Helium recovery by membrane gas separation using poly(o-acyloxyamide)s

Abstract

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/N2 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/N2 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.