The quest for optimal water quantity in the synthesis of metal-organic framework MOF-5

Efficient and simple room temperature synthesis of pure phase metal-organic framework MOF-5 has been developed, based on the use of anhydrous zinc acetate, Zn(OAc)2, as a precursor, instead of zinc acetate dihydrate. Crucial influence of water on a reaction pathway was revealed. In order to obtain MOF-5, different amounts of water have been added into the solutions of Zn(OAc)2 in N,N-dimethylformamide (DMF) to prepare in situ zinc acetate hydrates with 0.25, 0.5, and 1.0 mol of water. Commercially available zinc acetate dihydrate was also used as a precursor for comparison. These solutions were mixed at room temperature with the solution of 1,4-benzenedicarboxylic acid in DMF in the absence of any base. Based on XRD, FTIR, and SEM measurements, it was shown that the optimal amount of water for the synthesis of completely pure, crystalline phase MOF-5 is 0.25–0.5 mol of water per one mole of Zn. The reaction systems with 1.0 and 2.0 mol of water per one mole of Zn also led to solids with MOF-5 as the dominant phase, but they also contain small amounts of another phase, formed due to the decomposition (hydrolysis) and/or distortion of the MOF-5 framework in the presence of excess amounts of water. The product synthesized in the system without any added water contains MOF-5 phase in a very small amount, while main phase is zinc 1,4-benzenedicarboxylate and/or zinc hydrogen 1,4-benzenedicarboxylate. Regular cubic submicro/microcrystal morphology exhibited the samples synthesized using 0.5 and 0.25 mol water per one mole of Zn (pure MOF-5), while for the samples synthesized at mole ratios H2O/Zn2+ = 1.0 and 2.0 other particle shapes are also seen. By nitrogen sorption measurements it was found that the highest values of BET specific surface area (1937 m2 g−1), micropore volume (0.83 cm3 g−1), and micropore area (1590 m2 g−1) showed MOF-5 prepared at mole ratio H2O/Zn2+ = 0.5, while the highest yield of MOF-5 is obtained with the theoretical mole ratio H2O/Zn2+ = 0.25. Thermal stability of synthesized materials was investigated by TGA.