Modification of covalent organic frameworks with dual functions ionic liquids for membrane-based biogas upgrading

https://doi.org/10.1016/j.memsci.2020.117841Get rights and content

Highlights

  • Covalent organic framework (COF) modified by ionic liquid (IL) is prepared.

  • Such modification improves the CO2 solubility and reduces the pore size of COF.

  • IL@COF-300 composite is incorporated to fabricate MMMs for biogas upgrading.

  • Gas separation performance of MMMs is enhanced due to the dual functions of IL.

  • Gas separation performance of MMMs is stable over more than two months of testing.

Abstract

Development of high-performance membranes for biogas upgrading is an urgent demand for the application of membrane technology in the field of renewable energy. Covalent organic frameworks (COFs) exhibit promising potential in membrane-based separation for their highly ordered crystalline porous structure, total organic backbone and tailored functionality. However, the limited functional groups on frameworks and relatively larger pore size of existing COFs restrict further improvement in the separation efficiency especially for gas mixtures. This work reports a novel strategy for modifying the pore of COF-300 with imidazolium-based ionic liquid [bmim][Tf2N] by post-impregnation and then incorporate the composite particles IL@COF-300 into Pebax matrix to prepare mixed matrix membranes (MMMs). The IL decreases the pore size of COF-300 from 1.28 nm to 1.09 nm and increases the diffusion coefficient difference (DCO2/DCH4) between CO2 and CH4. Moreover, the presence of IL with high CO2 solubility endows the COF-300 pores with CO2-facilitating ability and thus increasing the solubility difference (SCO2/SCH4). The dual functions of IL lead to an enhanced separation performance of the resultant IL@COF-300/Pebax MMMs with an optimal permeability of 1601 Barrer and a CO2/CH4 gas selectivity of ~39, i.e. 209% and 87% higher than the pristine Pebax membrane, respectively, breaking the trade-off between permeability and selectivity and surpassing the Robeson 2008 upper-bound. The membrane also exhibits superior long-term operation stability during two months.

Introduction

Biogas is an alternative renewable and clean energy source that can alleviate the problem of depletion of fossil energy resources and environmental pollution [1]. Among the various upgrading technologies, membrane separation has attracted much attention for its relatively low energy consumption and small footprint. High-performance membrane materials are urgently demanded for efficient membrane-based biogas upgrading [2]. Covalent organic frameworks (COFs) are a kind of burgeoning crystalline material with permanent porosity, which are constructed from pure organic building blocks through strong covalent bonds by light elements [3]. COFs have drawn immense attention in recent years for gas storage and separation because of their unique properties such as ordered and adjustable pore structure, large surface area and facilely tailorable functionality [4]. Schiff-based COFs, formed by Schiff-base reaction, is a major type of COFs with outstanding thermal and chemical stability such as COF-300 and CTF-1 [[5], [6], [7]]. These merits make COFs suitable for applications under harsh conditions and excellent candidates for fabrication of highly efficient gas separation membranes. Gascon et al. introduced an azine-linked COF (ACOF-1) into Matrimid® 5218 to prepare MMMs for CO2/CH4 separation [8]. Compared to the pristine membrane, the ACOF-1/Matrimid® 5218 membrane showed a one-fold increase in CO2 permeability because of fast transport of gases through the filler pores, coupled with a slightly increased CO2/CH4 selectivity owing to the CO2-philic properties of these porous fillers. However, further improving separation performance of COFs-based MMMs is a challenge due to the limited functional groups on the frameworks and relatively larger pore size (0.6–4.8 nm) [9] than gas molecules (0.26–0.5 nm). The appropriate modification of COFs aiming towards target separation mixtures is considered as an effective strategy, which could remedy the shortages of the individual materials and generate new properties that can never be reached through the single component.

Taken both the diffusivity and solubility of gas molecules into consideration, modification of the pore walls of COFs with appropriate species or functional groups offers a promising alternative. Jiang et al. demonstrated a strategy to enhance the CO2 adsorption on COF by converting the hydroxyl groups in COF ([HO]100%-H2P-COF) into carboxylic acid groups ([HO2C]100%-H2P-COF) [10]. The pore size was reduced from 2.5 nm to 1.4 nm after conversion, and the stronger affinity of carboxylic acid groups towards CO2 led to a 5-fold dramatic increase of adsorption capacity. The [HO2C]100%-H2P-COF showed a high CO2/N2 selectivity of 77, whereas [HO]100%-H2P-COF exhibited a selectivity of only 8 at 100 kPa. Although this chemical conversion/modification is effective, it might lead to transformation of crystal structure and dissolution of parent materials during the bond breaking-forming process [11]. Therefore, modification of COFs with a simple and effective method to regulate physiochemical microenvironment of COFs is highly desired.

