Preparation of a Pickering emulsion by modification of an amine-functionalized graphene oxide surface with organosilane: efficient catalyst for the Knoevenagel condensation of malononitrile with aldehydes at mild temperature
In this study, a series of Pickering emulsions for catalysis of Knoevenagel condensations of malononitrile with aldehydes were prepared by surface modification of amine-functionalized graphene oxide (GO-NH2) with trimethoxymethylsilane (MTMS). The catalytic activity of the prepared Pickering emulsion system in a Knoevenagel condensation was mainly determined by the active sites and properties (namely droplet size, density, and distribution) of the emulsion. The results show that the mass ratio of 3-aminopropyltrimethoxysilane to MTMS affects the active catalytic sites and properties of the Pickering emulsion. Under conditions that provided a large number of active catalytic sites in the Pickering emulsion, modification of the surface of amine-functionalized GO with an organosilane enhanced substrate adsorption on the catalyst and improved the properties of water-in-oil Pickering emulsions. This increased the Picking emulsion catalytic activity. The Pickering emulsion prepared with GO-NH2 : Si = 1.8 : 0.2 gave the highest catalytic activity in a Knoevenagel condensation at 40 °C, and its activity was much higher than that of GO-NH2. The Pickering emulsion with GO-NH2(1.8)–Si(0.2) still showed high catalytic activity after six cycles and therefore has good reusability.