Aylin
Kertik
a,
Lik H.
Wee
*a,
Martin
Pfannmöller
b,
Sara
Bals
b,
Johan A.
Martens
a and
Ivo F. J.
Vankelecom
*a
aCentre for Surface Chemistry and Catalysis, University of Leuven, Celestijnenlaan 200f, B3001, Heverlee, Leuven, Belgium. E-mail: likhong.wee@kuleuven.be; ivo.vankelecom@kuleuven.be
bElectron Microscopy for Materials Science, University of Antwerp, Groenenborgerlaan 171, B2020, Antwerp, Belgium
First published on 9th August 2017
Conventional carbon dioxide (CO2) separation in the petrochemical industry via cryogenic distillation is energy intensive and environmentally unfriendly. Alternatively, polymer membrane-based separations are of significant interest owing to low production cost, low-energy consumption and ease of upscaling. However, the implementation of commercial polymeric membranes is limited by their permeability and selectivity trade-off and the insufficient thermal and chemical stability. Herein, a novel type of amorphous mixed matrix membrane (MMM) able to separate CO2/CH4 mixtures with the highest selectivities ever reported for MOF based MMMs is presented. The MMM consists of an amorphised metal–organic framework (MOF) dispersed in an oxidatively cross-linked matrix achieved by fine tuning of the thermal treatment temperature in air up to 350 °C which drastically boosts the separation properties of the MMM. Thanks to the protection of the surrounding polymer, full oxidation of this MOF (i.e. ZIF-8) is prevented, and amorphisation of the MOF is realized instead, thus in situ creating a molecular sieve network. In addition, the treatment also improves the filler-polymer adhesion and induces an oxidative cross-linking of the polyimide matrix, resulting in MMMs with increased stability or plasticization resistance at high pressure up to 40 bar, marking a new milestone as new molecular sieve MOF MMMs for challenging natural gas purification applications. A new field for the use of amorphised MOFs and a variety of separation opportunities for such MMMs are thus opened.
Broader contextRemoval of CO2 from natural and biogas is of growing concern with respect to harmful effects of greenhouse gas emissions and technological interests in CO2 as alternative building block for renewable fuels and chemicals. Even though polymer membranes are attractive for molecular-level separations in these industrial-scale processes, commercially available membranes still need to be improved for effective CO2 separations mainly due to their intrinsic permeability/selectivity trade-off, limited thermal stability and high susceptibility to plasticization. Mixed matrix membranes (MMMs), consisting of well-dispersed MOF fillers in a polymeric matrix, could potentially solve these issues. Present work demonstrates that thermally induced oxidative cross-linking of the polymer matrix and improved interactions between the polymer and the amorphised zeolitic imidazolate framework form plasticization-resistant MMMs for highly-selective separations of CO2/CH4 mixed-gas feeds. Obtained membranes could also be of use as molecular sieves for other separation processes, such as hydrocarbons (e.g. olefin/paraffin isomers) separation, or liquid phase separations via pervaporation or solvent resistant nanofiltration. |
An alternative approach is mixed matrix membranes (MMMs) consisting of fillers dispersed in a polymer. While the polymer offers mechanical stability and processibility,2 a filler with well-defined sorption properties provide more selective permeation.9 As filler, zeolitic imidazolate frameworks (ZIFs) as a sub-family of MOFs analogous to inorganic zeolites, are particularly interesting.10 ZIF-8 is an archetypal ZIF, consisting of zinc ions and 2-methylimidazole linkers coordinated in a sodalite topology having interconnected pore cavities and a pore aperture of 11.6 Å and 3.4 Å, respectively, capable of separating various gas mixtures.11–14 Poor compatibility between filler and polymer is a notorious problem in preparing MMMs.15,16 Methods such as above-Tg annealing,17 filler surface modification,15 melt-processing,18 priming9 and using low-molecular-weight additives,19 or plasticizers20 have been applied to overcome this problem.
Some MOFs, and specifically ZIFs tend to exhibit a solid–solid phase transformation known as amorphisation. Amorphous MOFs (aMOFs) are networks that retain their basic building blocks but exhibit no long-range ordering.4 aMOFs are obtained by introducing disorder into their parent crystalline frameworks through heating, pressure or mechanical ball-milling.4 Amorphisation is a well-known concept for inorganic materials, but the application of aMOFs is largely unexplored.4,21,22 The current potential application is limited to drug-release23,24 and entrapping harmful substances.25,26 In this work, we show that amorphised MOFs can be created in situ in a ZIF-containing polyimide membrane by a controlled thermal treatment in air. Combined with remarkable changes in polymer properties, this MOF transformation generates MMMs with the highest CO2/CH4 mixed-gas selectivities reported so far for membranes based on commercial polymers, marking a new milestone in the latest Robeson's upper-bound plot, and offering new opportunities to highly selective separations in a variety of related applications.
