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Abstract
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.
Highlights
This article has been edited by the Royal Society of Chemistry, including the commissioning, peer review process and editorial aspects up to the point of acceptance
With the incremental amount of metal source, product structures undergo the transformation from MIL-68 to MIL-53 or QMOF-2
Metal/ligand molar ratios were modulated in three different synthesis systems, which were respectively M : L 1⁄4 3 : 1, 1 : 1, 1 : 3 or 1 : 6. The MIL-68(In) sample was prepared according to the reported recipe [19]
Summary
All reagents and solvents received from commercial suppliers were used without further purification (indium nitrate hydrate, In(NO3)3.xH2O, Alfa Aesar, 99%; terephthalic acid (H2BDC), Alfa Aesar, 98%; 2-Aminoterephthalic acid (H2BDC-NH2), Alfa Aesar, 99%; 2-Bromoterephthalic acid (H2BDC-Br), Alfa Aesar, 97%; Nitroterephthalic acid (H2BDC-NO2), Alfa Aesar, 99%; N,NDimethylformamide (DMF), Aldrich, greater than 99%; anhydrous ethanol, Prolabo, 99.8%; anhydrous methanol, Aldrich, 99.9%)
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