Facile synthesis and gas adsorption behavior of new functionalized Al-MIL-101-X (X = –CH3, –NO2, –OCH3, –C6H4, –F2, –(CH3)2, –(OCH3)2) materials

Abstract

The synthesis of seven new functionalized aluminum terephthalates [Al3OCl(DEF)2(BDC-X)3] (DEF = N,N-diethylformamide; BDC = 1,4-benzene-dicarboxylate; X = –CH3, 1–CH3; –NO2, 2–NO2; –OCH3, 3–OCH3; –C6H4, 4–C6H4; –F2, 5–F2; –(CH3)2, 6-(CH3)2, –(OCH3)2, 7–(OCH3)2) having MIL-101 framework topology was carried out under solvothermal conditions (130 °C, 3–12 h) with conventional electric (CE) heating by using stochiometric mixtures of Al(ClO4)3·9H2O and H2BDC-X linkers in DEF. The previously reported Al-MIL-101–NH2 compound was also prepared under similar solvothermal conditions. Moreover, four of the compounds, namely 1–CH3, 3–OCH3, 4–C6H4 and 7–(OCH3)2, was synthesized under microwave-assisted solvothermal conditions (170 °C, 10 min, 150 W) by using similar reaction mixtures as the syntheses with CE heating. The compounds were fully characterized by X-ray powder diffraction (XRPD), Fourier transform infrared (FT-IR) spectroscopy, thermogravimetric (TG) and elemental analysis. The thermally activated forms of the as-synthesized compounds were obtained by exchange of the guest molecules with methanol, followed by heating the methanol-exchanged solids under dynamic vacuum. TG analyses carried out with the thermally activated compounds suggest high thermal stability up to 260–430 °C in an air atmosphere. All of the thermally activated compounds exhibit significant porosity, as confirmed by N2 and CO2 sorption experiments. The specific BET surface areas of the compounds fall in the 1328–2398 m2 g−1 range. The CO2 uptake values of the compounds lie in the 1.6–3.0 mmol g−1 range at 0 °C and 1 bar. The N2 adsorption capacities of the compounds are dependent on the size of the attached functional groups. On the other hand, the CO2 uptake values are dependent both on the size and the nature of functional groups attached to the terephthalate linker molecules.