From Nanoparticles to Single Crystals of Al‐MOFs: Synergistic Coordination and pH Modulation, and Rapid Sorption Kinetics Assessment by Optical Calorimetry

The properties of metal-organic frameworks (MOFs), such as sorption kinetics or mechanical and chemical stability, not only depend on their composition and chemical structure, but also on their crystal size and morphology. However, the tunability of the crystallite size of aluminum-based MOFs (Al-MOFs) is still a long-standing challenge. In this study, we present systematic high-throughput investigations elucidating the synergistic effects of different mono- and dicarboxylic acids (acetic acid, malonic acid, and oxalic acid) as coordination modulators and NaOH as a pH modulator on the crystal size of various Al-MOFs, with a focus on Al-MIL-53-NO₂. By varying the type and amount of coordination modulators, we successfully extended the range of achievable particle sizes to nanoparticles as well as large crystals (d_max ≈ 100 nm - 800 µm) compared to traditional synthesis methods using only coordination modulators. Thus, large crystals as well as nanoparticles of different Al-MOFs could be obtained by simply varying the molar ratio of the different modulators. Additionally, we explored the influence of particle size on CO₂ sorption properties using InfraSORP technology (optical calorimetry), revealing the increase in adsorption rates with decreasing particle size.