After a prolonged treatment at 350 °C for 24 h, a significant change in membrane color was noted as shown in Fig. 1a–d. The membranes changed from light yellow to dark brown when the temperature was increased. Interestingly, the onset of darkening for the MMMs was 250 °C, whereas it was 350 °C for the unfilled polymer, suggesting that the embedded ZIF-8 has significant influence on the thermal behaviour of the MMMs. According to the TG analysis (Fig. 1e and f), the Matrimid® membrane annealed at 100 °C shows a significant weight loss (10.3 wt%) from 170–470 °C, whereas the unfilled Matrimid® membrane treated at 350 °C only shows a negligible weight loss (1.2 wt%), pointing to an improved thermal stability. A similar trend is observed for the thermally treated MMM (18.2 wt% for 100 °C and 4.2 wt% for 350 °C). More importantly, the embedded ZIF-8 is not degraded upon heating to 340 °C, in contrast to the bulk ZIF-8 powder under similar conditions. The significant enhancement of the thermal stabilities and the change in the visual properties of the unfilled membranes suggests thermo-oxidative cross-linking of the polymer. To confirm this hypothesis, solubility tests were performed and the results are presented in Fig. S2 (ESI†) for the unfilled Matrimid®, and in Fig. 1g–i for the MMMs. Obviously, all membranes treated above 160 °C became largely insoluble in a good solvent for the polymer such as chloroform, with a gel content above 95% after 2 days (Fig. 1h and i).
In order to elucidate the underlying chemistry of the thermo-oxidative cross-linking of Matrimid®, the membranes were characterized by ATR-FTIR (Fig. 2a). The adsorption bands at 2960, 2927, and 2864 cm−1 assigned to the aliphatic C–H stretching vibration decreased in intensity with increasing temperature in line with the observed darker color of Matrimid®. It has been previously reported that oxidative cross-linking occurs at the methylene bridges of the polymer, which is associated with the ketone and aromatic CO stretching vibrations at 1600 and 930 cm−1, respectively.28 A decrease at 823 cm−1 and an increase at 1604 cm−1 are now observed, which could be assigned to methyl and CO oxidation, respectively, with increasing thermal treatment temperature. In general, thermal oxidation of polymers involves free-radical chain reactions. In the present study, a prolonged thermal treatment at 350 °C in the presence of oxygen induces oxidation and cross-linking. The methyl groups in the polyimide are oxidised to provide readily available cross-linking sites via the formation of –CH2 radicals. These free radicals react with O2 to form peroxy radicals (ROO˙), which can further abstract a hydrogen from adjacent polymer chains to generate hydroperoxide (ROOH) moieties. These hydroperoxides then undergo sequential termination steps to induce inter-chain cross-linking.29 The concise mechanism for thermo-oxidative cross-linking is shown in Fig. 2c. A complete chain reaction mechanism including the initiation, propagation and termination steps is illustrated in Fig. S3 and S4 (ESI†). The excess water content found in the cross-linked MMMs (2.6 wt%) versus 0.7 wt% in the MMMs annealed at 100 °C according to TGA (Fig. 1e, f and Fig. S5, S6, ESI†) is mainly due to the water formation as a result of the chain termination reaction. Glass transition temperatures (Tg) increased as the thermal oxidative cross-linking of methylene bridging groups became more prominent.30,31 While the Tg of the unfilled polymer increased to 340 °C, it reached 350 °C for the MMM treated at the same temperature. The tensile strength of the membranes evolved in line with this cross-linking, as shown in Table S1 and Fig. S7 (ESI†). Due to the MOF loading, the tensile strength of the MMMs was overall below that of Matrimid®. However, it still exhibited an exponential increase with thermal treatment temperature, i.e. with increasing cross-linking.
