Chinese Chemical Letters  2018, Vol. 29 Issue (6): 854-856   PDF    
Efficient CO2/N2 separation by mixed matrix membrane with amide functionalized porous coordination polymer filler
Qianqian Li, Jingui Duan, Wanqin Jin    
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, Nanjing 210009, China
Abstract: Mixed matrix membrane used to selective removal of CO2 was considered as an efficient solution to energy and environmental sustainability. In this study, a MMM that consists of amide functionalized porous coordination polymer filler (MIL-53-NH2) was successfully prepared, which sharply promotes the CO2/N2 selectivity from 44 (neat polymeric membrane) to 75. Remarkably, the positive effect of amide group and nanochannel of MIL-53-NH2 filler was illustrated by decreased selectivity of the MMM with formic acid modified MIL-53-NH2 filler (MIL-53-NHCOH).
Key words: Porous coordination polymer     Mixed matrix membrane     CO2/N2 separation     Amide group    

As the main greenhouse gas, CO2 is considered to be a threat in the context of global warming. The development of efficient technology for selective CO2 capture is a scientific challenge in a highest order [1]. Traditionally, the strategies of chemisorption by amide solvent and physical adsorption by porous absorbents have been widely investigated [2, 3]. However, due to the group of benefits in lower energy consumption, ease operation, environmental friendliness and mechanical simplicity, membrane process was believed as one of the most attractive strategies for continuous and efficient CO2 capture [4, 5].

Generally, membrane material played crucial roles in final separation performance [6-8]. For example, since the first report of organic polymer membrane in 2002 [9], various polymeric membranes have drawn widespread attention for gas separations [10-12]. However, the common problem of trade-off between permeability and selectivity of organic membranes limited their further development [11]. In parallel to the development of polymeric membrane, much research effort has been devoted to establish inorganic membranes [13]. Inorganic membranes with good separation performance are expensive, brittle and difficult to upscale. Thus, in order to overcome the above limitations, mixed matrix membrane (MMM) with combined advantages of good separation performance of inorganic materials and mechanical stability of organic polymers provides an alternation to go beyond the limitations of organic and inorganic membranes [14].

To date, a group of inorganic particles, such as carbon nanotubes [15], zeolites [16], silica [17] and attapulgite [17] have been employed as functional filler for MMM preparation. Indeed, each filler has its own advantages for the formation of MMM with controlled free volume or/and improved gas permeation, which are of great importance and considered to be the key factors in optimizing the performance in MMMs. In recent years, porous coordination polymer (PCP) materials, featuring well-defined pores, high crystalline and labile surface chemistry, exhibited intriguing advantages in gas storage and separation [18-23]. Therefore, the involved PCP filler with pre-designed function site will lead to bright future for rapid MMMs development and efficient CO2 selective capture.

Inspired by enhanced capability of CO2 capture by PCPs, herein, MIL-53-NH2, one of the water stable PCPs with accessible amide group and one dimensional (1D) channel, was selected as filler for MMM fabrication (Fig. 1). As expected, the MMM (15 wt% loading) showed sharply improved CO2/N2 selectivity from 44 (neat polymeric membrane) to 75. Furthermore, by changing the MIL- 53-NH2 particles to formic acid modified MIL-53-NH2 filler (MIL- 53-NHCOH), the decreased selectivity (from 75 to 65) of the MMM reflects the positive effect of amide group and assessable 1D channel in MIL-53-NH2 filler for faster CO2 permeation.

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Fig. 1. (a) View of 1D channel with rich amide groups (blue ball) in MIL-53-NH2; (b) Structure of Pebax; (c) Top and cross-sectional view of MIL-53-NH2@Pebax membrane (15 wt%).

MIL-53-NH2 was synthesized according to previous literatures [24, 25]. The crystalline structure and phase purity of as-synthesized MIL-53-NH2 were confirmed by powder X-ray diffraction (XRD) (Fig. S1 in Supporting information). Transmission electron microscopy (TEM) image showed that the average size of MIL-53- NH2 nanoparticles is around 150 nm (Fig. S2 in Supporting information). Further gas adsorption isotherms showed that MIL-53-NH2 has significant higher CO2 gas uptake (50 cm3/g) than that of N2 (7 cm3/g) at 1 bar, 298 K (Fig. S3a in Supporting information). The predicted gas selectivity by ideal adsorbed solution theory (IAST) is as high as 135 at low pressure range, similar as the result in previous report [26]. In other words, the amide groups in the channel of MIL-53-NH2 work well for selective CO2 capture (Fig. S4 in Supporting information).

