Selective Gas Permeation in Mixed Matrix Membranes Accelerated by

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Selective Gas Permeation in Mixed Matrix Membranes Accelerated by Hollow Ionic Covalent Organic Polymers Youdong Cheng, Linzhi Zhai, Minman Tong, Tanay Kundu, Guoliang Liu, Yunpan Ying, Jinqiao Dong, Yuxiang Wang, and Dan Zhao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05333 • Publication Date (Web): 13 Dec 2018 Downloaded from http://pubs.acs.org on December 16, 2018

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ACS Sustainable Chemistry & Engineering

Selective Gas Permeation in Mixed Matrix Membranes Accelerated by Hollow Ionic Covalent Organic Polymers Youdong Cheng,†,# Linzhi Zhai,†,‡,# Minman Tong,§ Tanay Kundu,† Guoliang Liu,† Yunpan Ying,† Jinqiao Dong,† Yuxiang Wang† and Dan Zhao*,†

†Department

of Chemical and Biomolecular Engineering, National University of

Singapore, 4 Engineering Drive 4, 117585, Singapore ‡School

of Environmental and Chemical Engineering, Jiangsu University of Science and

Technology, No. 2 of Mengxi Road, Zhenjiang 212003, China §School

of Chemistry and Materials Science, Jiangsu Normal University, No. 101 of

Shanghai Road, Xuzhou 221116, China

Corresponding Author

ACS Paragon Plus Environment

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*E-mail: [email protected]

ABSTRACT: Mixed matrix membranes (MMMs) have gained great attention for the efficient CO2 removal from raw nature gas or biogas (CO2/CH4 separation) and flue gas (CO2/N2 separation). Nevertheless, the development of high-performance MMMs for industrial applications is largely limited by the lack of suitable porous fillers. Herein, a novel ionic covalent organic polymer (ICOP-1) consisting of gas selective pores and hollow cavities is facilely fabricated using a metal triflate catalysed condensation reaction. Considering its unique structural properties, ICOP-1 is explored as a novel filler to enhance the gas separation properties of polysulfone (PSf) membranes. Defect-free MMMs are successfully prepared owing to the high polymer-filler affinity originating from the organic nature of these two phases. Besides, the large cavities and size-selective pores of ICOP-1 lead to a simultaneous increase in membrane CO2 permeability and CO2/CH4, CO2/N2 selectivities. With the addition of only 0.5 wt% of ICOP-1 fillers, the asprepared MMM demonstrates the optimal gas separation performance with a CO2/CH4


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selectivity of 39.7 (at a CO2 permeability of 6.19 Barrer) and a CO2/N2 selectivity of 36.7 (at a CO2 permeability of 6.85 Barrer), opening new opportunities in membrane-based industrial CO2 capture applications.

KEYWORDS: CO2 capture, mixed matrix membrane, polysulfone, hollow structure, ionic covalent organic filler

INTRODUCTION The urgent demand of mitigating global warming by curtailing greenhouse gas emissions prompts the advancement of separation technologies for CO2 removal from natural gas or biogas (CO2/CH4 separation) and flue gas (CO2/N2 separation). Membrane technology has been considered as a possible alternative to conventional technologies, such as amine scrubbing and pressure swing adsorption, owing to its good energy efficiency, low capital cost, environmental friendliness, simple and continuous operation.14

Despite the recent exciting advancements in designing novel membrane materials with

impressive gas separation performance,5-7 polymers still play the most important role in


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the current gas separation market mainly due to their good processability and strong mechanical strength that set the foundation of their industrial deployment. However, the gas separation performance of pure polymeric membranes is largely restricted by the Robeson upper bound limits, which depict that highly selective membranes are less permeable and vice versa.8,9 Numerous strategies have been reported to promote the separation performance of pure polymeric membranes, including polymer blending,10 membrane structure control,11 novel polymer design,12 thermal treatment,13 chemical cross-linking,14 reduction of membrane thickness,15 and fabrication of mixed matrix membranes (MMMs).16 Among them, the fabrication of MMMs is a simple and straightforward strategy that involves the addition of highly permeable fillers with gas selective capabilities into polymer matrixes, leading to advanced composite membranes with both good processability and superior separation performance. Over the past decades, a broad range of porous materials, such as zeolites,17 mesoporous silica,18 metal–organic frameworks (MOFs)19 and covalent organic frameworks (COFs),20 have been examined as potential fillers in improving the membrane performance. However, several challenges still exist, limiting the development of


