Improving ammonia uptake performance of zirconium-based metal-organic frameworks through open metal site insertion strategy

This study reports the development of a new type of MOFs by inserting transition metals into the bipyridine dicarboxylate ligand-containing Zr-based MOF, UiO-67-bpy, through one-pot synthesis (OPS) and post-synthesis functionalization (PSF) methods. Transition metals including Ni, Co, Mn, Fe and Cu were inserted into MOF to create open metal site (OMS). PXRD, FTIR, BET, SEM, EDS, UV-Vis and ICP were used to characterize the obtained MOFs. The ammonia and hydrogen uptake capacity of all samples was evaluated by static measurements. Among all samples, the Mn-containing UiO-67-bpy modified by PSF method, PSF-Mn, could achieve a maximum ammonia adsorption capacity of 245.90 mg/g at 298 K (Langmuir model). Among the samples processed by OPS, the MOF containing Mn, namely OPS-Mn, has a maximum adsorption capacity of 224.50 mg/g, showing the highest capacity. Moreover, Co-containing UiO-67-bpy modified by OPS method, OPS-Co, showed the hydrogen adsorption capacity of 20.78 mg/g at 77 K, which is comparable or even superior to the previously reported value. According to the values of isosteric enthalpy of adsorption, H2 adsorption behaviors of the UiO-67-bpy-M were strongly dependent on the metal species, so that the isosteric heats of H2 adsorption for these MOFs were in the range 0.672-1.002 kJ mol- 1, where OPS-Co gained the highest (1.002 kJ/mol). The good results obtained in the current work clearly show that the combination of PSF and OPS techniques is a promising approach to inserting OMS into MOFs. Although the surface area of the MOF is reduced, compared to the original MOF with a higher surface area, the use of MOFs with inserted OMS can achieve a higher gas uptake capacity.