Impact of the functionalization onto structure transformation and gas adsorption of MIL-68(In)

A series of functionalization -NH₂, -Br and -NO₂ has been performed on MIL-68(In) material in order to improve the porosity features of the pristine material. The functional groups grafted onto the ligand and the molar ratios of the ingredient indicate a profound influence on product formation. With the incremental amount of metal source, product structures undergo the transformation from MIL-68 to MIL-53 or QMOF-2. The situation is different depending on the variation of the ligands. Gas (N₂, Ar, H₂ and CO₂) adsorption-desorption isotherms were systematically investigated to explore the impact of the functionalization on the porous prototypical framework. Comparison of adsorption behaviour of N₂ and Ar indicates that the polar molecule exhibits striking interaction to N₂ molecule, which has a considerable quadrupole moment. Therefore, as a probe molecule, Ar with no quadrupole moment is more suitable to characterize the surface area with the polar groups. Meanwhile, Ar adsorption result confirms that the negative influence on the surface area stems from the size of the substituting groups. The uptake of H₂ and CO₂ indicates that the introduction of appropriate polar organic groups can effectively enhance the adsorption enthalpy of relative gases and improve the gas adsorption capacity apparently at low pressure. The introduction of -NO₂ is in favour of improving the H₂ adsorption capacity, while the grafted -NH₂ groups can most effectively enhance the CO₂ adsorption capacity.