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

Abstract

The reaction of the V‐shaped linker molecule 5‐hydroxyisophthalic acid (H ₂L ⁰), 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, 2, 4, 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 ¹H 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‐H ₂BDC‐NO ₂) was employed which exclusively led to the formation of [Ga(OH)(C ₈H ₃NO ₆)] (Ga‐MIL‐53‐ m‐BDC‐NO ₂), 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.

Abstract

In the frame: The nitration of the V‐shaped linker 5‐hydroxyisophthalic acid by Al(NO ₃) ₃ and Ga(NO ₃) ₃ prior to crystallization of the MOF leads selectively to Al‐CAU‐10 and Ga‐MIL‐53 type compounds. In situ IR spectroscopy and light scattering experiments in combination with ¹H‐NMR spectroscopy were used to study the product formation. DFT and GCMC calculations were employed to understand the differences in framework formation and water adsorption properties.