Xylene adsorption behaviors of Co-MOF-74(X) synthesized from Co(II) salt with different anions

Xylene, a typical volatile organic compound (VOC) and primary raw material in the chemical industry, has been found to be a key component in ozone formation. Metal organic frameworks (MOFs) containing open metal sites are capable of binding to xylene. To recover xylene from waste solvent and to mitigate the environmental risks associated with it, three 1D coordination polymers (CPs), Co-MOF-74(X) (X = OAc–, Cl–, or NO3–), were synthesized using various cobalt salts of Co(OAc)2·4H2O, CoCl2·6H2O and Co(NO3)2·6H2O to investigate the influence of anions. Moreover, the liquid phase adsorptive performance for xylene was evaluated. The morphology features and structure differences of the resulting Co-MOF-74(X) were determined by SEM-EDS, XRD, N2/CO2 adsorption, FTIR and XPS. In situ DRIFT was applied to further distinguish the adsorption performance of Co-MOF-74(X) for xylene. The results indicated that Co-MOF-74(X) exhibits a different morphology. Co-MOF-74(OAc–) is flaky and agglomerated severely, while both Co-MOF-74(Cl–) and Co-MOF-74(NO3–) are hexagonal rod–like crystals. All of Co-MOF-74(X) showed microporous structures with high specific surface areas. A small amount of Cl– was retained in Co-MOF-74(Cl–), forming a Co–Cl bond with Co(II), and the active site of adsorption increased. Co-MOF-74(Cl–) shows the highest adsorption capacity of 1317.65 mg/g for the xylene mixture with excellent cyclic stability reported so far, which is 2.17 or 2.78 times more than Co-MOF-74(NO3–) or Co-MOF-74(OAc–). Therefore, it is anticipated to be a highly efficient adsorbent for removal or recovery of xylene.