Facilitated transport effect of Ag + ion exchanged halloysite nanotubes on the performance of polyetherimide mixed matrix membrane for gas separation

Abstract

This study investigated the facilitated transport effect of Ag + ion exchanged halloysite nanotubes (HNTs) as filler on the gas separation performance of asymmetric mixed matrix membranes (MMMs). The polymer matrix employed in this study was commercial polyetherimide (PEI) Ultem 1000. The modified HNTs were prepared by treating HNTs with N-β-(aminoethyl)-γ-aminopropyltrimethoxy silane (AEAPTMS) and silver nitrate. FESEM, XRD, FTIR, TGA, DSC, EDX and pure gas permeation testing were used to characterise the modified HNTs and the fabricated MMMs. Three protocols were performed: (i) S-HNT MMM (no Ag + ion exchange treatment), (ii) S-Ag-HNT MMM (first Ag + ion exchanging and then silylation of HNTs), and (iii) Ag-S-HNT MMM (first silylation and then Ag + ion exchanging of HNTs). FTIR and TGA showed that silylation occurred successfully. From XRD we found out that, the Ag + ion exchanging did not affect the HNT crystalline structure. EDX revealed that, Ag + ion exchanging after silylation of HNTs resulted in much higher concentration of Ag + ions in the Ag-S-HNT product. This in turn showed that AEAPTMS could successfully enhance the HNTs cation exchange capacity (CEC), which resulted in higher concentration of Ag + ions in the modified HNTs. DSC disclosed depression in the glass transition temperature ( T g) of MMMs possessed Ag + ions. Three major factors were discussed: (i) facilitated transport affect of Ag + ions, (ii) Knudsen diffusion and (iii) T g depression. By increasing the fillers loading, all of the factors exhibited an additive influence on the permeability. The order of closeness of the resultant MMMs to the ideal morphology was as follows: Ag-S-HNT MMM > S-Ag-HNT MMM > S-HNT MMM. Ag-S-HNT MMM exhibited outstanding performance among the three protocols.