Fig. 2 Thermo-oxidative cross-linking of the membranes. The ATR-FTIR patterns of the (a) Matrimid® and (b) MMMs with 40 wt% ZIF-8 loading as a function of temperature. (c) The suggested cross-linking mechanism for the PI and the MMM based on the FTIR results. (d) The characteristic XRD peaks of ZIF-8-containing MMMs. Enlarged versions of ATR-FTIR spectra of (a) Matrimid® and (b) the MMMs with assigned peaks are provided in Fig. S8 (ESI†). |
The Matrimid® membranes thermally treated at 250 °C were completely dissolved after 2 days whereas the MMMs treated at the same temperature were more resistant to solvation, especially at high ZIF-8 loadings. The observation suggests that the embedded ZIF-8 actually acts as an additional chemical cross-linker upon heating. Chemical cross-linking is known to realize the reduction of the plasticization tendency of Matrimid® membranes,32 which is critical for their application in CO2 separations. There have also been reports of plasticization-resistant MMMs exhibiting direct cross-linking between MOF and polyimide.33,34 According to ATR-FTIR spectra (Fig. 2b), apart from the previously observed enhanced absorption bands for CO and the decline in C–H stretching vibration, an additional band largely masked by Matrimid® at 1538 cm−1 was noted. This band can be tentatively assigned to the aromatic N–H stretching band of the amide group, confirming the cross-linking of Matrimid® with the imidazolate in ZIF-8. The cross-linking of imidazole from aZIF-8 with Matrimid®, as based on the ATR-FTIR studies, is tentatively shown in Fig. 2c.
XPS analysis clearly illustrated the presence of the peaks assigned to Zn2+ Zn2p3 and Zn2p1 (Fig. S9, ESI†). As ZIF-8 is amorphised, the Zn–N bonds are the first to brake and create unsaturated Zn2+ sites, as reflected in the increased intensity of the XPS peaks for the MMM treated at 350 °C. Recently, cross-linking of the organic ligand in MOF with polymer gel has been reported via acidification with concentrated HCl for breaking the coordinative bonding between the Zn(II) ion and the carboxylate anion.35 However, for our system, the cross-linking of Matrimid® with the molecular building units of ZIF-8 hints of a phase transition upon thermal treatment. Despite the structural transformation of ZIF-8 has been extensively studied by Cheetham21 and Friščić,36,37 amorphisation of ZIF-8 by thermal treatment has not been realised to date mainly due to direct oxidation of ZIF-8 to ZnO in air. In order to evaluate the crystallinity of the thermally treated membranes, XRD was performed (Fig. 2d and Fig. S10a–c, ESI†). For the MMMs thermally treated at 100 °C and 160 °C, the crystalline structures of the ZIF-8 nanoparticles remained intact, but when treated at higher temperatures (250 and 350 °C), the typical XRD diffractions were no longer observed (Fig. 2d). The disappearance of the crystalline peaks suggest the amorphisation of the embedded ZIF-8. The amorphous state of the ZIF-8 embedded in the Matrimid® matrix was further confirmed by TEM electron diffraction (Fig. 3a). No diffraction spots at high camera length that could indicate the presence of ZIF-8 crystals were observed. Electron diffraction only reveals rings representative of amorphous phases at low camera length. Nevertheless, the absorption band at 750 cm−1 assigned to the out-of-plane bending of the imide 5-membered ring is preserved, indicating that the imidazole is thus not oxidized during this amorphisation process. The combined XRD, ATR-FTIR and electron diffraction results confirm that the basic building blocks and the connectivity of the amorphised ZIF-8 counterparts is retained, but the long-range periodic ordering is lacking. From previous studies by Lee et al.,38 direct thermal treatment of ZIF-8 at 350 °C induces ZnO formation at high temperatures. In our MMMs, no ZnO was formed as evidenced from the XRD patterns (Fig. 2d) by the absence of the characteristic diffraction peaks in the region between 30–40° (2θ). In addition, the ZnO band at 500 cm−1 is not observed from the FTIR spectra, which further excludes its formation at high temperature.38 Although ZIF-8 has been reported to undergo amorphisation via ball-milling or compression,4 amorphisation through thermal treatment has not been documented to date yet, since bulk ZIF-8 powder is more likely to undergo a direct phase transition to ZnO upon thermal treatment (Fig. S11, ESI†).38 We assume that the Matrimid® might provide thermochemical protection for the embedded ZIF-8 under a relatively slow heating rate and a prolonged 24 h thermal treatment which is in full agreement with the TGA results.
