Template-free amine-bridged silsesquioxane with dangling amino groups and its CO2 adsorption performance

Abstract

In this study, amine-bridged silsesquioxane (ABS) and ABS with dangling amino groups were synthesized by a facile method, without using any structure directing agents, and tested for CO₂ adsorption. The covalent attachment of dangling aminopropyl groups into ABS improves CO₂ adsorption significantly; CO₂ loading at 30 °C increases from 83.7 to 129.6 mg g⁻¹ by adding 3-aminopropyltriethoxysilane (APTES) to bis[3-(trimethoxysilyl)propyl]amine (BTMSPA) precursor at a molar ratio of APTES/BTMSPA = 0.5. The properties of CO₂ adsorbent can be controlled by the solvent water content and aging conditions. The water content of 25 wt% produces optimal solvent conditions for a resulting adsorbent with the highest surface area (333 m² g⁻¹), pore volume (0.3 cm³ g⁻¹), and CO₂ loading (132 mg g⁻¹ at 30 °C) with an aging time is 48 h in the same solvent at 60 °C. Aging in water has more effect on structural transition than ethanol; adsorbent possesses the highest pore volume (0.85 cm³ g⁻¹) and largest pore size (15.5 nm) when it was aged in water but it has the largest surface area (447 m² g⁻¹) when it was aged in ethanol. Aging in ethanol results in the best sorbent with the high CO₂ loading and stable CO₂ adsorption activity. An adsorbent aged for 144 h in ethanol shows excellent CO₂ adsorption performance in a packed bed reactor; it was relatively stable after 50 adsorption/desorption cycles with a CO₂ loading capacity of 144.6 mg g⁻¹. The success of this research offers a novel route for synthesize of ABS with minimized waste, while maintaining the quality of produced adsorbent.