Ionic liquids (ILs) are organic salts which consist of cations and anions in liquid phase [12]. ILs have become promising candidates in CO2 capture and separation, owing to their unique properties such as high CO2 solubility and nonvolatility [13]. The solubility of CO2 in some imidazolium-based ILs can exceed 1 mol/mol ILs, more than three orders of magnitude higher than that in water (only 0.0007 mol/mol water) at 20 °C, 1 bar [12], due to a weak anion Lewis acid/base complexation between CO2 and IL [14]. Compared with the conventional methyldiethanolamine (MDEA) absorption technology, the IL-based CO2 capture process could reduce the total energy by 66%, including electricity and thermal energy [15]. Besides direct application as absorbent, combination of membranes and ILs are expected to play a leading role in cost-effective, energy efficient and simple CO2 separation technology. Jonathan Albo et al. [14,[16], [17], [18]] have extensively studied various ILs with different supports for CO2 separation, which exhibited high permeation and selectivity towards CO2 and good long-term stability. ILs have also been incorporated into porous materials as cavity occupants or binding agents to improve the physicochemical property and gas affinity [19]. Ban et al. [20] and Li et al. [21] reported a strategy to synchronously improve CO2 permeability and selectivity through combining ILs with MOF cages. The design of modification in fillers has been demonstrated to be an effective method in enhancing the MMMs separation performance. However, as far as we know, there is no previous report about COFs with ILs to form COF composites to enable the fabrication of high-performance MMMs.

Herein, we envisage that COFs modified with ILs will afford a superior separation performance for the CO2-based gas mixture based on the dual functions of narrowing pore size and elevating CO2 solubility. Modification of the porous COF-300 with an IL, 1-butyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl)imide ([bmim][Tf2N]), via a post-impregnation method is demonstrated. The [bmim][Tf2N] is featured by its intrinsic high CO2 solubility. The resultant IL@COF-300 composite particles are incorporated into Pebax matrix to prepare MMMs for CO2/CH4 separation from biogas, which is an important process for sustainable and energy-saving biofuel manufacturing in order to replace fossil fuels [22]. These membranes show high performance for CO2/CH4 mixture separation and a good long-term operating stability.

Section snippets

Materials

Terephthaldehyde and tetra-(4-anilyl)-methane are purchased from Changchun Sanbang Co., Ltd (China). Anhydrous dioxane, acetic acid and anhydrous tetrahydrofuran are bought from Beijing Bailingwei Co., Ltd (China). 1-butyl-3-methylimidazolum bis(trifluoromethylsulfonyl)imide ([bmim][Tf2N]) is obtained from Aladdin Industrial Co. (China). Pebax® MH 1657 consisted of polyamide 6 (PA6, 40 wt%) and polyethylene oxide (PEO, 60 wt%) is offered by Tianjin Kelaite Co., Ltd. (China). Methanol and

Modification of COF-300 with [bmim][Tf2N]

The IL@COF-300 particles are synthesized by post-impregnation method. Notably, [bmim][Tf2N] is diluted by methanol before impregnation in order to decrease their viscosity and to facilitate gases diffusion [19]. As shown in Fig. 1, the morphology of the COF-300 and IL@COF-300 particles is characterized by SEM and TEM. The COF-300 particles reveal an oblong shape and the average size of particles is in the range of 1–2 μm approximately, as confirmed by Fig. 1 (a) and Fig. 1 (b). The IL@COF-300

Conclusion

We propose a strategy of modifying COF-300 by utilizing IL with high CO2 solubility through post-impregnation to fabricate the IL@COF-300/Pebax membranes. The pore size of COF-300 decreases from 1.28 nm to 1.09 nm after IL modification, which restricts the diffusion of the larger CH4 molecules more than CO2 molecules, and thus increasing the diffusion difference between CO2 and CH4. In addition, the IL has strong interaction with CO2 on the COF pore walls and thus remarkably elevating the CO2

CRediT authorship contribution statement

Rui Zhao: Conceptualization, Methodology, Software, Investigation, Writing - original draft. Hong Wu: Supervision. Leixin Yang: Formal analysis. Yanxiong Ren: Formal analysis. Yutao Liu: Visualization. Zihan Qu: Data curation. Yingzhen Wu: Data curation. Li Cao: Validation. Zan Chen: Validation. Zhongyi Jiang: Writing - review & editing, Project administration.

Declaration of competing interest

The authors declare no conflict of interest.

Acknowledgements

Greatly acknowledge the financial support of National Natural Science Foundation of China (2187821521621004), National Key R & D Program of China (2017YFB0603400), National Key Laboratory of United Laboratory for Chemical Engineering (SKL-ChE-17B01) and State Key Laboratory of Organic–Inorganic Composites (oic-201701004).

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