In Prof. Tiefeng Wang’s analysis group at Tsinghua College, our focus lies in designing the high-performance catalysts with industrial utility prospects. Whereas engaged on a mission of heterogeneous hydroformylation reactions, we serendipitously discovered that the electron microscopy can be utilized to characterize the linker flexibility in MOFs. Due to this fact, we work with Dr. Xiao Chen to analyze the rotation properties of benzene rings in UiO-66 and analyze the affect of various useful teams on native flexibility.

The dynamic properties of MOFs

Steel natural frameworks (MOFs) have been extensively studied and have potential purposes in separation, catalysis and pharmacy fields. The modifiability of ligands in MOFs expands the kinds of these supplies, allows extra utility eventualities, and ends in completely different macroscopic performances. Nonetheless, the connection between ligand functionalization and completely different properties stays imprecise. Earlier researches attributed the completely different efficiency primarily to the static influences of ligand functionalization, like digital modification of both MOFs construction or supported metals. It stays inadequate concerning the understanding of dynamic influences, that are important in molecular machines  and gasoline separations. Few research reported whether or not the functionalization can change the macroscopic efficiency by way of dynamic results, partially due to the shortage of appropriate characterization strategies, particularly for the microscopic rotation properties of natural linkers in MOFs. Solely ²H-NMR can present some statistical data of obvious activation energies, whereas direct data, similar to real-space photos, has not been obtained. 

Developments in electron microscopy know-how now enable researchers to straight visualize the atomic construction of MOFs. Nonetheless, the imaging of their dynamic properties stays clean though MOFs are extensively thought of versatile. Earlier research centered on the atomic-level imaging of the static construction, such because the modifications of Zr nodes after dehydroxylation and the lacking defects of natural linkers and Zr nodes. The linker dynamics haven’t been imaged in the actual house, though the respiratory and different flexibility properties are extremely correlated to the adsorption and catalysis performances. 

What’s new in our research

On this work, we research the dynamic affect of useful teams on UiO-66 kind MOFs. Utilizing iDPC-STEM (built-in differential section distinction scanning transmission electron microscopy) know-how, we straight ‘see’ the rotation properties of benzene ring within the BDC-X (p-benzenedicarboxylic acid) linkers of UiO-66-X. 

    Utilizing iDPC-STEM (built-in differential section distinction scanning transmission electron microscopy), we straight picture the BDC-X linkers in UiO-66-X. With the change of funtional teams, the rotation properties of benzene rings in BDC-X modifications accordingly, leading to important variations within the depth profiles alongside the lengthy axis. The complete width at half most (FWHM) of BDC-X linkers considerably broadens, which is attrbuted to the rotation of the benzene ring. Due to this fact, the rigidity of MOFs is extremely associated to the useful teams, which follows the sequence of -OH > -NH₂ > -H > -CH₃ ~ -F > -Cl > -Br. 

    The rotation energies of UiO-66-X with completely different useful teams are additionally calculates, and the outcomes are in keeping with the iDPC-STEM photos that UiO-66-OH exhibits essentially the most rigidity whereas UiO-66-Br exhibits the most important flexibility. The intramolecular hydrogen bonds between -OH and carboxyl O contributes to the improved regidity of UiO-66-OH. 

    The functionalization of linkers can have an effect on the macroscopic efficiency of MOFs by way of dynamic results by altering the native rigidity, which is a complement evaluation of static digital results. Utilizing the reported experimental information of CO2 seize, we discovered a optimistic relationship between rigidity of UiO-66-X and CO₂ uptake, which is probably attributed to the bigger variety of equivalent porous items. 

Conclusion and outlook

In conclusion, we studied the dynamic affect of useful teams on UiO-66-X samples. Utilizing iDPC-STEM know-how, we’re capable of straight ‘see’ the rotation properties of benzene rings in BDC-X linkers, and the rigidity in opposition to π-flipping is extremely associated to the useful teams. To one of the best of our data that is the primary use of electron microscopy to picture the rotation properties of natural linkers in MOFs, which is a vital complement to the spectral methodology and gives an method to grasp the native flexibility of MOFs from a extra direct perspective. Among the many UiO-66-X samples, UiO-66-OH pattern exhibits the very best native rigidity, which is attributed to the robust intramolecular hydrogen bond. The benzene rings in UiO-66-OH and UiO-66-NH₂ confirmed mainly the identical orientation at RT, which has not been reported. Furthermore, the distinction within the dynamic properties can be answerable for the completely different macroscopic properties, and we observe a optimistic relationship between CO₂ uptake and native rigidity of UiO-66-X. These outcomes of assorted rotation properties of UiO-66-X pave the best way for his or her potential purposes in capturing of small molecules, separation of natural compounds and molecular machines.