Regulation of the surface area and surface charge property of MOFs by multivariate strategy: Synthesis, characterization, selective dye adsorption and separation

By using multivariate strategy, a series of multivariate UiO-66-type metal - organic frameworks with mixed 1,4-benzendicarboxylic acid (BDC) and 1,2,4,5- benzentetracarboxylic acid (BTC) ligands in different ratio, namely UiO-66-n (COOH)(2) (n = 0, 0.25, 0.50, 0.75 and 1), had been successfully prepared. The products were characterized by H-1 NMR, powder X-ray diffraction, field emission scanning electron microscopy, thermogravimetric analysis, N-2 adsorption-desorption, and zeta potential. It can be observed that with the increase of BTC ratio, the surface area decreases from 1620 to 179 cm(3)/g, and the zeta potential decreases from 36.11 to - 29.78 mV. Therefore, this strategy can effectively regulate the surface area and charge of the frameworks, thereby regulate their adsorption behaviors towards selected dye molecules. In contrast to the pristine UiO-66 that only adsorption anionic dyes, the multivariate MOFs shows an excellent selective dye adsorption to cationic dyes, where the adsorption capacities of MB and RhB on UiO-66-0.25(COOH)(2), UiO-66-0.50(COOH)(2), and UiO-660.75(COOH)(2) are 87, 90, 94 mg/g and 84, 47, 46 mg/g, respectively. In addition, the selective adsorption to mixed cationic and anionic dyes (MB and MO) and cationic dyes with different size (MB and RhB) were also achieved by UiO-66-0.25(COOH)(2) and UiO-66-0.75(COOH)(2), respectively. This adsorption selectively is attribute to the electrostatic attraction between the charged surface of MOFs and the dyes, as well as the surface area of the MOFs.