Tuning the mechanical properties of sol-gel monolithic metal-organic frameworks by ligand engineering

The sol-gel monolithic MOFs has come to prominent attention for industrial application owing to the higher powder packing density, enhanced processabilities and mechanical stabilities compared to the powder counterpart. The mechanical properties are particularly important during machine shaping processing because of porous framework structure. We used ligand engineering to design and synthesize _monoUiO-66-type materials modified different chemical functional groups (-NH₂, -2OH, -2COOH) by sol-gel method, with the aim to assess the impact of different functional groups on the mechanical properties of these monolithic materials based on nanoindentation technology. We observe larger mass and sterically bulky functional groups (-2COOH) can significantly decrease the BET areas and pore volume of _monoUiO-66 through N₂ adsorption isotherms at 77 K. Hence, the two -COOH groups modified _monoUiO-66 tends to exhibit the higher H of 0.589 ± 0.018 GPa and E of 15.471 ± 0.250 GPa compared with _monoUiO-66 modified with -NH₂ (0.334 ± 0.009 GPa/11.959 ± 0.243 GPa) and -2OH (0.331 ± 0.008 GPa/10.251 ± 0.142 GPa) groups. The creep indentation tests and the jump indentation tests further demonstrate the modification by larger functional groups -COOH on _monoUiO-66 could resist irreversible plastic deformation. Furthermore, the _monoUiO-66-(COOH)₂ has significantly smaller the activation volume of 0.34 ∼ 0.43 nm³, highlighting the introduction of -COOH groups reduced the pore volume and restrict the number of pores involved in one collapse event. Our results demonstrate the larger mass and sterically bulky functional groups have significant influence on the mechanical properties of the _monoMOFs materials.