Preparation, characterization, and hydrogen storage capacity of MIL-53 metal-organic frameworks.

Metal organic frameworks (MOFs) are considered as most promising candidate for hydrogen storage material for practical application. MIL-53(Cr) MOFs were synthesized from Cr(NO3)3 x 9H2O combined with terephthalic acid organic linker. MIL-53(Cr) MOFs are octahedral in shape and the particle size was around 10 microm identified by FE-SEM. The cleaning of the MOFs crystals with different solvents at different warm temperature were found effective and approved to increase the specific surface area and porosity of MIL-53(Cr) MOFs. The XRD patterns represented that MIL-53(Cr) MOFs had well crystalline structures. Nitrogen adsorption isotherms show that Mil-53(Cr) has approximately type-I isotherm with a highest BET specific surface area of 1946 m2 g(-1) after treated with hot methanol. Hydrogen adsorption study shows that this material can store 0.45 wt.% of H2 measured at 303 K and 32 bar. The pre-edge XANES spectra confirm the existence of Cr(III) in crystalline framework of MIL-53(Cr) and the sharp feature at 6007 eV in XANES spectra represents the dipole-allowed electron transition from 1s to 4p(xy). In addition, EXAFS spectra indicate that MIL-53(Cr) metal organic frameworks structure has the Cr-O bond distance of 1.96 angstroms with a coordination number of 5.4.