Efficient CO₂ capture by metallo-supramolecular polymers as fillers to fabricate a polymeric blend membrane

Abstract

Novel fillers based on metallo-supramolecular polymers were incorporated into PEBA2533 to obtain gas separation membranes, which exhibit excellent CO₂ permeability and CO₂/N₂ selectivity.

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Carbon dioxide capture from power plant flue gas and subsequent sequestration is expected to play a key role in mitigating global climate change. Membrane separation is one of the most potential methods to remove CO₂ from flue gas,^1,2 due to the merits of low energy consumption, mechanical simplicity, ease to scale up and smaller footprint.^3,4 At present, the membranes most commonly used in gas separation are classified into polymer–organic additive systems and polymer–inorganic additive systems. The former usually contains ethylene oxide (EO) additives which possess excellent affinity with CO₂,⁵ such as PEG,⁶ Triton X-100,⁷ PEGDA,⁸ Tween⁹ and GTA.^10,11 However, these organic additives are usually low molecular weight and liquid except for high molecular weight PEG, which inevitably cause decline of mechanical strength of composite membranes with increasing additives content. Moreover, these additives may even exude from surface of membranes if their contents are overhigh. As for the high molecular weight PEG, they have a strong tendency to crystallize, causing decline of CO₂ permeability.⁸ All drawbacks mentioned above limit application of this kind of membrane in gas separation. The latter is another effective method used for CO₂/N₂ separation. Because it combines the merits of polymer membrane of easy processability and inorganic membrane of superior permeability and selectivity.¹² Accordingly, various types of fillers, such as SiO₂,¹³ metal oxide,¹⁴ graphene oxide (GO),¹⁵ zeolite,¹⁶ carbon molecular sieve,¹⁷ carbon nanotube (CNT)¹⁸ and MOF,¹⁹ have been attempted. But their performance is limited due to inevitably non-selective voids generating at the interface of polymer and inorganic particles due to the incompatibility of them.²⁰ Therefore, successful design and development of additives which combine the merits of organic and inorganic materials, is required to obtain novel composite membranes for effective CO₂ separation.

Metallo-supramolecular polymers, synthesized by coordination of metal ions with organic ditopic ligands, can be regarded as a novel type of polymer materials.²¹ This novel organic/inorganic hybrid polymer contains the natural characters of the original materials but also present novel properties because of newly created metal–ligand coordination among the reactants.²² Such polymers have shown widely potential applications including energy and information storage,^23,24 sensors,²⁵ self-healing materials,²⁶ stimuli-responsive gels,²⁷ photoluminescent devices^28,29 and electrochromic displays.^30,31 In the present work, we incorporated this novel hybrid polymer into thermoplastic copolymer PEBA2533 to prepare a new high permeable and selective membrane for CO₂ capture from N₂. Why we choose metallo-supramolecular polymer as novel additive is based on the three points. (1) To some extent, metallo-supramolecular polymer can form layers structure through self-assembly, which has been reported by Beck et al.³² and Schott et al.³³ This layer structure can increase interchain space, facilitating better diffusion of gas molecules. (2) The metal centers of metallo-supramolecular polymers have strong interaction with gas molecules,³⁴ accelerating the adsorption of gas molecules in membrane. Incorporation of metal ions can enhance free volume of membrane, also increasing diffusion of gas molecules. (3) Compared with inorganic fillers, metallo-supramolecular polymer has excellent solubility. As a result, good compatibility with polymer is considered to be achieved to effectively reduce the agglomeration and interface defect in the polymer–inorganic additive systems. On the other hand, compared with low molecular weight organic fillers, it is macromolecule, which can keep long-term operation stability of membrane. Therefore, composite membranes in which the metallo-supramolecular polymer containing EO group is added as novel additives, could effectively improve CO₂ separation performance. To the best of our knowledge, there is no report about metallo-supramolecular polymer incorporation into membranes for gas separation up to now.