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membranes suitable for industrial applications. The first challenge is the poor polymerfiller compatibility originating from the differences in physiochemical properties of the two phases, especially for inorganic filler-based MMMs. Such a compatibility issue may easily result in filler agglomeration and nonselective voids at the interfaces that deteriorate the membrane selectivity. The second challenge is the lack of delicate design on fillers with gas selective pores. For example, most COFs reported in the literature possess pores larger than 8 Å, therefore size-selective gas separation process is yet to be achieved in COF-based MMMs.20-22 Finally, it is crucial to selectively orientate the pores of fillers in MMMs, especially these one-dimensional (1D) pore channels, parallel to the gas concentration gradient to maximize the efficacy of dispersed fillers for membrane performance enhancement. Applying external forces, such as shear force23 and centrifugal force,19 has been demonstrated to be successful in selectively orientating these pores in MMMs, while the practicability of these methods is severely restricted by their operational complexity. Bearing these challenges in mind, herein, we propose to design and prepare a hollow ionic covalent organic polymer (denoted as ICOP-1) with size-discriminative pores that allow fast and selective transport of CO2 over CH4 and N2


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in MMMs for efficient CO2 separation (Scheme 1). It is worth mentioning that a large number of porous fillers are prepared as solid particles and sometimes their preparation process can be energy-intensive (e.g., heating) and time-consuming (e.g., several days).16,22,24 However, hollow ICOP-1 cylinders can be easily produced after 8 h stirring at room temperature, which ensures their large-scale production for industrial applications. The pure organic nature of ICOP-1 resembles that of the polymer matrix, paving the way for realizing good polymer-filler compatibility and eliminating interfacial voids. Moreover, the hollow cylinder structure of ICOP-1 not only greatly increases the membrane fractional free volume (FFV), but also allows the CO2 permeation through its selective pores regardless of its dispersion states. Notably, with the incorporation of only 0.5 wt% of ICOP-1 into a commercially available polysulfone (PSf) matrix, the resultant MMM exhibits a simultaneous increase in CO2 permeability (ca. 30%) and CO2/CH4 or CO2/N2 selectivities (ca. 50%), greatly advancing the application of membrane technology for industrial CO2 separation.


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Scheme 1. Synthesis of the Hollow Ionic Covalent Organic Polymer (ICOP-1) and Fabrication of ICOP-1/PSf MMMs for Gas Separations.

EXPERIMENTAL SECTION Materials. Triaminoguanidinium chloride was prepared from guanidine hydrochloride and hydrazine hydrate based on the literature procedure.25 All other reagents were purchased from commercial suppliers and used without further purification. Scandium(III) triflate [Sc(OTf)3, 99%], PSf (average Mw ca. 35,000 and average Mn ca. 16,000), hydrazine hydrate (reagent grade), 1,3,5-triformylbenzene (97%), ethanol (99.5%), tetrahydrofuran (THF, 99.9%) and chloroform (99%) were purchased from Sigma-Aldrich.


