A general post-synthetic modification approach of amino-tagged metal–organic frameworks to access efficient catalysts for the Knoevenagel condensation reaction

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Summary:
The authors from Beijing Key Laboratory of Function Materials for Molecule & Structure Construction, University of Science and Technology Beijing, developed a series of primary-alkyl-amine-grafted MOFs (UiO-66-NH-RNH₂, Cr-MIL-101-NH-RNH₂, Al-MIL-53-NH-RNH₂) that exhibit 396 h⁻¹ TOF and 99 % yield in room-temperature Knoevenagel condensation.

Background:
1. Previous solid base catalysts (MCM-41, zeolites, amine-MOFs) required high loadings or suffered from leaching; aromatic-amine-MOFs showed low basicity due to electron-withdrawing carboxylates.
2. The authors propose a one-step post-synthetic aziridine ring-opening to install primary alkyl amines, greatly enhancing basicity while retaining MOF porosity.

Research Content:
1. Synthesis: Amino-tagged MOFs were treated with 2-methylaziridine (1.5 eq) in CHCl₃ at 45 °C for 6 h to give MOF-NH-RNH₂.
2. Characterizations:
   1) BET: Cr-MIL-101-NH-RNH₂ 1175 m² g⁻¹, UiO-66-NH-RNH₂ 336 m² g⁻¹; pore sizes 12–34 Å.
   2) SEM: octahedral UiO-66-NH-RNH₂ ~160 nm; Cr-MIL-101-NH-RNH₂ ~160 nm spheres.
   3) PXRD, FTIR, TGA, ESI-MS confirm framework integrity and 15–36 % amine loading.
3. Application: 1 mol % catalyst, toluene, 23 °C, 2 h; 97–99 % conversion for 11 aldehydes; Henry reaction 96 % yield; size-selectivity up to 9.6 Å substrates.
4. Mechanism: Primary alkyl amine (pKb ≈ 10.6) activates aldehyde via imine intermediate; large pores & cages of Cr-MIL-101 accelerate diffusion, giving TOF 396 h⁻¹.

Outlook:
The work delivers recyclable, highly active heterogeneous base catalysts under mild conditions, providing a general postsynthetic route to robust amine-MOFs for C–C and C–N condensations.

A general post-synthetic modification approach of amino-tagged metal–organic frameworks to access efficient catalysts for the Knoevenagel condensation reaction
Authors: Yi Luan, Yue Qi, Hongyi Gao, Radoelizo S. Andriamitantsoa, Nannan Zheng, Ge Wang
DOI: 10.1039/c5ta00816f
Link: https://pubs.rsc.org/en/content/articlelanding/2015/ta/c5ta00816f

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