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.

Postsynthetic modification (PSM) of amino-functionalized metal-organic frameworks (MOFs) to those bearing pendant β-amidoketone arms using diketene is herein reported. Three unique MOF families demonstrate the scope of this transformation, which both is atom-economical and yields high conversions. In each case, the crystallinity was retained, and instances of exceptional solid-state ordering were observed in the PSM products, which has allowed detailed crystallographic characterization in multiple instances.

UiO-66-NH2 reacts with acrylonitrile, acrylic acid, methyl acrylate and methyl vinyl ketone leading to post-synthetic modification of the MOF through C–N bond formation. The acrylonitrile-modified MOF undergoes further reaction to form a tetrazolate-modified MOF.

The Mannich reaction of the zirconium MOF [Zr6O4(OH)4(bdc‐NH2)6] (UiO‐66‐NH2, bdc‐NH2=2‐amino‐1,4‐benzenedicarboxylate) with paraformaldehyde and pyrazole, imidazole or 2‐mercaptoimidazole led to post‐synthetic modification (PSM) through C−N bond formation. The reaction with imidazole (Him) goes to completion whereas those with pyrazole (Hpyz) and 2‐mercaptoimidazole (HimSH) give up to 41 and 36 % conversion, respectively. The BET surface areas for the Mannich products are reduced from that of UiO‐66‐NH2, but the compounds show enhanced selectivity for adsorption of CO2 over N2 at 273 K. The thiol‐containing MOFs adsorb mercury(II) ions from aqueous solution, removing up to 99 %. The Mannich reaction with pyrazole succeeds on [Zn4O(bdc‐NH2)3] (IRMOF‐3), but a similar reaction on [Zn2(bdc‐NH2)2(dabco)] (dabco=1,4‐diazabicyclo[2.2.2]octane) gave [Zn3(bdc‐NH2)1.32(bdc‐NHCH2pyz)1.68(dabco)]⋅2 C7H8 5, whereas the reaction with imidazole gave the expected PSM product. Compound 5 forms via a dissolution–recrystallisation process that is triggered by the “free” pyrazolate nitrogen atom competing with dabco for coordination to the zinc(II) centre. In contrast, the “free” nitrogen atom on the imidazolate is too far away to compete in this way. Mannich reactions on [In(OH)(bdc‐NH2)] (MIL‐68(In)‐NH2) stop after the first step, and the product was identified as [In(OH)(bdc‐NH2)0.41(bdc‐NHCH2OCH3)0.30(bdc‐N=CH2)0.29], with addition of the heterocycle prevented by steric interactions.

Zn 4 O(bdc-NH 2 ) 3 ], IRMOF-3 (bdc-NH 2 = 2-amino-1,4-benzenedicarboxylate), and [Cr 3 O(H 2 O) 2 F(bdc-NH 2 ) 3 ], MIL-101(Cr)-NH 2 , undergo tandem post-synthetic modification reactions with aldehydes and NaCNBH 3 to form secondary amine-functionalised metal-organic frameworks (MOFs). The degree of conversion ranges from 17-74% for IRMOF-3 (2a-i) and from 35-51% for MIL-101(Cr)-NH 2 (4a-d), and alkene, sulfide, ferrocenyl and pyridyl substituents can be successfully introduced into the zinc MOFs. For both the zinc and chromium MOFs, an increase in the steric bulk of the aldehyde leads to a reduction in the degree of conversion. Low conversion with bulky aldehydes can be exploited to generate products containing two different secondary amine substituents, such as [Zn 4 O(bdc-NH 2 ) 1.93 (bdc-NHCH 2 CH 2 CH 2 SMe) 0.47 (bdc-NHEt) 0.59 ] 2k in which sequential tandem modifications have taken place. N 2 adsorption experiments reveal that the post-synthetically modified MOFs display lower than anticipated BET surface areas together with hysteresis. This is consistent with some degradation of crystallinity occuring on treatment with NaCNBH 3 , as verified by control experiments, which leads to the formation of mesopores. Reactions of H 2 bdc-NH 2 with aldehydes and NaCNBH 3 afforded a series of secondary amine functionalised dicarboxylic acids H 2 L 1-8 after acid work up. These acids were reacted with Zn(NO 3 ) 2 ·6H 2 O to form the secondary aminefunctionalised MOFs 5a-h through direct synthesis. 1 H NMR analysis of 5a-g showed the presence of some bdc-NH 2 linkers in addition to the secondary amine-containing linkers, and indeed the presence of H 2 bdc-NH 2 was required for the reaction with H 2 bdc-NHCH 2 Fc (Fc = ferrocenyl) to form a crystalline MOF. In the case of the zinc MOFs, N 2 adsorption experiments generally showed greater BET surface areas for the products of the direct synthesis reactions than for those from post-synthetic modifications suggesting that, in this case, the former is the best approach to these functionalised MOFs. In contrast, post-synthetic modification is the optimal approach to form crystalline derivatives of MIL-101(Cr)-NH 2 as direct synthesis gave amorphous products. CrystEngCommThis journal is