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Imaging the versatile linkers in UiO-66

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Imaging the versatile linkers in UiO-66

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    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 challenge of heterogeneous hydroformylation reactions, we serendipitously discovered that the electron microscopy can be utilized to characterize the linker flexibility in MOFs. Subsequently, we work with Dr. Xiao Chen to research the rotation properties of benzene rings in UiO-66 and analyze the affect of various purposeful 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 sorts of these supplies, permits extra utility situations, and leads to totally different macroscopic performances. Nonetheless, the connection between ligand functionalization and totally different properties stays imprecise. Earlier researches attributed the totally 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 fuel separations. Few research reported whether or not the functionalization can change the macroscopic efficiency by 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, reminiscent of real-space photographs, has not been obtained. 

Developments in electron microscopy expertise now permit researchers to instantly 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 purposeful teams on UiO-66 sort MOFs. Utilizing iDPC-STEM (built-in differential section distinction scanning transmission electron microscopy) expertise, we instantly ‘see’ the rotation properties of benzene ring within the BDC-X (p-benzenedicarboxylic acid) linkers of UiO-66-X. 

Imaging the dynamic properties of linkers in UiO-66

iDPC-STEM photographs of UiO-66-NH2 and UiO-66-Br

    Utilizing iDPC-STEM (built-in differential section distinction scanning transmission electron microscopy), we instantly 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. Subsequently, the rigidity of MOFs is very associated to the purposeful teams, which follows the sequence of -OH > -NH2 > -H > -CH3 ~ -F > -Cl > -Br. 

Calculated rotation energies of UiO-66-X

    The rotation energies of UiO-66-X with totally different purposeful teams are additionally calculates, and the outcomes are per the iDPC-STEM photographs that UiO-66-OH exhibits probably 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. 

Affect of rotation energies on the experimental FWHM and CO2 adsorption properties

    The functionalization of linkers can have an effect on the macroscopic efficiency of MOFs by 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 constructive relationship between rigidity of UiO-66-X and CO2 uptake, which is presumably attributed to the bigger variety of equivalent porous models. 

Conclusion and outlook

In conclusion, we studied the dynamic affect of purposeful teams on UiO-66-X samples. Utilizing iDPC-STEM expertise, we’re in a position to instantly ‘see’ the rotation properties of benzene rings in BDC-X linkers, and the rigidity in opposition to π-flipping is very associated to the purposeful teams. To the most effective of our information 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 technique and offers an strategy to know the native flexibility of MOFs from a extra direct perspective. Among the many UiO-66-X samples, UiO-66-OH pattern exhibits the 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 could be accountable for the totally different macroscopic properties, and we observe a constructive relationship between CO2 uptake and native rigidity of UiO-66-X. These outcomes of assorted rotation properties of UiO-66-X pave the way in which for his or her potential purposes in capturing of small molecules, separation of natural compounds and molecular machines.

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