Investigation of cross-linked and additive containing polymer materials for membranes with improved performance in pervaporation and gas separation

Abstract

Pervaporation and gas separation performances of polymer membranes can be improved by crosslinking or addition of metal-organic frameworks (MOFs). Crosslinked copolyimide membranes show higher plasticization resistance and no significant loss in selectivity compared to non-crosslinked membranes when exposed to mixtures of CO2/CH4 or toluene/cyclohexane. Covalently crosslinked membranes reveal better separation performances than ionically crosslinked systems. Covalent interlacing with 3-hydroxypropyldimethylmaleimide as photocrosslinker can be investigated in situ in solution as well as in films, using transient UV/Vis and FTIR spectroscopy. The photocrosslinking yield can be determined from the FTIR-spectra. It is restricted by the stiffness of the copolyimide backbone, which inhibits the photoreaction due to spatial separation of the crosslinker side chains. Mixed-matrix membranes (MMMs) with MOFs as additives (fillers) have increased permeabilities and often also selectivities compared to the pure polymer. Incorporation of MOFs into polysulfone and Matrimid® polymers for MMMs gives defect-free membranes with performances similar to the best polymer membranes for gas mixtures, such as O2/N2 H2/CH4, CO2/CH4, H2/CO2, CH4/N2 and CO2/N2 (preferentially permeating gas is named first). The MOF porosity, its particle size and content in the MMM are factors to influence the permeability and the separation performance of the membranes.

Figure 1

Principles of pervaporation. The liquid feed mixture flows along the membrane and the feed components diffuse into and through the membrane at different rates. The liquid retentate is depleted and the vaporous permeate enriched in the preferentially permeating component [12].

Figure 2

Different approaches to improve polymeric membrane materials.

Figure 3

Schematic representation of a mixed-matrix membrane indicating the different possible sizes, shapes and components for the inorganic filler materials (e.g., MOFs, ZIFs and nanotubes).

Figure 4

Schematic presentation of the trade-off between permeability and selectivity with the 1991 and 2008 Robeson upper bounds [20,21]. In most cases the technologically attractive region lies around or above the Robeson upper bound.

Figure 5

Possible crosslinking methods in polymers containing carboxyl groups. In ionic crosslinked polymers the metal ions form a complex with several deprotonated carboxyl groups. Covalent crosslinking can be achieved thermally or photochemically.

Figure 6

Mechanism for photocrosslinking of PEMAA modified with 3-hydroxypropyldimethylmaleimide. Upon excitation to the bright singlet state some population is transferred to the triplet state by intersystem crossing. Electron transfer leads to quenching of the triplet population and allows spectroscopic identification of the maleimide anion. Photocrosslinking occurs by recombination of the ions to the dimer. Our spectroscopic results do not exclude a parallel reaction path via direct cross linking of favorably oriented maleimides in the excited singlet state, see text. Transient species which were detected spectroscopically (triplet state, radical anion) are marked in green.

Figure 7

Transient absorption spectra obtained at different time delays to the excitation laser pulse of (a) PEMAA esterified with 3-hydroxypropyl-dimethylmaleimide (esterification degree of 6%) in THF upon excitation at 266 nm. The broad absorption at ~340 nm is due to triplet-triplet excitation in maleimide [154]; (b) PEMAA films upon excitation at 266 nm. Absorption between 264 nm and 268 nm is dominated by scattered light and therefore the measured absorption in this range is omitted from the spectral average (five data points: ~8 nm).

Figure 8

Chemical structures of the copolyimid components discussed in this review.

Figure 9

(a) Chemical structure of 6FDA-ODA/6FDA-DABA 4:1 copolyimide functionalized with 3-hydroxypropyldimetylmaleimide. Depicted in green is the maleimide side group; (b) Laser-induced FTIR difference spectra of 3-hydroxypropyldimethyl-maleimide (green), 6FDA-ODA/6FDA-DABA 4:1 copolyimide (black) and the copolyimide with 3-hydroxypropyldimethylmaleimide (green); (c) Difference spectrum of 3-hydroxypropyldimethylmaleimide (green) in the carbonyl region. The positive feature can be reproduced by the sum of three Gauss curves (blue).

Figure 10

(a) Pervaporation results for conditioned 6FDA-4MPD/6FDA-DABA 4:1 copolyimide membranes, crosslinked with 1,4-butanediol (orange) and crosslinked with zircon(IV)-acetylacetonate (blue) and non-crosslinked (black) using a toluene/cyclohexane mixture at 60 °C. Permeate pressure was kept between 20 and 25 bar [9]; (b) CO₂/CH₄ separation characteristics for the 6FDA-6FpDA/6FDA-4MPD/6FDA-DABA 3:1:1 copolyimide ionically crosslinked with aluminium(III)-acetylacetonate (blue) and covalently crosslinked with ethylene glycol (orange) at 35 °C using a 50:50 CO₂/CH₄ feed gas mixture [9].

Figure 11

Linkers and sections of the packing diagrams of (a) Cu-BPY-HFS, [Cu(µ-SiF₆)(µ-4,4'-bipy)₂] (BPY = 4,4'-bipyridine, HFS = hexafluorosilicate), (b) Cu-BTC, [Cu₃(BTC)₂(H₂O)₃] (BTC = benzene-1,3,5-tricarboxylate) and (c) MOF-5 (IRMOF-1), [Zn₄O(BDC)₃] (BDC = benzene-1,4-dicarboxylate, terephthalate).

Figure 12

Linkers and sections of the packing diagrams emphasizing the cuboctahedral b-cage which is depicted by blue topological lines connecting the Zn atoms of (a) ZIF-8, [Zn(2-methylimidazolate)₂], (b) ZIF-20, [Zn(purinate)₂] and (c) ZIF-90, [Zn(2-carboxyaldehyde imidazolate)₂].

Figure 13

Linkers and sections of the packing diagrams of (a) Mn(HCOO)₂, (b) MIL-53(Al), [Al(BDC)(µ-OH)] (BDC = benzene-1,4-dicarboxylate, tere-phthalate) and (c) MIL-101, [Cr₃(O)(BDC)₃(F,OH)(H₂O)₂].

Figure 14

Single-gas CO₂, CH₄ and N₂ permeabilities and ideal (a) CO₂/CH₄ and (b) CO₂/N₂ selectivities of pure PSF and MIL-101/PSF membranes with different MIL wt.% loadings (averaged values for different membrane thicknesses).

Figure 15

(a) Single-gas CO₂; (b) CH₄ and N₂ permeabilities; (c) ideal CO₂/CH₄ and CO₂/N₂ selectivities for MIL-101/PSF membranes at different MIL wt.% loadings and membrane thicknesses.