Crosslinkable copolyimides for the membrane-based separation of p-/ o-xylene mixtures

Abstract

For the membrane based separation of p-/ o-xylene mixtures, pervaporation properties of several crosslinked and non-crosslinked 6FDA (4,4′-(hexafluoro-isopropylidene)-diphthalic anhydride))-based copolyimide membranes have been investigated. The copolyimides were synthesized by polycondensation of 6FDA with different diamino monomers, such as 4MPD (2,3,5,6-tetramethyl-1,4-phenylene-diamine), 6FpDA (4,4′-(hexafluoro-isopropylidene)-dianiline)) and DABA (3,5-diaminobenzoic acid). Thereby DABA was used as a monomer containing a crosslinkable carboxylic acid group. By variation of the diamino monomer ratio, different crosslinkable copolyimides were synthesized. The crosslinking of the free carboxylic acid groups was performed by the reaction with ethylene glycol (EG) in order to form diester crosslinks, or using hexamethylene diamine (HMDA) as crosslinking agent which then leads to diamide crosslinks. All membranes used in this study showed excellent chemical and thermal stability in pervaporation experiments at 60°C using a 50:50 p-/ o-xylene feed mixture. It has been found that the normalized flux through the different membranes strongly depends on the polymer backbone structure, but it is also influenced by the crosslinking agent. For p-/ o-xylene mixtures, normalized fluxes between 0.04 kg μm m −2 h −1 and 25 kg μm m −2 h −1 were reached with separation factors between 1.15 and 1.47.