Overcoming Synthetic Challenges: Solvent-Free Construction of a Cr–Phosphonate Framework with High Stability and Proton Conductivity

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Abstract
The sustainable, green, solvent-free synthesis offers new pathways for constructing high-performance metal-organic frameworks (MOFs) that are challenging to access via conventional solution chemistry. Herein, using this approach, we synthesize a highly stable crystalline chromium-phosphonate MOF, TGU-20, while a different counterpart, TGU-30, was obtained in the solution with the same ligand and Cr3+ ion. Interestingly, the three-dimensional electron diffraction technique reveals that TGU-20 adopts a monodeprotonated bidentate phosphonate coordination mode, forming ultranarrow channels densely lined with proton-donating acid P-OH groups. In contrast, TGU-30 features a mixed mono/dideprotonated coordination, resulting in large channels with sparse P-OH sites. As a result, TGU-20 displays exceptional proton conductivity of over 10-2 S cm-1 at 100% RH with a very low activation energy of 0.10 eV, reaching record-high room-temperature proton conductivity among phosphonate MOFs and 2 orders of magnitude higher than that of TGU-30. Additionally, both MOFs exhibit high stability in harsh conditions, including highly concentrated strong acid. The solvent-free synthesis not only directly yields a chromium-phosphonate MOF for the first time but also opens a mild and green avenue for selectively tuning metal-ligand coordination in reticular chemistry.
Funding Information
  • National Key Research and Development Program of China (2021YFB3802200)
  • National Natural Science Foundation of China (22478304)

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