Thermal post-synthetic modification of Al-MIL-53–COOH: systematic investigation of the decarboxylation and condensation reaction

Abstract

Aluminium trimellitate [Al(OH)(BDC–COOH)]·0.9H₂O (1), the Al-MIL-53–COOH derivative, was discovered under solvothermal conditions using a high-throughput set-up suitable for microwave (MW)-assisted heating. The compound shows high structural flexibility. The large-pore (lp) form of the framework is obtained under large excess of H₂O or after solvothermal treatment with N,N-dimethylformamide upon which [Al(OH)(BDC–COOH)]·0.7DMF (2) is obtained. Exposure of the water rich lp form of 1 to ambient conditions leads to the transformation to the narrow-pore (np) form. Thermal activation of both compounds results in the formation of the empty lp form and the activation of 1 was studied in detail by in situ IR-spectroscopy. Depending on the activation temperature and time two post-synthetic modification (PSM) processes are observed: the partial decarboxylation and the formation of acid anhydride groups. Thus at high temperatures and long activation times [Al(OH)(BDC–OCOCO–BDC)_x/2(BDC)_y] (x + y = 1) is formed. Upon cooling in air the anhydride functionality still remains intact, but the np form is obtained in air due to the adsorption of H₂O molecules. Sorption measurements of 1 confirm a preference for polar gases like H₂O and CO₂ in comparison to N₂, H₂, and CH₄. The N₂ capacity depends strongly on the degree of decarboxylation.