Characterization of Lewis acidity of cation-exchanged montmorillonite K-10 clay as effective heterogeneous catalyst for acetylation of alcohol

Catalytic activity of 14 types of metal ion-exchanged montmorillonite K-10 clay (Mn+-mont) is tested for the acetylation of cyclohexanol with acetic anhydride at room temperature. The activity increases with an increase in the charge/radius ratio of exchanged cation. The activity of Fe3+-mont is the highest and is higher than those of other Fe3+ catalysts, FeCl3·6H2O and FeCl3/SiO2, conventional acid catalysts, Al2O3 and HZSM5, and homogeneous Lewis acid, Sc(OTf)3. Fe3+-mont exhibits a high turnover numbers (TON = 49,500) and can be recycled without a loss in activity. Acidity of Fe3+-mont, Yb3+-mont and FeCl3/SiO2 was characterized by IR spectroscopy using three probe molecules: pyridine, acetonitrile and acetone. These catalysts dominantly act as Lewis acid, and the Lewis acid strength changes in the order of Fe3+-mont > Yb3+-mont > FeCl3/SiO2, as suggested by acetone adsorption microcalorimety. From the relationship between the activity and the acidic character, factors affecting the catalytic activity for the acetylation are discussed.

Among metal ion-exchanged montmorillonite (Mn+-mont) and various Fe3+ catalysts, Fe3+-mont was most effective for the acetylation of cyclohexanol with acetic anhydride. Using probe molecules sensitive to Lewis acid, IR and microcalorimetric experiments were conducted for characterizing acidity of each catalyst, and the relationship between the acidity and catalytic activity is discussed.Figure optionsDownload as PowerPoint slide