Synthesis Modulation as a Tool To Increase the Catalytic Activity of Metal–Organic Frameworks: The Unique Case of UiO-66(Zr)

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Summary:
The authors from KU Leuven, ENSICAEN-CNRS, and Ghent University developed highly active UiO-66(Zr) MOFs by co-modulation with trifluoroacetic acid (TFA) and HCl, achieving up to 2.4-fold higher Lewis-acid catalytic activity in citronellal cyclization and Meerwein–Ponndorf–Verley reduction.

Background:
1. To address the low catalytic activity of zirconium terephthalate UiO-66, prior work created missing-linker defects or post-synthetic acid treatments, yet suffered from limited site density and stability.
2. The authors proposed a one-pot modulation strategy using TFA and HCl to simultaneously tune linker deficiency and generate extra open Zr sites, yielding unprecedented activity.

Research Content:
1. Synthesis: UiO-66-X (X = 0–20 equiv TFA) was solvothermally synthesized from ZrCl₄ and terephthalic acid in DMF, optionally adding 1 equiv HCl to enhance TFA incorporation.
2. Characterizations:
   1) BET: surface areas up to ~1100 m² g⁻¹; pore size grows from 0.78 nm to 1–1.1 nm with 20 equiv TFA.
   2) SEM/TEM: crystallite size remains 300–600 nm irrespective of TFA.
   3) ¹⁹F MAS NMR, TGA, FTIR, CD₃CN-IR, and DFT-D calculations verified partial TFA substitution and post-thermal removal.
3. Application:
   – Citronellal cyclization: 75 % conversion in 10 h (vs 34 % for pristine UiO-66).
   – Meerwein–Ponndorf–Verley reduction of 4-tert-butylcyclohexanone: 93 % conversion in 24 h.
4. Mechanism: TFA competes with terephthalate during crystal growth, generating [Zr₆O₇(OH)](BDC)₄(CF₃COO) clusters; thermal activation at 320 °C removes CF₃COO⁻ and the last OH⁻, creating two open Zr⁴⁺ Lewis acid sites per cluster.

Outlook:
The co-modulation approach provides a general, scalable route to engineer high-density open metal sites in robust Zr-MOFs, unlocking their full catalytic potential for Lewis-acid-mediated transformations.

Synthesis Modulation as a Tool To Increase the Catalytic Activity of Metal–Organic Frameworks: The Unique Case of UiO-66(Zr)
Authors: Frederik Vermoortele, Bart Bueken, Gaëlle Le Bars, Ben Van de Voorde, Matthias Vandichel, Kristof Houthoofd, Alexandre Vimont, Marco Daturi, Michel Waroquier, Veronique Van Speybroeck, Christine Kirschhock, Dirk E. De Vos
DOI: 10.1021/ja405078u
Link: https://pubs.acs.org/doi/10.1021/ja405078u

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