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. 2018 Dec 12;5(12):181378.
doi: 10.1098/rsos.181378. eCollection 2018 Dec.

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

Lei Wu  1   2 , Weifeng Wang  1   2 , Rong Liu  1   2 , Gang Wu  3 , Huaxin Chen  2
Affiliations

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

Lei Wu et al. R Soc Open Sci. .

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.

Keywords: MIL-68(In)_X; MOFs; functionalization; gas adsorption; structure transformation.

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Conflict of interest statement

We declare we have no competing interests.

Figures

Figure 1.
Figure 1.
PXRD patterns for the samples synthesized from different synthesis systems: organic ligand is (a) H2BDC-Br; (b) H2BDC- NO2 and (c) H2BDC-NH2 (M : L = 3 : 1, 1 : 1) and (d) H2BDC-NH2 (M : L = 1 : 3, 1 : 6).
Figure 2.
Figure 2.
SEM photographs of the samples synthesized from different synthesis systems: organic ligand is (a) H2BDC-Br; (b) H2BDC- NO2 and (c) H2BDC-NH2.
Figure 3.
Figure 3.
(a) At 77 K, the N2 sorption isotherms and (b) at 87 K, the Ar sorption isotherms of MIL-68(In) (black); MIL-68(In)_NH2 (red); MIL-68(In)_Br (blue); MIL-68(In)_NO2 (green) (adsorption, solid; desorption, empty).
Figure 4.
Figure 4.
At 77 and 87 K, H2 sorption isotherms of MIL-68(In) (black); MIL-68(In)_NH2 (red); MIL-68(In)_Br (blue) and MIL-68(In)_NO2 (green) (a) in gravimetric percentage (77 K); (b) normalized per unit cell (77 K); (c) in gravimetric percentage (87 K) and (d) normalized per unit cell (87 K). (The inset is an enlargement of the low pressure region of the H2 isotherms.)
Figure 5.
Figure 5.
(a) H2 adsorption enthalpy, (b) CO2 adsorption enthalpy of MIL-68(In) (black); MIL-68(In)_NH2 (red); MIL-68(In)_Br (blue) and MIL-68(In)_NO2 (green).
Figure 6.
Figure 6.
At 273 and 298 K, CO2 sorption isotherms of MIL-68(In) (black); MIL-68(In)_NH2 (red); MIL-68(In)_Br (blue) and MIL-68(In)_NO2 (green) (a) in gravimetric percentage (273 K); (b) normalized per unit cell (273 K); (c) in gravimetric percentage (298 K) and (d) normalized per unit cell (298 K).
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