(219g) Impact of the Addition of Polyethylene Glycol (PEG) on Amine Efficiency and Heat of Sorption in Polyethylenimine (PEI)/Silica Sorbents

In CO₂ capture, solid amine sorbents have lower heat capacity and require less energy for regeneration than aqueous amine solutions. However, due to significant diffusional limitations that increase up during CO₂ sorption due viscosity excursions, the buried amine sites are inaccessible and therefore not utilized, resulting in amine efficiencies significantly lower than theoretical limits. To increases amine efficiency, efforts have been made to decrease diffusional barriers and alter theoretical CO₂: amine from 0.5 to 1. The latter requires a change in mechanism of adsorption, while the former requires a reduction in intermolecular interactions between amines.

We examined the impact of polyethylene glycol (PEG) addition on the performance of PEI sorbents for CO₂ capture, with an emphasis on determining the impact diffusion on measured capacity and heat of sorption values. A combined breakthrough reactor (BTR) with mass spectrometer (MS) and differential scanning calorimeter (DSC) enabled simultaneous measurement of both sorption capacity and heat flow in a packed bed column configuration, simulating a cycle in the pressure/temperature swing adsorption (PTSA) process. We synthesized a series of PEI-PEG/SiO₂ sorbents where the total polymer content is 40 wt.%, and performed breakthrough experiment at five different temperatures. Breakthrough results illustrated the influence of PEG addition on CO₂ diffusion, amine efficiency and the heat of CO₂ binding to confined amines. The CO₂ breakthrough curve becomes less steep with increasing PEG content or at elevated temperatures, indicating a reduced diffusional barrier and CO₂ capability to access buried amine sites. Amine efficiency increases with PEG content, and this increasing effect is much more significant at low temperatures. The sample with the highest PEG content exhibits amine efficiency two times higher than 40 wt% PEI/SiO₂ at 25℃. The heat of sorption data also reveals energetically less favorable interactions at lower PEI content and low temperatures, indicating the possible formation of the less stable carbamic acid and the shift in the CO₂: amine stoichiometric ratio. At high temperatures, the unstable species no longer exists and the heat of sorption is similar amongst samples. This work studied the role of PEG in promoting CO₂ diffusion and possible formation of less stable carbamic acid, which greatly increases the amine efficiency of PEI-impregnated silica. The breakthrough instrument with the simultaneous capability to measure heat flow approximates the industrial PTSA separation process, and the combination of capacity and heat flow data enables more robust spatiotemporal modeling of CO₂ sorption in a packed bed.