Poly(vinylidene fluoride) (PVDF) ultrafiltration membrane was selected as support (average pore size: 450 nm) to prepare MMMs by a method of solution casting [27]. With varied filler loading, four MIL-53-NH2@Pebax MMMs (named as MMM-5-NH2, MMM-10- NH2, MMM-15-NH2 and MMM-20-NH2) were prepared. PXRD of them showed that the characteristic diffraction peaks are same as that of as-synthesized MIL-53-NH2, indicating good chemical stability of MIL-53-NH2 filler (Fig. 2). Evidenced by SEM images and EDX mapping, the membrane thickness is around 5 μm, while the fillers distributed uniformly (Fig. 3 and Fig. S5 in Supporting information). The results of thermal gravimetric (TG) analysis confirmed high thermal stability of these MMMs, up to 350 ℃ (Fig. S6 in Supporting information) [24, 28].

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Fig. 2. PXRD of series MMMs with varied MIL-53-NH2 loading.

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Fig. 3. Cross-sectional view of series MMMs with varied MIL-53-NH2 loading: (a) 5%, (b) 10%, (c) 15%, (d) 20%.

After activation, the MMMs were sealed into the sample cell. Then, binary gas permeation experiments were performed on each of them by method of Wicke-Kallen bach at 10 ℃. In order to avoid machine error, each measurement was repeated three times. The flow rate of binary gas feed was consistent with a volumetric flow rate of 100 mL/min (each gas of 50 mL/min). For these MMMs, CO2/N2 selectivity increases with filler loading, and reaches to 75 at the loading of 15 wt%, which is 70% improvement than that of pure Pebax membrane (44). This is because the loaded MIL-53-NH2 filler exhibits not only the enhanced affinity towards selective CO2 capture, but also the regular 1D channel for CO2 permeation. However, when the filler loading reached to 20 wt%, the gradually increased non-selective interface defect within the MMM body results in decreased gas selectivity (58) (Fig. 4a), similar as other PCP-based MMMs [29, 30]. The non-selective interface defect here means the nano-space between the fillers. Despite the uniform distribution of MOF fillers in polymer solution, such kind space cannot be fully filled by polymer chains.

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Fig. 4. (a) CO2/N2 selectivity of neat Pebax membrane and series of MIL-53-NH2 based MMMs with varied filler loading; CO2/N2 selectivity of MIL-53-NHCOH@Pebax membrane (15 wt%) was highlighted by a red star; (b) Effect of operation temperature on CO2/N2 selectivity of MMM-15-NH2.

In order to validate the positive effect of amide group and micro-channel, the formic acid modified MIL-53-NH2 filler, named as MIL-53-NHCOH, was prepared. After modification, the relative intensity of IR absorption peak at 1670 cm-1 that belongs to the bending vibration of N-H sharply decreased (Fig. S6 in Supporting information), reflecting successful condensation between carboxylate and amide group [31]. In addition, the X-Ray diffractions of (110) and (200) plane are same as that of MIL-53-NH2, while the peak of (220) plane shifted to lower angle. This means a slight channel expansion was generated after introduction of additional HCO- group (Fig. S1). The particle size (150 nm) of MIL-53-NHCOH is same as that of MIL-53-NH2 (Fig. S7 in Supporting information), while its micro-porosity was confirmed by gas adsorption experiments. As shown in Fig. S3, the CO2 uptakes at 15 kPa of MIL-53-NH2 (28 cm3/g) is over nine times higher than that of MIL- 53-NHCOH (3 cm3/g). Based on this, a new MMM (MIL-53- NHCOH@Pebax) with 15 wt% fresh filler and same thickness was prepared (Fig. S8 in Supporting information). Binary gas permeation experiments showed that CO2/N2 separation selectivity of MIL-53-NHCOH@Pebax decreased to 65, demonstrating high importance of exposed amide group and micropore for preferred CO2 permeation (Fig. 4a).

In addition, effect of operation temperature on CO2/N2 selectivity was studied on MMM-15-NH2. As shown in Fig. 4b, the CO2/N2 selectivity decreases following elevated temperature from 10 ℃ to 50 ℃, but all of them are far higher than 8 [32], a value indicates the possibility for practical separation. In addition, due to the none-porosity of Pebax polymer, CO2 permeation of MMM-15- NH2 is close to that of Pebax-based MMMs, but is lower than that of MMMs with porous polymer [33].

In summary, by incorporating amide functionalized MIL-53- NH2 filler, Pebax-based MMMs showed significantly improved CO2/N2 selectivity. In addition, the decreased gas selectivity of the MMM with MIL-53-NHCOH filler demonstrates the importance of amide group and nanochannel in MIL-53-NH2 filler for improved CO2 permeation.

Acknowledgments

We thank the financial support of the National Natural Science Foundation of China (No. 21671102), Natural Science Foundation of Jiangsu Province (No. BK20161538), Innovative Research Team Program by the Ministry of Education of China (No. IRT17R54), Six Talent Peaks Project in Jiangsu Province (No. JY-030) and State Key Laboratory of Materials-Oriented Chemical Engineering (No. ZK201406).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.cclet.2017.11.008.

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