Helium Recovery by Membrane Gas Separation Using Poly(o-acyloxyamide)s

Poly(o-acyloxyamide)s (PORAs) derived from a poly(o-hydroxyamide) from 2,2'-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (APAF) diamine, APAF-POHA, have been synthesized and evaluated as gas separation materials. PORAs were synthesized from poly(o-hydroxyamide)s by derivatization of the free OH group with an aliphatic acid dichloride or dianhydride that allowed the introduction of acetyl or pivaloyl groups on their structure. All of them showed low medium thermal stability due to a degradation of the amide moieties, and most of them (those synthesized from isophthaloyl dichloride (IPC), 2,2'-bis(4-carboxyphenyl) hexafluoropropane dichloride (6FC), and 4,4'-dicarboxydiphenylsulfone dichloride (DPSC)) showed good mechanical properties. The poly(o-acyloxyamide)s synthesized have shown good permselectivities for He/N-2 and He/CH4 separations and very good permeability selectivity balance for He/CO2 separation. The acetyl PORAs showed better permeability versus selectivity balances than the pivaloyl ones. Moreover, selectivity increased while permeability decreased in the sequence 6F-APAF > IP-APAF > DPS-APAF. Intersegmental distances, d-spacing, were shown to decrease along this sequence, obtaining a nice correlation between permeability and d-spacing. For the He/N-2 and the He/CH4 pairs of gases, the best permeability selectivity values are obtained for 6F-APAF-Ac with a He permeability of 33 barrer and selectivities of 84 and 106, respectively. DPS-APAF-Ac gives a He permeability of 13 barrer and a selectivity of 21 for the He/CO2 separation.