The use of a dynamic solvent system as a novel approach to sustainable MOF crystallization

Abstract

Herein, we report a novel and green synthetic approach for pyrazolate-based metal–organic frameworks (MOFs) as demonstrated by the three prominent examples of this material class: [Ni(bdp)]_n and two supramolecular isomers [Zn(bdp)]_n and BUT-58 (H₂bdp = 1,4-bis(1H-pyrazol-4-yl)benzene). The replacement of dimethylformamide (DMF) with a dynamic solvent system (DSS) - a reactive mixture of 1-butanol and acetic acid - in the synthesis allows us to avoid toxic chemicals, as well as precisely control the crystal size and morphology of the products. Unlike DMF, which decomposes into lower-value byproducts during synthesis, DSS produces a value-added ester (butyl acetate). Furthermore, a sustainable washing procedure fully eliminates the need for DMF, while ensuring a high-porosity product. Improved material crystallinity leads to a more pronounced breathing behavior during nitrogen physisorption. Moreover, flexibility modulation through crystal size engineering becomes within reach. Due to the accessibility to highly crystalline materials, single-crystal X-ray diffraction on the pristine crystals could be performed to elucidate the preferred adsorption sites in the studied frameworks.