Preparation of Pickering emulsion by modification of amine-functionalized graphene oxide surface with organosilane: Efficient catalyst for Knoevenagel condensation of malononitrile with aldehydes at mild temperature
In this study, a series of Pickering emulsions for catalysis of Knoevenagel condensation were prepared by modification of the surface of amine-functionalized graphene oxide (GO-NH2) with trimethoxymethylsilane (MTMS). It was found that the catalytic activity of as-prepared Pickering emulsion system for Knoevenagel condensation was mainly determined by the active sites and properties (including droplets size, density and distribution) of emulsion. The results showed that the mass ratio of 3-aminopropyltrimethoxysilane (APTMS) to MTMS was an important factor affecting catalytic active sites and properties of Pickering emulsion. Under condition of maintaining more catalytic active sites in Pickering emulsion, modification of the surface of amine-functionalized GO with organosilane not only enhanced the adsorption of catalyst to substrate but also improved the properties of Pickering emulsion of W/O , thus leading to an increase in catalytic activity of Picking emulsion. Pickering emulsion prepared by GO-NH2(1.8)-Si(0.2) exhibited the highest catalytic activity for Knoevenagel condensation at room temperature (40 ℃) and its activity was much higher than that prepared by GO-NH2. The Pickering emulsion prepared by GO-NH2(1.8)-Si(0.2) still showed high catalytic activity after six cycles, demonstrating its good reusability.