UiO-66-polyether block amide mixed matrix membranes for CO2 separation
Graphical abstract
Introduction
The excessive emission of greenhouse gases (especially CO2) from fossil fuel combustion and other mankind economic and social activities has been causing serious environmental problems. Global attention has been focused on efficient CO2 capture and storage (CCS) from a huge amount of energy-intensive industrial gases, including natural gas, biogas, flue gas and so on [1], [2], [3], [4]. Compared with traditional technologies such as absorption, cryogenic purification and adsorption, membrane based separation has been considered to be a prospective alternative, with potentially high efficiency, low energy consumption, ease of scale-up and environmental friendliness. Polymeric materials are the most widely used membrane separation materials. Many classes of polymers were applied for CO2 separation, such as polyamides [5], polyimides [6], polycarbonates [7], polysilicone [8], poly(ethylene oxides) [9], etc. However, these polymeric membranes often suffer from the trade-off between permeability and selectivity [10], [11], which significantly restrict the separation performance. Many efforts have been taken to further improve the permeability and selectivity by developing next-generation membranes through tuning cavity size of membrane materials and chemical affinity towards guest molecules [12].
Metal–organic frameworks (MOFs) are a large emerging class of hybrid materials with porous crystalline structures that combine the connectivity of metal centers with the bridging ability of organic ligands. Judicious choice of metal and linker allows new structures to be designed and synthesized with the desired functionalities, pore sizes and pore shapes [13] for a vast range of potential applications such as gas storage [14], catalysis [15], sensing [16] and drug delivery [17]. Particularly, with the unique properties of tunable pore size, large surface areas and specific adsorption affinities, MOFs can be developed to be potential membrane materials for molecular separation [18], [19], [20]. It has been reported that pure MOFs membranes can show excellent gas separation performances [21], [22], [23], [24], [25], [26], [27], [28]. However, pure MOFs membranes are often subjected to the difficulty of being satisfactory for industrial-scale applications. Defects in the membranes such as cracks and inter-crystal gaps can cause non-selective permeation of gases. Another challenge is the large-area fabrication of pure MOFs membranes for being applied to industrial-scale gas separation processes. Mixed matrix membrane is a promising alternative route to develop applicable membranes, which combines the high separation performance of the incorporated fillers with the good processability and mechanical stability of polymers [29], [30], [31], [32], [33], [34], [35], [36]. Incorporating CO2-selective porous frameworks into polymer membranes has been proven to be an effective strategy to develop MOFs-based mixed matrix membranes owing to the wide range of application such as CO2 capture, natural gas treatment, biogas separation and hydrogen purification [1], [37], [38]. ZIF-90 with pore size of about 3.5 Å was added into polymers to improve the membranes CO2 permeability [31]. Also, Song et al. [39] fabricated nanocomposite membranes by incorporating ZIF-8 (with pore size of 3.4 Å) nanoparticles into Matrimid®, showing enhanced CO2 permeability and CO2/CH4 selectivity even at high ZIF-8 loading. These works focused on incorporating MOFs with small pore size to tuning the molecular sieving characteristic of the membranes. In addition, CO2-philic frameworks were also selected to prepare mixed matrix membranes because of the strong interaction between the functionalized groups of MOFs and CO2 molecules. Long et al. [40] chose the synthesized Mg2(dobdc) as the fillers, which exhibited excellent CO2 adsorption capacity. Hence, the CO2/N2 selectivity of Mg2(dobdc)-polymer mixed matrix membranes was improved obviously compared with those of pure polymer membranes. Recently, Zn(pyrz)2(SiF6) metal-organic framework with pore size of 3.8 Å was demonstrated to show strong affinity to CO2 molecules [41]. After introducing into polyethylene oxide matrix, the mixed matrix membranes showed improved CO2 permeability as well as CO2/N2 and CO2/CH4 selectivity [42]. It can be found that CO2-philic metal-organic frameworks with appropriate pore size are promising for developing highly efficient CO2 separation membranes.
