Hydrophilic and Morphological Modification of Nanoporous Polyethersulfone Substrates for Composite Membranes in CO2 Separation

A composite membrane with superior performance can be employed for CO₂ separation from flue gas. The separation performance of a composite membrane can be improved by tuning the selective layer and porous substrate. The optimal substrate should have suitable interfacial property and surface morphology, as well as minimal gas permeation resistance. In this paper, nanoporous polyethersulfone (PES) substrates were fabricated by the vapor- and non-solvent-induced phase inversion technique. N-methyl-2-pyrrolidone (NMP) and water were employed as solvent and non-solvent, respectively. 2-Methoxyethanol (2-ME) was incorporated into the casting solution as a pore-forming additive because of its high affinity with water, leading to an interconnected spongy structure in the bulk of the substrate. Approaches are described to improve the hydrophilicity and morphology of the PES substrate by incorporating a small amount of hydrophilic moieties, such as hydroxylated polyethersulfone (PES-OH) and polyvinylpyrrolidone (PVP). The surface morphologies and gas permeation resistances of the PES substrates were characterized by scanning electron microscopy (SEM) and a gas permeation measurement, respectively. The optimal hydrophilic additive was identified based on the substrate fabrication complexity and composite membrane performance improvement. The pristine PES substrate, with a pore size of 41 nm and a surface porosity of 12%, showed a CO₂ permeance of 9200 GPU (1 GPU = 10^-6 cm³ (STP)·cm^-2·s^-1·cmHg^-1) at 57°C and 1 atm. The addition of hydrophilic polymers reduced the gas permeation resistances of the PES substrates because of a more open morphology and a thinner dense layer. By incorporating appropriate amounts of PES-OH and PVP in the casting solutions, modified PES substrates showed CO₂ permeances of 14100 GPU and 21000 GPU, respectively, indicating a significant reduction of gas permeation resistance. The developed PES substrates were applied for the preparation of composite membranes for CO₂ separation. Amine-containing selective layers, with a thickness of approximately 175 nm, were coated on the developed pristine, PES-OH-modified, and PVP-modified PES substrates to form composite membranes. From the gas transport measurements by using a simulated flue gas, these 3 composite membranes showed CO₂ permeances of 785 GPU, 836 GPU, and 843 GPU, respectively, with CO₂/N₂ selectivities >140. A resistance-in-series model was employed to assess the effect of substrate morphology on the CO₂ permeance of a composite membrane. For the selective layer material in this study, 25 – 30% of the total transport resistance for CO₂ was caused by pore restriction, while 5% of the total resistance was due to the Knudsen diffusion in the substrate. This paper describes an efficient and cost-effective preparation process to fabricate improved PES substrates that can be applied for the preparation of composite membranes in gas separation.