Siloxane-based metal–organic frameworks with remarkable catalytic activity in mild environmental photodegradation of azo dyes

Abstract

Three new metal–organic frameworks (MOFs) were obtained from a siloxane dicarboxylic acid (CX), 4,4′-bipyridyne (BPy) or 1,2-bis(4-pyridyl)ethylene (EBPy) and copper or cobalt acetates. The compounds were characterized by IR spectroscopy, powder X-ray diffraction, energy-dispersive X-ray fluorescence (XRF), thermogravimetric analysis (TGA), dynamic water vapor sorption (DVS), electron microscopy (TEM, SEM), UV–vis diffuse reflectance and fluorescence measurements. These hydrophobic MOFs acted as efficient catalysts in two processes occurring in aqueous media. The best result in alkaline decomposition of hydrogen peroxide was 90% conversion after 30min. The catalytic activity in photodecomposition of Congo Red (CR) was followed in mild environmental conditions, i.e. under natural sunlight, without additional oxidation agents or pH adjustments. Unprecedented catalytic activity was found, especially for the Co complex with CX and EBPy, with discoloration efficiency of ca. 82% after 80min of exposure to sunlight. The obtained results, in terms of rate constants and efficiency are comparable with those reported for other catalytic systems in photoreactors or in the presence of H2O2. The best catalyst was also tested in a Fenton-like process and it was found that the addition of H2O2 doubles the rate constant and increases the efficiency to 97%. On the other hand, the remaining waters after CR photodecomposition were analyzed by mass spectrometry (ESI–MS) and it was concluded that “cleaner” water is obtained when the MOFs are used without addition of H2O2.