Control of availability of poly(vinylamine)-derived basic sites, catalytically active in Knoevenagel condensation, by deposition on various mesoporous silicas

Three mesoporous silicas (SBA-16, SBA-15 and MCF) with different pore sizes and architectures were used as supports for deposition of poly(vinylamine) (PVAm) by the "grafting from" technique. To attach chemically polymer to the silica surface, the support was initially functionalized with vinyltriethoxysilane (VTS). Subsequently, vinyl functionalities were copolymerized with N-vinylformamide. The obtained poly(N-vinylformamide) (PNVF) supported on the silica surface was hydrolyzed under acid conditions followed by neutralization at pH = 9.0 to form PVAm. It was found that porosity of the used silicas strongly influenced the content and distribution of introduced PVAm as was shown by XRD, TEM, low-temperature adsorption of nitrogen, TG and elemental analysis. The highest amounts of polymer were deposited on the foam-like MCF support characterized by the presence of the widest mesopores. The decrease in the pore size resulted in difficulties in the homogeneous dispersion of PVAm, which partially blocked the mesochannels. The XPS and DRIFT measurements revealed that three various types of N-containing species (primary amino groups, residual formamide groups and protonated amino moieties) were identified on the surface of the PVAm/SiO2 hybrid materials. Nevertheless, the content and accessibility of NH2 functionalities affected the catalytic performance of the synthesized materials in the Knoevenagel condensation. The highest catalytic activity showed the samples based on MCF, which possessed the highest dispersion of PVAm active phase. (C) 2016 Elsevier Inc. All rights reserved.