Mixed-linker MOFs with CAU-10 structure: synthesis and gas sorption characteristics

Abstract

The metal–organic framework compound [Al(OH)(BDC-Br)] (1) (BDC-Br²⁻ = 5-bromo-1,3-benzenedicarboxylate) denoted CAU-10-Br was synthesised under solvothermal reaction conditions. Its structure was successfully refined by Rietveld methods. The framework is based on the connection of infinite helical chains of cis-corner-sharing AlO₆-polyhedra via BDC-Br²⁻ ions. Thus non-intersecting parallel channels are formed, each periodically varying in diameter between 1.1 and 6.6 Å. Nevertheless 1 adsorbs CO₂ at 298 K, while it is non-porous towards H₂ and N₂ at 77 K. Employing high-throughput (HT) methods we identified synthesis conditions that lead to the formation of mixed-linker MOFs with CAU-10 topology. Starting with a molar ratio H₂BDC : H₂BDC-Br = 3 : 1 we established a synthesis procedure for the partially bromo-functionalised mixed-linker-MOF [Al(OH)(BDC)_0.8(BDC-Br)_0.2] (2) denoted as CAU-10-H/Br. Starting with a molar ratio H₂BDC-NH₂ : H₂BDC-NO₂ = 1 : 1 the partially NO₂- and NH₂-functionalised mixed-linker MOF [Al(OH)(BDC-NO₂)_0.55(BDC-NH₂)_0.23(BDC-NHCHO)_0.22] (3) denoted as CAU-10-NO₂/NH₂ was synthesised, in which the NH₂-groups were found to be partially formylated. The partial bromo-functionalisation in CAU-10-H/Br (2) leads to a lower sorption capacity in comparison with the parent structure CAU-10-H, while the pore accessibility is comparable. The incorporation of NO₂-, NH₂- and NHCHO-groups in CAU-10-NO₂/NH₂ (3) does not only affect the accessibility of the pores for N₂, but results also in an increased capacity for H₂ and CO₂ in comparison with the parent structure CAU-10-NO₂. The reproducibility of the synthesis procedures was tested regarding the composition of the MOFs and the resulting gas sorption properties. In 2 a molar fraction of (BDC)²⁻ = 80 ± 2% is found, while in 3 a molar fraction of (BDC-NO₂)²⁻ = 55 ± 2% is observed. Starting from the molar ratio H₂BDC : H₂BDC-CH₃ = 1 : 1 mixed linker MOFs denoted as CAU-10-H/CH₃ (4) were also obtained. In contrast to 2 and 3 the molar ratios of BDC²⁻/BDC-CH₃²⁻ and especially the sorption properties differ substantially for every batch. The compounds were further characterised by X-ray powder diffraction, thermogravimetric/elemental analysis, NMR/Vis/IR-spectroscopy, and gas sorption measurements.