One-pot synthesis of necklace-like MOF@CNTs: a universal strategy for enhancing molecular separation performance in mixed-matrix membranes†

Abstract

Metal–organic frameworks (MOFs) are usually employed as fillers within a polymer matrix to fabricate mixed matrix membranes (MMMs). However, the aggregation of MOFs, particularly at the nanoscale, frequently leads to defects at the filler–polymer interface, making it difficult to form continuous molecular transport channels. Here, a series of necklace-like MOF@carbon nanotubes (CNTs) were synthesized using a straightforward one-pot technique. Specifically, CNTs as a “lead wire” with substantial aspect ratios were first used to induce the self-assembly growth of MOFs as “beads” along the CNTs' longitudinal axis, for the preparation of necklace-like MOF@CNTs. Subsequently, necklace-like MOF@CNTs MMMs were obtained by interfacial polymerization or solvent evaporation. MOF@CNTs have good dispersibility in the polymer matrix and the MOFs within the necklace are in close contact and effectively link the MOF window to create micrometer-scale continuous molecular transport channels that can improve the separation performance of MMMs. For ZIF-90/PA mixed matrix nanofiltration membranes, the water permeability of M-ZIF-90@CNTs (27.15 L m⁻² h⁻¹ bar⁻¹) is greater than that of M-0 (8.11 L m⁻² h⁻¹ bar⁻¹). Furthermore, the dye rejection efficiency has been increased from 96.60% of M-0 to 99.55% of M-ZIF-90@CNTs and the rejection of NaCl remains relatively low across all nanofiltration membranes, at less than 10%, which has significant advantages in the field of dye/salt separation. For ZIF-90/PSf mixed matrix gas separation membranes, MPSf-ZIF-90@CNTs also exhibit remarkable CO₂/N₂ separation selectivity. Importantly, the proposed strategy for preparing necklace-like MOF@CNTs is universally applicable, and can be easily extended to other MOFs, such as ZIF-8, MOF-801, UiO-66, and UiO-66-NH₂, representing a universal strategy for constructing necklace-like MOF@CNTs structures.