Introduction
Target-oriented design and modification of CO2-philic MOF materials
Fig.1 Illustration of the conceptional framework from material design and separation application for CO2 capture, involving strategies towards design of CO2-philic MOFs and construction of crack-free membranes to achieve high-efficient CO2 capture. The images in the first column from top to bottom: reproduced with permission [13], copyright 2014, American Chemical Society; reproduced with permission [14], copyright 2015, Wiley-VCH; reproduced with permission [15], copyright 2015, American Chemical Society; reproduced with permission [16], copyright 2018, American Chemical Society. The images in the second column from top to bottom: reproduced with permission [17], copyright 2015, Wiley-VCH; reproduced with permission [18], copyright 2018, American Chemical Society; reproduced with permission [19], copyright 2017, Wiley-VCH; reproduced with permission [20], copyright 2018, Elsevier. |
Secondary building units
Metal-containing SBUs
Tab.1 CO2 adsorption capacity, heat and selectivity of typical MOFs |
MOFs | CO2 uptake capacity/(mmol·g–1) | Condition | Qst/(kJ·mol–1) | CO2/CH4 | CO2/N2 | Method | Ref. |
---|---|---|---|---|---|---|---|
HKUST-1 | 4.44 | 295 K, 1 bar | – | 5.5 | 20.2 | Henry’s law | [26] |
Mg-MOF-74 | 8.0 | 296 K, 1 bar | 47 (41.4)a) | – | – | – | [27,28] |
Co-MOF-74 | 7.0 | 296 K, 1 bar | 37 | – | – | – | [27] |
Zn-MOF-74 | 5.1 | 296 K, 1 bar | – | – | – | – | [27] |
Ni-MOF-74 | 5.8 | 296 K, 1 bar | 41 | – | – | – | [27] |
Ti-MOF-74 | – | – | (50.1)a) | – | – | – | [28] |
V-MOF-74 | – | – | (47.3)a) | – | – | – | [28] |
CPM-200-Fe/Mg | 5.68 | 298 K, 1 bar | 34.3 | – | 201 (50:50) (273 K) | IAST | [29] |
CPM-200-V/Mg | 3.59 | 298 K, 1 bar | 79.6 | – | 406 (50:50) (273 K) | IAST | [29] |
HHU-5 | 4.78 | 298 K, 1 bar | 25.6 | 6.2 (20:80)b) | 21.2 (20:80)b) | IAST | [39] |
IFMC-1 | 2.7 | 298 K, 1 bar | 30.7 | – | 26.9 | Capacity ratioc) | [40] |
Cu-TDPAT | 5.89 | 298 K, 1 bar | 42.2 | – | 79 (10:90)b) | IAST | [41] |
bio-MOF-11 | 4.1 | 298 K, 1 bar | 45 | – | 75 | Henry’s law | [45] |
ZIF-78 | 2.7d) | 298 K, 1 bar | 10.6 | 50 | Henry’s law | [46] | |
ZIF-81 | 2.2d) | 298 K, 1 bar | 5.7 | 24 | Henry’s law | [46] | |
ZIF-79 | 1.6d) | 298 K, 1 bar | 5.4 | 23 | Henry’s law | [46] | |
ZIF-69 | 2.2d) | 298 K, 1 bar | 5.1 | 20 | Henry’s law | [46] | |
ZIF-68 | 1.7d) | 298 K, 1 bar | 5.0 | 18 | Henry’s law | [46] | |
ZIF-82 | 2.2d) | 298 K, 1 bar | 9.6 | 35 | Henry’s law | [46] | |
ZIF-70 | 1.1d) | 298 K, 1 bar | 5.2 | 17 | Henry’s law | [46] |
a) Calculated by PBE-D2 method. b) Molar ratio of CO2 to N2 in a model mixture. c) Determined by the ratio of adsorbed amount of CO2 at 0.15 bar to N2 at 0.75 bar from isotherms. d) mmol·cm−3 as capacity unit. |
Organic linker SBUs
Fig.3 (a) Synthetic pathway for the functionalized organic linkers used in the synthesis of IRMOF-74-III. This methodology allowed us to prepare -CH3, -NH2, -CH2NHBoc and -CH2NMeBoc (5a–5d) functionalized linkers. On the right is shown a schematic representation of the IRMOF-74-III pore functionalized with the organic linkers 5a-5d and post-synthetic deprotection of Boc groups. Color code: C in gray, O in red, functional groups in purple, Mg as blue polyhedra. (b) Comparison of CO2 uptake at 25°C for IRMOF-74-III-CH3 (gray), -NH2 (green), -CH2NH2 (red), -CH2NHMe (blue), -CH2NHBoc (purple), and -H2NMeBoc (cyan). (c) Expansion of the low pressure range (<1 Torr). CO2 isotherms at 25°C for IRMOF-74-III-CH2NH2. (d) Breakthrough curves for IRMOF-74-III-CH3 under dry conditions (gray empty markers) and wet conditions (gray filled markers), and for IRMOF-74-III-CH2NH2 under dry conditions (red empty markers) and in the presence of water (red filled markers). Reproduced with permission [13], copyright 2014, American Chemical Society. |
Pore structure
Aperture/cage
Flexibility
Fig.4 (a) Illustration of pore space partition through symmetry matching regulated ligand insertion viewed along the c axis [56]. Copyright 2015, American Chemical Society. (b) Illustration of the cavity-occupying concept for tailoring the molecular sieving properties of ZIF-8 by incorporation of RTILs. The cut-off size shifts from the aperture size of six-membered ring to the reduced effective cage size by confinement of [bmim][Tf2N] in a ZIF-8’s SOD cage [14]. Copyright 2015, Wiley-VCH. |
Meso/micro-hierarchical pores
Topology
Hybridization
Cavity occupation and confinement
Surface coating
Chemical grafting