In the present work, we successfully synthesized four different metallo-supramolecular polymers via metal–ligand interaction between Pluronic F127 end modified with 4′-chloro-2′,2′:6′,2′′-terpyridine and Zn²⁺, Fe³⁺, Co²⁺ and Cu²⁺ ions. These four polymers as new additives are added into thermoplastic copolymer PEBA2533 to prepare a new high permeable and selective membrane for CO₂ capture from N₂. The terpyridine-terminated F127 (F127-Tpy) was synthesized as shown in Scheme 1. The FTTR (Fig. S1†) and ¹H NMR spectra (Fig. S2†) of F127-Tpy are in agreement with the previous literatures.³⁵ Then, metallo-supramolecular polymers (F127-Tpy–M) were prepared via coordination reaction between F127-Tpy and Zn²⁺, Fe³⁺, Co²⁺ and Cu²⁺ ions. UV/vis spectra (Fig. 1) show that after the end-group functionalization of F127 with Tpy, the terpyridine absorption bands appear at 204, 240 and 278 nm. The specific metal–ligand charge transfer (MLCT) band can be found in the Fig. 1, indicating formation of F127-Tpy–M complexation. In addition, the color variations of F127-Tpy solution added with four metal ions also prove the formation of the M–terpyridine bond.

Scheme 1 Synthesis of F127-Tpy.

Fig. 1 Absorption spectra of F127-Tpy solution and its complexes with Fe³⁺, Zn²⁺, Cu²⁺ and Co²⁺ ions. Inset photo shows the color change of F127-Tpy upon the addition of Fe³⁺, Zn²⁺, Cu²⁺ and Co²⁺ ions.

From FTIR spectra (Fig. S3†) of PEBA2533/F127-Tpy–M blend membranes, it can be clearly seen that F127-Tpy–M were physically blended within the polymer matrix. Moreover, the peaks at around 2800–3000 cm⁻¹, 1740 cm⁻¹ and 1097 cm⁻¹ of the PEBA2533 appear weaker and the peaks at around 1740 cm⁻¹ and 1097 cm⁻¹ show slight redshift after the addition of F127-Tpy, suggesting that new hydrogen bond is formed after disrupting the existing interchain hydrogen bonding in PEBA2533, and that they have a good compatibility.

As expected, EDAX mapping and SEM images of the membranes in (Fig. S4 and S5†) indicate that F127-Tpy achieves excellent dispersion in the PEBA2533. The thickness of all membranes is in a range of 100–110 μm. The cross section of pure PEBA2533 membrane is smooth while the cross sections of PEBA2533/F127-Tpy–M-50 membranes are rough. Rabiee et al. hypothesised that it was the result of the decline of crystallinity of membranes. The crystallinity reduction causes more amorphous and unsymmetrical structure in membrane body.³⁶ These structures are like scaffold and physical crosslinks in membranes to keep it uniform.¹¹ Moreover, no appreciable pore could be observed, indicating that defect-free dense membrane is prepared and a good compatibility between F127-Tpy–M and PEBA2533.³⁷

To study the crystallinity of PEBA2533/F127-Tpy–M blend membranes, DSC thermograms of PEBA2533, F127 and PEBA2533/F127-Tpy–M were shown in Fig. S6.† The low temperature melting point, T_m (PTMO), is ascribed to melting of crystals of the polyether blocks and occurs about 0–20 °C. The high temperature melting point, T_m (PA), is attributed to melting of polyamide crystals and exists approximately 140–160 °C. From DSC experiment, the heat of fusion (ΔH_m) and the degree of crystallization (f_c) of PEBA2533/F127-Tpy–M membranes were obtained, which were listed in Table S1.† From Table S1,† incorporation of F127-Tpy–M decreases crystallinity PA blocks and PEO of F127, which facilitates transport of gas in the membranes. In addition, the TGA analysis (Fig. S7†) shows that the blend membranes have good thermal stability. The mechanical properties (Fig. S8†) of blend membranes show that the PEBA2533/F127-Tpy–M membranes have excellent mechanical properties as gas separation membranes.

