Differences in Catalytic Activity of Poly(vinylamine) Introduced on Surface of Mesoporous SBA-15 by Grafting from and Grafting onto Methods in Knoevenagel Condensation

Various amounts of poly(vinylamine) (PVAm) were deposited on the surface of SBA-1S mesoporous silicas, prefunctionalized with suitable organosilanes, by two different grafting techniques: grafting from and grafting onto. In the grafting from route, the radical copolymerization between the vinyl-modified SBA-1S and N-vinylformamide, followed by acid hydrolysis and neutralization, was performed. The grafting onto pathway consisted of the reaction of preprepared PVAm chains with epoxide functionalities present on the silica surface. Diffuse reflectance Fourier transform spectroscopy confirmed the formation of covalent and hydrogen bonds between PVAm and the prefunctionalized SBA-15 surface. Low-temperature N, adsorption and X-ray photoelectron spectroscopy allowed the observation of differences in the distribution of formed PVAm in the pore system of SBA-1S. A higher efficiency of polymer deposition and its higher dispersion were achieved using the grafting from method. Furthermore, X-ray diffraction and transmission electron microscopy confirmed the stability of the hexagonal SBA-1S structure during all modification steps. The number of accessible NH2 surface species, considered as active sites, appeared to be a crucial parameter influencing the catalytic activity of the synthesized hybrid materials in the Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate.