In recent years, there has been a growing interest in the application of Organic Solvent Nanofiltration (OSN) to many areas including the fine chemical, pharmaceutical and petrochemical industries [1]. OSN applications include the purification and fractionation of natural products, homogeneous catalyst recovery, colour removal from pharmaceutical products, and monomer separation from oligomers. Although there is no shortage of polymeric materials for membrane fabrication, current challenges facing the further industrial application of OSN include: i) the lack of commercially viable membranes which are stable in a broad range of organic solvents including polar aprotic solvents, and ii) the ability to control the permeation pathways to give tunable molecular weight cutoff (MWCO) profiles in the molecular weight range 200 – 1000 g mol-1 [2]. This paper will describe a range of new membranes prepared by phase inversion which meet these challenges, and have been scaled up as far as small spiral wound elements capable of performing OSN in DMF and other aggressive solvents.

Available OSN membranes fall into two types (i) composites made from polydemethylsiloxane films on polyacrylonitrile supports, or integrally skinned asymmetric membranes made from polyimides (PI). PIs offer several attractive mechanical and physicochemical properties, including high glass transition temperatures, good thermal stability and chemical stability in many solvents and weak acids. Commercial PI OSN membranes have been shown to give good performances in several organic solvents (toluene, methanol and ethyl acetate), but have generally poor stability and performance in polar aprotic solvents such as dichloromethane (DCM), tetrahydrofuran (THF), dimethyl formamide (DMF) and n-methyl pyrrolidone (NMP), in which most of these membranes are soluble. Inorganic membranes have been developed which offer good stability in organic solvents, but they are often more expensive and difficult to handle [3]. To date, there are few reports of OSN in aggressive solvents such as THF, DMF and NMP, which are of particular industrial interest.

Control over the separation performance of OSN membranes is desirable to achieve improved species separation. Ideally, a single membrane formation platform might be used to make a range of membranes with varying, controlled separation performances. At present, although other properties such as solute-polymer interactions can affect the membrane separation performance, the selection of OSN membranes is still based upon the MWCO. This is defined as the molecular weight at which 90% rejection is obtained by interpolation from a plot of solute rejection against molecular weight [4]. Although some relatively small changes to the MWCO were reported through changes in some membrane formation parameters (evaporation time prior to phase inversion, dope solution polymer concentration, thermal annealing of the membrane and use of additives in dope solution), successful control over the membrane separation performance was by and large still not achieved [5].

This paper will present new, integrally skinned asymmetric polyimide membranes made from the PI polymer Lenzing P84. These membranes were prepared by immersion precipitation from a casting solution containing DMF as the solvent and 1,4-dioxane as the co-solvent. The variation of the solvent compositions during membrane formation allowed for control over the molecular weight cut off membranes for the use in organic solvent nanofiltration. The chemical stability of these membranes was improved through chemical crosslinking using aliphatic diamines. The resultant membranes had a spongy structure and were stable in many organic solvents including toluene, methanol, DCM, THF, DMF and NMP. Operation in DMF and THF for 120 h showed that the membranes had stable fluxes and good separation performances, with DMF permeability in the range of 1–8×10-5 Lm-2 h-1 Pa-1 (1–8 Lm-2 h-1 bar-1) and a MWCO between 250 and 400 g mol-1. Possible re-imidization and loss of crosslinking at elevated temperatures limited their range of application to temperatures <100 °C. Their ease of preparation makes the membranes easily scalable and opens up future possibilities for further applications in harsh solvent environments.

In conclusion, this presentation will show that chemical crosslinking of PI membranes improves the chemical stability of these membranes allowing extended operation in harsh solvents such as DMF; also, that different dope compositions allows for good control over the MWCO in the NF range.

References:

1. U. Razdan, S.V. Joshi, and V.J. Shah, Novel membrane processes for separation of organics, Curr. Sci. Ind., 85 (2003) 761.

2. P. Vandezande, L.E.M. Gevers, and I.F.J. Vankelekom, Solvent resistant nanofiltration: separating on a molecular level, Chem. Soc. Rev., 37 (2008) 365.

3. Y.H. See-Toh, F.W. Lim, and A. Livingston, Polymeric Membranes for Nanofiltration in Polar Aprotic Solvents, J. Membrane Sci., 301 (2007) 3.

4. Y.H. See Toh, X.X. Loh, K. Li, A. Bismarck, and A.G. Livingston, In search of a standard method for the characterisation of organic solvent nanofiltration membranes, J. Membrane Sci., 291 (2007) 120.

5. Y.H. See-Toh, F.C. Ferreira, and A.G. Livingston, The influence of membrane formation parameters on the functional performance of organic solvent nanofiltration membranes, J. Membrane Sci., 299 (2007) 236.