Transport, zwitterions, and the role of water for CO₂ adsorption in mesoporous silica-supported amine sorbents

The uptake of CO₂ in highly loaded, silica-supported, polyethylenimine (PEI)-impregnated sorbents was investigated in a reaction-diffusion model of the CO₂ adsorption process. The model successfully replicated the pseudoequilibrium behavior experimentally observed in thermogravimetry (TGA) experiments. A parametric study and sensitivity analysis of the model revealed that the stability and mobility of diffusive intermediates - assumed in the model to be zwitterions - effectively control the observable capacity of the sorbent. A subsequent quantum chemical study called into question the stability of zwitterions in PEI but suggested that physically bonded moieties involving water, amines, and CO₂ may be better candidates for diffusive intermediates. The implications are a strong dependence of the observable CO₂ capacity of the sorbent on the presence of water in the gas stream, which was found to be consistent with TGA results.