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.
Highlights
Membranes with high selectivity offer an attractive route to molecular separations, where technologies such as distillation and chromatography are energy intensive
We suggest that the interfacial synthesis occurs in four stages (Fig. 2g): Stage 1 (0–4 hours), interfacial polymerization of a continuous oligomeric film at the dichloromethane–water interface; Stage 2 (4–16 hours), self-sorting of the reactants and oligomers into the CC3α product and the formation of a partially reacted, semi-cage film; Stage 3 (24–48 hours), crystallization of CC3α and the formation of octahedral crystals in the film; and Stage 4 (48–96 hours), formation of defects in the film caused by larger octahedral crystals creating cracks and imperfections
Continuous, defect-free Porous organic cages (POCs) membranes can achieve high permeances for a range of organic solvents—in some cases exceeding upper performance bounds—while showing excellent separation performances. These highly ordered crystalline POC membranes exhibit a switchable phase transition between two crystalline forms, CC3α-PAN and CC3γ′-PAN. This allows graded sieving to separate a mixture of three organic dyes using a single, smart membrane and creates a membrane-based parallel to the widespread and highly effective use of solvent gradients in chromatography[52]
Summary
Manipulated the reagent concentrations from 0.2 to 2.5 wt%. We use the nomenclature CC3α-PAN-X h-Y% to refer to the membranes made with X hours of reaction time and Y weight percent of the reagents. From in situ GIXRD measurements on solvated CC3 films, while performing two consecutive cycles, we found that the composite membrane transformed cleanly between CC3α-PAN and CC3γ′-PAN when the solvent was switched between water and MeOH and back again (Fig. 4e). We attributed this switching phenomenon solely to the phase transition of CC3 films, rather than swelling of the membranes. Excess MeOH was used to flush any residual BB from the cell to leave only DR in the retentate, where it could be collected in pure form (Supplementary Section 1.4)
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