A new class of Zirconium-based porous MOF UiO-66 (UiO: University of Oslo), has attracted great interest recently [43]. In its face-centered-cubic crystal structure, each zirconium metal center is connected to 12 benzene-1,4-dicarboxylate (BDC) linkers to form the a highly connected framework, which is believed to be the main reason for the high structural stabilities. It has centric octahedral cages, which is linked with eight corner tetrahedral cages by means of triangular windows with the size of 6 Å or so. Many studies discovered that UiO-66 can exhibit strong affinity to CO2 molecules owing to the -OH groups coordinated to Zr cluster [44], [45], [46], [47]. A series of functionalized UiO-66 frameworks can be conveniently synthesized by simply incorporating different functional groups, as to finely tailor the pore structure and chemical properties [45], [46]. MMMs with Ti exchanged UiO-66 and polymers of intrinsic microporosity (PIMs) [48] have been proven to exhibit high CO2 permeability of 13540 Barrer for the separation of CO2/N2 mixture. Also, functionalized UiO-66 and polyimide (PI) MMMs showed enhanced CO2/CH4 separation performances [49], [50]. Amine-functionalization is an effective strategy for enhancing the CO2 affinity of MOFs crystals [49], [51], [52], [53]. As a result, UiO-66-NH2 was reported to show stronger CO2 adsorption capacity than that of pristine UiO-66 [46], which offers great potential for fabricating CO2-separation mixed matrix membranes. Su et al. [54] fabricated UiO-66-NH2/Polysulfone (PSF) membrane with CO2/N2 ideal selectivity of 26 and dramatically increased CO2 permeability of 46 Barrer. Moreover, phenyl acetyl functionalized UiO-66-NH2 particles were synthesized and incorporated into Matrimid® polymer [55]. The membrane showed CO2 permeability of about 37 Barrer (increased by 200%) and CO2/N2 ideal selectivity of about 30.
In this work, we report on the design and fabrication of UiO-66 based metal-organic frameworks mixed matrix membranes for CO2 capture application. Nano-sized UiO-66 crystals were synthesized and functionalized by amine-groups, which is aimed at maximizing the interaction between UiO-66 frameworks and polymer chains as well as improving the CO2 affinity of membranes. We chose polyether block amide (PEBA) as the polymer material. It is composed of ethylene oxide and amide groups in their soft and hard segments, respectively, which provide high CO2 permeability and mechanical strength at the same time [2]. Thereby, for the first time, we fabricated UiO-66 (NH2)-PEBA mixed matrix membranes as shown in Fig. 1. The membranes microstructures as well as MOF-polymer interaction were systematically characterized and studied. Compared with previous works of UiO-66 based mixed matrix membranes [49], [53], [54], [55], [56], our UiO-66 (NH2)-PEBA membranes showed more selective transport of CO2 molecules and simultaneous enhancement of CO2 permeability and CO2/N2 selectivity than those of pure PEBA membrane. The as-prepared membrane also showed excellent long term stability in humidified state, which shows promising potential for CO2 capture application.
Section snippets
Materials
For the synthesis of UiO-66 MOFs, zirconium (IV) chloride (ZrCl4) powder was supplied by Alfa Aesar, Terephthalic acid and 2-Aminoterephthalic acid were provided by Sigma-Aldrich, N,N-Dimethylformamide (DMF) was purchased by Sinopharm Chemical Reagent Co., Ltd, formic acid was received from Xilong Chemical Co., Ltd., ethanol was obtained by Wuxi City Yasheng Chemical Co., Ltd., PEBAX MH 1657 was purchased from Arkema, France. N2 and CO2 with purity 99.999% was supplied by Nanjing Special Gases
Physicochemical properties of UiO-66 MOFs
Scanning electron microscope (SEM) showed that the synthesized UiO-66 and UiO-66-NH2 both crystallized small octahedrally cubic nanocrystals with size in the range of 60–80 nm (Fig. 2). These nano-sized nanoparticles are considered to be excellent fillers to prepare thin and homogenous mixed matrix membranes [58]. EDXS was also carried out to analyze the elemental composition of synthesized UiO-66 MOFs samples. It can be seen that UiO-66 is composed of Zr, C and O elements (Fig. 2(b)), which is
Conclusion
In summary, we report the design and fabrication of UiO-66 based metal-organic frameworks-PEBA mixed matrix membranes. Compared with UiO-66, UiO-66-NH2 showed enhanced MOF-polymer interactions, which led to excellent dispersibility in polymer matrix. Moreover, stronger affinity with CO2 molecules was realized after functionalized by amine groups. The gas permeation behaviors of the as-prepared membranes were also investigated. The results showed that both of the UiO-66-PEBA and UiO-66-NH2-PEBA
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (Grant nos. 21476107, 21490585, 21406107), the Innovative Research Team Program by the Ministry of Education of China (Grant no. IRT13070), a Project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the Innovation Project of Graduate Research by Jiangsu Province (Grant no. KYLX_0764).
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