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). Replacement of dimethylformamide (DMF) by Dynamic Solvent System (a reactive mixture of 1-butanol and acetic acid) in the synthesis allows to avoid toxic chemicals, as well as precise control of crystal size and morphology of the products. Unlike DMF, which decomposes under the synthesis conditions into lower-value byproducts, DSS forms a value-added ester (butyl acetate) during the reaction. Furthermore, a sustainable washing procedure fully eliminates the need for DMF, while ensuring the 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.