Ultrasonic‐Assisted Linker Exchange (USALE): A Novel Post‐Synthesis Method for Controlling the Functionality, Porosity, and Morphology of MOFs

Abstract

The introduction of organic ligands into metal–organic frameworks (MOFs) with a specific topology and that cannot be attained by direct synthesis is a big challenge. To meet this challenge, different ligand exchange/incorporation methods have been employed. Here, a new method, called ultrasonic‐assisted linker exchange (USALE), has been developed to overcome the above‐mentioned problems. USALE is a novel method for ligand exchange based on the use of ultrasonic waves. The temperature and pressure caused by the USALE method in microscopic zones are so intense that the linker exchange process is much faster than with other methods. In addition to saving time during synthesis, the use of the USALE method leads to a higher surface area and pore volume compared with other methods such as solvent‐assisted linker exchange (SALE). In this way, improved gas adsorption capacity has been achieved for daughter frameworks synthesized by the USALE method. By using the USALE method, we have transformed a nonporous and easy‐to‐synthesize TMU framework ([Zn(OBA)(BPDB)0.5]n⋅2DMF (TMU‐4), in which H2OBA=4,4′‐oxybis(benzoic acid) and BPDB=1,4‐di(4‐pyridyl)‐2,3‐diaza‐1,3‐butadiene) into another porous framework ([Zn(OBA)(H2DPT)0.5]n⋅DMF (TMU‐34), in which H2DPT=3,6‐di(4‐pyridyl)‐1,4‐dihydro‐1,2,4,5‐tetrazine) that otherwise requires a relatively long time to synthesize. In addition to reducing the synthesis time for TMU‐34 (in comparison with both direct sonochemical synthesis and the indirect SALE method), the data obtained revealed that the daughter TMU‐34 framework synthesized by the USALE method has a higher surface area and accessible pore volume than TMU‐34 frameworks synthesized by SALE and direct methods. The application of SALE‐TMU‐34 and USALE‐TMU‐34 in a catalytic Henry condensation reaction and Congo Red adsorption experiments showed that the higher porosity of USALE‐TMU‐34 leads to a higher turn‐over frequency and saturation capacity compared with SALE‐TMU‐34.

Heterostructured hybrids of metal–organic frameworks (MOFs) and covalent–organic frameworks (COFs)

“…The post-synthesis ligand exchange method (PSE) exhibits some intrinsic advantages, such as operational simplicity, widespread generality, and thus extensive applications. 110 PSE is typically performed by incubating MOF crystals in a solution containing another pure ligand in the presence of a suitable solvent (DMF or water). Two possible structures of the core–shell and uniformly distributed MOF@MOF heterostructures can be constructed using this technique.…”

Turning Flexibility into Rigidity: Stepwise Locking of Interpenetrating Networks in a MOF Crystal through Click Reaction

“…All these structural features can be collectively manifested as inter- or intraframework movements. Significant changes in flexibility and adsorption properties of a given MOF platform have been observed for various metal ions, − linkers, − or their substituents. − These changes can be realized by rational selection of secondary building units, ,, post-assembly linker/metal exchange, , or by covalent post-synthetic modification (CPSM) of linkers. − Within the last approach, numerous studies have shown classical nucleophilic substitution , or substitution in a carbonyl group; , however, these reactions typically require chemically stable structures that are resistant to highly corrosive reagents, for example, acyl chlorides or acid anhydrides. Alternatively, to avoid framework degradation, a number of CPSMs have been carried out under mild conditions using Schiff base condensations, , reactions between amines and isocyanates, , or azide–alkyne click reactions. , On the other hand, these reactions suffer from limitations such as low conversion, products prone to hydrolysis, and necessity to use extra additives or catalysts.…”

Metal ions Directed Self‐assembly based on Mixed Ligands: From 2D hcb Net to 3D cds Framework

“…[ 19 ] Among N‐heterocyclic ligands, the bispyridyl‐ligand of 1,4‐bis(3‐pyridyl)‐2,3‐diaza‐1,3‐butadiene (3‐bpdb) is a good candidate for construction of coordination architectures. [ 20 ] For example, Arijit Halder and his team used N,N '‐donor ligands to obtain two isostructural two‐dimensional MOFs, namely, {[Zn(3‐bpdb)(suc)] · (H 2 O) 2 }n and {[Zn(3‐bpdb)(fum)(H 2 O) 2 ]}n, which show excellent fluorescence properties (suc = succinate and fum = fumarate). [ 21 ] Divyendu Singh's group reported a 2D CP base on a mixed ligand of 3bpdb and fdc, [Cd(fdc)(3bpdb)(H 2 O)], which has a 2D zig‐zag network structure and excellent fluorescence nature (fdc = 2,5‐furandicarboxylate dianion).…”