Enhanced selectivity of CO2 over CH4 in sulphonate-, carboxylate- and iodo-functionalized UiO-66 frameworks

Three new functionalized UiO-66-X (X = -SO3H, 1; -CO2H, 2; -I; 3) frameworks incorporating BDC-X (BDC: 1,4-benzenedicarboxylate) linkers have been synthesized by a solvothermal method using conventional electric heating. The as-synthesized (AS) as well as the thermally activated compounds were characterized by X-ray powder diffraction (XRPD), diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, thermogravimetric (TG), and elemental analysis. The occluded H2BDC-X molecules can be removed by exchange with polar solvent molecules followed by thermal treatment under vacuum leading to the empty-pore forms of the title compounds. Thermogravimetric analysis (TGA) and temperature- dependent XRPD (TDXRPD) experiments indicate that 1, 2 and 3 are stable up to 260, 340 and 360 degrees C, respectively. The compounds maintain their structural integrity in water, acetic acid and 1 M HCl, as verified by XRPD analysis of the samples recovered after suspending them in the respective liquids. As confirmed by N-2, CO2 and CH4 sorption analyses, all of the thermally activated compounds exhibit significant microporosity (S-Langmuir: 769-842 m(2) g(-1)), which are comparable to that of the parent UiO-66 compound. Compared to the unfunctionalized UiO-66 compound, all the three functionalized solids possess higher ideal selectivity in adsorption of CO2 over CH4 at 33 degrees C.