Aromatic polyamide nonporous membranes for gas separation application

Polymer membrane-based gas separation is a superior economical and energy-efficient separation technique over other conventional separation methods. Over the years, different classes of polymers are investigated for their membrane-based applications. The need to search for new polymers for membrane-based applications has been a continuous research challenge. Aromatic polyamides (PAs), a type of high-performance materials, are known for their high thermal and mechanical stability and excellent film-forming ability. However, their insolubility and processing difficulty impede their growth in membrane-based applications. In this review, we will focus on the PAs that are investigated for membrane-based gas separations applications. We will also address the polymer design principal and its effects on the polymer solubility and its gas separation properties. Accordingly, some of the aromatic PAs developed in the authors’ laboratory that showed significant improvement in the gas separation efficiency and placed them in the 2008 Robeson upper bound are also included in this review. This review will serve as a guide to the future design of PA membranes for gas separations.

Aromatic polyamides and copolyamides containing fluorene group

A series of new aromatic polyamides (PAs) and copolyamides (CPAs) containing fluorene group have been synthesized through polycondensation reaction. The chemical structure was confirmed by Fourier transform infrared and proton nuclear magnetic resonance (1H NMR). PAs and CPAs exhibited the higher thermal stability ( Td15 > 378°C in nitrogen), the higher glass transition temperature ( Tg > 345°C), and excellent solubility in polar solvent. Gas transport properties of the PA and CPA membranes were investigated using different single gases (hydrogen (H2), carbon dioxide (CO2), oxygen (O2), methane (CH4), and nitrogen (N2)). We discussed the effect of chemical structure and operating temperature on gas transport properties. The results show that PA-1 containing a hexafluoroisopropylidene moiety exhibited the highest gas permeability ( PH2 = 12.71 Barrer, PCO2 = 12.26 Barrer, and PO2 = 2.62 Barrer) and reasonably good selectivity ( α(H2/N2) = 27.63, α(CO2/N2) = 26.65, and α(O2/N2) = 5.70) at 25°C and 1 atm. For all the membranes, gas permeability gradually increased with the increase in operating temperature, while the selectivity gradually decreased. These gas permeation results were well correlated with fractional free volume, interchain d-spacing ( dsp), and intermolecular interaction.

Gas separation properties of Troeger’s base-bridged polyamides

A series of new polyamides (PAs) has been prepared from a Troeger base-bridged diamine (TB), 2,8- diamino-4,10-dimethyl-6H,12H-5,11-methanodibenzo[1,5]-diazocine and different commercially available diacid monomers via the conventional polycondensation method. Dense membranes were prepared from the PAs by solution casting and solvent evaporation techniques. The synthesized PAs showed high glass transition temperature (283–290°C), 10% weight loss up to temperature 431°C in air, and tensile strength up to 91 MPa. The PA membranes showed higher permeability than some commercially used glassy polymers (PCO2 up to 109 and PO2 up to 21 Barrer) and permselectivity (PCO2/PCH4 up to 53.7 and PO2/PN2 up to 7.52) in comparison to many other PAs published in the literature.