Evaluation of synthetic methods for microporous metal–organic frameworks exemplified by the competitive formation of [Cu2(btc)3(H2O)3] and [Cu2(btc)(OH)(H2O)]

Six different synthetic methods (solvothermal, microwave-assisted, atmospheric pressure and reflux, ultrasonic and mechanochemical conditions) for the metal–organic framework [Cu3(btc)2(H2O)3] (btc = benzene-1,3,5-tricarboxylate) were compared. The advantages and disadvantages of the various synthetic methods were discussed including the influence of different solvents and reaction conditions on product formation, microporosity and crystallisation. Therefore, pressure and temperature for the solvothermal synthesis (autoclave and microwave) were monitored during the reaction. The characterization of the samples was performed by X-ray powder diffraction, scanning electron microscopy, infrared-spectroscopy, thermogravimetric analysis and specific surface determination using the BET method. The experimental results show that microwave-assisted solvothermal synthesis is the best method to produce crystalline [Cu3(btc)2(H2O)3] in a short time associated with high purity, high specific pore volume (0.79 cm3/g) and quantitative yield. Noteworthy, the solvent-assisted mechanochemical approach gave a comparable specific pore volume of 0.74 cm3/g. Furthermore, synthesis conditions were explored which enable to control the product formation of either [Cu3(btc)2(H2O)3] or [Cu2(btc)(OH)(H2O)]. Finally, it is shown that water stability of [Cu3(btc)2(H2O)3] at higher temperatures is low, if the adsorbed and coordinated water is not removed rapidly upon heating.