Pervaporation of aromatic/non-aromatic hydrocarbon mixtures through crosslinked membranes of polyimide with pendant phosphonate ester groups

Polyimide from 3,3',4,3'-biphenyltetracarboxylic dianhydride (BPDA) and 2,4,6-trimethyl-1,3-phenylenediamine (TrMPD) was methyl-brominated and then phosphorylated by the reaction of -CH2Br with triethyl phosphite. The phosphorylation degree up to 200% (two phosphonate ester groups per repeating unit) was achieved. The phosphorylated polyimide membranes were crosslinked thermally and then with diamine. The membranes (P-PI(TAC)), after being conditioned with benzene, were subjected to pervaporation (PV) and sorption towards benzene/cyclohexane (Bz/Cx) and Bz/n-hexane (Hx) mixtures. The PV performance depended on the phosphorylation degree as well as the crosslinking degree. The P-PI(TAC) membranes displayed higher PV performance with excellent durability in comparison to the BPDA-TrMPD base polyimide and its homolog membranes. The higher PV selectivity was attributed to the higher solubility selectivity due to the affinity of phosphonate moieties to Bz. The diffusivity selectivity was around 2-3 for the Bz/Cx system at 343 K,but close to unity for the Bz/Hx system. The PV performance of the P-PI(TAC) membranes was comparable to that of the polymer alloy membranes of phosphorylated polystyrene and cellulose acetate. (C) 1999 Elsevier Science B.V. All rights reserved.