SRNF

Carbon-Carbon Bond-forming Reactions Using Srnf... 

The idea of separating homogeneous TMCs with solvent-resistant NF(SRNF) membranes has only emerged over the last decade with the advent of commercial SRNF membranes. A nondestructive, energy efficient separation and concentration of reusable catalysts from reaction products can thus be realized. The SRNF-coupled catalysis can be run continuously, or alternatively in a semibatch operation mode to lower reactor occupancy or when reaction conditions are too harsh to be membrane compatible. Precipitating by-products could be simply removed from the reactor after filtration before refilling it. 

With the advent of several commercially available types of SRNF membranes, this field seems to be finally re-activated since the decade-old pioneering work left... 

De Smet et al. introduced a continuous process combining SRNF with homogeneous catalysis. The concept was proven with two enantioselective hydrogenation catalysts, Rh-EtDUPHOS and Ru-BINAP, separated from a methanol reaction mixture by the commercial MPF-60 membrane with rejection values of 97-98%. Hydrogen played the role of reagent for the catalysis and its pressure provided the driving force for the separation [17]. 

Unfortunately at present not many of either type of membranes are available commercially, but undoubtedly many more will appear on the market in the near future. Corrrmercially available SRNF irKlude the SelRO membranes (Koch Membrane Systerrrs, USA), Starmem membranes (Membrane Extraction Technologies, UK), SolSep membranes (SolSep, The Netherlarrds), and Desal-5 and Desal-5-DK membranes (GE/Osmomics, USA) designed for aqueous applications, as well as for SRNF. 

SRNFs have found applications in food cherrristry, catalysis, chiral separatiorts, petrochemical industry arrd pharmaceutical manrrfacturing. The preparatiort, practical corrsiderations, arrd theoretical trarrsport mechanisrrts of SRNFs have been critically reviewed by P. Vanezarrde, L.E.M. Gevers arrd I.F. J. Vankelecom [Chem Soc Rev 37 365 2008]. 

L.E.M. Gevers, A.G. Livingston, D. Nair, S. Aerts, S. Kuypers, P.A. Jacobs, Physico-chemical interpretation of the SRNF transport mechanism for solvents through dense silicone membranes,... 

Basu, S. Maes, M. Cano-Odena, A. Alaerts, L. De Vos, D.E. Vankelecom, I. F. J. Solvent resistant nanofiltration (SRNF) membranes based on metal-organic frameworks. J. Membrane /., 2009, 344,190-198. 

The largest industrial SRNF plant is installed in the petrochemical industry (Bhore et al., 1999). Wax is a monoester of fatty acids that severely modifies the properties of lube oil and must therefore be removed (Hart et al., 1995). The traditional process of dewaxing involved the cooling of a hydrocarbon mixture in solvent or solvent mixtures (methyl ethyl keton, acetone) to temperatures typically ranging from —5 to — 18°C. In this chilling section, waxy components coagulated and were precipitated or filtered the solvent in the filtrate was removed by evaporation and reused in the process (Cuperus and Ebert, 2002). 

A typical application of SRNF is found in the vegetable oil industry. It is estimated that more than 2 million tons of extraction solvent is used in the United States alone. Starting from seed from plants as soybean or sunflower, oil is obtained by solvent extraction, eventually in combination with mechanical extraction. Hexane is by far the most common extraction solvent. Currently, evaporation is used to recover these solvents and reuse them in the process, which requires a considerable amount of energy, approximately 530 kJ/kg oil. In addition, the elevated temperatures hold a risk on thermal damage, and explosive vapors may create safety problems (Raman et al., 1996). These Umitations can be partially overcome by membrane technology. 

Oil-micelle mixtures are formed during hexane extraction and consist of triglycerides (oils), phospholipids, and solvent. Due to its polarity, phospholipids form very loose conglomerates that can be filtered off by membranes. The process results in a filtrate with clear oil and hexane and a phospholipid fraction that can be worked up more easily. Although hexane has to be removed by stripping, potential savings are found in the reduction of chemicals and improved quality of oil. Raman et al. (1996) reported that a mixture containing 20% of oil could be concentrated to 45% with a commercial SRNF membrane. 

Solvent-resistant NF (SRNF) represents a fairly new and interesting application of NF in different industrial fields (e.g., food, chemical, and pharmaceutical) for purification, recovery, or recycling of oligomers, catalysts, and solvents. This process requires membranes to be able to withstand aggressive environments, with high chemical resistance, coupled with desired permeability and selectivity. Not only TFC but also integrally skinned asymmetric membranes can be used for SRNF. The most widely used polymers for the preparation of SRNF membranes are polyimide, PAN, polyelectrolyte complex membranes (PECMs), and polydimethylsiloxane (PDMS). 

Recently, polymers belonging to the sulfone family, such as PSU, have also been reported for the preparation of SRNF membranes. Holda et al. [95] prepared SRNF membranes from PSU using a mixture of NMP and THF (70/30) as solvent. 

Polyphenylsulfone (PPSU) (Figure 1.11) shows great potential for the preparation of SRNF membranes, since it has high chemical and mechanical resistance and capacity to operate at high temperatures. Darvishmanesh et al. [97] prepared PPSU hollow-fiber NF membranes and studied their isopropanol (IPA) permeability and rejection to Rose Bengal and Bromothymol blue, as well as their resistance to several solvents. Although the produced fibers were visually stable in most of the solvents except MEK, permeability tests showed that the membranes were not stable in acetone and toluene. 

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