Cross-linked mixed matrix membranes consisting of carboxyl-functionalized multi-walled carbon nanotubes and P84 polyimide for organic solvent nanofiltration (OSN)

We have fabricated mixed matrix membranes (MMMs) consisting of carboxyl-functionalized multi-walled carbon nanotubes (MWCNTs-COOH) and P84 polyimide in this study for organic solvent nanofiltration (OSN) with the aid of chemical cross-linking by 1,6-hexanediamine (HDA). A moderate annealing is also used to manipulate the pore size for a better solute rejection. The incorporation of hydrophilic carbon nanotubes into P84 not only improves liquid sorption and transport but also increases membrane porosity and pore size. As a result, the permeances of water, ethanol, and isopropanol across the MMMs increases with an increase in MWCNTs-COOH loading up to 0.075 wt.%. However, a higher loading of MWCNTs-COOH reduces the separation performance. The cross-linked MMM comprising 0.05 wt.% MWCNTs-COOH has a rejection of 85% to rose bengal (1017.65 Da) while ethanol permeance is 9.6 LMH⋅bar⁻¹ at 5 bar. Interestingly, the rejection of rose bengal in isopropanol solutions is higher than that in ethanol solutions (i.e., 99 vs. 85%). After thermal annealing at 150 °C in a 3/1 EG/PEG400 (weight ratio) solution, the resultant membranes (MMM comprising 0.05 wt.% MWCNTs-COOH) show superlative rejections to small dye molecules (almost 100% to Safranin O dye molecules, 350.85 Da) in ethanol solutions. There is an obvious trade-off between rejection and permeance among the fabricated membranes, in which, the permeance enhancement and rejection diminishing occurred with the addition of MWCNTs-COOH; however, a vice-versa trend was observed in annealed membranes.