Studying the effect of reaction time on the properties of UiO-66-Ce MOF as an efficient adsorbent

In the present work, we report the effect of reaction time on the physico-chemical properties, adsorption performance, and structural stability of cerium-based metal-organic frameworks (UiO-66-Ce MOFs). The UiO-66-Ce MOFs were synthesized via a green room temperature (RT) synthetic method at different reaction times (from 3 to 24 h) and characterized by different techniques to examine their crystallinity, porosity, thermal and chemical stability as well as adsorption properties. It was found that the crystallinity, thermal and chemical stability as well as the porosity of UiO-66-Ce MOFs were increased with increasing the reaction time, in which the sample synthesized in 18 h exhibited the highest surface area of 968 m²/g. Accordingly, it can be inferred that the higher reaction time is better for the synthesis of highly stable UiO-66-Ce MOFs, and 18 h is the optimum time for this purpose. The synthesized UiO-66-Ce MOFs as well as the water-soaked samples (soaked in water for 10 days) were applied as potential adsorbents to remove a broad range of highly toxic pollutants such as malachite green (MG), methylene blue (MB), and hexavalent chromium (Cr(VI)) from water. It was found that all synthesized adsorbents have good adsorption performances towards these pollutants, in which their adsorption capacities for MB, MG, and Cr(VI) were in the range of 132-217, 91-120, and 49-58 mg/g, respectively. Although the Cr(VI) adsorption capacity of synthesized MOFs was reduced after soaking in water for 10 days, however, their adsorption performances towards cationic MB and MG dyes were significantly improved, in which their adsorption capacities were in the range of 173-254 and 164-183 mg/g, respectively. These findings indicate the applicability of UiO-66-Ce MOFs as stable adsorbents for long-time wastewater treatment applications.