Fig.7 (a) Proposed core-shell type [HEMIM][DCA]/ZIF-8 structure; (b) TEM images of [HEMIM][DCA]/ZIF-8 composite. Region in red-box in panel (i) is magnified in panel (ii). Panels (iii) and (iv) present higher magnification images at different locations focusing on the IL shell, respectively, where numbers on images represent the corresponding IL shell thickness at that location. (c) Ideal adsorption selectivity and IAST-predicted selectivities of ZIF-8 and [HEMIM][DCA]/ZIF-8 composite at room temperature. Reproduced with permission [16], copyright 2018, American Chemical Society. |
Computational simulations of MOF membrane materials
Construction of crack-free MOF membranes
Chemical epitaxy growth of active building blocks
Atomic/molecular-level ABBs
Fig.10 (a) Schematic diagram of the preparation of Ni2(L-Asp)2P (P= bipy or pz) membranes on nickel screens. Ni, cyan; C, gray; N, blue; O, red; the H atoms are omitted for clarity. (b) Large-area top-view SEM images of compound 1 and JUC-150 membranes showed on the left. Reproduced with permission [83], copyright 2014, The Royal Society of Chemistry. |
Nanometer-level ABBs
Fig.11 (a) Schematic illustration of the preparation procedure of highly oriented Zn2(bim)4 nanosheet membranes by epitaxy growth of ZnO as nanometer-level ABBs with assistance of GO [97]. Copyright 2018, The Royal Society of Chemistry. (b) Procedure for the preparation of highly c-oriented NH2-MIL-125 (Ti) membranes by epitaxy growth of NH2-MIL-125 (Ti) as nanometer-level ABBs (red spheres: Ti4+ ions; black rods: NH2-BDC (H2BDC= terephthalic acid)) [98]. Copyright 2018, Wiley-VCH. |
Micrometer-level ABBs
Fig.13 (a) Epitaxy growth based on LDH as micrometer-level ABBs into ZIF-8 membranes [17]. Copyright 2015, Wiley-VCH. (b) Epitaxy growth based on COF-300 as micrometer-level ABBs into Zn2(bdc)2(dabco) membranes [101]. Copyright 2016, American Chemical Society. (c) Epitaxy growth based on ZnO as micrometer-level ABBs into ZIF-8 membrane under ligand-vapor treatment [102]. Copyright 2018, American Association for the Advancement of Science. |
Interfacial assembly
Fig.14 Scheme depicting the interfacial assembly method coupling with microfluidic processing for MOF membranes in hollow fibers. (a) Side view of a series of fibers mounted in designed reactor. (b) The Zn2+ ions are supplied in a 1-octanol solution (light red) flowing through the bore of the fiber, whereas the methylimidazole linkers are supplied on the outer (shell) side of the fiber in an aqueous solution (light blue). (c) Magnified view of fiber support during synthesis. In this example, the membrane forms on the inner surface of the fiber by reaction of the two precursors to form a polycrystalline ZIF-8 layer (dark blue) [110]. Copyright 2014, American Association for the Advancement of Science. |
Ultrathin 2D nanosheet assembly
Fig.17 (a) Four-layered stacking diagram of Zn2(bim)3 precursors along the c-axis. Zn, green; N, orange; C, gray; H, white; O, red. The Zn coordination polyhedra are displayed in green, the layers with benzimidazole ligands along the c-axis are depicted in purple, and the others in yellow. (b) Two-layered Zn2(bim)3 structure highlighting the AB stacking mode. (c) Single-layered nanosheet with the benzo-rings upwards, highlighting the triply-linked coordination of Zn nodes with benzimdazole ligands (H atoms are omitted for clarity). Zn, green; N, orange; C, gray. (d) Binary gas separation performance of equimolar H2/CO2 through the Zn2(bim)3 nanosheet membranes prepared at different temperatures via hot-drop coating method. Reproduced with permission [19], copyright 2017, Wiley-VCH. |
Mixed matrix integration
Fig.20 (a) Post-synthetic modification of UiO-66-NH2 with methacrylic anhydride and subsequent polymerization with butyl methacrylate (BMA) by irradiation with UV light [127]. Copyright 2015, Wiley-VCH. (b) Formation of the ZIF-8–PDMS nanohybrid composite membrane by the simultaneous spray self-assembly technique [119]. Copyright 2014, Wiley-VCH. |
Fig.21 (A) Schematic illustration of the fabrication procedure of the ZIF-8-PMPS MMMs by the ‘‘plugging-filling’’ method. (B) SEM images of (a) top and (b) cross-sectional images of stainless-steel substrate with mesh pores; (c) top images of ZIF-8 nanoparticles plugged into the substrate; (d) top and (e) cross-sectional SEM images of ZIF-8-PMPS MMMs; (f)–(i) EDXS-mappings of (d) and (e) (Zn signal: purple; Fe signal: yellow; Si signal: cyan). Reproduced with permission [132], copyright 2012, Elsevier. |