionophore 89 in Figure 23a,b. The values of the photoinduced potential... 

Further progress will most likely include other types of reactions and new types of polymers as well as new solvent-stable nanofiltration membranes. Another promising field is the combination of chemo- and biocatalysis in cascade reactions... 

C. Linder, M. Nemas, M. Perry and R. Katraro, Silicone-derived Solvent Stable Membranes, US Patent 5,265,734 (November 1993). 

An interesting pore-filled composite membrane, made by photograft copolymerization onto a solvent-stable PAN UF membrane, has been established [47]. High flux and selectivity for PV separation of organic-organic mixtures were achieved by a very thin selective barrier and prevention of swelling of the selective polymer in the pores of the barrier. 

A difficult problem that prevented the use of nanofiltration in organic solvents for a long time was the limited solvent stability of polymeric nanofiltration membranes, and the lack of ceramic nanofiltration membranes. For polymeric membranes, different problems occurred zero flux due to membrane collapse [54], infinite nonselective flux due to membrane swelling [54], membrane deterioration [55], poor separation quality [ 5 6], etc. I n an early study of four membranes thought to be solvent stable (N30F, NF-PES-10, MPF 44 and MPF 50), it was observed that three of these showed visible defects after ten days exposure to one or more organic solvents, and the characteristics of all four membranes changed notably after exposure to the solvents [15]. This implies that these membranes should be denoted as semi-solvent-stable instead of solvent stable. 

Less information is available about the stability of ceramic membranes. It is generally thought that ceramic membranes have excellent solvent stability. Acid conditions may be more problematic it was shown [57] that an alumina nanofiltra-tion membrane was very sensitive to corrosion effects in dynamic experiments, whereas the performance of a similar titania membrane was stable in the pH range from 1.5 to 13. 

Moreover, this catalytic reaction could be employed in a continuously operated membrane reactor [105,106]. A stirred membrane reactor module equipped with a solvent-stable Koch MPF-50 membrane [107] was operated at 40 atm. After exchange of a few reactor volumes a steady conversion is achieved, e.g., 30% cyclohexene conversion for the example shown in Fig. 9 [32], corresponding to a catalytic activity of 1200 TO h 1. Over 30 exchanged reactor volumes, corresponding to a time of operation of 30 h, a productivity of a total of 29 000 turnovers was observed. 

Molecular structure and physicochemical properties of the carrier used are important for SLM stability, especially the lipophilicity, surface activity, and its solubihty in the membrane solvent. The membrane is less stable when a more surface-active compound is used as a carrier [89]. When the carrier loss is the main reason for the SLM instabihty, the membrane stabihty can be increased by attaching the carriers to a polymer or covalently hnked onto long ahphatic chains or polysiloxanes [95]. 

It was concluded that most of the ionic liquid from the external surface of the membrane disappeared during cell operation. However, comparison of the SEM-EDX spectra taken from membranes before (Fig. 11.3) and after (Fig. 11.5) immersion in the n-hexane/n-hexane solution showed similarity. The EDX spectra taken from a sample of np to a few micrometres thick/depth demonstrate the contribution of ionic liquid within the membrane pores which is more important than the accumulated liquid found on the surface. Consequently, from the SEM study, it was deduced that only the ionic liquid deposited on the external membrane surface has been stripped off during operation. The amount of ionic liquid retained in the membrane pores, however, was apparently kept constant, and consequently, the membrane was stable against the possible solvent action of n-hexane. 

An improved method for the preparation of solventless BLMs that is suitable for study of BLM transduction phenomena for the development of biosensors was recently reported [18] (figure 9.3). There is no need to simultaneously raise both electrolyte levels to compensate for the surface pressure of the membrane. Instead, a small amount (10 1) of the lipid solution in n-hexane is added dropwise onto the electrolyte surface in one cell compartment. The solvent is allowed to evaporate and then over a period of a few seconds the water level in one solution compartment is brought below the aperture and then raised again with a disposable syringe. Over 95% of the attempts to form BLMs are successful with this procedure (assuming the use of a freshly prepared dilute lipid solution) and these membranes are stable for periods of over 6 h. 

Cell membranes (elemental membranes, biomembranes) are made of a molecular bilayer (Figure 2.5.1) of water-insoluble lipids (solubility < 10 ° M ), cholesterol (30-50-mol%) and membrane proteins (20-50% of the membrane space). Cell membranes are stable only in bulk water. Organic solvents as well as lyophiliza-tion or other drying processes and contact with solid walls lead to destruction. In archaebacteria, one also finds molecular monolayers made of tetraether bolaam-phiphiles (Table 2.2.4). The thickness of biological cell membranes ranges usually from 4 to 6 nm. 

Industrial interest in soluble polymer-bound catalysts has been closely linked to the development of ultrafiltration membranes with sufficient long-term stability in organic solvents. Membranes fulfilling these requirements were prepared first in the late 1980s. Today, solvent-stable flat sheet membranes and membrane modules are available from several suppliers. As for the viability of ultrafiltration in organic solvents, rhodium-catalyzed hydroformylation of dicydopentadiene with continuous catalyst recovery and recycling has been demonstrated successfully on a pilot plant scale over an extended period of time [5]. The synthesis of other fine chemicals by asymmetric reduction and other reactions has also been carried out in continuously operated membrane reactors (also cf Section 7.5) [6-9]. The extent of commercial interest in catalysts bound to soluble polymers appears to fluctuate at intervals. Amongst other factors, the price of precious metals can be a driver. 

Van der Bruggen, B., Geens, J., and Vandecasteele, C. (2002) Fluxes and rejections for nanofiltration with solvent stable polymeric membranes in water, ethanol and n-hexane. Chemical Engineering Science 57, 2511-2518. 

In recent years, solvent-stable nanofiltration membranes with molecular weight cutoffs (MWCOs) ranging from 200-1000 g moP have emerged [1-3]. Applications have been proposed for a variety of industries including refining - e.g. hy-... 

NF technology is used in the pharmaceutical and biotechnology industry to recover antibiotics from fermentation processes. A schematic flow diagram of NF separation for recovery of 6-APA is shown in Figure 3.25 [10]. 6-AminopeniciUanic add (6-APA, MW 216 Da) is an intermediate in the manufacture of synthetic peniciUin and can be manufactured by an enzymatic process. A medium-sized plant produces 15—20 tons of mother liquor per day. NF (solvent stable membrane is used) separates the 6-APA (in the reten-tate) from the other Hquor at 60—80 kg/d and is recycled to the extraction unit, thereby minimising product losses (recovery is 90—95%). 

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