A comparative study on the microwave-assisted peroxidation of cyclohexane by a supracomplex as homogeneous catalyst

Abstract

A nanostructured supracomplexformulated as [K3FeIII (μ-ox)3(H2O)3]n (UC) where ox-2 = oxalate was synthesized by sonochemical process as an environmentally friendly method. Previously this complex was synthesized under hydrothermal conditions (HC) and characterized by single crystal X-ray structure (SC-XRD). Matching the data obtained from elemental analysis (CHN), energy-dispersive X-ray spectroscopy (EDS), Fourier-transform infrared spectroscopy)FT-IR(, thermogravimetric analysis (TGA),differential thermal analysis (DTA), and powder X-ray diffraction (PXRD) analysis confirmed that UC and HC have the same structure. But in another comparison, the results of scanning electron microscopy (SEM), andBrunner-Emmet-Teller (BET) analysis showed that UC and HC have different particles size, morphology, and texturalproperties. For the first time, the catalytic activity of the nanostructured Fe (III) supracomplex(UC) was compared with that obtained as single crystal (HC) in the peroxidation of cyclohexane in the absence and presence of microwave (MW) irradiation. Hydrogen peroxide was used as an ideal green oxidant because it does not produce toxic by-products. Also, microwave-assisted peroxidative oxidation of alkanes is an environmentally friendly and green method that avoids harsh reaction conditions such as high heat and contamination of products and reduces the amount of solvent used. Our findings showed that UC has higher catalytic activity, with a total product yield of 15 and 21% in the absence and presence of MW irradiation, respectively. For UC, by optimizing parameters such as temperature, solvent, catalyst, acid and oxidant, we were able to increase the yield up to 36.3%. A free radical mechanism is proposed for the peroxidation oxidation of cyclohexane in the presence of the supracomplex, involving the formation of cyclohexyl radical, its oxidation, and consequent hydroxylation by water.