Dual-layer polyethersulfone (PES)/BTDA-TDI/MDI co-polyimide (P84) hollow fiber membranes with a submicron PES-zeolite beta mixed matrix dense-selective layer for gas separation

Dual-layer PES/P84 hollow fiber membranes with a PES-zeolite beta mixed matrix dense-selective layer of 0.55 μm have been successfully fabricated in this study by adjusting the ratio of outer layer flow rate to inner layer flow rate during the spinning with the aid of heat-treatment at 235 °C and two-step coating. To our best knowledge, this is the thinnest thickness that has ever been reported for dual-layer hollow fiber membranes with the mixed matrix outer layer. The newly developed dual-layer hollow fibers exhibit enhanced O₂/N₂ and CO₂/CH₄ selectivity of around 10-20% compared with that of neat PES dense films. Both the polymer chain rigidification and partial pore blockage of zeolite beta may be the main causes for the selectivity increment. The outer surface of wet-spun hollow fibers shows fewer defects than that of hollow fibers spun at 1.5 cm air gap probably because of the overlarge elongation stress with an increase in air gap. Scanning electron microscope (SEM) images demonstrate that heat-treatment can effectively reduce or eliminate the voids induced by the unfavorable interaction between polymer and zeolite phases. Differential scanning calorimetry (DSC) results prove that heat-treated dual-layer hollow fiber membranes with a mixed matrix outer layer possess a higher glass transition temperature (T_g) over neat PES dense film. The performance of these newly developed dual-layer hollow fiber membranes has also been confirmed in mixed gas tests.