Fig. 3g shows core-loss energy-loss spectra taken for a polyimide area and two ZIF-8 areas as indicated in the HAADF image in Fig. 3h. Spectra are averages from STEM-SI data. A cross-section of 40 nm thickness was used to reduce effects from multiple scattering. To obtain a STEM-SI data set, the focused beam was scanned across the region outlined in Fig. 3h while recording an energy-loss spectrum for each scan position. The energy range is 275–650 eV so that core-loss signals contain information about the C–K edge (ca. 285 eV), N–K edge (ca. 397 eV), and O–K edge (ca. 532 eV). Spectra were corrected for multiple scattering (see Methods section) and the background signal was fitted by a power law function and subtracted. This allows quantitative comparison of the different element edges. It can be clearly seen that ZIF particles show much stronger signals for N and an elevated signal for π* in relation to σ* excitations compared to polyimide. The total integrated peak intensities remain the same. However, sharper peaks for the N-edge and the π* excitation for ZIF-8 nanoparticles indicate a better ordering of amorphous ZIF-8 building units than the amorphous polymer matrix. The O concentration is very low over the entire investigated region. Therefore, the spatially resolved spectra were used to quantify the local C and N compositions yielding a map of C/N ratios as shown in Fig. 3i. The map shows that the amorphous ZIF-8 regions in blue with high N concentrations exhibit a sharp boundary towards the polymer regions of high relative C content (red). Around the boundaries between polymer and ZIF-8 particles there is a small interfacial layer with intermediate C/N ratios (the grey layer around the ZIF-8 regions, Fig. 3i). Since the signals result from integration over the thickness of 40 nm, the interfacial profiles can be induced by the varying C/N ratios along the path of the electrons through the cross-section. However, as indicated by the black arrow in Fig. 3i, such a layer is also visible in a region where only a thin part of ZIF-8 is found. The constant occurrence of the same interfacial C/N ratio provides evidence of a crosslinking between the imidazole in amorphised ZIF-8 and polymer.
Fig. 5 The evolution of the gas selectivity of the thermally treated membranes. (a) Matrimid® and (b–d) MMMs with 20, 30 and 40% ZIF-8 loading with increasing annealing temperature. (e) The gas separation performance of the membranes prepared for this work, primary commercial polymers (black diamonds) and various MOF-based MMMs from literature (green dots) plotted against the Robeson plot of 1991 and 2008. The red and blue squares represent data from MMMs with ZIF-8 and ZIF-7, respectively. A fully detailed comparison of the data in this plot together with the measurements conditions can be found in the Table S3 (ESI†). |
However, the lower selectivities of 44 in comparison to our results are probably due to the shorter treatment times (15–30 minutes) applied. In a control experiment, the separation performance of a MMM prepared from pre-amorphised ZIF-8 filler was investigated for CO2/CH4 under similar operating conditions. The amorphised ZIF-8 sample was prepared via ball-milling, as reported by Cheetham and co-workers.40 The characterisation (XRD, SEM and N2 physisorption) of the amorphous ZIF-8 prepared by ball-milling is presented in Fig. S12–S14 and Table S2 (ESI†). The reported method for ZIF-8 amorphisation by ball-milling proved to be highly reproducible. The evidence of polymer–polymer cross-linking is confirmed by ATR-FTIR, as presented in Fig. S15 (ESI†). However, the separation reveals that this MMM with 30 wt% loading failed to achieve high CO2/CH4 selectivity in comparison to the crosslinked MMMs prepared via in situ thermal oxidative treatment (Table S3, ESI†). According to SEM imaging, significant agglomerations and large particles were observed in the pre-amorphised ZIF-8 sample compared to the as-synthesized ZIF-8 (Fig. S13, ESI†). Top view and cross-sectional SEM images show that large particle agglomerations and interfacial voids were clearly visible throughout the MMM (Fig. S16, ESI†). The results explicitly explain the poor separation performance of the MMM prepared from pre-amorphised ZIF-8 filler. The results reveal that thermally induced in situ amorphisation of ZIF-8 in the polymer matrix has distinct advantages over the use of pre-amorphised ZIF-8 for the preparation of defect-free MMMs.
