No abstract

Metal-organic frameworks (MOFs) have recently emerged as a new type of nanoporous materials with tailorable structures and functions. Usually, MOFs have uniform pores smaller than 2 nm in size, limiting their practical applications in some cases. Although a few approaches have been adopted to prepare MOFs with larger pores, it is still challenging to synthesize hierarchical-pore MOFs (H-MOFs) with high structural controllability and good stability. Here we demonstrate a facile and versatile method, an in situ self-assembly template strategy for fabricating stable H-MOFs, in which multi-scale soluble and/or acid-sensitive metal-organic assembly (MOA) fragments form during the reactions between metal ions and organic ligands (to construct MOFs), and act as removable dynamic chemical templates. This general strategy was successfully used to prepare various H-MOFs that show rich porous properties and potential applications, such as in large molecule adsorption. Notably, the mesopore sizes of the H-MOFs can be tuned by varying the amount of templates.

A microporous three-dimensional (3D) hydrogen-bonded organic framework (HOF-20) has been constructed from an aromatic-rich tetratopic carboxylic acid, 5-(2,6-bis­(4-carboxyphenyl)­pyridin-4-yl)­isophthalic acid (H4BCPIA). The activated HOF-20a has a moderately high Brunauer–Emmett–Teller (BET) surface area of 1323 m2 g–1 and excellent stability in water and HCl aqueous solution. HOF-20 exhibits highly efficient turn-up fluorescent sensing of aniline in water with a detection limit of 2.24 μM and is selective toward aniline in the presence of aromatic interferents, owing to the hydrogen bonding and edge-to-face π–π stacking interactions between the HOF-20 host and the guest aniline molecules, as demonstrated in the single-crystal X-ray structure of HOF-20⊃aniline. Density functional theory (DFT) calculations further demonstrate that the recognition of aniline molecules by HOF-20 could restrict the rotation of the aromatic rings in H4BCPIA linkers, reducing the nonradiative decay pathways upon photoexcitation and subsequently enhancing the fluorescence intensity.

Pursuing new proton-conducting materials has become a key issue to improve the performance of proton exchange membrane fuel cells for clean energy. As newly emerging materials, metal–organic frameworks (MOFs) have been attracting wide attention in this regard. Herein, using stable UiO-66 as a platform, through the modification of different functional groups of −SO3H, −2COOH, −NH2, and −Br in ligands, we explore the strategy to tune the proton conductivities of MOFs. With the highly acidic and strong hydrophilic functional groups −SO3H and −COOH, UiO-66-SO3H and UiO-66-2COOH show quite high proton conductivities of 0.34 × 10–2 and 0.10 × 10–2 S cm–1 at 303 K and ∼97% relative humidity, respectively while the −NH2, −H, and −Br represent comparatively low conductivities under the same conditions. Furthermore, water molecules adsorbed in the pores are proved to contribute greatly to the proton conductivities of these MOFs. Thermogravimetry-mass spectrometry (TG-MS) and molecular simulations are then used to analyze the interactions between the water molecules and MOFs. TG-MS analyses show two water molecule loss processes in UiO-66-SO3H and UiO-66-2COOH, but one in UiO-66-NH2, UiO-66, and UiO-66-Br during heating, which indicates the stronger affinity of −SO3H and −COOH functionalized UiO-66 toward water molecules than those with −NH2 and −Br, as well as UiO-66 itself. The isosteric heats (−Q st) of water adsorption and radial distribution functions (RDFs) in these MOFs are also evaluated by molecular simulations. It was found that UiO-66-SO3H and UiO-66-2COOH have higher −Q st of 86.50 and 52.10 kJ mol–1, whereas UiO-66-NH2, UiO-66, and UiO-66-Br have lower ones, 34.03, 19.04, and 36.36 kJ mol–1, respectively. The RDFs reveal the formation of hydrogen-bonding networks in UiO-66-SO3H, UiO-66-2COOH, UiO-66-NH2, and UiO-66, but not in UiO-66-Br.