Catalytic activity of MCM-41 and Al grafted MCM-41 for oxidative self and cross coupling of amines

Abstract

The catalytic activity of MCM-41 mesoporous silica (MS) and Al grafted MCM-41 (Al-MS) with varied amount of Al were studied for oxidative self and cross coupling of benzyl amines to imines. The role of Al, its amount, acidity of the catalyst, effect of reaction conditions (especially presence of solvent and excess air) and amines nature on the oxidative coupling reactions were investigated. Surface OH groups play an effective role in amine activation by hydrogen bonding, whereas the acid sites generated by incorporation of optimum amount of Al further improve the activity. The Al in optimum amount (Si/Al: 20) showed good catalytic activity in self coupling of benzyl amine as compared to MS and Al-MS with high Al loading. A suitable polar solvent and presence of excess air significantly promote the activity of MS and Al-MS for amine coupling reactions. The MS without Al was also found to be efficient catalyst for oxidative self and cross coupling of amines. The presence of Al in Al-MS (in optimum amount, i.e., Si/Al: 20) made the self coupling of benzyl amine faster than MS, whereas the cross coupling reactions with Al-MS were observed to be slightly slower than MS, which is probably due to competitive adsorption of amine substrate and amine nucleophile over acidic sites. The activity of MS and Al-MS catalysts for self and cross coupling reactions was also dependent of the amines nature (basicity and nucleophilicity).