Preparation of acid-base bifunctional mesoporous KIT-6 (KIT: Korea Advanced Institute of Science and Technology) and its catalytic performance in Knoevenagel reaction

Acid-base bifunctional mesoporous catalysts Al-KIT-6-NH2 containing different aluminum content have been synthesized through post synthetic grafting method. The materials were characterized by X-ray diffraction (XRD), scanning electron micrographs (SEM), transmission electron micrographs (TEM), Fourier-transform infrared spectroscopy (FTIR), IR spectra of pyridine adsorption, NH3-TPD and TG analysis. The characterization results indicated that the pore structure of KIT-6 was well kept after the addition of aluminum and grafting of aminopropyl groups. The acid amount of Al-KIT-6 increased with enhancing aluminum content. Catalytic results showed that weak acid and weak base favor the Knoevenagel reaction, while catalysts with strong acid and weak base exhibited worse catalytic behavior. - Graphical abstract: The postulated steps of mechanism for the acid-base catalyzed process are as follows: (1) the aldehyde gets activated by the surface acidic sites which allow the amine undergoes nucleophilic to attack the carbonyl carbon of benzaldehyde. (2) Water is released in the formation of imine intermediate. (3) The ethyl cyanoacetate reacts with the intermediate. (4) The benzylidene ethyl cyanoacetate is formed and the amine is regenerated. - Highlights: • KIT-6 and Al-KIT-6-NH2 with different Si/Al ratios has been successfully prepared. • 79.4% Yield was obtained over 46-Al-KIT-6-NH2 within 20 min in Knoevenagel reaction. • Low Al-content Al-KIT-6-NH2 shows better catalytic stability than high Al-content catalysts. • There is acid-base synergistic effect in Knoevenagel reaction