(66b) Two-Phase Synthesis of Mesoporous Silica Nanoparticles with Primary Amine Catalysts

This presentation describes a new type of mesoporous silica nanoparticles (MSNs) that is synthesized in liquid–liquid biphasic systems consisting of tetraethoxysilane (TEOS) and water in the presence of primary amines and cationic surfactants (cetyltrimethylammonium chloride, CTAC) at pH range of 11.1 to 11.5. The obtained MSNs are analyzed by scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), X-ray diffraction (XRD), and nitrogen adsorption-desorption. The results show that the MSNs with an average diameter in the range of 28–54 nm and a size polydispersity below ca. 15% have been achieved. Importantly, each MSN is composed of a number of tiny primary silica nanoparticles (PSNPs) forming 3D connected pore structure. The pore size of the MSNs can be tuned from 2.5 to 3.0 nm by changing the pH of catalyst stock solution, and larger pore sizes (3.1–4.5 nm) can be achieved by using pore swelling agents. Additionally, the pore swelling agent has an effect on the particle size of MSNs. The Brunauer-Emmett-Teller (BET) surface areas and total pore volumes vary from 550 to 750 m² g^-1 and from 1.2 to 1.7 cm³ g^-1, respectively. In comparison with conventional MCM-41-type MSNs, our new MSNs show outstanding colloidal and hydrothermal stabilities. They are colloidally stable at room temperature over one year, and their mesostructure was retained even after hydrothermal treatment at 120 °C for 24 h. Finally, based on the analysis of the morphology and structure of MSNs, a formation scheme based on the cooperative self-assembly of PSNPs and surfactant molecules is proposed.