Metal-Dependent and Selective Crystallization of CAU-10 and MIL-53 Frameworks through Linker Nitration

The reaction of the V-shaped linker molecule 5-hydroxyisophthalic acid (H2L0), with Al or Ga nitrate under almost identical reaction conditions leads to the nitration of the linker and subsequent formation of metal-organic frameworks (MOFs) with CAU-10 or MIL-53 type structure of composition [Al(OH)(L)], denoted as Al-CAU-10-L-0,L- 2,L- 4,L- 6 or [Ga(OH)(L)], denoted as Ga-MIL-53-L-2. The Al-MOF contains the original linker L-0 as well as three different nitration products (L-2, L-4 and L-4/6), whereas the Ga-MOF mainly incorporates the linker L-2. The compositions were deduced by H-1 NMR spectroscopy and confirmed by Rietveld refinement. In situ and ex situ studies were carried out to follow the nitration and crystallization, as well as the composition of the MOFs. The crystal structures were refined against powder X-ray diffraction (PXRD) data. As anticipated, the use of the V-shaped linker results in the formation of the CAU-10 type structure in the Al-MOF. Unexpectedly, the Ga-MOF crystallizes in a MIL-53 type structure, which is usually observed with linear or slightly bent linker molecules. To study the structure directing effect of the in situ nitrated linker, pure 2-nitrobenzene-1,3-dicarboxylic acid (m-H2BDC-NO2) was employed which exclusively led to the formation of [Ga(OH)(C8H3NO6)] (Ga-MIL-53-m-BDC-NO2), which is isoreticular to Ga-MIL-53-L-2. Density Functional Theory (DFT) calculations confirmed the higher stability of Ga-MIL-53-L-2 compared to Ga-CAU-10-L-2 and grand canonical Monte Carlo simulations (GCMC) are in agreement with the observed water adsorption isotherms of Ga-MIL-53-L-2.

Automated summary

The reaction of V-shaped linker molecule 5-hydroxyisophthalic acid with Al or Ga nitrate under similar conditions produced metal-organic frameworks with different structures compositions, identified through in situ and ex situ studies using H-1 NMR spectroscopy and Rietveld refinement. The Al-MOF contained original linker and various nitration products, while the Ga-MOF mainly incorporated a specific nitration product. Unexpectedly, Ga-MOF crystallized in a structure different from the anticipated CAU-10 type, with higher stability confirmed through Density Functional Theory calculations and agreement with water adsorption isotherms.