Hierarchical Metal–Organic Frameworks with Macroporosity: Synthesis, Achievements, and Challenges

Abstract

HIGHLIGHTS• The advantages of macroporous metal-organic frameworks (MOFs) in comparison with micro-and mesoporous MOFs are discussed.• A range of synthetic methods for the fabrication and characterisation of hierarchical MOFs with macroporosity are reviewed.• The applications, advancements, and challenges of each method are compared and assessed in detail.ABSTRACT Introduction of multiple pore size regimes into metalorganic frameworks (MOFs) to form hierarchical porous structures can lead to improved performance of the material in various applications.In many cases, where interactions with bulky molecules are involved, enlarging the pore size of typically microporous MOF adsorbents or MOF catalysts is crucial for enhancing both mass transfer and molecular accessibility. In this review, we examine the range of synthetic strategies which have been reported thus far to prepare hierarchical MOFs or MOF composites with added macroporosity. These fabrication techniques can be either pre-or post-synthetic and include using hard or soft structural template agents, defect formation, routes involving supercritical CO 2 , and 3D printing. We also discuss potential applications and some of the challenges involved with current techniques, which must be addressed if any of these approaches are to be taken forward for industrial applications.

Using Supercritical CO2 in the Preparation of Metal-Organic Frameworks: Investigating Effects on Crystallisation

“…Due to constraints on CO 2 pressure, MOF synthesis at low temperature was preferred. In this study, 40 • C was chosen as the synthesis temperature for Zn-MOF-74, Zn-BTC, [Cu 24 (OH-mBDC) 24…”

“…The blue solid was separated by filtration, repeatedly washed with methanol, yielding 1.9 g of solid after drying at room temperature. 24 ] n MOP was followed by the procedure reported for the synthesis of copper-based cuboctahedron metal-organic polyhedra by Lee et al [65]. The synthesis of this MOF is illustrated in Scheme 2.…”

“…Due to the high interest in these materials, various synthetic methods have been developed beyond the conventional solvothermal approaches which, though a straightforward way to achieve highly crystalline materials, have the potential to use large amounts of organic solvents. In addition, there has been a recent trend towards the creation of multiple porosities (so-called hierarchical pore structures) in these materials [23,24], leading to the development of a number of innovative synthetic strategies including macrostructural templating [25][26][27][28][29], gelation [30][31][32][33], acid etching [34][35][36][37], use of scCO 2 [38][39][40][41][42], and three-dimensional (3D) printing [43][44][45][46].…”

“…The scCO 2 synthetic method which was successfully employed for HKUST-1 [39,47] and Znbenzene-1,3,5-tricarboxylate (BTC) (in DMF) [38] has here been extended to the Zn-MOF-74, UiO-66 and Zn-BTC (in DMSO) MOF systems, [Cu 24 (OH-mBDC) 24 ] n MOPs, as well as to two new flexible MOF materials {[Zn 2 (L 1 )(DPE)]•4H 2 O} n and {[Zn 3 (L 1 ) 3 (4,4 -azopy)]•7.5H 2 O} n . The first four MOF/MOP materials were examined to see the effect of scCO 2 on the structure and yield of the products.…”

“…Zn-BTC was chosen as an example of a HKUST-1 MOF analogue that could not be synthesised using the mixed DMSO and MeOH solvent, which was reported previously as a crucial factor to obtain additional macropores from expanded solvent systems [53], even though this MOF was shown to be a successful candidate for scCO 2 templating [38]. [Cu 24 (OH-mBDC) 24 ] n (consisting of metal-organic polyhedral nanocages) was self-assembled from the coordination between copper ions and the angular bifunctional ligand benzene-1,3-dicarboxylate and N,N'-dimethylacetamide, forming a discrete structure [54]. Zn-MOF-74 was also synthesised with scCO 2 in the same manner as reported in the literature [47], in order to investigate the behaviour of CO 2 towards different solvent systems.…”