Green oxidation of benzyl alcohol by heterogeneous dioxidotungsten(VI) complex

Oxidation of alcohols is one of the important reactions in organic synthesis and industrial chemistry[۱]. There are many methods for oxidizing alcohols, but older methods (like used of potassium permanganate,potassium dichromate, periodate, etc.) are generally polluting methods that produce large amounts ofhazardous and toxic substances [۲]. Therefore, new methods have been developed for the oxidation ofalcohols and in these methods much attention has been paid to the environmental issues and green chemistrystandards. Due to many advantages of heterogeneous catalysts including high efficiency, great selectivity,ease of separation from the reaction mixture and reusability, the oxidation of alcohols in the presence ofheterogeneous catalysts has been developed [۳]. In this project, a new tungsten complex was synthesized bythe reaction of ONO-donor Schiff base ligand with tungsten(VI) chloride in methanol under reflux condition.The structure of the red crystals of this complex was determined by single crystal X-ray analysis and the resultsindicated that the product is a mononuclear dioxidotungsten(VI) complex. The molecular structure of thiscomplex is shown in Fig. ۱. In order to obtain a heterogeneous catalyst, this complex was reacted withpropionyl chloride-functionalized silica gel in dried acetonitrile. Heterogeneous catalyst was characterized byvarious analytical methods including XPS, XRD, EDX, SEM, DRS, TGA and FT-IR. All of these analysesconfirmed the successful supporting of complex on silica gel. The resulting heterogeneous catalyst was usedin the oxidation of benzyl alcohol and the effects of solvent, temperature and concentration of oxidant andcatalyst have been investigated. The results indicated that this heterogeneous catalyst can oxidize benzylalcohol to benzaldehyde and benzoic acid with high yield and its selectivity considerably depends on thereaction condition. Selectivity is considerably low in the presence of higher amounts of oxidant and also athigher temperatures. Nevertheless, the system is considerably selective toward benzaldehyde at lowtemperatures and controlled amounts of oxidant. The heterogeneous catalyst was recovered after theoxidation reaction and characterized by the same methods that used for supported catalyst. The resultindicated that supported heterogeneous catalyst has high stability and the recovered catalyst can be used foroxidation reaction several times without significant loss in this activity.

Keywords:

Tungsten complex , Supported catalyst , silica gel , Benzyl alcohol