Fig. 5b–d shows the MMM selectivity with regard to ZIF-8 loadings (20–40 wt%) and thermal treatment temperatures (100–350 °C). The thermally treated MMMs (20 wt%) at 100–160 °C exhibit permeabilities up to 20 Barrer which is more than double the permeabilities of the unfilled Matrimid® membranes (8 Barrer). The enhanced permeability is most probably due to the introduction of porosity by the MOF, leading to faster diffusion of the gas molecules by providing easy pathways for the penetrant gases. This phenomenon is more obvious at a higher ZIF-8 loading (30–40 wt%), achieving permeabilities up to 57 Barrer. Similar to the unfilled Matrimid® membranes, MMMs treated at low temperatures (100–160 °C) do not show an improved selectivity, instead an increased in CO2 permeabilities are observed (Fig. 5b), possibly due to the interfacial defects between ZIF-8 nanoparticles and the polymer as well as the residual DMF trapped in ZIF-8. However, with the increase in thermal treatment temperature to 250 °C and 350 °C, a significant increment in selectivities is still achieved. The cross-linked MMMs loaded with 30 wt% of amorphous ZIF-8 achieved the highest selectivity of 162 without sacrificing too much permeability. The superior CO2/CH4 selectivities achieved can be explained by several reasons. (1) Crosslinked polymer networks were achieved via thermal oxidative cross-linking reactions at 350 °C which serve as a molecular sieve network for CO2 separation over CH4 in a mixed-gas condition. (2) The polymer is also cross-linked with the imidazolate linker at the edges of the amorphous ZIF-8 particles, hence eliminating defects at the polymer–filler interface. The cross-linking at the interface of amorphous ZIF-8 with the polymer perfectly sealed the grain boundary at the interface which gave a defect-free MMM, as evidenced by the HAADF-STEM imaging. (3) The improved selectivity may also be attributed by the significant rigidification of the polymer around the filler. The densification of the polymer matrix after cross-linking decreases the CH4 permeability more than the CO2 permeability, which results overall in an improved CO2/CH4 selectivity. Similar phenomena have been reported for cross-linking polymers of intrinsic microporosity (PIMs) with improved selectivity for CO2/CH4 separation.41 Increasing Tg also suggests better interactions between the amorphous ZIF-8 nanoparticles with the polymer chains, restricting motions of the polymer chains, which also adds to the benefit of plasticization resistance at high pressure (40 bar) (vide infra). (4) The abundance of unsaturated Zn2+ and imidazolate linkers, originating from the building blocks of the amorphous ZIF-8 fillers, could also further promote stronger quadrupolar interactions with CO2.42 (5) Last but not least, the embedded crystalline ZIF-8 fillers in the polymer matrix were amorphised as a result of the thermal treatment. According to the EELS imaging, amorphisation of ZIF-8 also leads to a better ordering of the MOF structure in the cross-linked polymer matrix. The amorphous ZIF-8 possessing an interpenetrated, densely packed network structure can serve as an efficient molecular sieve for CH4 molecules.43 An earlier demonstration of the capability of ZIF-8 to trap molecules in the absence of long-range structural periodicity was reported by Chapman et al.,26 with structural evidence for the retention of I2 within the pore network of amorphised ZIF-8. ZIF-7 loaded MMMs show a similar trend in the separation of CO2/CH4 confirming that the demonstrated method is generic (see Table S3 and Fig. S17, S18, ESI†). Based on the separation performance of the MMMs for CO2/CH4 separation, the results suggest that the different chemical structures of ZIF-8 and ZIF-7 have an impact on the separation properties of the MMMs. ZIF-8 is composed of tetrahedral zinc(II) and 2-methylimidazolate, whereas ZIF-7 is composed of tetrahedral zinc(II) and benzimidazolate. The freely available methyl functional groups on the ZIF-8 linkers could enhance polymer-filler crosslinking. As a result, the amorphous MMMs prepared from ZIF-8 filler gave a slightly higher selectivity than ZIF-7 loaded MMMs. Fine-tuning of the surface properties of ZIF for further improvement in CO2/CH4 gas separation performance via e.g. incorporation of accessible amine functionality has been reported.44,45 The incorporation of those mixed-linker ZIFs into the polymer membranes proved to be effective for improving the ideal selectivity for CO2/CH4 separations.45
Over the years, extensive research on improving the performance of commercially available polymer membranes for CO2/CH4 separation via impregnation with MOFs aiming to exceed the Robeson 1991 upper-bound has taken place. Commercial polymers, such as PSF, Ultem®, PPEES or Matrimid® often lead to MMMs with separation properties well below the Robeson upper-bound.46,47 A typical Robeson plot of CO2/CH4 selectivity versus CO2 permeability is presented in Fig. 5e, revealing that the reported cross-linked MMMs containing amorphous ZIF-8 have a separation performance that reached the state-of-the-art upper-bound of 2008 showing the highest selectivities reported so far for MOF based MMMs CO2/CH4 separations. More importantly, gas separation measurements conducted at 40 bar feed pressure confirmed that the implemented post-synthesis treatment induced resistance to plasticization, with selectivities of 90.8 ± 4.2, 134 ± 7.6 and 140 ± 11.5 for the Matrimid®, ZIF-8 and ZIF-7 membranes with 40 wt% loading, respectively (Table S3, ESI†), as a logical consequence of the strong polymer cross-linking.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ee01872j |
This journal is © The Royal Society of Chemistry 2017 |