Phase Evolution During One-pot Synthesis of Amine Modified Mesoporous Silica Materials: Preparation, Properties, Carbon Dioxide Adsorption

Amino-functionalized mesoporous MCM-41 materials were synthesized by co-condensation of amino-organosiloxane precursors, namely 3-aminopropyltrimethoxysiloxane (APS), 3-(methylamino)propyltrimethoxysiloxane (MAPS) and 3-(phenylamino)propyltrimethoxysiloxane (PAPS) with tetraethyl orthosilicate (TEOS) in water solution containing ammonia and in the presence of surfactant as templating agent. The ratio of amino-organosiloxanes to TEOS in the synthesis mixture was 3, 5, 10, 20 and 30 wt%. The prepared samples were characterized by Small Angle X-ray Scattering (SAXS), High-Resolution Transmission Electron Microscopy (HRTEM), Thermogravimetric Analysis (TGA) and N2 adsorption/desorption. It followed from the measurements that the ordered porous materials of hexagonal symmetry were obtained at small ratios of APS and MAPS to TEOS (3, 5, 10 wt%). At larger ratios (20 and 30 wt%), disordered or non-porous amorphous amino-functionalized silica particles were obtained. For the PAPS precursor, hexagonally-packed structures were observed up to a PAPS to TEOS ratio of 30 wt%, when a transition to a lamellar phase occurs. The reasons of the structural changes are discussed in terms of modifications in the effective shapes of the cylindrical micelle aggregates, caused by different hydrophilicity/hydrophobicity of the used amine precursors and their interactions with surfactant micelles. To better understand the origin of this behavior, lattice Monte Carlo simulations of simple coarse-grained surfactant solutions were performed. All prepared samples were used to study CO2 adsorption. It was found that CO2 adsorption capacity in the samples increased with the increasing concentration of amino-organosiloxane precursor, independently of the surface area. These results imply that CO2 adsorption primarily takes place on amine active centers and physical adsorption, which is dependent on the high surface area, plays a less important role in CO2 adsorption on these materials.