Denitrogenation through Adsorption to Sulfonated Metal-Organic Frameworks

Sulfonic acid functionalized MIL-101(Cr) (S-MIL-101(Cr)) are obtained by sulfonation of MIL-101(Cr) (Cr3F(H2O)2O[(O2C)-C6H4-(CO2)]3·nH2O (n~25)) using triflic anhydride and sulfuric acid. The amount of sulfonic groups in the framework can be controlled by changing the molar ratio of MIL-101(Cr), triflic anhydride, and sulfuric acid. The sulfonated samples were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, nitrogen physical adsorption/desorption, acid-base potentiometric titration, and thermogravimetric analysis (TGA). The results show that the sulfonated samples retain the general structure of MIL-101(Cr), but the specific areas and pore diameters decrease. The sulfonated samples formed with between 0.21 and 0.42 mmol·g-1 of sulfonic acid groups. The adsorptive denitrogenation of a model fuel by different S-MIL-101(Cr) samples was investigated in batch adsorption experiments. Sulfonation can strengthen the interactions between nitrogen-containing compounds (NCCs) and the adsorbent. The sample obtained using a molar ratio of n(MIL-101(Cr)):n(H2SO4):n(Tf2O)=1:3:4.5 had the largest adsorption capacity for quinoline and indole. Compared with the bare MIL-101(Cr), this sulfonated material showed enhancement of the maximum adsorption capacity by 12.2% and 6.3% for quinoline and indole, respectively. Regeneration of the used adsorbent was conducted by washing with ethanol, and the adsorptive capacity for NCCs from the model-fuel showed no obvious decrease after three cycles of use.