Degradation of fluoride-free MIL-100(Fe) and MIL-53(Fe) in water: Effect of temperature and pH

The degradation in liquid water of two iron-containing MOFs MIL-100(Fe) and MIL-53(Fe) synthesized in fluoride-free conditions was studied. It was found that dispersing the MOFs in deionized water (1 mg/mL) results in the decrease of pH to 2.9 for MIL-100(Fe) and to 4.5 for MIL-53(Fe). Given this finding the stability of the MOFs in liquid water was characterized under two different sets of conditions: 1) reflux in water at 100 degrees C under obtained pH and 2) at ambient temperature under adjusted pH 7. After reflux of MIL-100(Fe) at 100 degrees C its XRD pattern remains unchanged, however a strong decrease of its BET surface area and appearance of alpha-Fe2O3 nanoparticles point out to a partial degradation of the MOF. Reflux of MIL-53(Fe) in water at 100 degrees C results in its partial degradation yielding H2BDC molecules intercalated in the pores of the remaining MOF particles. To study the degradation of MIL-100(Fe) and MIL-53(Fe) under adjusted pH 7 at room temperature the solids were dispersed in water and then NaOH solution was added in order to maintain a constant pH value. It was found that under such conditions the frameworks of both materials collapse yielding poorly crystallized 6-line ferrihydrite. This finding suggests that the notion of "stability in deionized water" frequently used for MOFs needs to be completed by specifying the exact pH value at which the tests are realized. (C) 2015 Elsevier Inc. All rights reserved.