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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 24618−24626
Constructing Unique Cross-Sectional Structured Mixed Matrix Membranes by Incorporating Ultrathin Microporous Nanosheets for Efficient CO2 Separation Xueqin Li, Jinpeng Hou, Ruili Guo,* Zhongming Wang, and Jianshu Zhang School of Chemistry and Chemical Engineering/Key Laboratory for Green Process of Chemical Engineering of Xinjiang Bingtuan, Shihezi University, Shihezi, Xinjiang 832003, China Downloaded via BUFFALO STATE on July 21, 2019 at 10:00:41 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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ABSTRACT: Ultrathin microporous nanosheets denoted as Zn-tetra-(4carboxyphenyl)porphyrin (Zn-TCPP) were synthesized and incorporated into a Pebax MH 1657 (Pebax) polymer to fabricate mixed matrix membranes (MMMs) for efficient CO2 separation. The Zn-TCPP nanosheets with a microporous structure provide high-speed channels for fast CO2 transport and shorten the diffusion pathways, both contributing toward high CO2 permeability. Furthermore, scanning electron microscopy results indicate that the ultrathin Zn-TCPP nanosheets with an ultrahigh aspect ratio (>200) tend to arrange horizontally in the Pebax matrix. The obtained unique cross-sectional structure of the MMMs functions as a selective barrier, allowing repeated discrimination of gases due to the tortuous interlayer of horizontal nanosheets, thus improving the selectivity of the MMMs. In addition, the horizontally arranged microporous nanosheets were found to strongly interact with the membrane matrix and endowed the MMMs with excellent interfacial compatibility, which improved the CO2 permeability and eliminated unselective permeation pathways. Significantly, the optimized CO2 separation performance of the MMMs surpassed the 2008 Robeson’s limit. KEYWORDS: Pebax, nanosheet, mixed matrix membrane, channel, CO2 separation
1. INTRODUCTION Currently, natural gas (mainly CH4) has become one of the most important clean energy gases. However, newly exploited natural gas is made up of impurity components, mainly acidic CO2 molecules, which affect its combustion heat value.1,2 Therefore, there is an urgent need to purify natural gas via the removal of CO2. At present, the membrane-based separation technology has become one of the most competitive CO2 separation technologies because of its relatively high efficiency and cost-effectiveness.3−6 However, the lack of new type of high-performance membranes (CO2 permeability >100 Barrer and CO2/N2 selectivity >70) limits their industrial application.7−11 Mixed matrix membranes (MMMs) have been deemed as promising membrane materials for overcoming the trade-off between gas permeability and selectivity.12−15 MMMs are fabricated by introducing a dispersed filler phase into a continuous polymer phase. The combination of the advantages of the designed filler and polymer material is considered to be a potential strategy for surpassing Robeson’s limit, achieving simultaneous high permeability and selectivity.16−22 Recent research studies have focused on the rational design of advanced two-dimensional (2D) fillers to achieve high permeability and selectivity of MMMs.23−26 The 2D fillers that have been embedded in MMMs include graphene oxide, porous carbon nanosheets, metal−organic framework (MOF) nanosheets, and so on.27−33 Among them, MOF nanosheets have attracted the interest of researchers because of the © 2019 American Chemical Society
advantages of their physical and chemical structures, which include nanosized ultrathin thickness, microporous apertures, regular in-plane arrays, abundant adsorption sites, and so on.34,35 MOF nanosheets are recognized as alternative fillers and promising candidates for fabricating MOF nanosheetbased MMMs to further improve their performance for CO2 separation. A recent literature work has reported advances in MOF nanosheet-based MMMs for CO2 separation. Yang et al.36 used 6FDA-DAM polyimide or PIM-1 as a polymer matrix paired with ns-CuBDC nanosheets (thickness: ∼40 nm, pore diameters: ∼0.52 nm, and aspect ratios: >25) to fabricate MMMs. In addition, the CO2/CH4 selectivity of MMMs could be improved by introducing ns-CuBDC, but the CO 2 permeability decreased upon an increase in the content of nanosheets. The as-prepared MMMs suffered from a trade-off effect. In addition, the diffusivity of CO2 and CH4 decreased upon the addition of ns-CuBDC with a thickness of ∼40 nm. Thus, it can be speculated that the decreased permeability resulted from the introduction of thick nanosheets in the MMMs. In addition, Rodenas et al.37 investigated MMMs doped with ns-CuBDC nanosheets (thickness: 5−25 nm, pore diameters: 20), which increased the CO2 permeability without any significant adverse effects on Received: May 5, 2019 Accepted: June 20, 2019 Published: June 20, 2019 24618
DOI: 10.1021/acsami.9b07815 ACS Appl. Mater. Interfaces 2019, 11, 24618−24626
Research Article
ACS Applied Materials & Interfaces the CO2/CH4 selectivity. This indicated that thin MOF nanosheets and suitable pore diameters are beneficial for improving the CO2 permeability, making them potential candidates as attractive nanofillers to overcome the trade-off in the properties of MMMs. On the basis of the abovementioned problems, ultrathin MOF nanosheets (Zn-TCPP nanosheets) were synthesized. Compared with other 2D nanosheets, Zn-TCPP nanosheets have the characteristics of a thickness of
