This work presents a study on mixed matrix membranes (MMMs) of the polymer of intrinsic microporosity PIM-1, embedding the crystalline Cr-terephthalate metal-organic framework (MOF), known as MIL-101. Different kinds of MIL-101 were used: MIL-101 with an average particle size of ca. 0.2 µm, NanoMIL-101 (ca. 50 nm), ED-MIL-101 (MIL-101 functionalized with ethylene diamine) and NH 2 -MIL-101 (MIL-101 synthesized using 2aminoterephthalic acid instead of terephthalic acid). Permeability, diffusion and solubility coefficients and their corresponding ideal selectivities were determined for the gases He, H 2 , O 2 , N 2 , CH 4 and CO 2 on the "as-cast" samples and after alcohol treatment. The performance of the MMMs was evaluated in relation to the Maxwell model. The addition of NH 2 -MIL-101 and ED-MIL-101 does not increase the membrane performance for the CO 2 /N 2 and CO 2 /CH 4 separation because of an initial decrease in selectivity at low MOF content, whereas the O 2 and N 2 permeability both increase for NH 2 -MIL-101. In contrast, MIL-101 and NanoMIL-101 cause a strong shift to higher permeability in the Robeson diagrams for all gas pairs, especially for CO 2 , without significant change in selectivity. Unprecedented CO 2 permeabilities up to 35,600 Barrer were achieved, which are among the highest values reached with PIM-1 based mixed matrix membranes. For various gas pairs, the permeability and selectivity were far above the Robeson upper bound after alcohol treatment. Short to 21/11/2018 16.36.31 PIM-MIL-101 manuscript_SEPPUR_Final_revised_clean.docx p. 2/24 medium time aging shows that alcohol treated samples with MIL-101 maintain a systematically higher permeability in time. Mixed gas permeation experiments on an aged ascast sample with 47 vol% MIL-101 reveal that the MMM sample maintains an excellent combination of permeability and selectivity, far above the Robeson upper bound (CO 2 =3,500-3,800 Barrer, CO 2 /N 2 = 25-27; CO 2 /CH 4 =21 -24). This suggests good perspectives for these materials in thin film composite membranes for real applications.
There is an urgent need to develop efficient and economic CO 2 purification technologies to upgrade waste CO 2 to a reusable purity. Membrane-based separation processes are seen as one of the possible solutions to this problem. [1] For large-volume membrane applications, such as CO 2 recovery, high permeability is essential to minimize the membrane area, in combination with good selectivity.For membrane applications, high free-volume polymers [2] exhibit good processability, but they are prone to physical ageing. As transport depends on free volume, physical ageing leads to loss of permeability over time. [3] Porous crystalline solids can give good transport properties, but are less easily fabricated into mechanically stable membranes. Combinations of polymers with inorganic or metal-organic particles in composite or mixed-matrix membranes (MMMs) [4] may give synergistic enhancements in performance, but difficulties are encountered in achieving good dispersion within the membrane. [5] Largely unexplored is the potential of purely organic dispersed phases, comprising only C, H, N, and O atoms, which should show better compatibility with a continuous polymeric matrix and which offer scope for tailoring the physical properties through organic synthesis.Herein we demonstrate a novel route to MMMs in which the dispersed phase is generated by in situ crystallization of porous organic cage molecules from a single homogeneous, molecular solution. The incorporation of porous organic cages significantly enhances permeability, whereas chemically reduced, nonporous cage molecules have an opposite effect.We also compare the gas separation performance of membranes where crystals were generated by in situ crystallization against membranes where pre-formed nanocrystals were dispersed by co-casting with the polymer.The crystallizable precursor is CC3 (Figure 1 a), which has approximately triangular windows of effective diameter 0.6 nm, which is large enough to admit gases and small organic molecules. [6] The imine-linked CC3 was prepared as a powder with a Brunauer-Emmett-Teller (BET) surface area of 620 m 2 g À1 , based on N 2 adsorption at 77 K. A suspension of racemic CC3 nanocrystals (nanoCC3) in dichloromethane was also prepared. The isolated nanocrystalline CC3 had a BET surface area of 770 m 2 g À1 . To examine the importance of rigidity and shape persistence in CC3, its reduced amine form was prepared. Complete reduction with sodium borohydride of all 12 imine linkages in CC3 results in transformation to a much less rigid dodecaamine molecule (redCC3), which does not exhibit permanent porosity in the solid state and which is amorphous in powder form. 5,5',6,6'-tetrahydroxy-3,3,3',3'-tetramethyl-1,1'-spirobisindane and 1,4-dicyanotetrafluorobenzene by a step polymerization involving a double aromatic nucleophilic substitution. c) SEM image of a cross-section of a PIM-1/CC3 composite membrane (weight ratio 10:2).
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