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1,4-Dioxane (99%), guanidine hydrochloride (98%) and benzaldehyde (98%) and were obtained from Tokyo Chemical Industry Co., Ltd. Synthesis of ICOP-1. Hollow ICOP-1 was synthesized via Schiff base condensation between 1,3,5-triformylbenzene and triaminoguanidinium chloride at room temperature. Specifically, 1,3,5-triformylbenzene (32 mg, 0.2 mmol) was dissolved in THF (5 mL), which was added into a THF/water mixture (1 mL : 2 mL) containing triaminoguanidinium chloride (28 mg, 0.2 mmol) and Sc(OTf)3 (2 mg, 4 µmol) under continuous stirring. The resultant suspension was continuously stirred at 500 rpm for 8 h. The final product was recovered by centrifuge, washed thoroughly with fresh THF and water, and dried at 120 ˚C under vacuum overnight to obtain ca. 80% isolated yield. The control experiment was performed using the same procedure without the Sc(OTf)3 catalyst. Synthesis of the model compound BdTGCl. Triaminoguanidinium chloride (1.17 g, 8.5 mmol) was sonicated and dissolved in the mixture of water (30 mL) and ethanol (50 mL). A solution of benzaldehyde (2.7 g, 25.5 mmol) in ethanol (10 mL) was slowly added to it. The resultant mixture was refluxed overnight and naturally cooled down to room


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temperature. The product was collected by centrifuge and crystallized by dissolving in hot ethanol. Crystallography of the model compound BdTGCl. The model compound BdTGCl was dissolved in a minimum amount of hot ethanol by heating and kept aside in an open glass vial. The crystals were harvested after 2 days, collected by filtration and subjected to single crystal X-ray diffraction measurements. The as-synthesized crystal of model compound was taken out from the mother solution, coated with paratone-N, placed on top of a nylon cryoloop (Hampton research) and then mounted in the Bruker SMART APEX single crystal X-ray diffractometer. The data collection was performed at 100 K. The data can be retrieved from the CCDC website using the number 1853076. The structure was solved in the trigonal P 31 1 2 space group, with Z = 6, using direct method. All non-hydrogen atoms were refined anisotropically, except the solvent carbon atoms, which were refined isotropically. Very high displacement parameters, high esd’s and partial occupancy due to the disorder make it impossible to determine the possible position of hydrogen in solvent molecules. The ellipsoids in ORTEP diagram is displayed at the 50% probability level.


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Preparation of membranes. PSf powder was heated at 100 °C under vacuum for 12 h to remove the adsorbed water and then completely dissolved in THF under continuous stirring to get an 8 wt% homogenous solution. A predetermined amount of filler (0.2, 0.5, 1 and 3 wt%) was dispersed in THF under sonication for 5 min to get the filler suspension. Next, the filler suspension was mixed with the polymer solution, followed by a continuous stirring for 12 h. The resultant suspension was then sonicated for 5 min and transferred into a glassy petri dish, slowly evaporated at room temperature for two days. The membranes were dried at 120 °C under vacuum for 12 h. Pure PSf membrane was fabricated by the same method without filler addition. The thicknesses of all membranes were 60–80 µm determined by the digital micrometer. Characterizations. Membrane cross-sectional morphology was investigated with a field emission scanning electron microscope (FESEM, Quanta 600). Samples were coated with Pt before observation. The crystallinity of ICOP-1 and all membranes were measured using an X-ray diffractometer (Rigaku MiniFlex) with a Cu X-ray source (λ = 1.54178 Å). The size as well as morphology of ICOP-1 were investigated by a transmission electron microscope (TEM, JEOL-JEM 2010F). Atomic force microscopy (AFM, Bruker Dimension


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Icon) was conducted to probe the morphology of ICOP-1. Thermogravimetric analysis (TGA) was performed in the range of 25–800 °C on a Shimadzu DTG-60AH at a ramp rate of 10 °C min−1. Differential scanning calorimetry (DSC) results were collected with Perkin Elmer DSC 8000 in the range of 60–300 °C. X-ray photoelectron spectroscopy (XPS, Kratos AXIS Ultra DLD) tests were performed to detect the elements inside ICOP1. The attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra were performed by the FTS-3500 ARX spectrometer. The sorption isotherms of CO2, CH4 and N2 of the sample were recorded with the Micromeritics ASAP 2020. The sample (∼50 mg) was activated at 120 °C under low pressure (

Selective Gas Permeation in Mixed Matrix Membranes Accelerated by

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