CO ₂ -Triggered Sedimentation in Concentrated Dispersions of Aminated Silica for CO ₂ Capture

This study focused on CO2-triggered phase separation in concentrated dispersions of mono-, di-, and triaminated silica for CO2 capture, based on the hypothesis of reduced regeneration energy coupled with the formation of a CO2-rich, water-lean sediment. The sedimentation of the CO2-rich and CO2-lean dispersions was studied using time-resolved optical transmittance measurements. The CO2 capacity, reaction rate, diffusivity, and solubility of the dispersions were also studied. The involved CO2-amine chemistry was studied by using infrared (IR) and cross-polarization solid-state 13C and 15N NMR spectroscopy, and the fluid behavior of the dispersions was studied by rheology. The aminated silica was of the SBA-15 type and characterized by N2 adsorption and desorption experiments, thermogravimetric analysis, powder X-ray diffraction, scanning and transmission electron microscopy, and crosspolarization solid-state 29Si NMR spectroscopy. The derived energy balances for a simplified process indicated that the regeneration of the CO2-rich sediments results in an energy demand similar to that of 30 wt % ethanolamine (MEA) in water. The bounds of the energy balances were found to be limited by the somewhat low CO2 capacities of the dispersions, which underscores the need for increasing the amino group density of the dispersions in future efforts. The observed changes in transmittance between the CO2-lean and CO2-loaded dispersions showed that sedimentation occurred within the first 10 min for the CO2-loaded dispersions, while the CO2-lean dispersions exhibited no change in transmittance after 60 min. The analysis of the pressure decay curves of the partial CO2 pressure showed that the absorption rates of the dispersions were smaller than those of monoethanolamine (MEA) in water but were similar to the absorption rates of 2-amino-2-methyl-1-propanol (AMP) in water. The IR spectroscopic analysis was consistent with the formation of ammonium carbamates at a low CO2 loading and the subsequent formation of HCO3– at a higher loading. The flow curves displayed rich and complex fluid behavior, which was strongly affected by the capture of CO2 by the dispersions. Phenomena such as shear thinning, jamming, and thixotropy were observed.