A smart and responsive crystalline porous organic cage membrane with switchable pore apertures for graded molecular sieving

Abstract

Membranes with high selectivity offer an attractive route to molecular separations, where technologies such as distillation and chromatography are energy intensive. However, it remains challenging to fine tune the structure and porosity in membranes, particularly to separate molecules of similar size. Here, we report a process for producing composite membranes that comprise crystalline porous organic cage films fabricated by interfacial synthesis on a polyacrylonitrile support. These membranes exhibit ultrafast solvent permeance and high rejection of organic dyes with molecular weights over 600 g mol^-1. The crystalline cage film is dynamic, and its pore aperture can be switched in methanol to generate larger pores that provide increased methanol permeance and higher molecular weight cut-offs (1,400 g mol^-1). By varying the water/methanol ratio, the film can be switched between two phases that have different selectivities, such that a single, 'smart' crystalline membrane can perform graded molecular sieving. We exemplify this by separating three organic dyes in a single-stage, single-membrane process.

Fig. 1. Synthesis of a crystalline CC3 film and its crystal structures.

a, Scheme showing the interfacial synthesis method used to fabricate crystalline CC3 films, which were subsequently adhered to a PAN support. These crystalline cage films can cycle between two different forms, CC3α-PAN and CC3γ′-PAN, by cycling the solvent between water and MeOH. CH₂Cl₂, dichloromethane. b, CC3α structure with its 3D pore network shown in yellow. c, The CC3γ′ structure, formed by soaking in MeOH, has additional extrinsic solvent-filled channels, shown here in orange, that open up additional porosity in the membrane in response to the MeOH solvent.

Fig. 2. Characterization of a CC3α film.

a, Photograph of composite membrane CC3α-PAN with a diameter of 7.4 cm. b, SEM image of CC3α-PAN showing the surface morphology of the CC3α film. Shown below is the cross-sectional FIB-SEM image of CC3α-PAN. c, AFM height image (top) and the height profile (bottom) of CC3α film transferred onto a silicon (Si) wafer. d, SEM image of a free-standing CC3α film, where the film was deliberately buckled to show its thickness. e,f, Raman microscope image (e) and Raman map (f) of a CC3α film on a glass support, where we purposely scratched the film before the measurement to expose the glass support (black stripe in f). The red regions on a CC3α film had comparable Raman spectra to the crystalline CC3α reference sample. g, SEM images of CC3α-PAN-X h-0.8% membranes formed at different reaction times, showing four stages of CC3α film formation. h, Out-of-plane GIXRD (wavelength, λ = 0.689 Å) patterns of CC3α-PAN-X h-0.8% membranes fabricated using reaction times between 4 and 48 hours (2θ refers to the scattering angle). i, Raman spectra of CC3α film, a crystalline CC3α reference and an amorphous CC3 reference. Source data

Fig. 3. Nanofiltration performance of CC3α membranes.

a, Plot showing pure solvent permeances versus their combined solvent properties (viscosity η, molar diameter d_m and solubility parameter δ_d) for CC3α-PAN, where R² is the coefficient of determination for the function. Hansen solubility parameter (δ) and the physical properties of each organic solvent are listed in Supplementary Table 2. b, Water permeance for CC3α-PAN-X h-0.8% membranes fabricated using reaction times that ranged between 4 and 96 hours. c, Dye rejection measurements for CC3α-PAN-X h-0.8% membranes in water. d,e, Water flux (d) and dye rejections (e) of a CC3α-PAN membrane under a range of applied pressures. f, Ultraviolet–visible absorption spectra of Congo red in water before (feed) and after (permeate and retentate) selectivity tests performed with CC3α-PAN. Insets show photographs of the feed, permeate and retentate solutions and the molecular structure of Congo red. Dye rejection was calculated using the intensity of the maximum absorption peak in the permeate and the feed, and equation (3) in the Methods. Mass balance calculations were performed using the maximum absorption peak values of the feed, permeate and retentate, with equation (4). All error bars depict the standard deviation (s.d.) of the data points obtained from at least three independent membranes. Source data

Fig. 4. X-ray diffraction characterization and switchable separation performance of CC3-PAN membranes.

a, GIXRD of CC3α-PAN in air and water, and CC3γ′-PAN in MeOH. Experimental PXRD patterns of CC3α and CC3γ′ powders are included as references. b, MWCO curve for CC3α-PAN in water and CC3γ′-PAN in MeOH containing 20 ppm dye solutes. The MWCO was determined by interpolating from the plot of rejection against the molecular weight of the dyes and corresponds to the molecular weight for which rejection reaches 90%. All error bars depict the s.d. of the data points obtained from at least three independent membranes. c, Reversible dye rejection of BB and solvent permeance of the CC3-PAN membrane observed upon switching the feedstock solvent between water and MeOH. All error bars denote the s.d. for measurements from at least three independent membranes. d, Photographs of CC3-PAN filtration dead-end cell captured from Supplementary Video 1 during the different cycles; BB is rejected in water by CC3α-PAN while CC3γ′-PAN does not reject BB in MeOH. e, In situ GIXRD patterns showing the reversible phase transition between CC3α-PAN and CC3γ′-PAN, by cycling between water and MeOH. f, Acetone permeance versus MWCO of general solutes in acetone for nanofiltration membranes reported in the literature and CC3α-PAN. MOF, metal–organic framework; MC, macrocycle; NPs, nanoparticles; GO, graphene oxide; PTMSP, poly(1-(trimethylsilyl)-1-propyne); PA, polyamide; PEI, polyethyleneimine; PANI, polyaniline; PI, polyimide; PE, polyethylene; PEEK, poly(ether ether ketone); PEO, poly(ethylene oxide); PIM, polymers of intrinsic microporosity; PIP, piperazine; PAR, polyacrylate (Supplementary Table 9 for full details). Source data

Fig. 5. Mixture fitting and graded sieving using a single switchable membrane.

a, BB rejection in mixtures of water and MeOH (v/v) for CC3-PAN (top), and photographs of the permeates (bottom). All error bars depict the s.d. of the data points obtained from at least three independent membranes. The red dashed line was fitted as the logistic function (y = 1/(1 + exp(−16.1(x – 0.617))); Supplementary Section 1.4). b, Photographs showing the ternary molecular separation in a filtration dead-end cell, the nascent mixture feedstock, the permeate (P) collected in the first and second step, and the retentate (R) collected in the second step. c, Scheme showing ternary molecular separation of three dyes (DR, BB and NP) using one single membrane (CC3-PAN) in a continuous process: Step 1, CC3α-PAN in water (blue background) allows permeation of only NP, leaving BB and DR in the retentate. Step 2, 90 vol% MeOH (green background) was added into the retentate to transform the membrane structure to CC3γ′-PAN, which allows permeation of only BB, leaving DR in the retentate. d, Ultraviolet–visible absorption spectra of the mixture containing three molecules in water, permeate from water, mixture and permeate from 90 vol% of MeOH in water and the remaining retentate. Note, the maximum absorbance wavelength for BB is 551 nm in water and 587 nm in MeOH; BB also shows absorbance at 305 nm in MeOH, while NP shows its maximum absorbance at 312 nm in the same solvent. Source data