Mn/Al2O3 and Mn/ZrO2 as selective catalysts for the synthesis of bis(indolyl)methanes: The role of surface acidity and particle morphology

Mn/Al2O3 and Mn/ZrO2 were prepared by precipitation method. Their catalytic activity was investigated in the synthesis of biologically important bis(indolyl)methanes via, a condensation reaction between an indole and different aldehydes. The materials exhibited excellent catalytic activity (TOF in the range 180–380 h−1) and produced the expected product in good yield and selectivity. Isolation of product from the reaction mixture was simple and neat. The catalysts were found to be recyclable and used up to five cycles without any loss in their catalytic activity. Mn/ZrO2 was also found to be a better catalyst than Mn/Al2O3 in terms of isolated yield of the product and recyclability. Mn/ZrO2 catalyst was found to be selective for aldehydes and not for ketones. In order to investigate the relationship between the catalytic activity and physico-chemical properties, Mn/Al2O3 and Mn/ZrO2 were characterized for their surface and bulk properties by PXRD, BET, TPD-NH3 and SEM techniques. A good correlation between the catalytic activity of the materials and the concentration of the acid sites with intermediate strength was observed. Mn/ZrO2 exhibited rod like structure. This material was found to be a general catalyst for the synthesis of a number of bis(indolyl)methane derivatives with good yield and selectivity.

Graphical abstractThe protocol is highly chemoselective for aldehydes in the synthesis of bis(indolyl)methanes. The presence of acid sites with intermediate acid strength and morphology plays a key role. The methodology is simple and greener.Figure optionsDownload full-size imageDownload as PowerPoint slide