Amino-functionalized mesostructured cellular foam silica: a highly efficient and recyclable catalyst in the Knoevenagel condensation reaction

Abstract

Mesostructured cellular foam silica was functionalized by an amino group (MCF-NH$_{2})$ using the postsynthesis grafting method and utilized as a recyclable catalyst for the Knoevenagel reaction to afford $\alpha $, $\beta $-unsaturated compounds. The catalyst was systematically characterized by various analyses such as nitrogen adsorption, X-ray diffraction, Fourier transform infrared spectrophotometry, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, and Hammett indicator method. The results showed that MCF-NH$_{2}$ catalyst was successfully prepared with a high surface area of 215 m$^{2}$ g$^{-1}$ and a large pore volume of 1.37 cm$^{3}$ g$^{-1}$. Compared with the nonfunctionalized MCF sample, the window and cell sizes of MCF-NH$_{2}$ decreased after amino group modification, whose NH$_{2}$-loading density was 1.56 mmol g$^{-1}$. The optimal condition was investigated and the highest catalytic activity was obtained in the presence of 0.1 g of MCF-NH$_{2}$ catalyst under ultrasound irradiation at room temperature in the mixed solvent of ethanol and water with a volume ratio of 3:1. Various aromatic aldehydes and active methylene compounds were converted to their desired products with excellent yields within a short time. Moreover, MCF-NH$_{2}$ catalyst showed high recyclability, being used four times without any significant decrease in activity.