Separation of aromatics/aliphatics with crosslinked 6FDA-based copolyimides

For the separation of benzene/cyclohexane mixtures the pervaporation proper ties of different crosslinked 4,4'-hexafluoro-isopropylidene dianhydrid (6F DA) based copolyimide membranes have been investigated. The copolyimides we re obtained by polycondensation of the 6FDA with various diamines. In order to obtain high permeability as well as high selectivity a combination of t he diamines 2,3,5,6 tetramethyl-1, 4-phenylene diamine (4MPD), 4,4'hexafluo ro-isopropylidene dianiline (6FpDA) and 3,5-diaminobenzoic acid (DABA) as a monomer with a crosslinkable group was used. Crosslinking is necessary to prevent undesirable swelling effects, which generally occur with non-crossl inked polyimides, e.g. if high benzene concentrations are reached in pervap oration. In this study the degree of crosslinking was kept constant at 20%, whereas the ratio of the diamino monomers 6FpDA and 4MPD varied. The sorpt ion properties of the crosslinked copolyimides as well as those of the non- crosslinkable reference polymers 6FDA-4MPD and 6FDA-6FpDA were investigated at 60 degreesC using benzene and toluene as aromatic components and cycloh exane, hexane and heptane as aliphatic components. Pervaporation experiment s were performed at 60 degreesC with all the polymeric materials synthesize d using benzene/cyclohexane mixtures with benzene concentrations covering t he whole concentration range. It has been found, that all the crosslinked p olymers have excellent chemical and thermal stability in pervaporation expe riments. In all the cases, conditioning of the membrane samples with pure b enzene is a suitable pretreatment to enhance flux without decreasing select ivity significantly. For the most promising membrane material, the 6FDA-4MP D/DABA 4:1 crosslinked with ethylene glycol, pervaporation experiments were performed with a 50:50 wt.% benzene/cyclohexane feed mixture in the temper ature range between 60 and 110 degreesC in order to determine the influence of temperature on the separation characteristics. (C) 2001 Elsevier Scienc e B.V. All rights reserved.