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

Abstract

Abstract For the separation of benzene/cyclohexane mixtures the pervaporation properties of different crosslinked 4,4′-hexafluoro-isopropylidene dianhydrid (6FDA) based copolyimide membranes have been investigated. The copolyimides were obtained by polycondensation of the 6FDA with various diamines. In order to obtain high permeability as well as high selectivity a combination of the diamines 2,3,5,6 tetramethyl-1, 4-phenylene diamine (4MPD), 4,4′hexafluoro-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-crosslinked polyimides, e.g. if high benzene concentrations are reached in pervaporation. In this study the degree of crosslinking was kept constant at 20%, whereas the ratio of the diamino monomers 6FpDA and 4MPD varied. The sorption properties of the crosslinked copolyimides as well as those of the non-crosslinkable reference polymers 6FDA–4MPD and 6FDA–6FpDA were investigated at 60°C using benzene and toluene as aromatic components and cyclohexane, hexane and heptane as aliphatic components. Pervaporation experiments were performed at 60°C with all the polymeric materials synthesized using benzene/cyclohexane mixtures with benzene concentrations covering the whole concentration range. It has been found, that all the crosslinked polymers have excellent chemical and thermal stability in pervaporation experiments. In all the cases, conditioning of the membrane samples with pure benzene is a suitable pretreatment to enhance flux without decreasing selectivity significantly. For the most promising membrane material, the 6FDA–4MPD/DABA 4:1 crosslinked with ethylene glycol, pervaporation experiments were performed with a 50:50 wt.% benzene/cyclohexane feed mixture in the temperature range between 60 and 110°C in order to determine the influence of temperature on the separation characteristics.