To test the separation performance of PEBA2533/F127-Tpy–M membranes, the pure gas permeabilities and selectivities are shown in Fig. 2 and S10.† All PEBA2533/F127-Tpy–M membranes have significantly higher permeabilities for the CO₂ and CO₂/N₂ selectivities. The permeability of CO₂ first increased then decreased with the increase of F127-Tpy–M content, the maximum values were in 60 w% content of F127-Tpy–M, about P_CO2 = 386.95 Barrer (PEBA2533/F127-Tpy–Cu-60), P_CO2 = 250.11 Barrer (PEBA2533/F127-Tpy–Zn-60), P_CO2 = 382.95 Barrer (PEBA2533/F127-Tpy–Co-60), P_CO2 = 228.12 Barrer (PEBA2533/F127-Tpy–Fe-60), respectively.

Fig. 2 Pure gas CO₂, O₂, N₂ and H₂ permeabilities: (a) PEBA2533/F127-Tpy–Co membranes, (b) PEBA2533/F127-Tpy–Fe membranes, (c) PEBA2533/F127-Tpy–Cu membranes, (d) PEBA2533/F127-Tpy–Zn membranes.

Fig. 3 is hypothetical structures of our prepared membranes, based on above experimental results and three points. As shown in Fig. 3, 2,2′:6′,2′′-terpyridines of F127-Tpy can form a well-defined octahedral structure with transition metal ions,³² expanding the inner space of the polymer matrix, then promoting gas permeability. The layers structure formed through self-assembly of F127-Tpy–M also accelerates gas permeability. What's more, in term of molecular dynamics, CO₂ has a smaller kinetic diameter (0.330 nm) than N₂ (0.364 nm), leading to higher CO₂ permeability than N₂ permeability. The improved CO₂/N₂ selectivity is ascribed to the EO units which have Lewis acid–base interaction with CO₂ (ref. 9) and metal ions of F127-Tpy–M as gas adsorbing centers.³⁸ Therefore, more CO₂ molecules gather in the membranes to form high concentration gradient and thus enhance CO₂/N₂ selectivity.³⁹

Fig. 3 Schematic representation of morphology of (a) PEB2533/F127-Tpy–M, (b) PEB2533/F127-Tpy and (c) PEB2533/F127.

In order to verify our assumption, we also investigated the separation performance of PEBA2533/F127-Tpy and PEBA2533/F127 membranes where there were not metal ions. Fig. S9† shows the separation performance of PEBA2533/F127-Tpy–M-60, PEBA2533/F127-Tpy-60 and PEBA2533/F127-60 membranes. From the Fig. S11,† the CO₂ permeability and CO₂/N₂ selectivity of PEBA2533/F127-Tpy–M-60 and PEBA2533/F127-Tpy-60 blend membranes are lower than that of PEBA2533/F127-Tpy–M-60 membranes. Because of absence of metal ions, there are no gas adsorbing centers and layers structure in blend membranes. Accordingly, bulk density of blend density of blend membranes increased. As a result, gas permeability and selectivity of the membranes decreased.

The obtained results from the prepared PEBA2533/F127-Tpy–M membranes together with those previously reported⁴⁰ were listed in Robeson's upper bound graphs in Fig. 4. The anti-trade-off phenomenon in gas separation for CO₂/N₂ by the PEBA2533/F127-Tpy–M membranes were observed, which displays that the α_CO2/N2 increases with an elevation in the P_CO2 values. Specifically, at high F127-Tpy–M content, the performance of membrane ultimately surpasses the upper bound, indicating that the prepared membranes are desirable gas separation membranes for CO₂ capture and separation of CO₂ from light gases.

Fig. 4 Robeson plots for CO₂/N₂ of PEBA2533/F127-Tpy–M membranes.

In summary, we prepared novel polymer blend membranes using metallo-supramolecular polymers (F127-Tpy–M) as filler. The incorporation of F127-Tpy–M effectively improved the permeability and selectivity of CO_2, compared with the pure polymer membranes. As far as we know, this is the first example which studies the effect of metallo-supramolecular polymer on gas separation performance of the membrane. We believe that our present work will draw people's attention to metallo-supramolecular polymers for gas separation. Moreover, our strategy can be beneficial to prepare other excellent membrane materials.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (No. 51173072), the Fundamental Research Funds for the Central Universities (JUSRP51408B), and MOE & SAFEA for the 111 Project (B13025), the National Nature Science Foundation of China (Grant No. 21106053).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra12913c

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