Synthesis and characterization of the interpenetrated MOF-5

Abstract

MOF-5 is an important metal–organic framework and has been intensely studied, especially in its hydrogen storage properties. In this study, we obtained the interpenetrated MOF-5 materials (MOF-5-int) using N,N′-dimethylformamide (DMF) or N,N′-diethylformamide (DEF) as solvents. The Langmuir surface area of MOF-5-int determined by N₂ adsorption is 950–1100 m² g⁻¹, much lower than the non-penetrated MOF-5 (3000 m² g⁻¹). However, it can store 1.54–1.82 wt% by volumetric method hydrogen at 77 K and 1 atm, which is higher than the amount stored by the non-penetrated MOF-5. The MOF-5-int was also characterized by XRD-powder diffraction, thermogravimetric analysis (TGA), nitrogen adsorption/desorption analysis, scanning electron microscope (SEM) and X-ray single-crystal structure diffraction. In addition, we found grinding greatly facilitates the decomposition of the MOF-5-int material by H₂O to a nonporous phase ZnBDC·xH₂O (within 2–5 min, BDC = 1,4-benzenedicarboxylate), even under low humidity (30%), which calls for careful handling of the MOF-5 material. The effects of the water content, reaction time, reaction temperature, molar ratio of Zn(NO₃)₂ to H₂BDC, addition of H₂O₂, rapid stirring and dilution on the synthesis of MOF-5-int were studied and the synthetic conditions were optimized. Moreover, Hafizovic et al. (J. Am. Chem. Soc., 2007, 129, 3612) found the intensity ratio of the powder XRD peak at 9.7° to that at 6.8° (referred to as the R₁ value) of MOF-5 can be used to predict its porosity. The lower the intensity ratio, the more porous it is. In this study, we showed that MOF-5-int can have a very low R₁ value but also a low porosity. The low specific surface area (SSA) is mainly due to its interpenetrated structure instead of the entrapped zinc species or the mesopores in the material, as previously proposed in the literature, and associated with the characteristic, very strong peak at 13.8° in its XRD-powder diffraction pattern. A high R₂ value (the ratio of the intensity of the peak at 13.8° to that at 6.8°) suggests an interpenetrated structure, especially when the R₁ value is low. In addition, we found that although entrapped ZnO or solvent molecules can increase the R₁ value, and a low R₁ value implies no zinc species or solvent molecules entrapped in the MOF-5 framework, a high R₁ value does not necessarily suggest the presence of entrapped molecules.