The Ionic Liquid–H₂O Interface: A New Platform for the Synthesis of Highly Crystalline and Molecular Sieving Covalent Organic Framework Membranes

Abstract

Covalent organic frameworks (COFs) are highly porous crystalline polymers with uniform pores and large surface areas. Combined with their modular design principle and excellent properties, COFs are an ideal candidate for separation membranes. Liquid−liquid interfacial polymerization is a well-known approach to synthesize membranes by reacting two monomers at the interface. However, volatile organic solvents are usually used, which may disturb the liquid−liquid interface and affect the COF membrane crystallinity due to solvent evaporation. Simultaneously, the domain size of the organic solvent−water interface, named the reaction zone, can hardly be regulated, and the diffusion control of monomers for favorable crystallinity is only achieved in the water phase. These drawbacks may limit the widespread applications of liquid−liquid interfacial polymerization to synthesize diverse COF membranes with different functionalities. Here, we report a facile strategy to synthesize a series of imine-linked freestanding COF membranes with different thicknesses and morphologies at tunable ionic liquid (IL)−H 2 O interfaces. Due to the H-bonding of the catalysts with amine monomers and the high viscosity of the ILs, the diffusion of the monomers was simultaneously controlled in water and in ILs. This resulted in the exceptionally high crystallinity of freestanding COF membranes with a Brunauer−Emmett− Teller (BET) surface area up to 4.3 times of that synthesized at a dichloromethane−H 2 O interface. By varying the alkyl chain length of cations in the ILs, the interfacial region size and interfacial tension could be regulated to further improve the crystallinity of the COF membranes. As a result, the as-fabricated COF membranes exhibited ultrahigh permeance toward water and organic solvents and excellent selective rejection of dyes.

Assembling covalent organic framework membranes via phase switching for ultrafast molecular transport

“…Majority of the reported COF membranes are assembled using this one-step procedure, predominantly in liquid phase using interfacial (oil–water biphasic system) or in-situ solvothermal (monophasic system) methods 30 . In liquids, controlling the concurrent polymerization and crystallization during membrane formation is quite challenging; primarily due to the high surface tension (2 × 10 −2 −5 × 10 −2 N m −1 )) and viscosity (0.3–4 cp) of liquids which makes the removal of by-product from the reaction site extremely difficult 31 , 32 . Consequently, the concentration of reacting monomers is low near the polymerization sites whereas that of byproducts is higher, leading to hindered reaction reversibility 20 , 33 , 34 .…”

2D Polymer Nanosheets for Membrane Separation

“…[39] This technique could be used in the homogeneous solution, [40][41][42] or at the interface, such as the liquid/solid, [43] air/water, [44][45][46] and liquid/liquid interfaces. [47] In homogeneous solution synthesis, 2D polymer nanosheets are very prone to stacking into bulk materials due to various noncovalent interactions. [48,49] To synthesize 2D polymer nanosheets in one step, introducing repulsive interactions to disrupt the stacking is a very operative strategy.…”

“…[ 39 ] This technique could be used in the homogeneous solution, [ 40 , 41 , 42 ] or at the interface, such as the liquid/solid, [ 43 ] air/water, [ 44 , 45 , 46 ] and liquid/liquid interfaces. [ 47 ]…”

Two‐Dimensional Metal‐Organic Frameworks and Covalent Organic Frameworks

“…Wang et al . [ 58 ] reported a simple method to synthesize a series of imine‐based self‐supporting membranes with different thicknesses and morphologies at the tunable ionic liquid (IL)/H 2 O interface. Due to the hydrogen bonding between the catalyst and the amine monomer and the high viscosity of the ionic liquid, the diffusion of the monomer in water and ionic liquid was controlled at the same time.…”