Lanthanide Metal-Organic Frameworks with Six-Coordinated Ln(III) Ions and Free Functional Organic Sites for Adsorptions and Extensive Catalytic Activities

Abstract

Three chelating-amino-functionalized lanthanide metal-organic frameworks, Y-DDQ, Dy-DDQ and Eu-DDQ, were synthesized with a flexible dicarboxylate ligand based on quinoxaline (H2DDQ = N, N'-dibenzoic acid-2,3-diaminoquinoxaline). The three-dimensional framework is constructed by the H2DDQ linkers connecting the zigzag ladders, showing a net of sra topology. In the structures, one kind of Ln(III) ions metal centers are six-coordinated and thus can potentially behave as open metal sites (OMSs), while the free chelating amino groups can act as free functional organic sites (FOSs). The N2 and Ar adsorption behaviors indicate that these Ln-DDQ exhibits stable microporous frameworks with high surface area after remove of the solvents. Owing to presence of OMSs and FOSs, these MOFs show good ability of CO2, dyes captures and Lewis acid catalyst for cyanosilylation reaction. In view of the existing FOSs in the framework, Pd NPs were immobilized onto the MOFs through graft interactions between free chelating amino groups and metal ions precursor using postsynthetic modification. The well dispersed Pd@Ln-DDQs exhibit efficient and recyclable catalytic reduction of 4-nitrophenol to 4-aminophenol, and they can also act as an excellent catalyst for Suzuki-Miyaura cross-coupling reactions with the exposed Pd NPs.

Figure 1. View of the asymmetric unit of Y-DDQ.

Figure 2. An infinite 3D framework with 13.798 × 12.422 Å dimensions along the a-axis.

Figure 3

(a) 1D metal chain. (b) A zigzag ladder simplified from the metal chain. (c) A simplified three-dimensional net of the sra topology of Y-DDQ.

Figure 4. N₂ and Ar adsorption isotherms for Y-DDQ, Dy-DDQ and Eu-DDQ.

Figure 5. CO₂ adsorption isotherms in Y-DDQ at 273 and 298 K.

Figure 6. Isosteric heats of CO₂ adsorption (Qst) value for Y-DDQ calculated using isotherms collected at 273 and 298 K.

Figure 7. UV-vis absorption spectra of MB solution and the relationship between C_t/C₀ and reaction time (t) in the absorption of MB with Y-DDQ.

Figure 8. UV-vis absorption spectra of CV solution and the relationship between C_t/C₀ and reaction time (t) in the absorption of CV with Y-DDQ.

Figure 9

(Left) HRTEM images of Y-DDQ. (Right) HRTEM images of Pd@Y-DDQ.

Figure 10. (Left) UV-vis spectra of 4-nitrophenol reduction. (Right) Effect of Pd@Y-DDQ on the reduction rate of 4-NP.

Figure 11. Kinetic profile for Suzuki-Miyaura coupling reactions reaction catalyzed by Pd@Y-DDQ, removal of Pd@Y-DDQ after 2 h.